Excitation control apparatus for synchronous machine

Abstract: An excitation control apparatus for synchronous machine which coincides voltage/frequency ratio of a synchronous machine (6) with a limited value without setting gains of an automatic voltage regulator (AVR; 6) considering the harmony between the automatic voltage regulator (AVR; 6) and voltage/frequency ratio limiter (8) by means of adding the integrated output of a value corresponding to the deference between the limited value and obtained voltage/frequency ratio of the synchronous machine (1) to a desired control value of the synchronous machine (1) by the automatic voltage regulator (AVR; 6).

Description: This invention relates to an excitation control apparatus for a synchronous machine. Fig. 1 is a structural block diagram of a known excitation control system disclosed, for example, in Japanese Patent Application Laid-Open No 58-176000 (1983). Referring to this Fig. 1, reference numerals respectively represent: 1 a synchronous machine, 2 field windings of the machine, 3 an exciter for feeding a field current to the field windings 2, 4 a potential transformer (PT) for detecting the voltage of the synchronous machine, 5 a current transformer (CT) for detecting the current of the synchronous machine 1, and 6 an automatic voltage regulator (AVR) for controlling the voltage of the synchronous machine 1 to be constant. The AVR 6 is specifically constituted of a voltage detection circuit 61 which detects the output of the PT 4, a reference circuit 62 which generates a desired control value of the voltage of the synchronous machine 1, an amplifier circuit A 63 which amplifies the difference between the reference circuit 62 and the voltage detection circuit 61, a phase compensation circuit 64 which enhances the controlling stability of the voltage, an adding circuit 65 which adds signals from the other supplemental functional devices to be described later, and an amplifier circuit B 66 which amplifies the output of the adding circuit 65 and generates a signal supplied to the exciter 3. A power system stabilizer 7 detects the electric power from output signals of the PT 4 and CT 5 and supplies a supplementary signal to the AVR 6 thereby to enhance the stability of the system. Meanwhile, a V/Hz limiter 8 detects the V/Hz value(the ratio of the voltage V to the frequency Hz) from the output signals of the PT 4 and CT 5, and feeds a supplementary signal to the AVR 6, thereby controlling the V/Hz not to exceed a predetermined value. A poor excitation limiter 9, by detecting the power and reactive power from the output signals of the PT 4 and CT 5, supplies a supplementary signal to the AVR 6 so as to drive the synchronous machine 1 at no lower than a predetermined level. The current flowing in the field windings 2 is detected by a shunt 10 which in turn outputs a signal to an overexcitation limiter 11. When the overexcitation limiter 11 detects the overexciting state not smaller than a predetermined value, it supplies a supplementary signal to the AVR 6 thereby to restrict the overexcitation. The operation of the conventional controlling system in the structure described hereinbefore will now be depicted. The output voltage from the synchronous machine 1 is decreased by the PT 4. The signal of the PT 4 is input to the AVR 6 to be converted to a signal easy to amplify, add, etc. (converted generally to a direct current signal) by the voltage detection circuit 61. The value from the reference circuit 62 is a desired control value for the AVR 6 to control the voltage of the synchronous machine 1. The output difference of the reference circuit 62 from the voltage detection circuit 61 is amplified to an appropriate value by the amplifier circuit A 63. If the amplified signal is positive, the output voltage of the synchronous machine 1 is lower than the desired control value of the voltage set by the reference circuit 62, and therefore, the AVR 6 outputs a control signal to raise the output of the exciter 3. As a result, the current flowing in the field windings 2 is increased and the output voltage of the synchronous machine 1 is eventually raised. In the phase compensation circuit 64, the output of the amplifier circuit A 63 is compensated to increase the controlling stability, and the output signal from this phase compensation circuit 64 is added to outputs of various kinds of functional devices such as the power system stabilizer 7, V/Hz limiter 8 and the like in the adding circuit 65. The amplifier circuit B 66 amplifies the output of the adding circuit 65 and generates a signal proportional to the field current which is the output of the exciter 3 (i.e., the exciter 3 has an amplifying function as well). The output voltage of the synchronous machine 1 is controlled in the manner as above to the desired control value set by the reference circuit 62 within the AVR 6. The amplifier circuit A 63 and B 66 and phase compensation circuit 64 are provided so that the output voltage of the synchronous machine 1 coincides with the desired control value set by the reference circuit 62 in the AVR 6 as often as possible. Although it is ideal, needless to say, that the desired control value should be coincident with the actual value at all times, the synchronous machine 1 itself has the time lag which produces a difference between the desired control value and actual value caused by an external disturbance to the controlling system, for example, when an accident happens at the output side of the synchronous machine 1 or when the desired control value is changed. Therefore, the AVR 6 intends to perform control so as to remove the above difference as quickly as possible. In the meantime, the V/Hz limiter 8 restricts the V/Hz value of the synchronous machine 1 under a predetermined value, with noting the fact that the current running in the synchronous machine 1 and coils of the transformers (not shown) is proportional to the V/Hz value. The V/Hz limiter 8 controls the current in the coils. Fig. 2 is a block diagram of the transfer function representing the controlling characteristic of the AVR 6 and V/Hz limiter 8. Each of the exciter 3 and synchronous machine 1 is generally expressed by gains and first-order lag functions. The AVR 6, amplifier circuit A 63, phase compensation circuit 64 and amplifier circuit B 66 are respectively represented by gains and first-order lead/lag functions. Among those, the constants K E , T E of the exciter 3 and constants K G , T G of the synchronous machine 1 are fixed values of the apparatus. A gain K₂ of the amplifier circuit B 66 is generally an output amplifying gain of the AVR 6 and therefore a fixed value of the regulator. On the other hand, the constants of the amplifier circuit A 63 and phase compensation circuit 64 are variable which can be set, by a variable resistor or the like. The voltage controllability of the AVR 6 is determined by these variable parameters K₁, T lead , T lag . These parameters are designed beforehand on the basis of the constants of the synchronous machine 1 and exciter 3, but finally determined through individual adjustment. The V/Hz limiter 8 can be represented by gains and phase compensation similar to the AVR 6. Because the conventional controller has the construction as above, when the frequency of the synchronous machine 1 is lowered, for instance, the V/Hz limiter 8 outputs a signal to lower the excitation. On the contrary, the AVR 6 outputs a signal to raise the excitation to make the voltage from the synchronous machine 1 constant. Therefore, it is necessary to set the respective gains to harmonize the V/Hz limiter 8 with the AVR 6. It is also a drawback of the prior art that the desired V/Hz value of the V/Hz limiter 8 may not be perfectly coincident with the actual V/Hz, that is, an offset error increases. Moreover, since the gain K₁ of the amplifier circuit A 63 and the time parameters T lead , T lag of the phase compensation circuit 64 should be set one by one, and if each constant of the exciter 3 or the synchronous machine 1 is different in actual operation from the design stage, the above gain and time parameters K ₁, T lead and T lag should be individually adjusted, so that the values vary depending on the adjusting people. The optimum parameters cannot always be obtained and, finding the proper adjusting becomes difficult. This invention has been devised to solve the above-discussed problems, and has the essential object to provide an excitation control apparatus for a synchronous machine which does not require to set gains considering harmonization of an AVR with a V/Hz limiter and which agrees a desired V/Hz value with the actual V/Hz value. A further object of this invention is to provide an excitation control apparatus for a synchronous machine capable of automatically obtain a control constant of an AVR simply by setting the constants of the synchronous machine and the exciter. A yet further object of this invention is to provide an excitation control apparatus for a synchronous machine capable of operating/setting amplification gains and time constants, instead of setting them one by one, from the actual values of constants of the exciter and synchronous machine by automatically measuring the actual values of the constants. The above and further objects and advantages of the invention will more fully be apparent from the following detailed description of embodiments shown in the accompanying drawings. According to the present invention, an excitation control apparatus is provided as defined in claims 1, 2 and 3. Fig. 1 is a structural block diagram of a conventional excitation controlling system; Fig. 2 is a block diagram of the transfer function of the conventional excitation controlling system; Fig. 3 is a structural block diagram of an excitation controlling system according to a first embodiment of this invention; Fig. 4 is a block diagram of the transfer function of the first embodiment; Fig. 5 is a structural diagram of an excitation controlling system according to a second embodiment of this invention; Fig. 6 is a flowchart of the operating process in the second embodiment; Fig. 7 is a block diagram of the transfer function of the second embodiment; Fig. 8 is a structural block diagram of an excitation controlling system according to a third embodiment of this invention; Fig. 9 is a flowchart of the operating process in the third embodiment; Fig. 10 is a block diagram of the transfer function of the third embodiment; Fig. 11 is a diagram of the step response waveform of an output current of an exciter; and Fig. 12 is a diagram of the step response waveform of an output voltage of a synchronous machine. Embodiments of the invention will be discussed more in detail hereinafter with reference to the accompanying drawings. [Embodiment 1] Fig. 3 is a structural block diagram of an excitation control apparatus according to a first embodiment of this invention, in which the same parts are designated by the same reference numerals as in Fig. 1, and the description thereof will be abbreviated here. One structural difference of the first embodiment from Fig. 1 is to provide an analog transducer 601 which detects a voltage V, a power P, a frequency F and an reactive power Q from the signals of the PT 4 and CT 5 of the synchronous machine 1, and an adding circuit 602 which adds (actually subtracts) the output of the V/Hz limiter 8 to (from) the output of the reference circuit 62. Fig. 4 is a block diagram indicative of the operation of the V/Hz limiter 8 and the connecting state of the V/Hz limiter 8 with the AVR 6 in the exciting system of Fig. 3. In Fig. 4, a comparator 82 compares a desired V/Hz control value 81 of the V/Hz limiter 8 with the actual V/Hz value. A switching device 83 outputs either the value 0.0 when the actual V/Hz value is not larger than the desired control value 81, or the difference between the actual value and the desired control value when the actual value is larger. An integrator 84 is activated according to the output of the switching device 83. A reference numeral 85 denotes a divider for calculating the V/Hz value from the V and F values. Now, the operation of the controlling system will be depicted. Referring again to Fig. 4, when the V/Hz value detected by the divider 85 is larger than the desired control value 81, the output of the switching device 83 changes from 0.0 to a difference value (= desired control value - V/Hz value). Supposing that the desired control value is 1.1pu, V = 1.2pu and F = 1.0pu, the switching device 83 outputs 1.1pu - 1.2pu = -0.1pu, so that the output of the integrator 84 is changed to one direction in accordance with an inclination of the integrating time constant T. The output of the integrator 84 is added to the desired control value 62 of the AVR 6. The initial desired control value V of the AVR 6 is 1.2pu and therefore, it assumes V = 1.2pu. When the sum of the desired control value 62 (1.2pu) and the output of the integrator 84 (-0.lpu) becomes 1.1pu, the output of the synchronous machine 1 becomes 1.1pu. At this time, V = 1.1pu and F = 1.0pu, and it results in V/Hz = 1.1pu so that the output of the switching device 83 turns to be 0.0. Since the output value of the integrator does not change any more when the input thereof becomes 0.0, the system is balanced. At this point, the V/Hz value is perfectly coincident with the desired value 1.1pu, thus completing the V/Hz control in a desired manner. Although the exciter 3 is a rotary exciter having a gain and a first-order lag characteristic in the first embodiment, a static exciter with an almost negligible first-order lag may be used. Moreover, although the control function of the AVR 6 is represented by gains and first-order lead/lag functions, a feedback loop for feeding back the output of the exciter may be provided to compensate the lag of the exciter. Further, the present control apparatus is digital, but this invention is applicable to an analog AVR with the same effect as above. [Embodiment 2] Fig. 5 is a structural block diagram of an excitation controlling system according to a second embodiment of this invention, wherein the same parts are denoted by the same reference numerals as in Figs. 1-3, with the description thereof being abbreviated. In Fig. 5, reference numerals are respectively: 100 a control constant operation circuit which determines control constants of the amplifier circuit A 63 and the phase compensation circuit 64, 101 a gain K G of the synchronous machine 1, 102 a time constant T G of the synchronous machine 1, 103 a gain K E of the exciter 3, 104 a time constant T E of the exciter 3, and 105 a tuning switch to instruct the operation of the control constant operation circuit 100. The above gains and time constants are set by a setting means (not shown) respectively. The operation of the controlling system of Fig. 5 will be described with reference to a flowchart in Fig. 6. A zero cross frequency ωc and a phase margin m are internally set within the control constant operation circuit 100. The gain K G , time constant T G , gain K E and time constant T E are set to the operation circuit 100 from outside (S51). Whether the tuning switch 105 is turned ON or OFF is judged (S52), and if the tuning switch 105 is ON, the following equations (1), (3) and (4) are operated (S53). T lead = T E The forward transfer function in a transfer function block diagram in Fig. 7 is represented as follows; K 1 · K 2 · K E · K G · 1 1 + T lag · S · 1 1 + T G · S T lag is determined according to the formula (3) below so that the function of the formula (2) has such characteristic that the zero cross frequency is ωc and the phase margin is m. T lag = tan [180 o - m - tan -1 ωcT G ωc Further, K₁ is determined similarly. K 1 = 1+ ωcT lag 1/2 1+ ωcT G 1/2 K 2 ·K E ·K G The AVR 6 of the instant embodiment operates (S55) with use of the above-obtained control parameters K₁, T lead and T lag thereby to control the exciter 3 (S56). Accordingly, if only the gain K G and time constant T G of the synchronous machine 1, gain K E and time constant T E of the exciter 3 are set, the control parameters K₁, T lead , T lag are automatically determined, thus making it easy to adjust the control to obtain the optimum. The time constant naturally do not depend on the controlling people. Even in such a structure as to set the gain K G , time constant T G of the synchronous machine 1, gain K E and time constant T E of the exciter 3 by hardware (setting device), an internal parameter may be used in the case where the AVR is of a digital type. [Embodiment 3] An excitation control apparatus according to a third embodiment of this invention is indicated in a block diagram of Fig. 8. The same parts of the third embodiment as in Fig. 1 and in the first and second embodiments will be designated by the same reference numerals, the description of which will be omitted. In Fig. 8, references show respectively: 110 a transfer characteristic detection/operation circuit for detecting/operating gains and time constants of the synchronous machine 1 and exciter 3, K G , T G , K E and T E gains and time constants of the synchronous machine 1 and the exciter 3 operated by the transfer characteristic detection/operation circuit 110 respectively, 111 a voltage signal outputted from the synchronous machine 1 and detected by the PT 4, 112 a current signal outputted from the exciter 3 and detected by the shunt 10, 113 an auto-tuning instruction switch for instructing the transfer characteristic detection/operation circuit 110 and control constant operation circuit 100 to operate or not, and 114 a step signal generation circuit for applying a step signal to the output of the amplifier circuit B 66 in response to the instruction from the transfer characteristic detection/operation circuit 110. The operation of the controlling system in the above-described structure will be explained with reference to a flowchart of Fig. 9. Fig. 9 indicates the timing of the operations by the transfer characteristic detection/operation circuit 110 and control constant operation circuit 100 in the whole of the AVR 6. Before the detection/operation of the transfer characteristic, whether or not the auto-tuning instruction switch 113 is turned ON is detected (S92). If the switch 113 is in the ON state, the transfer characteristic is operated (S93). The transfer characteristic detection/operation circuit 110 generates an instruction to the step signal generation circuit 114, thereby to apply a stepped disturbance signal to the output of the amplifier circuit B 66. As the output current signal 112 of the exciter detected by the shunt 10 is inputted to the transfer characteristic detection/operation circuit 110 (S91), the data when the changing volume of the peak value is within a predetermined range is selected and denoted as K E through analysis of the data stored in a built-in memory as shown in Fig. 11. Then, the time until the value reaches 63% of the final value K e is rendered T E . In other words, the step response at a detection point A115 of Fig. 10 is measured and, the gain K E and time constant T E of the exciter 3 are obtained. Fig. 12 is a waveform diagram when the time and peak value are stored in a memory by inputting the voltage signal from the synchronous machine 1 detected by the PT 4 to the transfer characteristic detection/operation circuit 110 (S91). The data when the changing volume of the peak value is within a predetermined range is selected as K X by analyzing the data in the memory. Moreover, the time T X until the value reaches 63% of the final value K X is obtained. That is, the response at a detection point B116 is measured. The response at the detection point B116 is expressed by a second-order function as in a formula (5) below; G S = K E 1+T E ·S · K G 1+T G ·S = K E ·K G 1 + T E + T G S + T E ·T G ·S 2 A term of high-order is negligible in general, and therefore the term S² can be omitted as follows; G S = K E ·K G 1+ T E +T G S = K X 1+T X S As a result, the gain K G and time constant T G of the synchronous machine 1 are obtained according to the formulae (7) and (8); T G = T X - T E K G = K X /K E The control constant operation circuit 100 determines the control parameters K₁, T lead , T lag of the AVR 6 in the same manner as in the second embodiment with using the gain K G , time constant T G of the synchronous machine,and the gain K E , and time constant T E of the exciter 3 determined by the transfer characteristic detection/operation circuit 110 along with the zero cross frequency ωc and phase margin m which are factors to determine the specifications of the control system incorporated inside (S94). T lead = T E The forward transfer function of the voltage function block diagram of Fig. 10 becomes as follows from the formula (9); K 1 · K 2 · K E · K G · [1/(1 + T lag · S)]·[1/(1 + T G ·S)] T lag is determined as below so that the function of the formula (10) has such characteristic that the zero cross frequency is ωc and phase margin is m. T lag = tan [180 o -m-tan -1 ωcT G ] ω Moreover, K₁ is determined likewise. K 1 = 1+ ωcT lag 1+ ωcT G K 2 ·K E ·K G As indicated by the above formulae (9), (11) and (12), if the constants K G , T G , K E and T E and ωc and m indicating the specifications of the controlling system are determined, the control parameters K₁, T lead , T lag are automatically determined. The operations according to the formulae (9), (11) and (12) are conducted by the control constant operation circuit 100 (S95), thereby to output a control signal for the exciter 3 (S96). In the foregoing second and third embodiments, the exciter 3 is a rotary exciter having a gain and a first-order lag characteristic. However, a static exciter having a negligible first-order lag characteristic can be employed. Moreover, although the control function of the AVR 6 is represented by gains and first-order lead/lag functions, a minor loop, i.e., an output feedback loop can be provided to compensate the lag of the exciter. Since the scope of the invention is defined by the appended claims, all changes that fall within the bounds of the claims, or equivalence of such bounds are therefore intended to be embraced by the claims.

Claims: An excitation control apparatus for a synchronous machine (1) comprising: an exciter (3), an automatic voltage regulator (AVR; 6) for controlling the output voltage from the synchronous machine (1) so as to coincide with a desired control value (62) by controlling the current output from the exciter (3) to the synchronous machine (1); a voltage/frequency ratio limiter (8) for limiting a ratio (V/Hz) of voltage to frequency of the synchronous machine (1) so as not to exceed a predetermined ratio value (81) by outputting a signal to the automatic voltage regulator (AVR; 6) on the basis of the detected output voltage and frequency from the synchronous machine (1); characterized by an integrator (84) for time integrating a value corresponding to the difference between the detected voltage/frequency ratio (V/Hz) and said predetermined ratio value (81); adding means (601) for correcting said desired control value (62) by adding the integrated value to said desired control value. An excitation control apparatus for a synchronous machine (1) comprising: an exciter (3); an automatic voltage regulator (AV 3; 6) for controlling the output voltage from the synchronous machine (1) so as to coincide with a desired control value (62) by controlling the current output from the exciter (3) to the synchronous machine (1); characterized by detection/operation circuit (110) for determining respective constants (K G , T G , K E , T E ) of the transfer characteristic of the exciter (3) and the synchronous machine (1) by detecting the output current of the exciter (3) and the output voltage of the synchronous machine (1); an operation circuit (100) for determining control parameters (K₁, T lead , T lag ) for automatic voltage regulation from said determined constants of the exciter (3) and synchronous machine (1), and from factors (ω c ,  m ) for defining the control specification. An excitation control apparatus for a synchronous machine comprising: an exciter (3); an automatic voltage regulator (AVR; 6) for controlling the output voltage from the synchronous machine (1) so as to coincide with a desired control value by controlling current amount output from the exciter (3) to the synchronous machine (1); characterized by setting means (101 - 104) for setting respective constants (K G , T G , K E , T E ) of the transfer characteristic of the exciter (3) and the synchronous machine (1); an operation circuit (100) for determining control parameters (K₁, T lead , T lag ) for automatic voltage regulation from said set constants of the exciter (3) and synchronous machine (1), and from factors (ω c ,  m ) for defining the control specification. An excitation control apparatus as forth in one of Claims 1 to 3, wherein the control parameters of the automatic voltage regulator (AVR; 6) are defined by gains and first-order lead/lag functions. An excitation control apparatus as set forth in one of Claims 1 to 4, wherein a feedback loop for compensating delay of the exciter (3) is provided. An excitation control apparatus as set forth in one of Claims 1 to 5, wherein said exciter (3) is a rotary exciter having a gain and a first-order lag characteristic. An excitation control apparatus as set forth in one of Claims 1 to 5, wherein said exciter (3) is a static exciter whose first-order lag is almost negligible. An excitation control apparatus as set forth in one of Claims 2 or 3, further comprising a switch (115, 113) for directing said operation circuits (100, 110) to start their operation. An excitation control apparatus as set forth in Claim 2, further comprising a signal generating circuit (114) for outputting step signals to the excitation system of the synchronous machine.

Inventors: FUJIKAWA MEGUMI C O SEIGYO SEI, TANAKA SEIICHI C O SEIGYO SEIS, FUJIKAWA, MEGUMI, C/O SEIGYO SEISAKUSHO, TANAKA, SEIICHI, C/O SEIGYO SEISAKUSHO
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H02P9
IPC codes: H02P9