Abstract:
A voltage stabilization control method for controlling a voltage of a power system that is connected to an adjustable speed machine includes detecting an active power change of the power system; outputting a control signal indicative of the amount of adjustment when the adjustment of the active power of the adjustable speed machine is required on the basis of the active power change; and receiving a steady operation command value with respect to the adjustable speed machine, generating a stabilization control signal resulting from adding the control signal to the steady operation command value and outputting the stabilization control signal to the adjustable speed machine. The present invention also resides in a voltage stabilization control apparatus.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a voltage stabilization control method, and more particularly to an improvement in voltage stability of a power system in a voltage stabilization control method for the power system due to an adjustable speed machine. 
     2. Description of the Related Art 
     Hitherto, in order to improve voltage stability of the power system, there are used a control unit and a control system in which a transmission voltage is controlled to a constant value by an excitation control unit such as a PSVR (power system voltage regulator) or an HSVC (high side voltage control), or a reactive power is compensated by a phase modifier such as an SVG (statcom). In general, a relation between the transmission voltage and a terminal voltage of the power generator is represented by the following expression. 
     
       
           V   H   =V   g   −I   q   ·X   t   (1) 
       
     
     Where V H  is a transmission voltage at a higher voltage side of a main transformer connected with a terminal of a synchronous machine, V g  is a terminal voltage of the synchronous machine, I q  is a reactive current, and X t  is a leakage reactance of the main transformer. 
     Since a normal voltage control controls the terminal voltage of the synchronous machine constantly, when the system voltage is lowered, the transmission voltage V H  drops together with a drop of the system voltage as the reactive current I q  increases. On the contrary, according to the transmission voltage constant control method such as the PSVR or the HSVC, the transmission voltage can be maintained so as to compensate an amount as large as the dropped amount corresponding to the reactance of the transformer by increasing the terminal voltage of the synchronous machine. Also, in the phase modifier such as the SVG, the reactive current is compensated, thereby being capable of preventing the transmission voltage from dropping. 
     FIG. 8 is a simplified circuit diagram showing a known general transmission system. In the figure, reference numeral  100  denotes a synchronous machine of a sending end,  101  is a transmission line that connects a transmission end and a receiving end,  102  is a load of the receiving end, and  103  is a transmission line impedance. FIG. 9 shows a characteristic (P-V curve) of an active power to a voltage of the transmission system shown in FIG.  8 . Referring to FIG. 9, reference symbol C 11  is a PV characteristic of the transmission system, and C 21  is a load characteristic. In a normal state, the power system operates at an equilibrium point of an intersection A of C 11  and C 12 . When the transmission line impedance increases due to a transmission line fault or the like, the characteristic of the transmission system largely changes. For example, in the case where the transmission characteristic is reduced to C 12  after the fault but the load characteristic cannot be extended over C 22 , the transmission characteristic and the load characteristic cannot intersect with each other, with the result that the operation equilibrium point is lost and a voltage drop or a voltage breakdown. In the case where a component of a non-linear load such as a constant power load or an inductor load is large, or in the case where a tap changer such as an LTC is limited, the possibility that such a situation may occur is high. In order to prevent this circumstance, the transmission characteristic, after the fault is extended up to C 13 , can intersect with the load characteristic. In other words, when the active power which is short circuited at the load is quickly supplied, the transmission system can not be saved from the voltage drop or the voltage breakdown. However, in the conventional voltage control and reactive power control, the active power cannot be supplied. Also, in the case of a synchronous machine, because control of the active power can be conducted only by a speed control system, the power cannot be controlled at a high speed. 
     As shown in FIG. 9, it is generally known that there is a voltage stability limit H in the transmission characteristic C 11  that is in a steady operation state. However, when the operation equilibrium point becomes lower than H, the voltage fluctuates. The value of the stability limit H changes by using the HSVC, the PSVR or the phase modifier. There is a fear that the operation equilibrium point enters an unstable region due to acceleration of the generators connected to the transmission system after an accident has been removed, or a rapid request at the load side. In this case, it is necessary to suppress the power that rapidly increases in the transmission system, but control cannot be conducted so as to absorb an increasing power of the transmission system and maintain the voltage of the transmission system by only the conventional voltage control and reactive power compensation. Also, in case of a synchronous machine, because the control of the active power can be controlled by only the speed control system, the power cannot be controlled at a high speed. 
     As described above, in the case where the power is short circuited at the load side due to the transmission line fault or the load is rapidly changing, if the active power that is short circuited at the load can be rapidly supplied, the voltage drop of the transmission system or the voltage breakdown can be prevented. However, in conventional voltage control and reactive power control, because the active power cannot be supplied, when a fault or the like occurs, a voltage drop and a voltage collapse occurs in the transmission system. 
     Also, there is a fear that the operation equilibrium point enters an unstable region due to acceleration of the generators connected to the transmission system after a fault has been removed, or upon a rapid request for power at the load side. In this case, because it is impossible to absorb the increasing power of the transmission system at a high speed and maintain the voltage of the transmission system by conventional voltage control and reactive power compensation, there arises a voltage drop of the transmission system and a voltage collapse occurs. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above-mentioned problems, and therefore an object of the present invention is to obtain a voltage stabilization control method and apparatus each of which is capable of stopping a voltage drop of the transmission system and preventing the voltage collapse by supplying an active power to a load at a high speed when the active power is short at the load, and absorbing the power when the power rapidly increases in the transmission system. 
     With the above object in view, the voltage stabilization control method of the present invention for controlling a voltage of a power system that is connected with an adjustable speed machine, comprises the steps of: detecting a change of a condition of the power system; outputting a control signal indicative of the amount of adjustment when the adjustment of the active power of the adjustable speed machine is required on the basis of the change of the condition of the power system; and receiving a steady operation command value with respect to the adjustable speed machine, generating a stabilization control signal resulting from adding the control signal to the steady operation command value and outputting the stabilization control signal to the adjustable speed machine. 
     Therefore, the active power of the adjustable speed machine connected to the power system is controlled at a high speed in correspondence with the active power change of the power system, thereby making it possible to suck or supply the power at the control target position and to stop the voltage drop of the transmission system, whereby the voltage collapse can be prevented. 
     Also, the change of the condition of the power system may comprise an active power change of the power system, a frequency change of the power system or a voltage change of the power system. 
     The present invention resides in a voltage stabilization control system for controlling a voltage of a power system that is connected with an adjustable speed machine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of this invention will become more fully apparent from the following detailed description taken with the accompanying drawings in which: 
     FIG. 1 is a diagram for explanation of an operation of a voltage stabilization control method in accordance with a first embodiment of the present invention; 
     FIG. 2 is a diagram for explanation of an operation of a voltage stabilization control method in accordance with a second embodiment of the present invention; 
     FIG. 3 is a diagram for explanation of an operation of a voltage stabilization control method in accordance with a third embodiment of the present invention; 
     FIG. 4 is a diagram for explanation of an operation of a voltage stabilization control method in accordance with a fourth embodiment of the present invention; 
     FIG. 5 is a diagram for explanation of an operation of a voltage stabilization control method in accordance with a fifth embodiment of the present invention; 
     FIG. 6 is a diagram for explanation of an operation of a voltage stabilization control method in accordance with a sixth embodiment of the present invention; 
     FIG. 7 is a diagram for explanation of an operation of a voltage stabilization control method in accordance with a seventh embodiment of the present invention; 
     FIG. 8 is a circuit diagram showing the outline of a known transmission system; and 
     FIG. 9 is an explanatory diagram showing a known active power to a voltage P-V characteristic curve. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, a description will be given in more detail of the preferred embodiments of the present invention with reference to the accompanying drawings. 
     First Embodiment 
     FIG. 1 is a block diagram for explanation of a power system voltage stabilizing method in accordance with a first embodiment of the present invention. In the figure, reference numeral  11  denotes a state detecting circuit for detecting a system frequency change or an active power change at a control target point,  12  denotes a power control circuit for outputting a control signal for restoring an increase or a decrease in the active power at the control target point, and  13  denotes a signal adder. 
     Subsequently, a description will be given of an operation principle of the power system voltage stabilizing method according to the first embodiment of the present invention. 
     The state detecting circuit  11  always monitors an operation state and outputs to the power control circuit  12 , a state indicative signal I of 0 when the state is normal, a state indicative signal I of 1 when the system frequency or the active power at the control target point increases to a rated value or more, a state indicative signal I of −1 when the system frequency or the active power at the control target point decreases to the rated value or less, and a variation amount X (represented by a predetermined function X=f(U)) such as a deviation of the frequency or the active power or a change speed as represented by the following expressions. 
     
       
         I=0 when Uu&gt;U&gt;Ul, 
       
     
     
       
         I=1 when U&gt;Uu, 
       
     
     
       
           I =−1 when  U&lt;Ul,   
       
     
     and 
     
       
           X=f ( U ) 
       
     
     Where I is a state indicative signal, U is an active power or frequency, Ul is a lower limit rated value of the active power or the frequency, and Uu is an upper limit rated value of the active power or the frequency. 
     As represented by the following expressions, the power control circuit  12  outputs 0.0 to the signal adder  13  when receiving I=‘0’ from the state detecting circuit  11 . The power control circuit  12  outputs to the signal adder  13 , a control signal Y that allows an increase in the active power at the control target point to be restored at a high speed by a variation amount X detected by the state detecting circuit  11  or a function prepared in advance when receiving I=‘1’. The power control circuit  12  outputs to the signal adder  13 , the control signal Y that allows a decrease in the active power at the control target point to be restored at a high speed by a variation amount X detected by the state detecting circuit  11  or a function prepared in advance when receiving I=‘−1’. 
     Y=F(x, I, t) (example: PID), or 
     Y=f(I, t) (example: step function). 
     The signal adder  13  receives a steady operation command value P* of an adjustable speed machine and adds an output of the power control circuit  12  to a value of the P* to output the added value to an excitation control system of the adjustable speed machine. 
     As described above, in this embodiment, the excitation control system of the adjustable speed machine controls the active power of the adjustable speed machine in accordance with the steady command value when the excitation control system of the adjustable speed machine normally operates. However, when an increase or a decrease in the power at the control target point is detected, the power control signal is supplied to the excitation control system of the adjustable speed machine to adjust (absorb or supply) the active power of the adjustable speed machine, thereby making it possible to restore the increase or the decrease in the transmission power of the transmission system at a high speed and to stop a voltage drop in the transmission system, whereby the voltage collapse can be prevented. 
     Second Embodiment 
     FIG. 2 is a block diagram for explanation of a power system voltage stabilizing method in accordance with a second embodiment of the present invention. In FIG. 2, reference numeral  21  denotes a state detecting circuit for detecting a fault signal indicative of a system frequency change or an active power change at a control target point and a fault occurrence or a load rapid change,  22  denotes a power control circuit for outputting a control signal for restoring an increase or a decrease in the active power at the control target point, and  23  denotes a signal adder. 
     Subsequently, a description will be given of an operation principle of the power system voltage stabilizing method according to the second embodiment of the present invention. 
     The state detecting circuit  21  always monitors an operation state and outputs to the power control circuit  22 , a state indicative signal I of 0 when the state is normal, a state indicative signal I of 1 when the system frequency or the active power at the control target point increases to a rated value or more and receiving the abnormality information of a power increase or an operation command, a state indicative signal I of −1 when the system frequency or the active power at the control target point decreases to a rated value or less, and the state detecting circuit  21  receives the abnormality information of the power increase or the operation command, and a variation amount X such as a deviation of the frequency or the active power or a change speed as represented by the following expressions. 
     
       
         I=0 when Uu&gt;U&gt;Ul and E=0, 
       
     
     
       
         I=1 when U&gt;Uu and E=1, 
       
     
     
       
           I =−1 when  U&lt;Ul  and  E =− 1 , 
       
     
     
       
           X=f ( U ) 
       
     
     Where E is abnormality information or an operation command, I is a state indicative signal, U is an active power or frequency, Ul is a lower limit rated value of the active power or the frequency, and Uu is an upper limit rated value of the active power or the frequency. 
     As represented by the following expressions, the power control circuit  22  outputs 0.0 to the signal adder  23  when receiving I=‘0’ from the state detecting circuit  21 . The power control circuit  22  outputs to the signal adder  23 , a control signal Y that allows an increase in the active power at the control target point to be restored at a high speed by a variation amount X detected by the state detecting circuit  21  or a function prepared in advance when receiving I=‘1’. The power control circuit  22  outputs to the signal adder  23 , the control signal Y that allows a decrease in the active power at the control target point to be restored at a high speed by a variation amount X detected by the state detecting circuit  21  or a function prepared in advance when receiving I=‘−1’. 
     Y=F(x, I, t) (example: PID), or 
     Y=f(I, t) (example: step function). 
     The signal adder  23  adds an output of the power control circuit  22  to a steady operation command value P* of the adjustable speed machine to output the added value to an excitation control system of the adjustable speed machine. 
     As described above, according to this embodiment, the excitation control system of the adjustable speed machine controls the active power of the adjustable speed machine in accordance with the steady command value when the excitation control system of the adjustable speed machine normally operates. However, when an increase or a decrease in the power at the control target point is detected, the power control signal is supplied to the excitation control system of the adjustable speed machine to adjust (absorb or supply) the active power of the adjustable speed machine, thereby making it possible to restore the increase or the decrease in the transmission power of the transmission system at a high speed and to stop a voltage drop in the transmission system, whereby the voltage collapse can be prevented. 
     Third Embodiment 
     FIG. 3 is a block diagram for explanation of a power system voltage stabilizing method in accordance with a third embodiment of the present invention. In FIG. 3, reference numeral  31  denotes a state detecting circuit for detecting a system frequency change or an active power change and the voltage change at a control target point,  32  denotes a power control circuit for outputting a control signal for restoring an increase or a decrease in the active power at the control target point, and  33  denotes a signal adder. 
     Subsequently, a description will be given of an operation principle of the power system voltage stabilizing method according to the third embodiment of the present invention. 
     As represented by the following expressions, the state detecting circuit  31  always monitors an operation state and outputs to the power control circuit  32 , a state indicative signal I of 0 when the state is normal, a state indicative signal I of 1 when the system frequency or the active power at the control target point increases to a rated value or more and the voltage at the control target point decreases to the rated value or less, and a state indicative signal I of −1 when the system frequency or the active power at the control target point decreases to the rated value or less, and the voltage at the control target point decreases to the rated value or less. 
     
       
         I=0 when Uu&gt;U&gt;Ul and V&gt;Vc, 
       
     
     
       
         I=1 when U&gt;Uu and V&lt;Vc, 
       
     
     
       
           I =−1 when  U&lt;Ul  and  V&lt;Vc,   
       
     
     
       
           X=f ( U ) 
       
     
     Where V is a voltage, Vc is a voltage lower limit rated value, I is a state indicative signal, U is an active power or frequency, Ul is a lower limit rated value of the active power or the frequency, and Uu is an upper limit rated value of the active power or the frequency. 
     As represented by the following expressions, the power control circuit  32  outputs 0.0 to the signal adder  33  when receiving I=‘0’ from the state detecting circuit  31 . The power control circuit  32  outputs to the signal adder  33 , a control signal Y that allows an increase in the active power at the control target point to be restored at a high speed by a variation amount X detected by the state detecting circuit  31  or a function prepared in advance when receiving I=1′. The power control circuit  32  outputs to the signal adder  33 , the control signal Y that allows a decrease in the active power at the control target point to be restored at a high speed by a variation amount X detected by the state detecting circuit  31  or a function prepared in advance when receiving I=‘−1’. 
     Y=F(x, I, t) (example: PID), or 
     Y=f (I, t) (example: step function). 
     The signal adder  33  adds an output of the power control circuit  32  to a steady operation command value P* of the adjustable speed machine to output the added value to an excitation control system of the adjustable speed machine. 
     As described above, according to this embodiment, the excitation control system of the adjustable speed machine controls the active power of the adjustable speed machine in accordance with the steady command value when the excitation control system of the adjustable speed machine normally operates. However, when an increase or a decrease in the power at the control target point is detected, the power control signal is supplied to the excitation control system of the adjustable speed machine to adjust (absorb or supply) the active power of the adjustable speed machine, thereby making it possible to restore the increase or the decrease in the transmission power of the transmission system at a high speed and to stop a voltage drop in the transmission system, whereby the voltage collapse can be prevented. 
     Fourth Embodiment 
     FIG. 4 is a block diagram for explanation of a power system voltage stabilizing method in accordance with a fourth embodiment of the present invention. In FIG. 4, reference numeral  41  denotes a state detecting circuit for detecting a system frequency change or an active power change and a voltage change at a control target point, and abnormality information indicative of a fault occurrence or a load rapid change or an operation command,  42  denotes a power control circuit for outputting a control signal for restoring an increase or a decrease in the active power at the control target point, and  43  denotes a signal adder. 
     Subsequently, a description will be given of an operation principle of the power system voltage stabilizing method according to the fourth embodiment of the present invention. 
     As represented by the following expressions, the state detecting circuit  41  always monitors an operation state and outputs to the power control circuit  42 , a state indicative signal I of 0 when the state is normal, a state indicative signal I of 1 when the system frequency or the active power at the control target point increases to a rated value or more, a voltage at the control target point decreases to the rated value or less, and the state detecting circuit  41  receives abnormality information of a power increase or an operation command, and a state indicative signal I of −1 when the system frequency or the active power at the control target point decreases to the rated value or less, the voltage at the control target point decreases to the rated value or less, and the state detecting circuit  41  receives the abnormality information of a power decrease or an operation command. 
     
       
         I=0 when Uu&gt;U&gt;Ul, V&gt;Vc and E=0, 
       
     
     
       
         I=1 when U&gt;Uu, V&lt;Vc and E=1, 
       
     
     
       
           I =−1 when  U&lt;Ul, V&lt;Vc  and  E =−1, 
       
     
     
       
           X=f ( U ) 
       
     
     Where V is a voltage, Vc is a voltage lower limit rated value, E is abnormality information or an operation command, I is a state indicative signal, U is an active power or frequency, Ul is a lower limit rated value of the active power or the frequency, and Uu is an upper limit rated value of the active power or the frequency. 
     The power control circuit  42  outputs 0.0 to the signal adder  43  when receiving I=‘0’ from the state detecting circuit  41 . The power control circuit  42  outputs to the signal adder  43 , a control signal Y that allows an increase in the active power at the control target point to be restored at a high speed by a variation amount X detected by the state detecting circuit  41  or a function prepared in advance when receiving I=‘1’. The power control circuit  42  outputs to the signal adder  43 , the control signal Y that allows a decrease in the active power at the control target point to be restored at a high speed by a variation amount X detected by the state detecting circuit  41  or a function prepared in advance when receiving I=‘−1’. 
     Y=F(x, I, t) (example: PID), or 
     Y=f(I, t) (example: step function). 
     The signal adder  43  adds an output of the power control circuit  42  to a steady operation command value P* of the adjustable speed machine to output the added value to an excitation control system of the adjustable speed machine. 
     As described above, according to this embodiment, the excitation control system of the adjustable speed machine controls the active power of the adjustable speed machine in accordance with the steady command value when the excitation control system of the adjustable speed machine normally operates. However, when an increase or a decrease in the power at the control target point is detected, the power control signal is supplied to the excitation control system of the adjustable speed machine to adjust (absorb or supply) the active power of the adjustable speed machine, thereby making it possible to restore the increase or the decrease in the transmission power of the transmission system at a high speed and to stop a voltage drop in the transmission system, whereby the voltage collapse can be prevented. 
     Fifth Embodiment 
     FIG. 5 is a block diagram for explanation of a power system voltage stabilizing method in accordance with a fifth embodiment of the present invention. In FIG. 5, reference numeral  51  denotes an abnormality information or operation command detecting circuit for detecting an accident or a load rapid change,  52  denotes a power control circuit for outputting a control signal for restoring an increase or a decrease in the active power at the control target point, and  53  denotes a signal adder. 
     Subsequently, a description will be given of an operation principle of the power system voltage stabilizing method according to the fifth embodiment of the present invention. 
     As represented by the following expressions, the abnormality information or operation command detecting circuit  51  always monitors an operation state and outputs to the power control circuit  52 , a state indicative signal I of 0 when the state is normal, a state indicative signal I of 1 when receiving the abnormality information of the power increase or the operation command, and a state indicative signal I of −1 when receiving the abnormality information of the power decrease or the operation command. 
     Normal: I=0, 
     Abnormality of the power increase: I=1, and 
     Abnormality of the power decrease: I=−1. 
     As shown in FIG. 5, the power control circuit  52  outputs 0.0 to the signal adder  53  when receiving I=‘0’ from the abnormality information or operation command detecting circuit  51 . The power control circuit  52  outputs to the signal adder  53 , a control signal Y that allows an increase in the active power at the control target point to be restored at a high speed by a control function  1  prepared in advance when receiving I=‘1’ (example: I=1, control function I). The power control circuit  52  outputs to the signal adder  53 , the control signal Y that allows a decrease in the active power at the control target point to be restored at a high speed by a control function H prepared in advance when receiving I=‘−1’ (example: I=−1, control function II). 
     The signal adder  53  adds an output of the power control circuit  52  to a steady operation command value P* of the adjustable speed machine to output the added value to an excitation control system of the adjustable speed machine. 
     As described above, according to this embodiment, the excitation control system of the adjustable speed machine controls the active power of the adjustable speed machine in accordance with the steady command value when the excitation control system of the adjustable speed machine normally operates. However, when an increase or a decrease in the power at the control target point is detected, the power control signal is supplied to the excitation control system of the adjustable speed machine to adjust (absorb or supply) the active power of the adjustable speed machine, thereby making it possible to restore the increase or the decrease in the transmission power of the transmission system at a high speed and to stop a voltage drop in the transmission system, whereby the voltage collapse can be prevented. 
     Sixth Embodiment 
     FIG. 6 is a block diagram for explanation of a power system voltage stabilizing method in accordance with a sixth embodiment of the present invention. In FIG. 6, reference numeral  61  denotes a voltage stability margin estimating circuit that calculates a voltage stabilization margin of a transmission system in accordance with the operation information of a phase modifier for a system operation condition, located higher voltage side voltage control or SVG, or an LTC tap changer, and  62  denotes a rated value calculating circuit that automatically calculates and updates the rated value set forth in the above-mentioned first to fourth embodiments in accordance with the stabilization margin. 
     A description will be given of an operation principle of the control method of the adjustable speed machine in the power system voltage stabilizing method in accordance with the sixth embodiment of the present invention. 
     The voltage stability margin estimating circuit  61  calculates an active power limit value PH and a voltage stabilization limit value VH at the voltage stabilization limit of the transmission system in accordance with the operation information of a phase modifier for a system operation condition, located higher voltage side voltage control or SVG, or an LTC tap changer, and outputs a difference between an active power P and PH at the time of the transmission system steady operation, and a difference between the voltage V and VH to the rated value calculating circuit  62  as a stabilization margin. 
     
       
         
           P=PH−P 
         
       
     
     
       
         
           V=VH−V 
         
       
     
     
       
           PH=F 1( H, S, L, f ( x )) 
       
     
     
       
           VH=F 2( H, S, L, f ( x )) 
       
     
     Where P is an active power in steady operation at a control point, V is a voltage in the steady operation at the control point, PH is an active power stabilization limit, and VH is a voltage stabilization limit. 
     As represented by the following expressions, the rated value calculating circuit  62  automatically calculates and updates the rated value set forth in the above-mentioned first to fourth embodiments so as to set the frequency and the active power increase limit rated value to be higher when the active power stability margin from the voltage stability margin estimating circuit  61  is larger, and to set the frequency and the active power increase limit rated value to be lower when the active power stability margin is smaller, and also so as to set the voltage decrease limit rated value to be lower when the voltage stabilization margin from the voltage stability margin estimating circuit  61  is larger, and to set the voltage decrease limit rated value to be higher when the voltage stabilization margin is smaller. 
     Active power upper limit rated value: Pu=F1 (.P, V) 
     Frequency upper limit rated value: fu=F2 (.P, V) 
     Voltage lower limit rated value: Vc=F3 (.P, V) 
     As described above, according to this embodiment, since the voltage increase limit rated value is appropriately calculated and automatically updated in accordance with the stabilization margin of the transmission system, the effect of the voltage stabilization control can be further improved. 
     Seventh Embodiment 
     FIG. 7 is a block diagram for explanation of a power system voltage stabilizing method in accordance with a seventh embodiment of the present invention. In the figure, reference numeral  71  denotes a power control output adjusting circuit that adjusts the output of the power control in accordance with the operation information of a phase modifier for a system operation condition, located higher voltage side voltage control or SVG, or an LTC tap changer, and  72  denotes a power control circuit set forth in the above-mentioned first to fifth embodiments of the present invention. 
     A description will be given of an operation principle of the control method of the adjustable speed machine in the power system voltage stabilizing method in accordance with the seventh embodiment of the present invention. As represented by the following expressions, the power control output adjusting circuit  71  outputs the respective output adjustment gains K(1) and K(−1) to the power control circuit  72  in cases of the power increase and the power decrease so that the device is located in accordance with the operation information of a phase modifier for located higher voltage side voltage control or SVG, or an LTC tap changer, and the power control output is small and the device is not located when the operation is enabled, or the power control output is large when the function is limited. 
     Case of the power increase: K(1)=f1(H, S, L), 
     Case of the power decrease: K(−1)=f2(H, S, L) 
     In the power control circuit  72  according to the first to fifth embodiments, the output from the power control output adjusting circuit  71  adjusts the output of the power control by K(1) in case of the power increase, and adjusts the output of the power control by K(−1) in case of the power decrease. 
     Y=F(K(I), x, I, t) 
     Y=f(K(I), I, t) 
     As described above, according to this embodiment, if the output of the power control of the adjustable speed machine is appropriately adjusted in accordance with the circumstances such as the voltage control and the reactive power control of the transmission system, the effect of the voltage stabilization control can be further improved. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.