Abstract:
A generating system having a rotating type generator to interconnect a power system in order to output power of the generator to the power system comprises a generator, an exciting circuit and an excitation control section. It also comprises short-cycle stabilizing sections for outputting a short-cycle stabilizing signal for suppressing short-cycle power fluctuations according to at least one of an electric parameter and a mechanical parameter of the generator and a long-cycle stabilizing section for outputting a long-cycle stabilizing signal for suppressing long-cycle power fluctuations having a cycle time longer than short-cycle power fluctuations according to the mechanical parameter of the generator. The outputs of the short-cycle stabilizing sections and the long-cycle stabilizing section are sent to the excitation control section.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of Application No. PCT/JP99/00087, filed Jan. 13, 1999. 
    
    
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 10-005043, filed Jan. 13, 1998, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to a generating system interconnected to a power system. More particularly, it relates to a stabilizing apparatus to be incorporated into the magnetic excitation control system of a rotating type generator such as an alternator and designed to attenuate power fluctuations and enhance the stability of the power system. 
     Magnetic excitation systems for exciting field circuits of generators such as alternators that are rotating type generators can be generally and roughly classified into AC excitation systems, DC excitation systems and static excitation systems. The AC excitation system uses an AC exciter. The DC excitation system uses a DC exciter. The static excitation system uses a semiconductor switching element such as a thyristor. 
     A thyristor excitation system, which is a typical static excitation system being popularly used at present as excitation system, will be described below. Also, a power stabilizing system (PSS) adapted to be used in a thyristor excitation system will be explained. 
     FIG. 1 is a block diagram of an excitation system using a conventional PSS that can effectively attenuate power fluctuations of generator mode (power fluctuations of a short cycle of about 1 to 2 seconds). 
     As shown in FIG. 1, the generator excitation control system receives an input an AVR reference voltage  2  (hereinafter referred to as “ 90 OR”) and the output of transformer  3  (hereinafter referred to as “PT”) operating an instrument to an automatic voltage regulator  4  (hereinafter referred to as “AVR”) in order to maintain at a constant value the terminal voltage of generator  1  connected to a turbine T. The AVR reference voltage  2  serves to establish a generator voltage. The potential transformer  3  detects the generator voltage. The AVR  4  operates to control the generator voltage. 
     PSS  5  is provided to produce the generator  1  operate on a stable basis. The output signal of the PSS is input to the AVR  4  and used in the operation of controlling the generator voltage. The field voltage of the generator  1  is thereby regulated to control the transient active power of the generator  1  in order to suppress power fluctuations. 
     An excitation transformer  6  is arranged to get an excitation source out of the voltage of the generator  1 . The output voltage of the excitation transformer  6  is input to a thyristor bridge  7 . The field voltage of the generator  1  is modified to regulate the generator voltage according to the value set by said  90  R  2  by controlling the ignition angle of the thyrister bridge  7 . 
     The PSS  5 , which is currently commercially available, detects the active power P 8  of the generator  1  from the generator voltage detected by the PT  3  and the generator current detected by the CT. The PSS  5  then detects and calculates a change ΔP in the active power P 8 , a change Δω in the rotational speed ω 9  of the rotor of the generator  1 , or a change Δf in the generator voltage frequency corresponding to the change in the system side frequency (not shown). The PSS may use one of these signals or two or more of the signals (hereinafter referred to as “multivariable PSS”). 
     Of multi-variable PSSs, those of the type that use the change ΔP in the active power of the generator  1  as input and have an appropriate stabilization function (hereinafter referred to as “ΔP-PSS”) are most widely used at present. 
     The reason for this is that the change in the active power of the generator can be electrically detected and a stabilization function can be set into the PSS with ease because the PSS does not require phase compensation as much as a PSS (hereinafter referred to as “Δω-PSS”) that uses the change Δω in the rotational speed ω  9  of the rotor of the generator  1  as input, although the latter also has an appropriate stabilization function. 
     The multi-variable PSS  5  shown in FIG. 1 is a typical PSS adapted to cover a broader frequency band subject to power fluctuations than a ΔP-PSS and a Δω-PSS as it comprises both a ΔP-PSS and a Δω-PSS that can effectively suppress power fluctuations. This is why such a multi-variable PSS (hereinafter referred to as “(ΔP+Δω)-PSS” is used for a thyristor excitation system. 
     There are PSSs of other types that may also be used for thyrsitor excitation systems, including one (hereinafter referred to as “Δf-PSS”) that uses a frequency signal representing either the voltage or the current of the generator  1  as input and also has an appropriate stabilization function) and one (hereinafter referred to as “(ΔP+Δω)-PSS”) that comprises both a ΔP-PSS and a Δω-PSS. 
     Various PSSs as described above may also be used for AC/DC excitation systems. 
     The excitation system further comprises an excessive-excitation limiting device for preventing excessive excitation of the generator  1 , an inadequate-excitation limiting device for limiting inadequate excitation of the generator  1 , a V/F controlling device for excessive excitation of the exciting transformer  6  or the armature winding of the generator  1 , and the like, where V represents the generator voltage and F represents the generator frequency.) However, these devices do not exert any direct influence on the operation of the PSS  5  and, therefore, only the AVR  4  and the PSS  5  are discussed here in detail. 
     Both analog hardware and digital hardware are commercially available. The AVR  4  and the multi-variable PSS  5  are applicable to hardware of either type in functional terms. 
     While various types of excitation systems are available as pointed out above, the one shown in FIG. 1 is of the type that is mainly used at present. Therefore, the prior art technologies will be discussed below by way of this excitation system. 
     FIG. 2 is a block diagram of a conventional AVR  4 , illustrating its configuration. Referring to FIG. 2, PSS output signal  5 A of multi-variable PSS  5  is input to the AVR  4 . Adder A 1  adds the PSS output signal  5 A to the outcome of the computation for determining the deviation of the generator voltage Vg 3 A as detected by PT 3  from the  90 R  2 . The signal ΔV 70  obtained as a result of the addition is input to a voltage control section  11  operating on the basis of a gain and an advance/delay to be used to stabilize the voltage control loop. 
     The output of the voltage control section  11  is equivalent to the field voltage Efd  12  of the generator  1 . 
     FIG. 3 is a schematic block diagram of a known multi-variable PSS  5 . As shown in FIG. 3, the change −ΔP in the active power is made to pass through a stabilization function Gp(S)  13 , while the change Aco  9 A in the rotational speed ω  9  of the generator  1  is made to pass through a stabilization function Gw(S)  14  before they are added by adder A 2 . The sum of the addition is input from an output limiter  15  to the AVR  4  as PSS output signal  5 A. The stabilization functions Gp(S)  13  and G(w)  14  can remove noise from the input signal by passing the latter through a reset filter  16 , an advance/delay circuit  17  and a limiter  18 , as shown in FIG.  4 . 
     Due to the above described functional features, the multi-variable PSS  5  can eliminate any steady-state deviations for AVR control that arises when no power fluctuation occurs and correct the phase to output an appropriate voltage regulating signal. 
     Meanwhile, in recent years, the stability of power system is threatened than ever as the power system increases in scale. As a result, there occur not only local fluctuations that have been a main problem and are short-cycle fluctuations lasting for about 1 second but also inter-system fluctuations that are long-cycle fluctuations lasting for about 2 to 3 seconds. 
     The ΔP-PSS that is used in many generators in service at present effectively suppresses the local fluctuation. 
     The (ΔP+Δω)-PSS is also used in many generators to suppress the long-cycle power fluctuation. It is reported that the (ΔP+Δω)-PSS effectively raises the level of power that can be supplied on a stable basis. (See “Development of Pulse PSS for Suppressing Power Fluctuations in a Broad Area,” Meeting of Power and Energy Department, Society of Electricity, 1996, “Development of Multi-PSS for Suppressing Long-Cycle Fluctuation in Interconnected Systems,” Theory of Electricity B, Voltage. 115-B, No. 1, 1995.) As the amount of interchange power increases among power companies, the cycle period of long-cycle power fluctuations increases and it has become difficult for the existing PSS to suppress long-cycle power fluctuations. 
     Efforts are being made to interchange power among power companies to an enhanced extent in order to increase the efficiency of operation of the power plants of power companies and run the associated systems more flexibly. It is planned to interchange more power among power companies in the future. Further, as the sales of power to remote customers by IPPs (Independent Power Plants) rises along with the self delivery of power from house generators, power will be supplied over long distances on a huge scale. 
     Let us imagine, for example, power systems  68 A,  68 B and  69 , each comprising a plurality of generators G and a load, may be interconnected by power transmission lines  60 A and  60 B. Then, power may be supplied from the power system  68 A to the power system  68 B over a long distance through the lines  60 A and  60 B. 
     With such an arrangement, more power will be expectedly supplied from the power system  68 A to the power system  68 B than ever within a period of several years from now. 
     When this expected increase in the amount of power interchanged among power companies is taken into consideration, it seems difficult for the (ΔP+Δω)-PSS to maintain the stability of the power systems when large scale power fluctuations result from a severe accident such as three-phase earth fault induced by thunderbolt. Then, it will not possible for the (ΔP+Δω)-PSS to control power fluctuations of generator mode developing in the power systems  68 A,  68 B and  69 . Nor will it be possible for the (ΔP+Δω)-PSS to control power fluctuations of system mode developing between the power systems  68 A and  68 B. In other words, the limit to interchange power is defined by the limit to which the stability of the power systems can be maintained. 
     FIG. 6 is a graph illustrating the outcome of a stability simulation conducted on the assumption that an accident occurred as a result of a three-phase earth fault of a power system providing service over a broad area, involving long-distance power transmission. The simulation shows the waveform of fluctuating power that appears after the accident if a known PSS is used. The cycle of fluctuation of power caused by the accident is about 5.5 seconds. The power fluctuation remains even 40 seconds after the accident to prove it is almost getting to the stability limit. If the level of interchange power is raised under this condition, the power systems will no longer be able to secure its stability. 
     As pointed out above, it is known that fluctuating power occurs in generator mode and also system mode. More specifically, fluctuating power can appear in generator mode among the generators of a same power company with a cycle of about 1 second and also in system mode among the generators of different power companies with a long cycle (of about 2 to 10 seconds). Thus, it is necessary to develop a new large PSS that can effectively suppress power fluctuations in both modes. 
     While the ΔP-PSS that uses a change ΔP in the active power of a generator as a stabilizing signal is incorporated in many plants at present, it is theoretically adapted to suppress power fluctuations that last for about 1 second or less (between 0.5 seconds and 1 second). 
     However, it can hardly suppress long-lasting system-mode power fluctuations that continue for about 2 to 10 seconds. 
     On the other hand, the Δω-PSS that uses a change Δω in the rotational speed of the rotor of the generator  1  as a stabilizing signal can effectively suppress long-lasting system-mode power fluctuations that continue for about 2 seconds. 
     The Δf-PSS that uses a change Δf in the frequency as a stabilizing signal tends to operate almost in the same way as the Δω-PSS. 
     At present, a combination of the ΔP-PSS and the Δω-PSS, i.e., the (ΔP+Δω)-PSS, is employed for the purpose of suppressing power fluctuations that last for about 0.5 seconds to about 2 seconds. Actually, this system works effectively. 
     However, as more power is interchanged among power companies, long-lasting power fluctuations that continue for about 2 seconds or more occur more frequently and the period of power fluctuations becomes longer as a function of the amount of interchange power. The (ΔP+Δω)-PSS can suppress power fluctuations lasting for about 2 second or more only with low efficiency. 
     The exciting systems for generators that operate together with power systems, are roughly classified into two types, i.e., static excitation system and rotary exciting system. The thyristor exciting system is a typical static exciting system, whereas AC exciter is a typically rotary exciting system. 
     The object of the present invention is to provide a PSS that can quickly suppress power fluctuations that may usually occur over a broad cycle zone, ranging from fluctuations of generator mode (having a short cycle of about 0.5 seconds) to fluctuations of system mode (having a long cycle of about 10 seconds), in order to stabilize power systems and secure power interchange over a broad area on a stable basis and is applicable to both a static exciting system and a rotary exciting system, without adversely affecting the shaft-twisting vibration of the turbines or generators. 
     BRIEF SUMMARY OF THE INVENTION 
     The above object of the present invention is achieved by providing a generating system having a rotating type generator to interconnect a power system in order to output power of the generator to the power system, the generating system comprising: 
     an exciting circuit for exciting the field circuit of the generator; 
     an excitation control section for controlling the excitation of the exciting circuit in order to regulate the output of the generator; 
     a short-cycle stabilizing section for outputting a short-cycle stabilizing signal for suppressing short-cycle power fluctuations in accordance with at least one of an electric parameter and a mechanical parameter of the generator; 
     a long-cycle stabilizing section for outputting a long-cycle stabilizing signal for suppressing long-cycle cycle power fluctuations having a cycle time longer than short-cycle power fluctuations in accordance with the mechanical parameter of the generator; and 
     an output section for outputting the output of the short-cycle stabilizing section and that of the long-cycle cycle stabilizing section to the excitation control section. 
     In another aspect of the invention, there is also provided an apparatus for stabilizing a power system to be incorporated into the magnetic excitation control system of a rotating type generator in order to quickly attenuate power fluctuations and enhance the stability of the power system, the apparatus comprising: 
     a short-cycle stabilizing section for computationally determining a short-cycle stabilizing signal for suppressing short-cycle power fluctuations in accordance with at least one of an electric parameter and a mechanical parameter of the generator; 
     a long-cycle stabilizing section for computationally tationally determining a long-cycle stabilizing signal for suppressing long-cycle power fluctuations having a cycle time longer than short-cycle power fluctuations according to the mechanical parameter of the generator; and 
     an adding section for applying the output of the short-cycle stabilizing section and that of the long-cycle stabilizing section to the magnetic excitation control system. 
     While a rotating type generator according to the invention may typically be an alternator that can normally be used for a hydraulic power system, a thermal power system or an atomic power system, it can also be used for a generator-motor or an induction generator having distributed winding that is designed to be applicable to a pumping-up power system. 
     The electric parameter of the generator to be used for generating a short-cycle stabilizing signal and/or a long-cycle stabilizing signal may be an active power signal of the generator, a voltage signal of the generator or a signal equivalent to it, a current signal of the generator or a signal equivalent to it, a voltage frequency signal of the generator or a signal equivalent to it or a current frequency signal of the generator or a signal equivalent to it. 
     The mechanical parameter of the generator may be a rotational speed signal of the rotor of the generator or a signal equivalent to it, a phase angle signal of the rotor of the generator or a signal equivalent to it, an guide vane opening signal of the water wheel linked to the generator or a valve opening signal of the turbine coupled to the generator. 
     Thus, according to the invention, it is now possible to secure power interchange over a broad area on a stable basis by quickly suppressing power fluctuations that may usually occur over a broad cycle zone, ranging from short-cycle fluctuations (of generator mode) to long-cycle fluctuations (of system mode), thereby stabilizing power systems. 
     In still another aspect of the invention, there is also provided a generating system having a rotating type generator to interconnect a power system in order to output power of the generator to the power system, the generating system comprising: 
     an exciting circuit for exciting the field circuit of the generator; 
     an excitation control section for controlling the excitation of the exciting circuit in order to regulate the output of the generator; 
     a long-cycle stabilizing section for outputting a long-cycle stabilizing signal for suppressing long-cycle power fluctuations having a cycle time longer than short-cycle power fluctuations in accordance with a mechanical parameter of the generator; and 
     an output section for outputting the output of the long-cycle stabilizing section to the excitation control section. 
     In still another aspect of the invention, there is also provided an apparatus for stabilizing a power system to be incorporated into the magnetic excitation control system of a rotating type generator in order to quickly attenuate power fluctuations and enhance the stability of the power system, the apparatus comprising: 
     a long-cycle stabilizing section for computationally determining a long-cycle stabilizing signal for suppressing long-cycle power fluctuations according to the mechanical parameter of the generator. 
     Thus, according to the invention, it is now possible to secure power interchange on a stable basis by quickly suppressing long-cycle power fluctuations (of system mode), thereby stabilizing power systems. Differently stated, the generator mode practically does not give rise to any problem when there is no adjacently located generator or when a plurality of generators transmit power to a remote load by way of a system impedance so that it is only necessary to suppress power fluctuations of system mode. Then, the above described arrangement of PSS can effectively suppress power fluctuations of system mode as it comprises a long-cycle stabilizing section for computationally determining a long-cycle stabilizing signal. 
     While a rotating type generator according to the invention may typically be an alternator that can normally be used for a hydraulic power system, a thermal power system or an atomic power system, it can also be used for a generator-motor or an induction generator having distributed winding that is designed to be applicable to a pumping-up power system. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a block diagram illustrating the configuration of an exciting system using a conventional PSS that can effectively suppress power fluctuations of generator mode; 
     FIG. 2 is a block diagram illustrating the configuration of a conventional AVR  4 ; 
     FIG. 3 is a block diagram illustrating the configuration of a conventional multivariable PSS  5 ; 
     FIG. 4 is a block diagram illustrating the stabilization functions Gp(S)  13  and G(w)  14  provided in the multivariable PSS of FIG. 3; 
     FIG. 5 is a diagram illustrating a long-distance power transmission system that comprises a plurality of generators and a plurality of loads; 
     FIG. 6 is a graph illustrating the outcome of a stability simulation conducted on the assumption that an accident occurred as a result of a three-phase earth fault of a power system providing service over a broad area, involving long-distance power transmission; 
     FIG. 7 is a graph showing the outcome of a stability simulation conducted only by using a conventional PSS; 
     FIG. 8 is a block diagram illustrating the configuration of an exciting system using a PSS according to this invention; 
     FIG. 9 is a block diagram illustrating the configuration of a multiple PSS  5 ′, which is the first embodiment of the invention; 
     FIG. 10 is a graph illustrating the outcome of a stability simulation conducted on a long-distance, broad-area power transmission by the use of the multiple PSS  5 ′ of the first embodiment; 
     FIG. 11 is a graph illustrating the outcome of a stability simulation conducted by operating the multiple PSS  5 ′ according to the first embodiment in an exciter system; 
     FIG. 12 is a block diagram illustrating principal components of a multiple PSS  5 ′, which is the second embodiment of this invention; 
     FIG. 13 is a block diagram illustrating principal components of a multiple PSS  5 ′, which is the third embodiment of the present invention; 
     FIG. 14 is a block diagram illustrating principal components of a multiple PSS  5 ′, which is the fourth embodiment of the invention; 
     FIG. 15 is a block diagram illustrating principal components of a multiple PSS  5 ′, which is the fifth embodiment of the present invention; and 
     FIG. 16 is a block diagram illustrating principal components of a multiple PSS  5 ′, which is the sixth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the present invention will be described below, by referring to the accompanying drawing. 
     (First Embodiment) 
     FIG. 8 is a block diagram illustrating the configuration of an exciting system using a PSS according to this invention. The components identical to those shown in FIG. 1 are denoted respective by the same reference numerals and will not be described any further. Only the components different from those shown in FIG. 1 will be described below. 
     As shown in FIG. 8, the exciting system of this embodiment comprises a multiple PSS  5 ′, instead of the multi-variable PSS  5  shown in FIG.  1 . 
     FIG. 9 is a block diagram illustrating the configuration of a multiple PSS  5 ′ according to the present embodiment. The components identical to those shown in FIG. 3 are denoted respectively by the same reference numerals. 
     As illustrated in FIG. 9, the multiple PSS  5 ′ comprises a ΔP-PSS, a Δω-PSS, a Δδ-PSS, and an adder A 3 . The ΔP-PSS is a conventional PSS adapted to receive, as input, the change −ΔP in the active power P 8  of the generator  1  and having appropriate stabilization function Gp(S)  13  in order to suppress power fluctuations of generator mode showing a short cycle. The Δω-PSS is also a conventional PSS adapted to receive, as input, the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  and having appropriate stabilization function Gw(S)  14  in order to suppress power fluctuations of generator mode showing a short cycle. The Δδ-PSS is a PSS adapted to receive, as input, the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  and has stabilization function Gδ(S)  10  for compensating the phase advance/delay in order to put the phase of the phase angle signal of the rotor of the generator  1  in phase with the input signal. The adder A 3  is adapted to add the output signal S 1  of the ΔP-PSS, the output signal S 2  of the Δω-PSS and the output signal S of the Δδ-PSS. The multiple PSS  5 ′ is adapted to input its output signal S 5  produced from the adder A 1  to the AVR  4 . 
     The ΔP-PSS having the stabilization function Gp(S)  13  and the Δω-PSS having the stabilization function Gw(S)  14  are combined into a (ΔP+Δω)-PSS, which operates as section for calculating a short-cycle stabilizing signal. The Δε-PSS having the stabilization function Gδ(S)  10  operates as section for calculating a long-cycle stabilization signal. 
     While FIG. 9 does not show the limiters provided for limiting the effective range of controlling the generator voltage, they may be incorporated in the ΔP-PSS having the stabilization function Gp(S)  13 , the Δω-PSS having the stabilization function Gw(S)  14 , and the Δδ-PSS having the stabilization function Gδ(S)  10 , respectively. Alternatively, a single limiter may be provided to limit the output signal S 5  of the multiple PSS. 
     The stabilization functions Gp(S)  13 , Gw(S)  14  and Gδ(S)  10  of the three PSSs mentioned above same as those illustrated in FIG.  4 . The formulas (1), (2) and (3) shown below are examples that can be used respectively for these stabilization functions.              Gp   =         Kp   ·     Tp1S        (     1   +   Tp2S     )              (     1   +   Tp3S     )           (     1   +   Tp1S     )          (     1   +   Tp4S     )          (     1   +   Tp5S     )                 (     formula                 1     )               Gw   =         Kw   ·     Tw1S        (     1   +   Tw2S     )              (     1   +   Tw3S     )          (     1   +   Tw4S     )           (     1   +   Tw1S     )          (     1   +   Tw5S     )          (     1   +   Tw6S     )          (     1   +   Tw7S     )                 (     formula                 2     )                 G                 δ     =       K                   δ   ·   T                   δ                 1        S        (     1   +     T                 δ                 2      S       )            (     1   +     T                 δ                 3      S       )          (     1   +     T                 δ                 4      S       )           (     1   +     T                 δ                 1      S       )          (     1   +     T                 δ                 5      S       )          (     1   +     T                 δ                 6      S       )          (     1   +     T                 δ                 7      S       )                 (     formula                 3     )                                
     This embodiment of multiple PSS  5 ′ according to the present embodiment and having the above described configuration operates in a manner as described below. Note, however, that the description of the components same as those illustrated in FIGS. 1 and 2 is omitted and the operation of the components other than those shown in FIGS. 1 and 2 will be described below. 
     In the multiple PSS  5 ′, the change −ΔP in the active power P 8  of the generator  1  is supplied to the adder A 3  by way of the stabilization function Gp(S)  13 , as shown in FIG. 3, while the change Δω  9 A in the rotational speed ω  9  of the generator  1  is also supplied to the adder A 3  by way of the stabilization functions Gw(S)  14  and Gδ(S)  10 . The adder A 3  then adds these changes to generate PSS output signal  5 A. The PSS output signal  5 A is input to the AVR  4 . 
     With this arrangement, the (ΔP+Δω)-PSS that is a conventional PSS realized by combining the ΔP-PSS and Δω-PSS having stabilization functions Gp(S)  13  and Gw(S)  14 , respectively are adapted to suppress power fluctuations of adjacent generator mode and power fluctuations of generator mode that can occur a cross compound generating system or a generating system having low-voltage synchronous generators connected directly to each other with a short-cycle of 2 Hz, lasting only for 0.5 seconds. On the other hand, the Δδ-PSS that is a parallel type PSS having the stabilization function Gδ(S)  10  is adapted to suppress power fluctuations of system-mode. With this sharing arrangement, it is possible to quickly suppress power fluctuations occurring in operating power systems over a broad cycle zone ranging from fluctuations of generator mode to fluctuations of system mode in order to stabilize power systems and secure power interchange over a large area on a stable basis. 
     FIG. 10 is a graph obtained as a result of a stability simulation of a long-distance broad-area power transmission system, comprising the embodiment of multiple PSS  5 ′ according to the invention as shown in FIG.  9 . The PSS  5 ′ was operated in the same conditions as shown in FIG. 6 that summarily shows the outcome of the simulation conducted by the using a conventional PSS. In FIG. 10, time (in seconds) is plotted on the abscissa, and phase angle δ (in decrees) is plotted on the ordinate. 
     The multiple PSS  5 ′ used in the simulation, a result of which is shown in FIG. 10, has the following constants:                  Δ                 P     -     PSS                 corresponding                 to                   (     Equation                 1     )         =       0.8   ×   5      S       1   +     5      S                         Δ                 ω     -     PSS                 corresponding                 to                   (     Equation                 2     )         =       15   ×   10      S       1   +     10      S                         Δ                 δ     -     PSS                 corresponding                 to                   (     Equation                 3     )         =       100        (     20      S     )          (     1   +     3      S       )                 (     1   +     10      S       )          (     1   +     20      S       )                 (     1   +     0.02      S       )                                          
     These constants are changed if they differ from the constants, or conditions, selected for the generator  1  and AVR  4  used in the above-mentioned simulation. 
     Assume that the output capacity of all the generators used in the simulation is 100%. Then, the ratio of the generator  1 , for which the multiple PSS  5 ′ is sued, is 9.4%. It will be appreciated that the stability is improved as the ratio by which the PSS of FIG. 9 is used rises. However, as seen from FIG. 10, the stability is sufficiently high to make the operation of the system practically free from problems even if the PSS is used by 9.4% the output capacity of the system. 
     The multiple PSS  5 ′ shown in FIG. 9 performs well against various power fluctuations that can occur while the generator is operating, ranging from power fluctuations of system-mode resulting from disturbances such as a system failure as shown in FIG. 10 to power fluctuations of generator mode resulting from small disturbances such as a change in the load (not shown). 
     For example, the multiple PSS  5 ′ according to the present embodiment may be operated as follows in an exciter system:            Δ                 P     -     PSS                 corresponding                 to                   (     Equation                 1     )         =       0.3   ×   5        S        (     1   +     0.1      S       )            (     1   +     0.5      S       )           (     1   +     5      S       )          (     1   +     0.02      S       )          (     1   +     0.02      S       )                     Δ                 ω     -     PSS                 corresponding                 to                   (     Equation                 2     )         =       8   ×   10        S        (     1   +     0.4      S       )            (     1   +     0.06      S       )           (     1   +     10      S       )          (     1   +     0.02      S       )          (     1   +     0.02      S       )                     Δ                 δ     -     PSS                 corresponding                 to                   (     Equation                 3     )         =       80        (     20      S     )          (     1   +     0.7      S       )          (     1   +     0.7      S       )           (     1   +     10      S       )          (     1   +     20      S       )          (     1   +     0.02      S       )          (     1   +     0.02      S       )                                
     FIG. 11 is a graph obtained as a result of a stability simulation of the operation of the multiple PSS  5 ′ of this embodiment in an exciter system. FIG. 7 is a graph obtained as a result of a stability simulation of the operation of only a conventional PSS. As shown in FIG. 11, the phase-angle fluctuations, i.e., the power fluctuations caused by a system failure is suppressed in about 3 seconds if the embodiment of multiple PSS  5 ′ is used. 
     By contrast, the phase angle of the generator increases with time to make the power system unstable as shown in FIG. 7 if a conventional PSS is used. 
     As can be seen clearly from the graphs, the embodiment of multiple PSS  5 ′ according to the present embodiment can remarkably enhance the stability of a power system if it comprises both a thyristor exciting system and an exciter system. 
     (Second Embodiment) 
     FIG. 12 is a block diagram of a principal portion of the second embodiment of multiple PSS  5 ′ according to the invention. The components that are identical to those shown in FIG. 9 are denoted respectively by the same reference numerals and will not explained be any further. Thus, only the components different from those shown in FIG. 9 will be described below. 
     As shown in FIG.  12  and if compared with the PSS  5 ′ illustrated in FIG. 9, the multiple PSS  5 ′ of this embodiment of the invention has two additional components including a power-fluctuation frequency detecting section  51  and a constant selecting section  53 . 
     The power-fluctuation frequency detecting section  51  detects the frequency of power fluctuations from the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1 . 
     The constant selecting section  53  selects control constants out of a number of constants stored in advance by taking various system conditions into consideration according to the signal of the frequency detected by the power-fluctuation frequency detecting section  51  or a signal equivalent to it. 
     More specifically, there is provided a table prepared in advance and including stabilization constants Kδ, Tδ 1 , Tδ 2 , Tδ 3 , Tδ 4 , Tδ 5 , Tδ 6  and Tδ 7  selected for (formula 3) above for the parallel PSS on the basis of various possible system conditions, on the one hand, and frequencies of power fluctuations corresponding to these stabilization constants, on the other hand. The constant selecting section  53  automatically selects the stabilization constants that are closely related to the detected frequency of power fluctuations out of this table. Thus, the power-fluctuation frequency detecting section  51  and the constant selecting section  53  are made to have a control-constant regulating feature. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the first embodiment will be discussed here. Otherwise, the description of the operation of the first embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration in response to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS can suppress the power fluctuations for which the stabilization function is responsible. 
     (Third Embodiment) 
     FIG. 13 is a block diagram of a principal portion of the third embodiment of multiple PSS  5 ′ according to the invention. The components that are identical to those shown in FIG. 9 are denoted respectively by the same reference numerals and will not explained be any further. Thus, only the components different from those shown in FIG. 9 will be described below. 
     As shown in FIG.  13  and if compared with FIG. 9, the multiple PSS  5 ′ of this embodiment of the invention has two additional components including a power-fluctuation frequency detecting section  51  and a constant computing section  54 . 
     The power-fluctuation frequency detecting section  51  detects the frequency of power fluctuations from the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1 . 
     The constant calculating section  54  has the function of automatically regulating control constants in order to change the stabilization function of the parallel-type PSS, using the predetermined algorithm in response to the signal of the frequency of power fluctuations detected by the power-fluctuation frequency detecting section  51  or a signal equivalent to it. 
     More specifically, there is provided a table prepared in advance and including stabilization constants Kδ, Tδ 1 , Tδ 2 , Tδ 3 , Tδ 4 , Tδ 5 , Tδ 6  and Tδ 7  selected for (formula 3) above for the parallel PSS on the basis of various possible system conditions, on the one hand, and frequencies of power fluctuations corresponding to these stabilization constants, on the other hand. There is also provided an approximate expression for the frequency F of power fluctuations corresponding each of the above constants. For example, the section  54  may automatically compute the constant of each parallel-type PSS by substituting F in the approximate expression of the second degree as show below with the detected value of the frequency of power fluctuations: 
     
       
           K δ( F )= A   0   +A   1 × F+A   2 × F×F   (Equation 4) 
       
     
     where A 0 , A 1  and A 2  are coefficients for the equation of the second degree. 
     Thus, the power-fluctuation frequency detecting section  51  and the constant computing section  54  are made to have a control-constant regulating feature. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the first embodiment will be discussed here. Otherwise, the description of the operation of the first embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS are automatically computed by the constant computing section  54  using the formulas provided in advance by taking various system conditions into consideration in response to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS can suppress the power fluctuations for which the stabilization function is responsible. 
     (Fourth Embodiment) 
     FIG. 14 is a block diagram of a principal portion of the fourth embodiment of multiple PSS  5 ′ according to the invention. The components that are identical to those shown in FIG. 9 are denoted respectively by the same reference numerals and will not explained be any further. Thus, only the components different from those shown in FIG. 9 will be described below. 
     As illustrated in FIG. 14, the multiple PSS  5 ′ of this embodiment of the invention comprises a plurality of (N) Δδ-PSSs  10 A through  10 N that are parallel-type PSSs. Each of the Δδ-PSSs receives a signal obtained by compensating the phase delay of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  to make it in phase with the signal representing the phase angle of the rotor of the generator  1  and has stabilization function Gδ(S) for suppressing power fluctuations of system mode. An adder A 4  is provided to add output signals S 3 A through S 3 N of the parallel-type PSSs to produce a sum signal. Another adder A 3  is provided to add the sum signal and output signals S 1  and S 2  of the ΔP-PSS and Δω-PSS, or (ΔP+Δω)-PSS, which are PSSs of conventional type, to produce output signal S 5  of the multiple PSS. The output signal S 5  is then input to the AVR  4 . 
     While FIG. 14 does not show the limiters provided for limiting the effective range of controlling the generator voltage, they may be incorporated in the ΔP-PSS having the stabilization function Gp(S)  13 , the Δω-PSS having the stabilization function Gw(S)  14 , and the Δδ-PSSs having the stabilization functions Gδ(S)  10 A to  10 N, respectively. Alternatively, a single limiter may be provided to limit the output signal S 5  of the multiple PSS. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the first embodiment will be discussed here. Otherwise, the description of the operation of the tenth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     More specifically, this multiple PSS  5 ′ comprises a plurality of parallel-type PSSs having respective stabilization functions are differentiated to make themselves adaptable to power fluctuations existing in the system so that, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the first embodiment will be selected for the stabilization functions  10 A through  10 N of the parallel-type PSSs. Then, the output signals S 3 A through S 3 N of these parallel-type PSSs are added to the output signals S 1 , S 1  of the conventional type PSS to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes that are abundant in terms of number or mode are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization functions  10 A through  19 N. 
     With this sharing arrangement, it is possible to quickly suppress power fluctuations occurring in operating power systems over a broad cycle zone ranging from fluctuations of generator mode to fluctuations of system mode in order to stabilize power systems and secure power interchange over a large area on a stable basis. 
     (Fifth Embodiment) 
     FIG. 15 is a block diagram of a principal portion of the fifth embodiment of multiple PSS  5 ′ according to the invention. The components that are identical to those shown in FIG. 12 are denoted respectively by the same reference numerals and will not explained be any further. Thus, only the components different from those shown in FIG. 12 will be described below. 
     As illustrated in FIG. 15, the multiple PSS  5 ′ of this embodiment of the invention comprises a plurality of (a pair of) parallel-type PSSs. These parallel-type PSSs use either the signal of the frequency of power fluctuations detected from the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  or a signal equivalent to it. Then, they automatically select the constants most suited to the detected frequency of power fluctuations out of the constants obtained in advance by computation for the stabilization functions in response to the power fluctuations existing in the systems. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the second embodiment will be discussed here. Otherwise, the description of the operation of the second embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization functions  10  that are different from each other. 
     With this sharing arrangement, it is possible to quickly suppress power fluctuations occurring in operating power systems over a broad cycle zone ranging from fluctuations of generator mode to fluctuations of system mode in order to stabilize power systems and secure power interchange over a large area on a stable basis. 
     (Sixth Embodiment) 
     FIG. 16 is a block diagram of a principal portion of the sixth embodiment of multiple PSS  5 ′ according to the invention. The components that are identical to those shown in FIG. 13 are denoted respectively by the same reference numerals and will not explained be any further. Thus, only the components different from those shown in FIG. 13 will be described below. 
     As illustrated in FIG. 16, the multiple PSS  5 ′ of this embodiment uses either the signal of the frequency of power fluctuations detected from the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  or a signal equivalent to it. Then, they automatically determine the constants for controlling the stabilization functions most suited to the frequency of power fluctuations existing in the systems by computation using the predetermined algorithm. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the third embodiment will be discussed here. Otherwise, the description of the operation of the third embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization functions  10  that are different from each other. 
     With this sharing arrangement, it is possible to quickly suppress power fluctuations occurring in operating power systems over a broad cycle zone ranging from fluctuations of generator mode to fluctuations of system mode in order to stabilize power systems and secure power interchange over a large area on a stable basis. 
     (Seventh Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises only a Δδ-PSS that is a parallel-type PSS having stabilization function Gδ(S)  10  and is adapted to input the output of Δδ-PSS to said AVR  4 . Instead and unlike the third embodiment, this embodiment does not comprise a (ΔP+Δω)-PSS realized by combining a ΔP-PSS that is a conventional PSS having stabilization function Gp(S)  13  and Δω-PSS that is a conventional PSS having stabilization function Gw(S)  14  as described above by referring to FIG.  9 . 
     Otherwise, this embodiment is same as the above described first embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the first embodiment will be discussed here. Otherwise, the description of the operation of the first embodiment also applies to this embodiment. 
     In the multiple PSS  5 ′, the change Δω  9 A in the rotational speed ω  9  of the generator  1  is input to the AVR  4  by way of the stabilization function Gδ(S)  10  as output signal  5 A of the multiple PSS  5 ′. 
     Power fluctuations of system mode are suppressed by the Δδ-PSS that is a parallel-type PSS having stabilization function Gδ(S)  10  and adapted to suppress such power fluctuations of system mode. 
     More specifically, as for the generator  1 , the stabilization function Gδ(S)  10  shown in (formula 3) is selected for the parallel-type PSS of the multiple PSS  5 ′ so as to suppress power fluctuations of system mode because only such fluctuations are problematic to the generator  1 . 
     (Eighth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS having stabilization function Gδ(S)  10 , a power-fluctuation frequency detecting section  51  and a constant selecting section  53  and is adapted to input the output of Δδ-PSS to said AVR  4 . Instead and unlike the second embodiment, this embodiment does not comprise a (ΔP+Δω)-PSS realized by combining a ΔP-PSS that is a conventional PSS having stabilization function Gp(S)  13  and Δω-PSS that is a conventional PSS having stabilization function Gw(S)  14  as described above by referring to FIG.  12 . 
     Otherwise, this embodiment is same as the above described second embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the second embodiment will be discussed here. Otherwise, the description of the operation of the second embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS can suppress the power fluctuations for which the stabilization function is responsible. 
     (Ninth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS having stabilization function Gδ(S)  10 , a power-fluctuation frequency detecting section  51  and a constant computing section  54  and is adapted to input the output of Δδ-PSS to said AVR  4 . Instead and unlike the third embodiment, this embodiment does not comprise a (ΔP+Δω)-PSS realized by combining a ΔP-PSS that is a conventional PSS having stabilization function Gp(S)  13  and Δω-PSS that is a conventional PSS having stabilization function Gw(S)  14  as described above by referring to FIG.  13 . 
     Otherwise, this embodiment is same as the above described third embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the third embodiment will be discussed here. Otherwise, the description of the operation of the third embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS are automatically computed by the constant computing section  54  by using the formulas preselected according to the frequency as detected by the power-fluctuation frequency detecting section  1 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS can suppress the power fluctuations for which the stabilization function is responsible. 
     (Tenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a plurality of (a total of N) Δδ-PSSs that are parallel-type PSSs having respective stabilization functions Gδ(S)  10 A through  10 N and is adapted to input the sum signal obtained by adding the output signals S 3 A through S 3 N of Δδ-PSSs by means of adder A 4  to said AVR  4 . Instead and unlike the fourth embodiment, this embodiment does not comprise a (ΔP+Δω)-PSS realized by combining a ΔP-PSS that is a conventional PSS having stabilization function Gp(S)  13  and Δω-PSS that is a conventional PSS having stabilization function Gw(S)  14  as described above by referring to FIG.  14 . 
     Otherwise, this embodiment is same as the above described fourth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fourth embodiment will be discussed here. Otherwise, the description of the operation of the fourth embodiment also applies to this embodiment. 
     As the control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS can suppress the power fluctuations for which the stabilization functions are responsible. 
     (Eleventh Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a plurality of (more specifically a pair of) Δδ-PSSs that are parallel-type PSSs having stabilization function  10 , a plurality of (more specifically a pair of) power-fluctuation frequency detecting sections  51  and a plurality of (more specifically a pair of) constant selecting sections  53  and is adapted to input the sum signal obtained by adding the output signal S 3  of each of the Δδ-PSSs by means of adder A 5  to said AVR  4 . Instead and unlike the fifth embodiment, this embodiment does not comprise a (ΔP+Δω)-PSS realized by combining a ΔP-PSS that is a conventional PSS having stabilization function Gp(S)  13  and Δω-PSS that is a conventional PSS having stabilization function Gw(S)  14  as described above by referring to FIG.  15 . 
     Otherwise, this embodiment is same as the above described fifth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fifth embodiment will be discussed here. Otherwise, the description of the operation of the fifth embodiment also applies to this embodiment. 
     With this embodiment, if the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light differ greatly from each other and the influence of the difference is negligible or if the cycle of power fluctuations fluctuates greatly due to changes in the power interchange so that more rigorous requirements have to be selected for the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Twelfth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a plurality of (more specifically a pair of) Δδ-PSSs that are parallel-type PSSs having stabilization function  10 , a plurality of (more specifically a pair of) power-fluctuation frequency detecting sections  51  and a plurality of (more specifically a pair of) constant computing sections  54  and is adapted to input the sum signal obtained by adding the output signal S 3  of each of the Δδ-PSSs by means of adder A 6  to said AVR  4 . Instead and unlike the sixth embodiment, this embodiment does not comprise a (ΔP+Δω)-PSS realized by combining a ΔP-PSS that is a conventional PSS having stabilization function Gp(S)  13  and Δω-PSS that is a conventional PSS having stabilization function Gw(S)  14  as described above by referring to FIG.  16 . 
     Otherwise, this embodiment is same as the above described sixth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 17. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the sixth embodiment will be discussed here. Otherwise, the description of the operation of the sixth embodiment also applies to this embodiment. 
     With this embodiment, if the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light differ greatly from each other and the influence of the difference is negligible or if the cycle of power fluctuations fluctuates greatly due to changes in the power interchange so that more rigorous requirements have to be selected for the system, control constants that are more delicate than those of the third embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Thirteenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the frequency of the voltage or the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the first embodiment as illustrated in FIG.  9  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described first embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the first embodiment will be discussed here. Otherwise, the description of the operation of the first embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the change −ΔP of the active power  8 , the change Δω  9 A of the rotational speed ω  9  of the generator  1  and the signal of the frequency of the voltage or the current of the generator  1  are fed to the adder A 3  respectively by way of the stabilization function Gp(S)  13 , the stabilization function Gw(S)  14  and the stabilization function Gδ(S)  10  and added by the adder A 3  to obtain the output signal  5 A of the multiple PSS that is output to the AVR  4  as shown in FIG.  2 . 
     Then, power fluctuations of adjacent generator mode and those of generator mode are suppressed by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization functions Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of system mode are suppressed by the Δδ-PSS that is a parallel-type PSS having stabilization function Gδ(S)  10  and adapted to receive the signal of the frequency of the voltage or the current of the generator  1  as input. 
     With this sharing arrangement, it is possible to quickly suppress power fluctuations occurring in operating power systems over a broad cycle zone ranging from fluctuations of generator mode to fluctuations of system mode in order to stabilize power systems and secure power interchange over a large area on a stable basis. 
     (Fourteenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the frequency of the voltage or the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the second embodiment as illustrated in FIG.  10  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described second embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the second embodiment will be discussed here. Otherwise, the description of the operation of the second embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Fifteenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the frequency of the voltage or the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the third embodiment as illustrated in FIG.  11  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described third embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the third embodiment will be discussed here. Otherwise, the description of the operation of the third embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted as input to receive the signal of the frequency of the voltage or the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Sixteenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal of the frequency of the voltage or the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fourth embodiment as illustrated in FIG.  14  and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fourth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fourth embodiment will be discussed here. Otherwise, the description of the operation of the fourth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Seventeenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the frequency of the voltage or the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fifth embodiment as illustrated in FIG.  15  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to-suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fifth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fifth embodiment will be discussed here. Otherwise, the description of the operation of the fifth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal of the frequency of the voltage or the current of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Eighteenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the frequency of the voltage or the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the sixth embodiment as illustrated in FIG.  16  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described sixth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the sixth embodiment will be discussed here. Otherwise, the description of the operation of the sixth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal of the frequency of the voltage or the current of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Nineteenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the frequency of the voltage or the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the seventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described seventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the seventh embodiment will be discussed here. Otherwise, the description of the operation of the seventh embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the signal of the frequency of the voltage or the current of the generator  1  is input to the AVR  4  as output signal  5 A of the multiple PSS by way of the stabilization function Gp(S)  10 . 
     Then, power fluctuations of system mode are suppressed by the ΔP-PSS that is a parallel-type PSS adapted to use as input a signal of the frequency of the voltage or the current of the generator  1  and having stabilization function Gδ(S)  10  selected so as to be adaptable to such power-fluctuations. 
     More specifically, as for the generator  1 , the stabilization function Gδ(S)  10  shown in (formula 3) is selected for the parallel-type PSS of the multiple PSS  5 ′ adapted to use as input the signal of the frequency of the voltage or the current of the generator  1  so as to suppress power fluctuations of system mode because only such fluctuations are problematic to the generator  1 . 
     (Twentieth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the frequency of the voltage or the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eighth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eighth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  51 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eighth embodiment will be discussed here. Otherwise, the description of the operation of the eighth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     (Twenty-first Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the frequency of the voltage or the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the ninth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described ninth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the ninth embodiment will be discussed here. Otherwise, the description of the operation of the ninth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  are automatically computed by the constant computing section  54  by using the formulas selected in advance according to the frequency as detected by the power-fluctuation frequency detecting section  51 . 
     (Twenty-second Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal of the frequency of the voltage or the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the tenth embodiment and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described tenth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the tenth embodiment will be discussed here. Otherwise, the description of the operation of the tenth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the frequency of the voltage or the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Twenty-third Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the frequency of the voltage or the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eleventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eleventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eleventh embodiment will be discussed here. Otherwise, the description of the operation of the eleventh embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal of the frequency of the voltage or the current of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Twenty-fourth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the frequency of the voltage or the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the twelfth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described twelfth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the twelfth embodiment will be discussed here. Otherwise, the description of the operation of the twelfth embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal of the frequency of the voltage or the current of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Twenty-fifth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the first embodiment as illustrated in FIG.  9  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described first embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the first embodiment will be discussed here. Otherwise, the description of the operation of the first embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the change −ΔP of the active power  8 , the change Δω  9 A of the rotational speed ω  9  of the generator  1  and the signal of the active power P 8  of the generator  1  are fed to the adder A 3  respectively by way of the stabilization function Gp(S)  13 , the stabilization function Gw(S)  14  and the stabilization function  10  and added by the adder A 3  to obtain the output signal  5 A of the multiple PSS that is output to the AVR  4  as shown in FIG.  2 . 
     Then, power fluctuations of adjacent generator mode and those of generator mode are suppressed by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization functions Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of system mode are suppressed by the Δδ-PSS that is a parallel-type PSS having stabilization function Gδ(S)  10  and adapted to receive as input the signal of the active power P 8  of the generator  1 . 
     With this sharing arrangement, it is possible to quickly suppress power fluctuations occurring in operating power systems over a broad cycle zone ranging from fluctuations of generator mode to fluctuations of system mode in order to stabilize power systems and secure power interchange over a large area on a stable basis. 
     (Twenty-sixth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the second embodiment as illustrated in FIG.  10  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described second embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the second embodiment will be discussed here. Otherwise, the description of the operation of the second embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Twenty-seventh Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the third embodiment as illustrated in FIG.  11  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described third embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the third embodiment will be discussed here. Otherwise, the description of the operation of the third embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted as input to receive the signal of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Twenty-eighth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fourth embodiment as illustrated in FIG.  14  and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fourth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fourth embodiment will be discussed here. Otherwise, the description of the operation of the fourth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Twenty-ninth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fifth embodiment as illustrated in FIG.  15  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fifth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fifth embodiment will be discussed here. Otherwise, the description of the operation of the fifth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal of the active power P 8  of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Thirtieth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the sixth embodiment as illustrated in FIG.  16  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described sixth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the sixth embodiment will be discussed here. Otherwise, the description of the operation of the sixth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal of the active power P 8  of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Thirty-first Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the seventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described seventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the seventh embodiment will be discussed here. Otherwise, the description of the operation of the seventh embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the signal of the active power P 8  of the generator  1  is input to the AVR  4  as output signal  5 A of the multiple PSS by way of the stabilization function Gp(S)  10 . 
     Then, power fluctuations of system mode are suppressed by the ΔP-PSS that is a parallel-type PSS adapted to use as input a signal of the active power P 8  of the generator  1  and having stabilization function Gδ(S)  10  selected so as to be adaptable to such power-fluctuations. 
     More specifically, as for the generator  1 , the stabilization function Gδ(S)  10  shown in (formula 3) is selected for the parallel-type PSS of the multiple PSS  5 ′ adapted to use as input the signal of the active power P 8  of the generator  1  so as to suppress power fluctuations of system mode because only such fluctuations are problematic to the generator  1 . 
     (Thirty-second Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eighth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eighth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eighth embodiment will be discussed here. Otherwise, the description of the operation of the eighth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     (Thirty-third Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the ninth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described ninth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the ninth embodiment will be discussed here. Otherwise, the description of the operation of the ninth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  are automatically computed by the constant computing section  54  by using the formulas selected in advance according to the frequency as detected by the power-fluctuation frequency detecting section  51 . 
     (Thirty-fourth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the tenth embodiment and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described tenth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the tenth embodiment will be discussed here. Otherwise, the description of the operation of the tenth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Thirty-fifth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eleventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eleventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eleventh embodiment will be discussed here. Otherwise, the description of the operation of the eleventh embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal of the active power P 8  of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Thirty-sixth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the twelfth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described twelfth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the twelfth embodiment will be discussed here. Otherwise, the description of the operation of the twelfth embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal of the active power P 8  of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Thirty-seventh Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the first embodiment as illustrated in FIG.  9  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described first embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the first embodiment will be discussed here. Otherwise, the description of the operation of the first embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the change −ΔP of the active power  8 , the change Δω  9 A of the rotational speed ω  9  of the generator  1  and the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  are fed to the adder A 3  respectively by way of the stabilization function Gp(S)  13 , the stabilization function Gw(S)  14  and the stabilization function Gδ(S)  10  and added by the adder A 3  to obtain the output signal  5 A of the multiple PSS that is output to the AVR  4  as shown in FIG.  2 . 
     Then, power fluctuations of adjacent generator mode and those of generator mode are suppressed by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization functions Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of system mode are suppressed by the Δδ-PSS that is a parallel-type PSS having stabilization function Gδ(S)  10  and adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1 . 
     With this sharing arrangement, it is possible to quickly suppress power fluctuations occurring in operating power systems over a broad cycle zone ranging from fluctuations of generator mode to fluctuations of system mode in order to stabilize power systems and secure power interchange over a large area on a stable basis. 
     (Thirty-eighth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the second embodiment as illustrated in FIG.  10  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described second embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the second embodiment will be discussed here. Otherwise, the description of the operation of the second embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Thirty-ninth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the third embodiment as illustrated in FIG.  11  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described third embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the third embodiment will be discussed here. Otherwise, the description of the operation of the third embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted as input to receive the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Fortieth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fourth embodiment as illustrated in FIG.  14  and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fourth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fourth embodiment will be discussed here. Otherwise, the description of the operation of the fourth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Forty-first Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fifth embodiment as illustrated in FIG.  15  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fifth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fifth embodiment will be discussed here. Otherwise, the description of the operation of the fifth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Forty-second Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  in place of the change Δ 107   9 A in the rotational speed ω  9  of the rotor of the generator  1  of the sixth embodiment as illustrated in FIG.  16  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described sixth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the sixth embodiment will be discussed here. Otherwise, the description of the operation of the sixth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Forty-third Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the seventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described seventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the seventh embodiment will be discussed here. Otherwise, the description of the operation of the seventh embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  is input to the AVR  4  as output signal  5 A of the multiple PSS by way of the stabilization function Gp(S)  10 . 
     Then, power fluctuations of system mode are suppressed by the ΔP-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  and having stabilization function Gδ(S)  10  selected so as to be adaptable to such power-fluctuations. 
     More specifically, as for the generator  1 , the stabilization function Gδ(S)  10  shown in (formula 3) is selected for the parallel-type PSS of the multiple PSS  5 ′ adapted to use as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  so as to suppress power fluctuations of system mode because only such fluctuations are problematic to the generator  1 . 
     (Forty-fourth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eighth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eighth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eighth embodiment will be discussed here. Otherwise, the description of the operation of the eighth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     (Forty-fifth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the ninth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described ninth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the ninth embodiment will be discussed here. Otherwise, the description of the operation of the ninth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  are automatically computed by the constant computing section  54  by using the formulas selected in advance according to the frequency as detected by the power-fluctuation frequency detecting section  51 . 
     (Forty-sixth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the tenth embodiment and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described tenth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the tenth embodiment will be discussed here. Otherwise, the description of the operation of the tenth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Forty-seventh Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eleventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eleventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eleventh embodiment will be discussed here. Otherwise, the description of the operation of the eleventh embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Forty-eighth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the twelfth embodiment and having stabilization function G 5  (S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described twelfth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the twelfth embodiment will be discussed here. Otherwise, the description of the operation of the twelfth embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSS. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal equivalent to the rotational acceleration generated by combining the signal of the guide vane opening of the water wheel and that of the active power P 8  of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Forty-ninth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the first embodiment as illustrated in FIG.  9  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described first embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the first embodiment will be discussed here. Otherwise, the description of the operation of the first embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the change −ΔP of the active power  8 , the change Δω  9 A of the rotational speed ω  9  of the generator  1  and the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  are fed to the adder A 3  respectively by way of the stabilization function Gp(S)  13 , the stabilization function Gw(S)  14  and the stabilization function Gδ(S)  10  and added by the adder A 3  to obtain the output signal  5 A of the multiple PSS that is output to the AVR  4  as shown in FIG.  2 . 
     Then, power fluctuations of adjacent generator mode and those of generator mode are suppressed by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization functions Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of system mode are suppressed by the Δδ-PSS that is a parallel-type PSS having stabilization function Gδ(S)  10  and adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1 . 
     With this sharing arrangement, it is possible to quickly suppress power fluctuations occurring in operating power systems over a broad cycle zone ranging from fluctuations of generator mode to fluctuations of system mode in order to stabilize power systems and secure power interchange over a large area on a stable basis. 
     (Fiftieth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the second embodiment as illustrated in FIG.  10  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described second embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the second embodiment will be discussed here. Otherwise, the description of the operation of the second embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Fifty-first Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the third embodiment as illustrated in FIG.  11  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described third embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the third embodiment will be discussed here. Otherwise, the description of the operation of the third embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted as input to receive the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Fifty-second Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fourth embodiment as illustrated in FIG.  14  and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fourth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fourth embodiment will be discussed here. Otherwise, the description of the operation of the fourth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Fifty-third Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fifth embodiment as illustrated in FIG.  15  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fifth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fifth embodiment will be discussed here. Otherwise, the description of the operation of the fifth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Fifty-fourth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the sixth embodiment as illustrated in FIG.  16  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Aδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described sixth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the sixth embodiment will be discussed here. Otherwise, the description of the operation of the sixth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω -PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Fifty-fifth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the seventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described seventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the seventh embodiment will be discussed here. Otherwise, the description of the operation of the seventh embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  is input to the AVR  4  as output signal  5 A of the multiple PSS by way of the stabilization function Gp(S)  10 . 
     Then, power fluctuations of system mode are suppressed by the ΔP-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  and having stabilization function Gδ(S)  10  selected so as to be adaptable to such power-fluctuations. 
     More specifically, as for the generator  1 , the stabilization function Gδ(S)  10  shown in (formula 3) is selected for the parallel-type PSS of the multiple PSS  5 ′ adapted to use as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  so as to suppress power fluctuations of system mode because only such fluctuations are problematic to the generator  1 . 
     (Fifty-sixth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eighth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eighth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eighth embodiment will be discussed here. Otherwise, the description of the operation of the eighth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     (Fifty-seventh Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the ninth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described ninth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the ninth embodiment will be discussed here. Otherwise, the description of the operation of the ninth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  are automatically computed by the constant computing section  54  by using the formulas selected in advance according to the frequency as detected by the power-fluctuation frequency detecting section  51 . 
     (Fifty-eighth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the tenth embodiment and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described tenth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the tenth embodiment will be discussed here. Otherwise, the description of the operation of the tenth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Fifty-ninth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eleventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eleventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eleventh embodiment will be discussed here. Otherwise, the description of the operation of the eleventh embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Sixtieth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the twelfth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described twelfth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the twelfth embodiment will be discussed here. Otherwise, the description of the operation of the twelfth embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSS. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal equivalent to the rotational acceleration generated by combining the signal of the valve opening of the turbine directly linked to the generator  1  and that of the active power P 8  of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Sixty-first Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the first embodiment as illustrated in FIG.  9  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described first embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the first embodiment will be discussed here. Otherwise, the description of the operation of the first embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the change −ΔP of the active power  8 , the change Δω  9 A of the rotational speed ω  9  of the generator  1  and the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  are fed to the adder A 3  respectively by way of the stabilization function Gp(S)  13 , the stabilization function Gw(S)  14  and the stabilization function Gδ(S)  10  and added by the adder A 3  to obtain the output signal  5 A of the multiple PSS that is output to the AVR  4  as shown in FIG.  2 . 
     Then, power fluctuations of adjacent generator mode and those of generator mode are suppressed by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization functions Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of system mode are suppressed by the Δδ-PSS that is a parallel-type PSS having stabilization function Gδ(S)  10  and adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1 . 
     With this sharing arrangement, it is possible to quickly suppress power fluctuations occurring in operating power systems over a broad cycle zone ranging from fluctuations of generator mode to fluctuations of system mode in order to stabilize power systems and secure power interchange over a large area on a stable basis. 
     (Sixty-second Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the second embodiment as illustrated in FIG.  10  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described second embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the second embodiment will be discussed here. Otherwise, the description of the operation of the second embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Sixty-third Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the third embodiment as illustrated in FIG.  11  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described third embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the third embodiment will be discussed here. Otherwise, the description of the operation of the third embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted as input to receive the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Sixty-fourth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fourth embodiment as illustrated in FIG.  14  and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fourth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fourth embodiment will be discussed here. Otherwise, the description of the operation of the fourth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Sixty-fifth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fifth embodiment as illustrated in FIG.  15  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fifth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fifth embodiment will be discussed here. Otherwise, the description of the operation of the fifth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δδ-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Sixty-sixth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the sixth embodiment as illustrated in FIG.  16  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described sixth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the sixth embodiment will be discussed here. Otherwise, the description of the operation of the sixth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Sixty-seventh Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the seventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described seventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the seventh embodiment will be discussed here. Otherwise, the description of the operation of the seventh embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  is input to the AVR  4  as output signal  5 A of the multiple PSS by way of the stabilization function Gp(S)  10 . 
     Then, power fluctuations of system mode are suppressed by the ΔP-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  and having stabilization function Gδ(S)  10  selected so as to be adaptable to such power-fluctuations. 
     More specifically, as for the generator  1 , the stabilization function Gδ(S)  10  shown in (formula 3) is selected for the parallel-type PSS of the multiple PSS  5 ′ adapted to use as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  so as to suppress power fluctuations of system mode because only such fluctuations are problematic to the generator  1 . 
     (Sixty-eighth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eighth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eighth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eighth embodiment will be discussed here. Otherwise, the description of the operation of the eighth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     (Sixty-ninth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the ninth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described ninth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the ninth embodiment will be discussed here. Otherwise, the description of the operation of the ninth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  are automatically computed by the constant computing section  54  by using the formulas selected in advance according to the frequency as detected by the power-fluctuation frequency detecting section  51 . 
     (Seventies Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the tenth embodiment and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described tenth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the tenth embodiment will be discussed here. Otherwise, the description of the operation of the tenth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Seventy-first Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eleventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eleventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eleventh embodiment will be discussed here. Otherwise, the description of the operation of the eleventh embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Seventy-second Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the twelfth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described twelfth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the twelfth embodiment will be discussed here. Otherwise, the description of the operation of the twelfth embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal equivalent to the phase angle of the rotor of the generator  1  generated by combining the signal of the active power P 8  of the generator  1  and that of voltage Vg 3 A of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Seventy-third Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the first embodiment as illustrated in FIG.  9  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described first embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the first embodiment will be discussed here. Otherwise, the description of the operation of the first embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the change −ΔP of the active power  8 , the change Δω  9 A of the rotational speed ω  9  of the generator  1  and the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  are fed to the adder A 3  respectively by way of the stabilization function Gp(S)  13 , the stabilization function Gw(S)  14  and the stabilization function Gδ(S)  10  and added by the adder A 3  to obtain the output signal  5 A of the multiple PSS that is output to the AVR  4  as shown in FIG.  2 . 
     Then, power fluctuations of adjacent generator mode and those of generator mode are suppressed by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization functions Gp(S)  13  and Δδ-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of system mode are suppressed by the Δδ-PSS that is a parallel-type PSS having stabilization function Gδ(S)  10  and adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1 . 
     With this sharing arrangement, it is possible to quickly suppress power fluctuations occurring in operating power systems over a broad cycle zone ranging from fluctuations of generator mode to fluctuations of system mode in order to stabilize power systems and secure power interchange over a large area on a stable basis. 
     (Seventy-fourth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the second embodiment as illustrated in FIG.  10  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described second embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the second embodiment will be discussed here. Otherwise, the description of the operation of the second embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Seventy-fifth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the third embodiment as illustrated in FIG.  11  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described third embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the third embodiment will be discussed here. Otherwise, the description of the operation of the third embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted as input to receive the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Seventy-sixth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fourth embodiment as illustrated in FIG.  14  and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fourth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fourth embodiment will be discussed here. Otherwise, the description of the operation of the fourth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Seventy-seventh Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fifth embodiment as illustrated in FIG.  15  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fifth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fifth embodiment will be discussed here. Otherwise, the description of the operation of the fifth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Seventy-eighth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the sixth embodiment as illustrated in FIG.  16  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described sixth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the sixth embodiment will be discussed here. Otherwise, the description of the operation of the sixth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Seventy-ninth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the seventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described seventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the seventh embodiment will be discussed here. Otherwise, the description of the operation of the seventh embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  is input to the AVR  4  as output signal  5 A of the multiple PSS by way of the stabilization function Gp(S)  10 . 
     Then, power fluctuations of system mode are suppressed by the ΔP-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  and having stabilization function Gδ(S)  10  selected so as to be adaptable to such power-fluctuations. 
     More specifically, as for the generator  1 , the stabilization function Gδ(S)  10  shown in (formula 3) is selected for the parallel-type PSS of the multiple PSS  5 ′ adapted to use as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  so as to suppress power fluctuations of system mode because only such fluctuations are problematic to the generator  1 . 
     (Eightieth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eighth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eighth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eighth embodiment will be discussed here. Otherwise, the description of the operation of the eighth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     (Eighty-first Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the ninth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described ninth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the ninth embodiment will be discussed here. Otherwise, the description of the operation of the ninth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  are automatically computed by the constant computing section  54  by using the formulas selected in advance according to the frequency as detected by the power-fluctuation frequency detecting section  51 . 
     (Eighty-second Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the tenth embodiment and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described tenth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the tenth embodiment will be discussed here. Otherwise, the description of the operation of the tenth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Eighty-third Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eleventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eleventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eleventh embodiment will be discussed here. Otherwise, the description of the operation of the eleventh embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSS. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Eighty-fourth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the twelfth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described twelfth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the twelfth embodiment will be discussed here. Otherwise, the description of the operation of the twelfth embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the phase angle of the rotor of the generator  1  and that of voltage Vg 3 A of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Eighty-fifth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the first embodiment as illustrated in FIG.  9  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described first embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the first embodiment will be discussed here. Otherwise, the description of the operation of the first embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the change −ΔP of the active power  8 , the change Δω  9 A of the rotational speed ω  9  of the generator  1  and the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal are fed to the adder A 3  respectively by way of the stabilization function Gp(S)  13 , the stabilization function Gw(S)  14  and the stabilization function Gδ(S)  10  and added by the adder A 3  to obtain the output signal  5 A of the multiple PSS that is output to the AVR  4  as shown in FIG.  2 . 
     Then, power fluctuations of adjacent generator mode and those of generator mode are suppressed by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization functions Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of system mode are suppressed by the Δδ-PSS that is a parallel-type PSS having stabilization function Gδ(S)  10  and adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1 . 
     With this sharing arrangement, it is possible to quickly suppress power fluctuations occurring in operating power systems over a broad cycle zone ranging from fluctuations of generator mode to fluctuations of system mode in order to stabilize power systems and secure power interchange over a large area on a stable basis. 
     (Eighty-sixth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the second embodiment as illustrated in FIG.  10  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described second embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the second embodiment will be discussed here. Otherwise, the description of the operation of the second embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Eighty-seventh Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the third embodiment as illustrated in FIG.  11  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described third embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the third embodiment will be discussed here. Otherwise, the description of the operation of the third embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted as input to receive the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Eighty-eighth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fourth embodiment as illustrated in FIG.  14  and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fourth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fourth embodiment will be discussed here. Otherwise, the description of the operation of the fourth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Eighty-ninth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fifth embodiment as illustrated in FIG.  15  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fifth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fifth embodiment will be discussed here. Otherwise, the description of the operation of the fifth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Ninetieth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the sixth embodiment as illustrated in FIG.  16  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described sixth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the sixth embodiment will be discussed here. Otherwise, the description of the operation of the sixth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Ninety-first Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the seventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described seventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the seventh embodiment will be discussed here. Otherwise, the description of the operation of the seventh embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  is input to the AVR  4  as output signal  5 A of the multiple PSS by way of the stabilization function Gp(S)  10 . 
     Then, power fluctuations of system mode are suppressed by the ΔP-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  and having stabilization function Gδ(S)  10  selected so as to be adaptable to such power-fluctuations. 
     More specifically, as for the generator  1 , the stabilization function Gδ(S)  10  shown in (formula 3) is selected for the parallel-type PSS of the multiple PSS  5 ′ adapted to use as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  so as to suppress power fluctuations of system mode because only such fluctuations are problematic to the generator  1 . 
     (Ninety-second Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eighth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eighth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eighth embodiment will be discussed here. Otherwise, the description of the operation of the eighth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     (Ninety-third Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the ninth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described ninth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the ninth embodiment will be discussed here. Otherwise, the description of the operation of the ninth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  are automatically computed by the constant computing section  54  by using the formulas selected in advance according to the frequency as detected by the power-fluctuation frequency detecting section  51 . 
     (Ninety-fourth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the tenth embodiment and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described tenth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the tenth embodiment will be discussed here. Otherwise, the description of the operation of the tenth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Ninety-fifth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eleventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eleventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eleventh embodiment will be discussed here. Otherwise, the description of the operation of the eleventh embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Ninety-sixth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the twelfth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described twelfth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the twelfth embodiment will be discussed here. Otherwise, the description of the operation of the twelfth embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal of the internal phase angle of the generator  1  generated from the difference of the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and the signal of the voltage phase of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (Ninety-seventh Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the first embodiment as illustrated in FIG.  9  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described first embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the first embodiment will be discussed here. Otherwise, the description of the operation of the first embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the change −ΔP of the active power  8 , the change Δω  9 A of the rotational speed ω  9  of the generator  1  and the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  are fed to the adder A 3  respectively by way of the stabilization function Gp(S)  13 , the stabilization function Gw(S)  14  and the stabilization function Gδ(S)  10  and added by the adder A 3  to obtain the output signal  5 A of the multiple PSS that is output to the AVR  4  as shown in FIG.  2 . 
     Then, power fluctuations of adjacent generator mode and those of generator mode are suppressed by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization functions Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of system mode are suppressed by the Δδ-PSS that is a parallel-type PSS having stabilization function Gδ(S)  10  and adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1 . 
     With this sharing arrangement, it is possible to quickly suppress power fluctuations occurring in operating power systems over a broad cycle zone ranging from fluctuations of generator mode to fluctuations of system mode in order to stabilize power systems and secure power interchange over a large area on a stable basis. 
     (Ninety-eighth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the second embodiment as illustrated in FIG.  10  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described second embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the second embodiment will be discussed here. Otherwise, the description of the operation of the second embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (Ninety-ninth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the third embodiment as illustrated in FIG.  11  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described third embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the third embodiment will be discussed here. Otherwise, the description of the operation of the third embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted as input to receive the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (One Hundredth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fourth embodiment as illustrated in FIG.  14  and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fourth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fourth embodiment will be discussed here. Otherwise, the description of the operation of the fourth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (One Hundred-first Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fifth embodiment as illustrated in FIG.  15  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fifth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fifth embodiment will be discussed here. Otherwise, the description of the operation of the fifth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (One Hundred-second Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the sixth embodiment as illustrated in FIG.  16  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described sixth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the sixth embodiment will be discussed here. Otherwise, the description of the operation of the sixth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (One Hundred-third Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the seventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described seventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the seventh embodiment will be discussed here. Otherwise, the description of the operation of the seventh embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  is input to the AVR  4  as output signal  5 A of the multiple PSS by way of the stabilization function Gp(S)  10 . 
     Then, power fluctuations of system mode are suppressed by the ΔP-PSS that is a parallel-type PSS adapted to use as input a signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and having stabilization function Gδ(S)  10  selected so as to be adaptable to such power-fluctuations. 
     More specifically, as for the generator  1 , the stabilization function Gδ(S)  10  shown in (formula 3) is selected for the parallel-type PSS of the multiple PSS  5 ′ adapted to use as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  so as to suppress power fluctuations of system mode because only such fluctuations are problematic to the generator  1 . 
     (One Hundred-fourth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eighth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eighth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the ninth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described ninth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the ninth embodiment will be discussed here. Otherwise, the description of the operation of the ninth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eighth embodiment will be discussed here. Otherwise, the description of the operation of the eighth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     (One Hundred-fifth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  are automatically computed by the constant computing section  54  by using the formulas selected in advance according to the frequency as detected by the power-fluctuation frequency detecting section  51 . 
     (One Hundred-sixth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the tenth embodiment and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described tenth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the tenth embodiment will be discussed here. Otherwise, the description of the operation of the tenth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (One Hundred-seventh Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eleventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eleventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eleventh embodiment will be discussed here. Otherwise, the description of the operation of the eleventh embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (One Hundred-eighth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the twelfth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described twelfth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the twelfth embodiment will be discussed here. Otherwise, the description of the operation of the twelfth embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal of the internal voltage phase of the generator  1  generated by combining the signal of voltage Vg 3 A and that of the current of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (One Hundred-ninth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  in place of the change Δ 107   9 A in the rotational speed ω  9  of the rotor of the generator  1  of the first embodiment as illustrated in FIG.  9  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described first embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the first embodiment will be discussed here. Otherwise, the description of the operation of the first embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the change −ΔP of the active power  8 , the change Δω  9 A of the rotational speed ω  9  of the generator  1  and the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  are fed to the adder A 3  respectively by way of the stabilization function Gp(S)  13 , the stabilization function Gw(S)  14  and the stabilization function Gδ(S)  10  and added by the adder A 3  to obtain the output signal  5 A of the multiple PSS that is output to the AVR  4  as shown in FIG.  2 . 
     Then, power fluctuations of adjacent generator mode and those of generator mode are suppressed by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization functions Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of system mode are suppressed by the Δδ-PSS that is a parallel-type PSS having stabilization function Gδ(S)  10  and adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1 . 
     With this sharing arrangement, it is possible to quickly suppress power fluctuations occurring in operating power systems over a broad cycle zone ranging from fluctuations of generator mode to fluctuations of system mode in order to stabilize power systems and secure power interchange over a large area on a stable basis. 
     (One Hundred-tenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the second embodiment as illustrated in FIG.  10  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described second embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the second embodiment will be discussed here. Otherwise, the description of the operation of the second embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (One Hundred-eleventh Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the third embodiment as illustrated in FIG.  11  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described third embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the third embodiment will be discussed here. Otherwise, the description of the operation of the third embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     As the stabilization function  10  as shown in (formula 3) is so arranged for each parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted as input to receive the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1  that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (One Hundred-twelfth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fourth embodiment as illustrated in FIG.  14  and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fourth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fourth embodiment will be discussed here. Otherwise, the description of the operation of the fourth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through ION as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (One Hundred-thirteenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the fifth embodiment as illustrated in FIG.  15  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described fifth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the fifth embodiment will be discussed here. Otherwise, the description of the operation of the fifth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (One Hundred-fourteenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the sixth embodiment as illustrated in FIG.  16  and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described sixth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the sixth embodiment will be discussed here. Otherwise, the description of the operation of the sixth embodiment also applies to this embodiment. 
     Then, power fluctuations of generator mode are suppressed mainly by the (ΔP+Δω)-PSS that is a PSS of conventional type realized by combining a ΔP-PSS having stabilization function Gp(S)  13  and Δω-PSS having stabilization function Gw(S)  14 , whereas power fluctuations of a plurality of system modes showing much different frequencies are suppressed by the Δδ-PSS that comprises a plurality of parallel-type PSSs having stabilization function Gδ(S)  10  and receives as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1 . 
     The control constants of the stabilization function  10  as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (One Hundred-fifteenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the seventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described seventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 9. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the seventh embodiment will be discussed here. Otherwise, the description of the operation of the seventh embodiment also applies to this embodiment. 
     With this multiple PSS  5 ′, the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  is input to the AVR  4  as output signal  5 A of the multiple PSS by way of the stabilization function Gp(S)  10 . 
     Then, power fluctuations of system mode are suppressed by the ΔP-PSS that is a parallel-type PSS adapted to use as input a signal of the combination of the signal of the rotational speed ω 9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  and having stabilization function Gδ(S)  10  selected so as to be adaptable to such power-fluctuations. 
     More specifically, as for the generator  1 , the stabilization function Gδ(S)  10  shown in (formula 3) is selected for the parallel-type PSS of the multiple PSS  5 ′ adapted to use as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  so as to suppress power fluctuations of system mode because only such fluctuations are problematic to the generator  1 . 
     (One Hundred-sixteenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eighth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eighth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 12. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eighth embodiment will be discussed here. Otherwise, the description of the operation of the eighth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  are automatically selected by the constant selecting section  53  out of a number of constants stored in advance by taking various system conditions into consideration according to the frequency detected by the power-fluctuation frequency detecting section  51 . 
     (One Hundred-seventeenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises a Δδ-PSS that is a parallel-type PSS adapted to use as input a signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the ninth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described ninth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 13. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the ninth embodiment will be discussed here. Otherwise, the description of the operation of the ninth embodiment also applies to this embodiment. 
     When the power-fluctuation frequency of the generator  1  deviates from the expected frequency, the power-fluctuation frequency detecting section  51  detects the power-fluctuation frequency from the rotational speed ω  9  of the rotor of the generator  1  and the control constants including the gain and the advance/delay constant of the parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  are automatically computed by the constant computing section  54  by using the formulas selected in advance according to the frequency as detected by the power-fluctuation frequency detecting section  51 . 
     (One Hundred-eighteenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (N) Δδ-PSSs adapted to use as input a signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the tenth embodiment and having respective stabilization functions Gδ(S)  10 A through  10 N that include a phase advance/delay compensation function and compensate the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSSs being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described tenth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 14. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the tenth embodiment will be discussed here. Otherwise, the description of the operation of the tenth embodiment also applies to this embodiment. 
     The control constants of the stabilization functions  10 A through  10 N as shown in (formula 3) are so selected for each parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  as to suppress the target power fluctuations most effectively in response to the power fluctuations existing in the system. In this way, each parallel-type PSS adapted to receive as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  can suppress the power fluctuations for which the stabilization function is responsible. 
     (One Hundred-nineteenth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the eleventh embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described eleventh embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 15. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the eleventh embodiment will be discussed here. Otherwise, the description of the operation of the eleventh embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     (One Hundred-twentieth Embodiment) 
     This embodiment of multiple PSS  5 ′ comprises as parallel-type PSS a plurality of (more specifically a pair of) Δδ-PSSs adapted to use as input a signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  in place of the change Δω  9 A in the rotational speed ω  9  of the rotor of the generator  1  of the twelfth embodiment and having stabilization function Gδ(S)  10  that includes a phase advance/delay compensation function and compensates the phase delay in-phase with the phase angle signal of the rotor of the generator  1  for the input signal, said Δδ-PSS being also adapted to suppress long cycle power fluctuations of system mode. 
     Otherwise, this embodiment is same as the above described twelfth embodiment and hence will not be described here any further. Therefore, this embodiment will be understood by referring to FIGS. 8 and 16. 
     Now, the operation of the multiple PSS  5 ′ of this embodiment having the above described configuration will described below. 
     Note, however, that only the operation of the part of this embodiment that is different from the twelfth embodiment will be discussed here. Otherwise, the description of the operation of the twelfth embodiment also applies to this embodiment. 
     With this embodiment, if the difference between the cycle of power fluctuations occurring during the day when the load of the system is heavy and that of power fluctuations occurring during the night when the load of the system is light is large and its influence is severe or if the cycle of power fluctuations varies greatly due to changes in the power interchange so that more rigorous conditions have to be applied to the system, control constants that are more delicate than those of the second embodiment will be selected for the stabilization functions of the parallel-type PSSs. Then, the output signals S 3  of these parallel-type PSSs are added to obtain the output signal S 5  of the multiple PSS that is output to the AVR  4 . 
     The power-fluctuations of system mode of a plurality of systems that vary greatly in terms of frequency are suppressed by the Δδ-PSS adapted to use as input the signal of the combination of the signal of the rotational speed ω  9  of the rotor of the generator  1 , that of the frequency of the voltage of the generator  1 , that of the frequency of the current of the generator  1 , that of the active power P 8  of the generator  1 , that of the guide vane opening of the water wheel, that of the valve opening of the turbine directly linked to the generator  1 , that of the phase angle of the rotor of the generator  1 , that of the phase of voltage Vg 3 A of the generator  1 , that of voltage Vg 3 A of the generator  1  and that of the current of the generator  1  and comprising a plurality of parallel-type PSSs having respective stabilization functions  10  that are different from each other. 
     The present invention provides a PSS that can quickly suppress power fluctuations that may usually occur over a broad cycle zone, ranging from fluctuations of generator mode (having a short cycle of about 0.5 seconds) to fluctuations of system mode (having a long cycle of about 10 seconds), in order to stabilize power systems and secure power interchange over a broad area on a stable basis and is applicable to both a static exciting system and a rotary exciting system, without adversely affecting the shaft-twisting vibration of the turbines or generators. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.