Excitation controller and excitation control method for stabilizing voltage in electric power system

[0002] 1. Field of the Invention

[0003] The present invention relates to an excitation controller and an excitation control method for stabilizing voltage in an electric power system.

[0004] 2. Description of the Prior Art

[0005] Prior art excitation controllers detect a high-side voltage of a transformer connected to a synchronous machine or generator and then control the synchronous machine according to the detected voltage such that the high-side voltage has a constant value, in order to improve the stability of the electric power system, as disclosed in Japanese patent application publication (TOKKAIHEI) No.4-79798, for example.

[0006] Referring next to FIG. 23, there is illustrated a block diagram showing the structure of a prior art excitation controller as disclosed in Japanese patent application publication (TOKKAIHEI) No.4-79798. As shown in the figure, a first potential transformer or PT 102 detects an output terminal voltage V_Gof a synchronous machine 101, and a subtracter 104 subtracts the detected output terminal voltage V_Gfrom a reference voltage r_Gset by a voltage setter 103. A reduced gain circuit 105 then multiplies the result of the subtraction from the subtracter 104 by a gain β.

[0007] A second potential transformer or PT 109 detects a transmission voltage V_Hon a transmission bus 106 (i.e., a high-side voltage of a transformer 108 connected, by way of a breaker 107, to the transmission bus 106), and a second subtracter 111 subtracts the detected transmission voltage V_Hfrom a transmission reference voltage r_Hset by a high-side voltage setter 110. A high-voltage gain circuit 112 then multiplies the result of the subtraction from the second subtracter 111 by a gain K_H.

[0008] Then an adder 113 adds the result of the multiplication from the reduced gain circuit 105 and the result of the multiplication from the high-voltage gain circuit 112 together. An automatic voltage regulator or AVR 114 generates a timing signal for controlling the rectifying timing of an exciter 115 based on the result of the addition from the adder 113. The exciter 115 receives the timing signal and then supplies a field current to a field winding 116 of the synchronous machine 101 according to the timing signal.

[0009] In this way, the prior art excitation controller detects the high-side voltage of the transformer 108, and then controls the synchronous machine such that the transmission voltage on the transmission bus 106 remains constant.

[0010] A problem with a prior art excitation controller constructed as above is that although it can keep the high-side voltage of a transformer connected to a synchronous machine constant, in addition to an excitation control panel on which an AVR and an adder are mounted, another control panel is needed for mounting a high-side voltage setter, a potential transformer, a reference value correction circuit, and so on thereon, thus increasing the manufacturing cost of the electric power system.

[0011] Another problem is that since a transmission bus is located at a considerable distance from an electric power plant building for housing the synchronous machine, the transformer, and so on, a long cable susceptible to noise is needed to connect an excitation control panel located in the electric power plant building to a PT located in the vicinity of the transmission bus, thus decreasing the reliability of the electric power system.

[0012] A further problem is that an expensive PT is needed to detect the transmission voltage on the transmission bus and hence the manufacturing cost of the electric power system is increased.

[0013] A still further problem is that since when a transformer equipped with a tap changer for changing taps under load conditions is provided, the tap-changer-equipped transformer including an automatic tap control function, changing taps changes the transformation ratio, it is difficult to maintain the output terminal voltage of the generator at a nominal value and to keep the transmission voltage on the transmission bus constant. Another problem is that since the reactance X_tof the tap-changer-equipped transformer changes with a change in taps of the tap-changer-equipped transformer as this results in a change in the ratio of a cross-current prevention reactance X_DRfor preventing any cross current from flowing between the synchronous machine and another synchronous machine, to the reactance X_tof the tap-changer-equipped transformer, a cross current can flow among a plurality of synchronous machines including the above-mentioned synchronous machines, which operate in parallel with one another, if the plurality of synchronous machines have different transformation ratios.

[0014] The present invention is proposed to solve the above problems. It is therefore an object of the present invention to provide an excitation controller and an excitation control method capable of maintaining the output terminal voltage of a synchronous machine or generator, at a nominal value and keeping the transmission voltage on a transmission bus constant without having to detect the transmission voltage on the transmission bus.

[0015] In accordance with one aspect of the present invention, there is provided an excitation controller comprising: a reactive current detector for detecting a reactive current output from a synchronous machine connected, by way of a tap-changer-equipped transformer equipped with a tap changer for changing taps under load conditions, to a transmission system; a tap controller for setting a tap ratio of the tap-changer-equipped transformer according to an output terminal voltage of the synchronous machine; a voltage setter for setting a synchronous machine's-side reference voltage of the tap-changer-equipped transformer on the basis of a transmission system's-side reference voltage of the tap-changer-equipped transformer, the tap ratio set by the tap controller, and the reactive current detected by the reactive current detector; and a controlling unit for controlling an excitation system for exciting the synchronous machine according to the reference voltage set by the voltage setter.

[0016] In accordance with a preferred embodiment of the present invention, the voltage setter obtains the synchronous machine's-side reference voltage by dividing the transmission system's-side reference voltage of the tap-changer-equipped transformer by the tap ratio, and by adding a voltage drop in the tap-changer-equipped transformer to a result of the division, the voltage drop being obtained from the reactive current. Preferably, another synchronous machine can be connected to the transmission system, and the voltage setter can obtain the voltage drop on the basis of a reactance of the tap-changer-equipped transformer and a cross-current prevention reactance for preventing any cross current from flowing between the synchronous machine and the other synchronous machine.

[0017] In accordance with another preferred embodiment of the present invention, the voltage setter obtains the synchronous machine's-side reference voltage by dividing the transmission system's-side reference voltage of the tap-changer-equipped transformer by a step-up ratio corresponding to the tap ratio, and by adding a voltage drop in the tap-changer-equipped transformer to a result of the division, the voltage drop being obtained from a reactance change ratio corresponding to the tap ratio. Preferably, another synchronous machine can be connected to the transmission system, and the voltage setter can obtain the voltage drop on the basis of the step-up ratio, the reactance change ratio, and a cross-current prevention reactance for preventing any cross current from flowing between the synchronous machine and the other synchronous machine.

[0018] In accordance with another preferred embodiment of the present invention, when the reactive current has a predetermined reference value other than zero, the voltage setter sets the synchronous machine's-side reference voltage of the tap-changer-equipped transformer in such a manner that the voltage drop due to the cross-current prevention reactance becomes zero.

[0019] In accordance with another aspect of the present invention, there is provided an excitation control method comprising the steps of: setting a synchronous machine's-side reference voltage of a tap-changer-equipped transformer equipped with a tap changer for changing taps under load conditions on the basis of a reactive current output from a synchronous machine connected, by way of the tap-changer-equipped transformer, to a transmission system, a tap ratio of the tap-changer-equipped transformer, and a transmission system's-side reference voltage of the tap-changer-equipped transformer; and controlling an excitation system for exciting the synchronous machine according to the reference voltage set in the setting step.

[0020] In accordance with a preferred embodiment of the present invention, the setting step includes the step of obtaining the synchronous machine's-side reference voltage by dividing the transmission system's-side reference voltage of the tap-changer-equipped transformer by the tap ratio, and by adding a voltage drop in the tap-changer-equipped transformer to a result of the division, the voltage drop being obtained from the reactive current. Preferably, the setting step includes the step of obtaining the voltage drop on the basis of a reactance of the tap-changer-equipped transformer and a cross-current prevention reactance for preventing any cross current from flowing between the synchronous machine and another synchronous machine connected to the transmission system.

[0021] In accordance with another preferred embodiment of the present invention, the setting step includes the step of obtaining a step-up ratio and a reactance change ratio corresponding to the tap ratio, and the step of obtaining the synchronous machine's-side reference voltage by dividing the transmission system's-side reference voltage of the tap-changer-equipped transformer by the step-up ratio, and by adding a voltage drop in the tap-changer-equipped transformer to a result of the division, the voltage drop being obtained from the reactance change ratio. Preferably, the setting step includes the step of obtaining the voltage drop on the basis of the step-up ratio, the reactance change ratio, and a cross-current prevention reactance for preventing any cross current from flowing between the synchronous machine and another synchronous machine connected to the transmission system.

[0022] In accordance with another preferred embodiment of the present invention, the setting step is the step of setting the synchronous machine's-side reference voltage of the tap-changer-equipped transformer in such a manner that the voltage drop due to the cross-current prevention reactance becomes zero, when the reactive current has a predetermined reference value other than zero. The excitation control method can further comprise the step of setting the predetermined reference value according to a transmission system's-side voltage of the tap-changer-equipped transformer. As an alternative, the excitation control method can further comprise the step of setting the predetermined reference value according to a difference between a previous transmission system's-side reference voltage of the tap-changer-equipped transformer and a new transmission system's-side reference voltage of the tap-changer-equipped transformer, and a reactance of a transmission line of the transmission system. Preferably, the excitation control method further comprises the step of estimating the reactance of the transmission line of the transmission system, which can vary with time.

[0023] Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.

[0024] FIG. 1 is a block diagram showing the structure of an electric power system provided with an excitation controller according to a first embodiment of the present invention;

[0025] FIG. 2 is a system diagram showing the electric power system as shown in FIG. 1;

[0026] FIG. 3 is a block diagram showing the structure of an example of a voltage setter as shown in FIG. 1;

[0027] FIG. 4 is a block diagram showing the structure of another example of the voltage setter as shown in FIG. 1;

[0028] FIG. 5 is a flow diagram showing an excitation control method according to the first embodiment of the present invention;

[0029] FIG. 6 is a block diagram showing the structure of an electric power system provided with an excitation controller according to a second embodiment of the present invention;

[0030] FIG. 7 is a system diagram showing the electric power system as shown in FIG. 6;

[0031] FIG. 8 is a block diagram showing the structure of an example of a voltage setter as shown in FIG. 6;

[0032] FIG. 9 is a flow diagram showing an excitation control method according to the second embodiment of the present invention;

[0033] FIG. 10 is a block diagram showing the structure of an electric power system provided with an excitation controller according to a third embodiment of the present invention;

[0034] FIG. 11 is a flow diagram showing an excitation control method according to the third embodiment of the present invention;

[0035] FIG. 12 is a graph showing a relationship between a reactive current output from a synchronous machine and a high-side voltage of a tap-changer-equipped transformer;

[0036] FIG. 13 is a flow diagram showing an excitation control method according to a fourth embodiment of the present invention;

[0037] FIG. 14 is a block diagram showing the structure of an electric power system provided with an excitation controller according to a sixth embodiment of the present invention;

[0038] FIG. 15 is a flow diagram showing an excitation control method according to the sixth embodiment of the present invention;

[0039] FIG. 16 is a block diagram showing the structure of an electric power system provided with an excitation controller according to a seventh embodiment of the present invention;

[0040] FIG. 17 is a flow diagram showing an excitation control method according to the seventh embodiment of the present invention;

[0041] FIG. 18 is a graph showing a relationship between a high-side voltage of each of a plurality of tap-changer-equipped transformers of split winding type, which are connected to a plurality of synchronous machines, respectively, and a reactive current output from each of the plurality of synchronous machines;

[0042] FIG. 19 is a block diagram showing the structure of an electric power system provided with an excitation controller according to an eighth embodiment of the present invention;

[0043] FIG. 20 is a flow diagram showing an excitation control method according to the eighth embodiment of the present invention;

[0044] FIG. 21 is a flow diagram showing an excitation control method according to a ninth embodiment of the present invention;

[0045] FIG. 22 is a flow diagram showing an excitation control method according to a tenth embodiment of the present invention; and

[0046] FIG. 23 is a block diagram showing the structure of an electric power system provided with a prior art excitation controller.

[0047] First Embodiment

[0048] Referring next to FIG. 1, there is illustrated a block diagram showing the structure of an electric power system provided with an excitation controller according to a first embodiment of the present invention. FIG. 2 is a system diagram showing the electric power system shown in FIG. 1. In the figures, reference numeral 1 denotes a synchronous machine, numeral 2 denotes a tap-changer-equipped transformer equipped with a tap changer for changing a tap ratio according to an actual output terminal voltage V_Gof the synchronous machine 1 under load conditions, numeral 3 denotes a first potential transformer or PT for detecting the actual output terminal voltage V_Gof the synchronous machine 1, numeral 4 denotes a tap controller for controlling the tap ratio n of the tap-changer-equipped transformer 2 according to the output terminal voltage V_Gdetected by the first PT 3, numeral 5 denotes a breaker, numeral 6 denotes a transmission line, numeral 7 denotes a transmission bus of a power plant, numeral 8 denotes a second potential transformer or PT for detecting the actual output terminal voltage V_Gof the synchronous machine 1, and numeral 9 denotes a current transformer for detecting a reactive current I_Qoutput from the synchronous machine 1.

[0049] Furthermore, reference numeral 10 denotes a voltage setter for setting a synchronous machine's-side reference voltage V_Grefof the tap-changer-equipped transformer 2 (i.e., an output terminal reference voltage of the synchronous machine 1) on the basis of the reactive current I_Qdetected by the CT 9, the tap ratio n controlled by the tap controller 4, and a transmission's-side (i.e., high-side) reference voltageV_Hrefof the tap-changer-equipped transformer 2.

[0050] Reference numeral 11 denotes a subtracter for subtracting the output terminal voltage V_Gdetected by the second PT 8 from the output terminal reference voltage V_Grefset by the voltage setter 10, and for furnishing a deviation signal having a value corresponding to the result of the subtraction, and numeral 12 denotes an automatic voltage regulator or AVR for controlling the rectifying timing of an exciter 13 on the basis of the deviation signal fed from the subtracter 11. The exciter 13 can supply a field current to a field winding 14 of the synchronous machine 1 according to an instruction from the AVR 12. The AVR 12 and the exciter 13 control the excitation system for exciting the synchronous machine. In FIG. 2, X_tdesignates a reactance of the tap-changer-equipped transformer 2, X_Ldesignates a reactance of the transmission line, and V_Sdesignates an infinite voltage of the transmission bus.

[0051] Referring next to FIG. 3, there is illustrated a block diagram showing the structure of the voltage setter 10 as shown in FIG. 1. In the figure, reference numeral 10adenotes an arithmetic operator, and numeral 10bdenotes a memory in which the high-side reference voltage V_Href, the reactance X_tof the tap-changer-equipped transformer 2, and so on are stored. The arithmetic operator 10acan compute the output terminal reference voltage V_Grefusing the contents of the memory 10b.

[0052] The voltage setter 10 is not limited to the one as shown in FIG. 3. As an alternative, the voltage setter 10 can be implemented via hardware components such as a divider 10c,a multiplier 10d,and an adder 10e,as shown in FIG. 4.

[0053] Next, a description will be made as to the operation of the excitation controller of the first embodiment with reference to FIG. 5, a flowchart of an excitation control method in accordance with the first embodiment of the present invention.

[0054] The second PT 8, in step ST11, detects the actual output terminal voltage V_Gof the synchronous machine 1, and the CT 9, in step ST12, detects the reactive current I_Qoutput from the synchronous machine 1. Then the voltage setter 10, in step ST13, calculates and sets the output terminal reference voltage V_Greffrom the detected reactive current I_Q, the tap ratio n controlled by the tap controller 4, and the high-side reference voltage V_Hrefof the tap-changer-equipped transformer 2, according to the following equation (1):

[0055] V_Gref=V_Href/n+X_t*I_Q  (1)

[0056] where X_tis the reactance of the tap-changer-equipped transformer 2. The high-side reference voltage V_Hrefcan be determined according to desired operation conditions set for the electric power plant.

[0057] After the voltage setter 10 has set the output terminal reference voltage V_Grefof the synchronous machine 1 in this way, the subtracter 11, in step ST14, subtracts the actual output terminal voltage V_Gof the synchronous machine 1, which has been detected by the second PT 8, from the output terminal reference voltage V_Grefset by the voltage setter 10, and furnishes the deviation signal having a value corresponding to the result of the subtraction to the AVR 12.

[0058] The AVR 12 then, in step ST15, generates a timing signal on the basis of the deviation signal output from the subtracter 11 to control the rectifying timing of the exciter 13. The exciter 13, in step ST16, receives the timing signal from the AVR 12 and supplies the field current to the field winding 14 of the synchronous machine 1 according to the timing signal.

[0059] Incidentally, if the deviation signal output from the subtracter 11 has a positive value, the field current fed to the field winding 14 is increased and hence the output terminal voltage V_Gof the synchronous machine 1 increases, whereas if the deviation signal output from the subtracter 11 has a negative value, the field current fed to the field winding 14 is decreased and hence the output terminal voltage V_Gof the synchronous machine 1 decreases. In this manner, the AVR 12 and the exciter 13 control the synchronous machine 1 so that the actual output voltage V_Gof the synchronous machine 1 is equal to the output terminal reference voltage V_Gref.

[0060] As can be seen from the system diagram of FIG. 2, when the tap ratio of the tap-changer-equipped transformer 2 is n, there is a relationship, as expressed by the following equation (2), between the output terminal voltage V_Gof the synchronous machine 1 and the high-side voltage V_Hof the tap-changer-equipped transformer 2.

[0061] V_H=n(V_G−X_t*I_Q)  (2)

[0062] Accordingly, using the high-side reference voltage V_Href, the output terminal voltage V_Gof the synchronous machine 1 and the high-side voltage V_Hof the tap-changer-equipped transformer 2 can be given by the following equations (3) and (4):

[0063] V_G=V_Href/n+X_t*I_Q  (3)

[0064] V_H=V_Href  (4)

[0065] Thus, when the actual output terminal voltage V_Gof the synchronous 1 under normal operating conditions is high, by increasing the tap ratio n (n>1) by means of the tap controller 4, the excitation controller of the first embodiment can maintain the actual output terminal voltage V_Gat about a nominal value. In addition, even though the tap ratio n is changed, the excitation controller of the first embodiment can control the actual high-side voltage V_Hof the tap-changer-equipped transformer 2 so that it is equal to the high-side reference voltage V_Href.

[0066] As previously mentioned, the excitation controller in accordance with the first embodiment of the present invention sets the output terminal reference voltage V_Grefof the synchronous machine 1 from the reactive current I_Qoutput from the synchronous machine 1, the tap ratio n of the tap-changer-equipped transformer 2, the high-side reference voltage V_Hrefof the tap-changer-equipped transformer 2, and then controls the field current to be supplied to the field winding 14 of the synchronous machine 1 according to the difference between the output terminal reference voltage V_Grefand the actual output terminal voltage V_G. Accordingly, even though transformer taps are changed in the tap-changer-equipped transformer 2 and hence the transformation ratio is changed, the excitation controller can hold the output terminal voltage V_Gof the synchronous machine 1 at about a nominal value and therefore keep the transmission bus 7 at a certain voltage. As a result, the excitation controller of the first embodiment can make the most of the ability of the generator even in the event of a failure on the transmission system or even in the event of a rapid increase in the load, thus offering a significant improvement in the voltage stability.

[0067] Furthermore, since there is no need to detect the high-side voltage of the tap-changer-equipped transformer 2, only one excitation control panel is needed in the vicinity of the synchronous machine. In other words, there is no need to provide two control panels, and this results in space savings.

[0068] In addition, since there is no need to detect the high-side voltage of the tap-changer-equipped transformer 2, there is no need to connect the excitation control panel in which the AVR, the adder and so on are included with the transmission bus through a cable or the like. Accordingly, noise that has a deleterious effect on the excitation controller can be reduced, thereby improving the reliability.

[0069] Moreover, since there is no need to provide an expensive PT for detecting the high-side voltage of the tap-changer-equipped transformer 2, the manufacturing cost can be reduced.

[0070] Embodiment 2

[0071] In the aforementioned first embodiment, it is assumed that only one synchronous machine is connected to the transmission system. In contrast, an excitation controller in accordance with a second embodiment of the present invention can control a plurality of synchronous machines connected to the transmission system. For simplicity, an electric power system provided with the excitation controller according to the second embodiment of the present invention includes two synchronous machines.

[0072] Referring next to FIG. 6, there is illustrated a block diagram showing the structure of the electric power system provided with the excitation controller according to the second embodiment of the present invention. FIG. 7 is a system diagram showing the electric power system shown in FIG. 6. In the figures, reference numeral 1adenotes another synchronous machine that differs from a first synchronous machine 1, numeral 2adenotes a second tap-changer-equipped transformer that differs from a first tap-changer-equipped transformer 2, the second tap-changer-equipped transformer being equipped with a tap changer for changing tap ratios according to the output terminal voltage V_G1of the second synchronous machine 1aunder load conditions, and numeral 5adenotes a second breaker. The other structure of the excitation controller of the second embodiment is the same as that of the excitation controller of the aforementioned first embodiment as shown in FIG. 1, with the exception that a voltage setter 10 of the second embodiment has a different mechanism for setting the output terminal reference voltage V_Grefof the synchronous machine 1, and therefore the description of the same components will be omitted hereinafter. Also, the excitation controller according to the second embodiment includes first and second PTs for detecting the actual output terminal voltage V_G1of the second synchronous machine 1a,a CT for detecting the reactive current output from the second synchronous machine 1a,a voltage setter for setting the output terminal reference voltage of the second synchronous machine 1a,an exciter for exciting the second synchronous machine 1a,and a subtracter and an AVR for controlling the exciter, which are not shown in FIG. 6.

[0073] The voltage setter 10 can be implemented via hardware components including a second divider 10fand a subtracter 10g,as shown in FIG. 8, in addition to the hardware components of FIG. 3.

[0074] Next, a description will be made as to the operation of the excitation controller of the second embodiment with reference to FIG. 9, a flowchart of an excitation control method in accordance with the second embodiment of the present invention. All steps of FIG. 9 except step ST23, which takes the place of ST13 of FIG. 5, are already shown in FIG. 5, and therefore the description of the same steps can be omitted hereinafter.

[0075] The voltage setter 10, in step ST23, calculates and sets the output terminal reference voltage V_Grefof the first synchronous machine 1 from the detected reactive current I_Q, the tap ratio n controlled by the tap controller 4, and the high-side reference voltage V_Hrefof the first tap-changer-equipped transformer 2, according to the following equation:

[0076] V_Gref=V_Href/n+(X_t−X_DR/n)*I_Q  (5)

[0077] where X_tis the reactance of the tap-changer-equipped transformer 2, and X_DRis a cross-current prevention reactance for preventing any cross current from flowing, by way of the transmission bus 7, among the plurality of synchronous machines connected to the transmission system.

[0078] Similarly, the other voltage setter (not shown) intended for the second synchronous machine 1acalculates and sets the output terminal reference voltage V_Grefof the second synchronous machine 1afrom the detected reactive current I_Q, the tap ratio n controlled by the tap controller 4, and the high-side reference voltage V_Hrefof the second tap-changer-equipped transformer 2a,according to equation (5).

[0079] As shown in equation (5), since the output terminal reference voltage V_Grefis set in consideration with the cross-current prevention reactance X_DR, no cross current flows among the plurality of synchronous machines connected to the transmission system, and the actual output terminal voltage V_Gis controlled such that it agrees with the output terminal reference voltage V_Gref.

[0080] As can be seen from the system diagram of FIG. 7, when the current tap ratio of the tap-changer-equipped transformer 2 is n, there is a relationship, as expressed by the following equation (6), between the output terminal voltage V_Gof the first synchronous machine 1 and the high-side voltage V_Hof the first tap-changer-equipped transformer 2.

[0081] V_H=n(V_G−X_t*I_Q)  (6)

[0082] Accordingly, using the high-side reference voltage V_Href, the output terminal voltage V_Gof the first synchronous machine 1 and the high-side voltage V_Hof the first tap-changer-equipped transformer 2 can be given by the following equations (7) and (8):

[0083] V_G=V_Href/n+(X_t−X_DR/n)*I_Q  (7)

[0084] V_H=V_Href−X_DR*I_Q  (8)

[0085] Thus, when the actual output terminal voltage V_Gof the first synchronous machine 1 under normal operating conditions is high, by increasing the tap ratio n (i.e., n>1) by means of the tap controller 4, the excitation controller of the second embodiment can maintain the actual output terminal voltage V_Gof the first synchronous machine 1 at about a nominal value. In addition, even though the current tap ratio is changed, the excitation controller of the second embodiment can control the actual high-side voltage V_Hof the first tap-changer-equipped transformer 2 such that it agrees with the high-side reference voltage V_Href. Similarly, the excitation controller can maintain the actual output terminal voltage V_G1of the second synchronous machine 1aat about a nominal value, and control the actual high-side voltage V_Hof the second tap-changer-equipped transformer 2asuch that it agrees with the high-side reference voltage V_Href, even though the current tap ratio of the second transformer 2ais changed.

[0086] As previously mentioned, the excitation controller in accordance with the second embodiment of the present invention sets the actual output terminal reference voltage V_Grefof each of the plurality of synchronous machines in consideration with the cross-section prevention reactance X_DR. Accordingly, the present embodiment makes it possible to prevent any cross current from flowing among the plurality of synchronous machines that operate in parallel with one another even when the plurality of tap-changer-equipped transformers respectively connected to the plurality of synchronous machines have different transformation ratios.

[0087] Embodiment 3

[0088] The excitation controller in accordance with the second embodiment sets the output terminal reference voltage V_Grefof each of the plurality of synchronous machines from the reactive current I_Qoutput from each of the plurality of synchronous machines, the tap ratio n of the corresponding tap-changer-equipped transformer 2 or 2a,and the high-side reference voltage V_Hrefof the plurality of tap-changer-equipped transformers 2 and 2a,according to equation (5). In this case, since the output terminal reference voltage V_Grefof each of the plurality of synchronous machines is calculated in consideration with the cross-current prevention reactance X_DR, the high-side voltage V_Hof the plurality of tap-changer-equipped transformers 2 and 2aagrees with the reference voltage V_Hrefonly when the reactive current I_Q=0 (see equation (8)). However, when each of the plurality of generators operates under normal operating conditions, because of the reactive current I_Q≠0, it is impossible for the high-side voltage V_Hof the plurality of tap-changer-equipped transformers 2 and 2ato be made equal to the reference voltage V_Href.

[0089] In contrast, when all the generators operate under normal operating conditions, an excitation controller in accordance with a third embodiment sets the output terminal reference voltage V_Grefof each of the plurality of synchronous machines such that the high-side voltage V_Hof the plurality of tap-changer-equipped transformers agrees with the reference voltage V_Hrefwhen the reactive current I_Qoutput from each of the plurality of synchronous machine agrees with a reference value I_Q0other than zero.

[0090] An electric power system including the excitation controller of the third embodiment has the same structure as that as shown in FIG. 6, and therefore the description of the structure will be omitted hereinafter. The voltage setter 10 can be implemented via hardware components including a second subtracter 10hand a second multiplier 10i,as shown in FIG. 10, in addition to the hardware components of FIG. 8.

[0091] Next, a description will be made as to the operation of the excitation controller of the third embodiment with reference to FIG. 11, a flowchart of an excitation control method in accordance with the third embodiment of the present invention. All steps of FIG. 11 except step ST33, which takes the place of ST13 of FIG. 5, are already shown in FIG. 5, and therefore the description of the same steps can be omitted hereinafter.

[0092] The voltage setter 10, in step ST33, calculates and sets the output terminal reference voltage V_Grefof the first synchronous machine 1 from the reactive current I_Qoutput from the first synchronous machine 1, the reactive current reference value I_Q0other than 0, the tap ratio n of the first tap-changer-equipped transformer 2, and the high-side reference voltage V_Hrefof the first tap-changer-equipped transformer 2, according to the following equation (9):

[0093] V_Gref=V_Href/n+X_t*I_Q−X_DR/n*(I_Q−I_Q0)  (9)

[0094] where the reference value I_Q0can be given by I_Q0=Q_S/V_g, in which V_gis the voltage of each generator and Q_gis the reactive power in an operating state in which the high-side reference voltage=V_Href. V_gand Q_gare determined by the operating condition of each generator, system conditions and so on.

[0095] Similarly, the second voltage setter (not shown) intended for the second synchronous machine 1acalculates and sets the output terminal reference voltage V_Grefof the second synchronous machine 1afrom the reactive current I_Qoutput from the second synchronous machine 1a,the reactive current reference value I_Q0other than 0, the tap ratio n of the second tap-changer-equipped transformer 2a,and the high-side reference voltage V_Hrefof the second tap-changer-equipped transformer 2a,according to equation (9).

[0096] In this way, the output terminal reference voltage V_Grefof each of the first and second synchronous machines 1 and 1ais determined according to equation (9), and each of these synchronous machines is controlled according to the output terminal reference voltage V_Gref. As a result, the output voltage V_Gof each of the plurality of synchronous machines and the high-side voltage V_Hof the corresponding tap-changer-equipped transformer can be expressed as follows:

[0097] V_G=V_Href/n+X_t*I_Q−X_DR/n*(I_Q−I_Q0)  (10)

[0098] V_H=V_Href−X_DR*(I_Q−I_Q0)  (11)

[0099] Referring next to FIG. 12, there is illustrated a graph showing a relationship between the reactive current I_Qoutput from each of the plurality of synchronous machines and the high-side voltage V_Hof the corresponding tap-changer-equipped transformer. In the figure, reference numeral 21 denotes the high-side voltage V_Hof each of the plurality of tap-changer-equipped transformers 2 and 2awhich is given by equation (8) as explained in Embodiment 2, and numeral 22 denotes the high-side voltage V_Hof each of the plurality of tap-changer-equipped transformers 2 and 2awhich is given by equation (11) of the present embodiment. The horizontal axis shows the reactive current I_Qand the vertical axis shows the high-side voltage V_H.

[0100] As shown in FIG. 12, the relationship designated by 21 (equation (8)) indicates that the high-side voltage V_Hof the corresponding tap-changer-equipped transformer 2 or 2ais equal to the reference voltage V_Hrefonly when the reactive current I_Q=0, whereas the relationship designated by 22 (equation (11)) indicates that the high-side voltage V_Hof the corresponding tap-changer-equipped transformer 2 or 2ais equal to the reference voltage V_Hrefwhen the reactive current I_Q=I_Q0other than zero. In other words, when the reactive current I_Qhas a certain reference value other than zero, the voltage drop due to the cross-current prevention reactance becomes zero.

[0101] As previously mentioned, the excitation controller in accordance with the third embodiment sets the output terminal reference voltage V_Grefof each of the plurality of synchronous machines such that the actual high-side voltage V_Hof the plurality of tap-changer-equipped transformers agrees with the reference voltage V_Hrefwhen the reactive current I_Qoutput from each of the plurality of synchronous machines is equal to its reference value I_Q0other than zero. Accordingly, the present embodiment offers an advantage of being able to make the actual high-side voltage V_Hof the plurality of tap-changer-equipped transformers agree with the reference voltage V_Hrefmore precisely than the aforementioned second embodiment does, and to maintain the actual output terminal voltage V_Gof each of the plurality of synchronous machines at about a nominal value.

[0102] Embodiment 4

[0103] The excitation controller of the aforementioned third embodiment controls each of the plurality of synchronous machines using the reactive current reference value I_Q0corresponding to the high-side reference voltage V_Hrefof the corresponding tap-changer-equipped transformer. In this case, if the high-side reference voltage of the plurality of tap-changer-equipped transformers is changed from V_Href1to V_Href2in an operating state in which the reactive current reference value I_Q0=I_Q01and the high-side voltage V_Hof each of the plurality of tap-changer-equipped transformers=V_Href1, the varied (i.e., new) high-side voltage V_H2of each of the plurality of tap-changer-equipped transformers is expressed as follows because the reactive current I_Qalso changes from I_Q1to I_Q2.

[0104] V_H2=V_Href2−X_DR*(I_Q2−I_Q01)  (12)

[0105] However, the high-side voltage V_H2of each of the plurality of tap-changer-equipped transformers does not agree with the varied reference voltage V_Href2because I_Q2≠I_Q01in equation (12).

[0106] In contrast, an excitation controller in accordance with a fourth embodiment can set a reactive current reference value I_Q0corresponding to the varied reference voltage V_Href2on the basis of the difference between the previous high-side reference voltage V_Href1and the varied or new reference voltage V_Href2and the reactance of the transmission line so as to make the high-side voltage V_H2of the plurality of tap-changer-equipped transformers agree with the varied reference voltage V_Href2.

[0107] An electric power system including the excitation controller of the fourth embodiment has the same structure as that as shown in FIG. 6, and therefore the description of the structure will be omitted hereinafter. The voltage setter 10 can be implemented via hardware components as shown in FIG. 10, so as to change the reactive current reference value I_Q0as the high-side reference voltage V_Hrefof the plurality of tap-changer-equipped transformers changes.

[0108] Next, a description will be made as to the operation of the excitation controller of the fourth embodiment with reference to FIG. 13, a flowchart of an excitation control method in accordance with the fourth embodiment of the present invention. In the flowchart of FIG. 13, steps ST41, ST42, and ST45 to ST47 are the same as steps ST11, ST12, and ST14 to ST16 of FIG. 5, and therefore the description of those steps will be omitted hereinafter.

[0109] The voltage setter 10, in step ST43, calculates a new reactive current reference value I_Q0by subtracting the previous reference voltage V_Href1from the varied or new reference voltage V_Href2, followed by dividing the result of the subtraction by the reactance X_Lof the transmission line 6, and by adding the result of the division to the previous reference value I_Q01, according to the following equation:

[0110] I_Q0=I_Q01+(V_Href2−V_Href1)/X_L  (13)

[0111] The voltage setter 10 then, in step ST44, calculates and sets a new output terminal reference voltage V_Grefof the first synchronous machine 1 according to the following equation (14):

[0112] V_Gref=V_Href2/n+X_t*I_Q−X_DR/n*(I_Q−I_Q0)  (14)

[0113] Similarly, the other voltage setter (not shown) intended for the second synchronous machine 1acalculates and sets a new output terminal reference voltage V_Grefof the second synchronous machine 1aaccording to equation (14).

[0114] In this way, the output terminal reference voltage V_Grefof each of the plurality of synchronous machines 1 and 2ais determined according to equation (14), and each of these synchronous machines 1 and 1ais controlled according to the output terminal reference voltage V_Gref. As a result, the output voltage V_Gof each of the plurality of synchronous machines and the high-side voltage V_Hof the corresponding tap-changer-equipped transformer can be expressed as follows:

[0115] V_G=V_Href2/n+X_t*I_Q−X_DR/n*[I_Q−{I_Q01+(V_Href2−V_Href1)/X_L}]  (15)

[0116] V_H=V_Href2−X_DR*[I_Q−{I_Q01+(V_Href2−V_Href1)/X_L}]  (16)

[0117] As previously mentioned, the excitation controller in accordance with the fourth embodiment can change the reactive current reference value I_Q0as the high-side reference voltage V_Hrefof the plurality of tap-changer-equipped transformers changes. Accordingly, the present embodiment offers an advantage of being able to keep the transmission voltage of the transmission system constant and to maintain the actual output terminal voltage V_Gof each of the plurality of synchronous machines at about a nominal value even if the high-side reference voltage V_Hrefof the plurality of tap-changer-equipped transformers is changed.

[0118] Embodiment 5

[0119] In the above-mentioned fourth embodiment, it is assumed that the reactance X_Lof the transmission line has a known value. However, the reactance X_Lof the transmission line can actually vary from moment to moment. In view of this, a voltage setter 10 according to a fifth embodiment is so constructed as to successively estimate the reactance X_Lof the transmission line that varies with time.

[0120] The effective power P and reactive power Q of each of the plurality of synchronous machines can be expressed by the following equations (17) and (18):

[0121] P=V_G*E_fd*sin δ/(X_d+X_L)  (17)

[0122] Q=E_fd²*X_L/(X_d+X_L)²−(X_L−X_d)*E_fd*cos δ/(X_d+X_L)²−X_d/(X_d+X_L)²  (18)

[0123] where X_dis a synchronous reactance, and Efd is an internal voltage behind the synchronous reactance. The reactance X_Lof the transmission line can be calculated by eliminating the phase angle δ from above equations (17) and (18).

[0124] Thus the voltage setter of the fifth embodiment can determine the reactive current reference value I_Q0using the accurate reactance X_Lof the transmission line at all times. Accordingly, the fifth embodiment offers an advantage of being to make the actual high-side voltage V_Hof the plurality of tap-changer-equipped transformers agree with the high-side reference voltage V_Hrefwith a high degree of accuracy, and to maintain the actual output terminal voltage V_Gof each of the plurality of synchronous machines at about a nominal value.

[0125] Embodiment 6

[0126] The excitation controller in accordance with the fourth embodiment calculates a new reactive current reference value I_Q0from the previous and new high-side reference voltages V_Href1and V_Href2of the plurality of tap-changer-equipped transformers according to equation (13). In contrast, an excitation controller in accordance with a sixth embodiment of the present invention is so constructed as to detect the high-side voltage V_Hof the plurality of tap-changer-equipped transformers and determine the reactive current reference value I_Q0from the detected high-side voltage V_H.

[0127] Referring next to FIG. 14, there is illustrated a block diagram showing the structure of an electric power system including the excitation controller according to the sixth embodiment. In the figure, reference numeral 15 denotes a voltage transformer or PT for measuring a voltage corresponding to the high-side voltage V_Hof the plurality of tap-changer-equipped transformers 2 and 2a,and numeral 16 denotes a V_Hdetector for determining the high-side voltage V_Hof the plurality of tap-changer-equipped transformers 2 and 2afrom the measurement result of the PT 15. The other structure of the excitation controller of the sixth embodiment is the same as that of the aforementioned fourth embodiment, with the exception that the voltage setter 10 uses a different method of calculating the reactive current reference value I_Q0, and therefore the description of the other structure will be omitted hereinafter.

[0128] Next, a description will be made as to the operation of the excitation controller of the sixth embodiment with reference to FIG. 15, a flowchart of an excitation control method in accordance with the sixth embodiment of the present invention. In the flowchart of FIG. 15, steps ST61, ST62, and ST65 to ST68 are the same as steps ST41, ST42, and ST44 to ST47 of FIG. 13, and therefore the description of those steps will be omitted hereinafter.

[0129] The V_Hdetector 16, in step ST63, determines the high-side voltage V_Hof the plurality of tap-changer-equipped transformers 2 and 2afrom the measurement result of the PD 15. The voltage setter 10 then, in step ST64, calculates a new reactive current reference value I_Q0from the varied (i.e., new) high-side reference voltage V_Href2, the detected high-side voltage V_H, the reactive current I_Qoutput from the corresponding synchronous machine 1, and the cross-current prevention reactance X_DR, according to the following equation (19):

[0130] I_Q0=I_Q−(V_Href2−V_H)/X_DR  (19)

[0131] Incidentally, when a signal indicating the detected high-side voltage V_His used as a feedback signal to make the high-side voltage V_Hagree with its reference value, the signal has to be a continuous one. In contrast, when the detected high-side voltage V_His used to calculate the reactive current reference value, like the present embodiment, there is no need to generate a continues signal indicating the detected high-side voltage V_H. Therefore, the detected high-side voltage V_Hcan be notified of the voltage setter 10 through a communications line such as a telephone line.

[0132] As previously mentioned, the excitation controller in accordance with the sixth embodiment determines a new reactive current reference value I_Q0from the detected high-side voltage V_Hof the plurality of tap-changer-equipped transformers and the reactive current I_Qoutput from each of the plurality of synchronous machines. Accordingly, the sixth embodiment offers an advantage of being able to make the actual high-side voltage V_Hof the plurality of tap-changer-equipped transformers agree with its reference voltage V_Hrefwith a high degree of accuracy and to maintain the actual output terminal voltage V_Gof each of the plurality of synchronous machines at about a nominal value.

[0133] Embodiment 7

[0134] In the above-mentioned first embodiment, it is assumed that even if the tap ratio n of the tap-changer-equipped transformer is changed, the step-up ratio n_vis the same as the reactance change ratio n_r. In contrast, an excitation controller in accordance with a seventh embodiment is so constructed as to control a synchronous machine connected, by way of a split winding type of tap-changer-equipped transformer whose step-up ratio n_vand reactance change ratio n_rdiffer from each other as the tap ratio n of the tap-changer-equipped transformer is changed, to a transmission system. The tap-changer-equipped transformer can be of core type, for example.

[0135] Referring next to FIG. 16, there is illustrated a block diagram showing the structure of an electric power system including the excitation controller according to the seventh embodiment. In the figure, reference numeral 2bdenotes the split winding type of tap-changer-equipped transformer connected to a synchronous machine 1. The step-up ratio n_vand reactance change ratio n_rof the tap-changer-equipped transformer 2bdiffer from each other as the tap ratio n of the tap-changer-equipped transformer 2bis changed. The other structure of the excitation controller of the seventh embodiment is the same as that of the aforementioned first embodiment as shown in FIG. 1, and therefore the description of the other structure will be omitted hereinafter.

[0136] Next, a description will be made as to the operation of the excitation controller of the seventh embodiment with reference to FIG. 17, a flowchart of an excitation control method in accordance with the seventh embodiment of the present invention. All steps of FIG. 17 except step ST73, which takes the place of ST13 of FIG. 5, are already shown in FIG. 5, and therefore the description of the same steps will be omitted hereinafter.

[0137] The voltage setter 10, in step ST73, calculates and sets the output terminal reference voltage V_Grefof the synchronous machine 1 from the detected reactive current I_Q, the step-up ratio n_v, the reactance change ratio n_r, and the high-side reference voltage V_Hrefof the tap-changer-equipped transformer 2b,according to the following equation (20):

[0138] V_Gref=V_Href/n_v+n_r/n_v*X_t*I_Q  (20)

[0139] Thus the output terminal voltage V_Gis controlled such that it agrees with the reference voltage V_Gref. There is a relationship, as expressed by the following equation (21), between the output terminal voltage V_Gof the synchronous machine 1 and the high-side voltage V_Hof the tap-changer-equipped transformer 2b.

[0140] V_H=n_v*V_G−n_r*X_t*I_Q  (21)

[0141] Accordingly, using the high-side reference voltage V_Href, the output terminal voltage V_Gof the synchronous machine 1 and the high-side voltage V_Hof the tap-changer-equipped transformer 2bcan be given by the following equations (22) and (23):

[0142] V_G=V_Href/n_v+n_r/n_v*X_t*I_Q  (22)

[0143] V_H=V_Href  (23)

[0144] Thus, when the actual output terminal voltage V_Gof the synchronous machine 1 under normal operating conditions is high, by increasing the tap ratio n (n>1) (i.e., increasing the reactance change ratio n_v) by means of the tap controller 4, the excitation controller of the seventh embodiment can maintain the actual output terminal voltage V_Gof the synchronous machine 1 at about a nominal value. In addition, even though the current tap ratio is changed, the excitation controller of the seventh embodiment can control the actual high-side voltage V_Hof the tap-changer-equipped transformer 2bsuch that it agrees with the high-side reference voltage V_Href.

[0145] As previously mentioned, when the step-up ratio n_vand reactance change ratio n_rof the tap-changer-equipped trans former 2bdiffer from each other as the tap ratio n of the tap-changer-equipped transformer 2bis changed, the excitation controller in accordance with the seventh embodiment of the present invention sets the output terminal reference voltage V_Grefof the synchronous machine 1 from the reactive current I_Qoutput from the synchronous machine, the step-up ratio n_v, the reactance change ratio n_r, and the high-side reference voltage V_Hrefof the tap-changer-equipped transformer, and then controls the field current to be supplied to the field winding of the synchronous machine according to the difference between the output terminal reference voltage V_Grefand actual output terminal voltage V_Gof the synchronous machine. Accordingly, the present embodiment makes it possible to maintain the actual output terminal voltage V_Gof the synchronous machine at about a nominal value and to keep the voltage on the transmission bus constant even if the step-up ratio n_vand reactance change ratio n_rof the tap-changer-equipped transformer 2bdiffer from each other as the tap ratio n of the tap-changer-equipped transformer 2bis changed.

[0146] Embodiment 8

[0147] In the above-mentioned second embodiment in which a plurality of synchronous machines are connected, by way of respective tap-changer-equipped transformers, to a transmission system, it is assumed that the step-up ratio n_vand reactance change ratio n_rof each of the plurality of tap-changer-equipped transformers are of equal value even if the tap ratio n of each of the plurality of tap-changer-equipped transformers is changed. In contrast, an excitation controller in accordance with an eighth embodiment is so constructed as to control each of a plurality of synchronous machines (e.g., two synchronous machines) connected, by way of respective tap-changer-equipped transformers of split winding type, each of them having step-up ratio n_vand reactance change ratio n_rthat differ from each other as the tap ratio n of each of the plurality of tap-changer-equipped transformers is changed, to a transmission system. Each tap-changer-equipped transformer can be of core type, for example.

[0148] For simplicity, the electric power system includes two synchronous machines. When the step-up ratio n_vand reactance change ratio n_rof each of the two tap-changer-equipped transformers differ from each other as the tap ratio n of each of the two tap-changer-equipped transformers is changed, there is a relationship between the high-side voltage V_Hof each of the two tap-changer-equipped transformers and the output terminal voltage V_Gi(i=1, 2) of each of the two synchronous machines, which is expressed by each of the following equations:

[0149] V_H=n_v*V_G1−n_r1*X_t*I_Q1  (24)

[0150] V_H=n_v*V_G2−n_r2*X_t*I_Q2  (25)

[0151] where n_vis the step-up ratio, n_r1and n_r2are the respective reactance change ratios of the two tap-changer-equipped transformers of split winding type respectively connected to the two synchronous machines, and I_Q1and I_Q2are the respective reactance currents output from the two synchronous machines.

[0152] When calculating the output terminal reference voltage of each of the two synchronous machines according to equation (5), as explained in Embodiment 2, the respective high-side voltages V_H1and V_H2of the two tap-changer-equipped transformers are given by the following equations:

[0153] V_H1=V_Href−{X_DR−(n_v−n_r1)*X_t}*I_Q1  (26)

[0154] V_H2=V_Href−{X_DR−(n_v−n_r2)*X_t}*I_Q2  (27)

[0155] Accordingly, when n_r1>n_r2, there are a relationship as shown in FIG. 18 between the high-side voltage V_Hof each of the first and second tap-changer-equipped transformers of split winding type connected to each of the first and second synchronous machines and the reactive current I_Q1or I_Q2. As can be seen from FIG. 18, if n_r1≠n_r2, it is impossible to control the two synchronous machines with a high degree of accuracy such that the high-side voltage V_Hof each of the two tap-changer-equipped transformers of split winding type agrees with its reference value V_Href, in addition to an unbalanced sharing of the total reactive current between the two synchronous machines.

[0156] In contrast, the excitation controller according to the eighth embodiment is so constructed as to properly control two or more synchronous machines, even if these synchronous machines are respectively connected to two or more tap-changer-equipped transformers of split winding type, each of them having step-up ratio n_vand reactance change ratio n_rthat differ from each other as the tap ratio n of each of the plurality of tap-changer-equipped transformers is changed.

[0157] Referring next to FIG. 19, there is illustrated a block diagram showing the structure of an electric power system including the excitation controller according to the eighth embodiment. In the figure, reference numeral 2bdenotes a tap-changer-equipped transformer of split winding type connected to a synchronous machine 1, and numeral 2cdenotes another tap-changer-equipped transformer of split winding type connected to another synchronous machine 1a.The step-up ratio n_vand reactance change ratio n_rlof the first tap-changer-equipped transformer 2bdiffer from each other as the tap ratio n of the tap-changer-equipped transformer 2bis changed. Also, the step-up ratio n_vand reactance change ratio n_r2of the second tap-changer-equipped transformer 2cdiffer from each other as the tap ratio n of the tap-changer-equipped transformer 2cis changed. The other structure of the excitation controller of the eighth embodiment is the same as that of the aforementioned second embodiment as shown in FIG. 6, and therefore the description of the other structure will be omitted hereinafter.

[0158] Next, a description will be made as to the operation of the excitation controller of the eighth embodiment with reference to FIG. 20, a flowchart of an excitation control method in accordance with the eighth embodiment of the present invention. All steps of FIG. 20 except step ST83, which takes the place of step ST13 of FIG. 5, are already shown in FIG. 5, and therefore the description of the same steps will be omitted hereinafter.

[0159] The voltage setter 10, in step ST83, calculates and sets the output terminal reference voltage V_Grefof the first synchronous machine 1 from the detected reactive current I_Q, the step-up ratio n_vand reactance change ratio n_r(=n_r1) of the first tap-changer-equipped transformer 2band the high-side reference voltage V_Href, according to the following equation (28):

[0160] V_Gref=V_Href/n_v+(n_r/n_v*X_t−X_DR/n_v)*I_Q  (28)

[0161] Similarly, another voltage setter (not shown) intended for the second synchronous machine 1acalculates and sets the output terminal reference voltage V_Grefof the second synchronous machine 1afrom the detected reactive current I_Q, the step-up ratio n_vand reactance change ratio n_r(=n_r2) of the second tap-changer-equipped transformer 2c,and the high-side reference voltage V_Href, according to equation (28).

[0162] Thus the actual output terminal voltage V_G1or V_G2of each of the two synchronous machines 1 and 1ais controlled such that it agrees with its reference value V_Gref. There is a relationship, as expressed by the following equation (29), between the output terminal voltage V_Gof the first synchronous machine 1 and the high-side voltage V_Hof the first tap-changer-equipped transformer 2b.

[0163] V_H=n_v*V_G−n_r*X_t*I_Q  (29)

[0164] Accordingly, using the high-side reference voltage V_Href, the output terminal voltage V_Gof the first synchronous machine 1 and the high-side voltage V_Hof the first tap-changer-equipped transformer 2bcan be given by the following equations (30) and (31):

[0165] V_G=V_Href/n_v+(n_r/n_v*X_t−X_DR/n_v)*I_Q  (30)

[0166] V_H=V_Href−X_DR*I_Q  (31)

[0167] Thus, when the actual output terminal voltage V_Gof the first synchronous machine 1 under normal operating conditions is high, by increasing the tap ratio n (n>1) by means of the tap controller 4, the excitation controller of the eighth embodiment can maintain the actual output terminal voltage V_Gof the first synchronous machine 1 at about a nominal value. Similarly, when the output terminal voltage V_Gof the second synchronous machine 1aunder normal operating conditions is high, by increasing the tap ratio n (n>1) by means of the tap controller 4, the excitation controller of the eighth embodiment can maintain the actual output terminal voltage V_Gof the second synchronous machine 1aat about a nominal value. In addition, even though the tap ratio of one tap-changer-equipped transformer 2bor 2cis changed, the excitation controller of the eighth embodiment can control the actual high-side voltage V_Hof the tap-changer-equipped transformer 2bor 2csuch that it agrees with the high-side reference voltage V_Href.

[0168] As previously mentioned, the excitation controller in accordance with the eighth embodiment of the present invention sets the output terminal reference voltage V_Grefof each of the plurality of synchronous machines from the reactive current I_Qoutput from each of the plurality of synchronous machines, the step-up ratio n_v, the reactance change ratio n_r, and the high-side reference voltage V_Hrefof each of the plurality of tap-changer-equipped transformers connected to each of the plurality of synchronous machines, and controls the field current to be supplied to the field winding of each of the plurality of synchronous machines according to the difference between the output terminal reference voltage V_Grefand the actual output terminal voltage V_G, when the step-up ratio n_vand reactance change ratio n_rof each of the plurality of tap-changer-equipped transformers differ from each other as the tap ratio n of each of the plurality of tap-changer-equipped transformers is changed. Accordingly, the present embodiment makes it possible to maintain the actual output terminal voltage V_Gof each of the plurality of synchronous machines at a nominal value and to keep the voltage on the transmission bus constant even if the step-up ratio n_vand reactance change ratio n_rof each of the plurality of tap-changer-equipped transformers differ from each other as the tap ratio n of each of the plurality of tap-changer-equipped transformers is changed. In addition, the present embodiment offers an advantage of being able to offer a more equivalent sharing of the total reactive current among the plurality of generators.

[0169] Embodiment 9

[0170] The excitation controller in accordance with the above-mentioned second embodiment, which employs a plurality of tap-changer-equipped transformers each of which has a step-up ratio n_vand a reactance change ratio n_rthat are the same as the tap ratio n, sets the output terminal reference voltage V_Grefof each synchronous machine from the reactive current I_Qoutput from each synchronous machine, the reactive current reference value I_Q0, the tap ratio n, and the high-side reference voltage V_Hrefof the plurality of tap-changer-equipped transformers, according to equation (5). In contrast, in accordance with a ninth embodiment, there is provided an excitation controller that employs a plurality of tap-changer-equipped transformers each of which has a step-up ratio n_vand a reactance change ratio n_rthat differ from each other as the tap ratio n is changed, and that sets the output terminal reference voltage V_Grefof each synchronous machine from the reactive current I_Qoutput from each synchronous machine, the reactive current reference value I_Q0, the step-up ratio n_vand reactance change ratio n_rof each of the plurality of tap-changer-equipped transformers, and the high-side reference voltage V_Hrefof the plurality of tap-changer-equipped transformers.

[0171] An electric power system including the excitation controller according to the ninth embodiment has the same structure as that of the aforementioned eighth embodiment as shown in FIG. 19, and therefore the description of the structure will be omitted hereinafter.

[0172] Next, a description will be made as to the operation of the excitation controller of the ninth embodiment with reference to FIG. 21, a flowchart of an excitation control method in accordance with the ninth embodiment of the present invention. All steps of FIG. 21 except step ST93, which takes the place of step ST13 of FIG. 5, are already shown in FIG. 5, and therefore the description of those steps will be omitted hereinafter.

[0173] The voltage setter 10, in step ST93, calculates and sets the output terminal reference voltage V_Grefof the first synchronous machine 1 from the reactive current I_Qoutput from the synchronous machine 1, the reactive current reference value I_Q0, the step-up ratio n_vand reactance change ratio n_r(=n_r1) of the first tap-changer-equipped transformer 2b,and the high-side reference voltage V_Hrefof the first tap-changer-equipped transformer 2b,according to the following equation (32):

[0174] V_Gref=V_Href/n_v+n_r/n_v*X_t*I_Q−X_DR/n_v*(I_Q−I_Q0)  (32)

[0175] Similarly, another voltage setter (not shown) intended for the second synchronous machine 1acalculates and sets the output terminal reference voltage V_Grefof the second synchronous machine 1afrom the reactive current I_Qoutput from the second synchronous machine 1a,the reactive current reference value I_Q0, the step-up ratio n_vand reactance change ratio n_r(=n_r2) of the second tap-changer-equipped transformer 2c,and the high-side reference voltage V_Hrefof the second tap-changer-equipped transformer 2c,according to equation (32).

[0176] In this way, the output terminal reference voltage V_Grefof each of the first and second synchronous machine 1 and 1ais determined according to equation (32), and each of these synchronous machines 1 and 1ais controlled according to the output terminal reference voltage V_Gref. As a result, the output terminal voltage V_Gof each of the plurality of synchronous machines and the high-side voltage V_Hof each of the plurality of tap-changer-equipped transformers can be given by the following equations:

[0177] V_G=V_Href/n_v+n_r/n_v*X_t*I_Q−X_DR/n_v*(I_Q−I_Q0)   (33)

[0178] V_H=V_Href−X_DR*(I_Q−I_Q0)  (34)

[0179] As previously mentioned, when the reactive current I_Qoutput from each of the plurality of synchronous machines agrees with its reference value I_Q0other than zero, the excitation controller in accordance with the ninth embodiment of the present invention sets the output terminal reference voltage V_Grefof each of the plurality of synchronous machines such that the actual high-side voltage V_Hof the plurality of tap-changer-equipped transformers agrees with the high-side reference voltage V_Href, even if the step-up ratio n_vand reactance change ratio n_rof the tap-changer-equipped transformer differ from each other. Accordingly, the excitation controller according to the present embodiment can make the actual high-side voltage V_Hof the plurality of tap-changer-equipped transformers agree with the high-side reference voltage V_Hrefmore precisely than the excitation controller according to the aforementioned eighth embodiment does. Furthermore, the excitation controller according to the present embodiment can maintain the actual output terminal voltage V_Gof each of the plurality of synchronous machines at about a nominal value. In addition, the present embodiment offers an advantage of being able to offer a more equivalent sharing of the total reactive current among the plurality of generators.

[0180] Embodiment 10

[0181] The excitation controller in accordance with the above-mentioned fourth embodiment, which employs a plurality of tap-changer-equipped transformers each of which has a step-up ratio n_vand a reactance change ratio n_rthat are the same as the tap ratio n, calculates a new reactive current reference value I_Q0corresponding to a varied (or new) high-side reference voltage V_Href2from the difference between a previous high-side reference voltage V_Href1and the varied high-side reference voltage V_Href2, and the reactance of the transmission line. In contrast, in accordance with a tenth embodiment, there is provided an excitation controller that employs a plurality of tap-changer-equipped transformers each of which has a step-up ratio n_vand a reactance change ratio n_rthat differ from each other as the tap ratio n is changed, and that calculates the new reactive current reference value I_Q0corresponding to the varied (or new) high-side reference voltage V_Href2from the difference between the previous high-side reference voltage V_Href1and the varied high-side reference voltage V_Href2, and the reactance of the transmission line and sets the output terminal reference voltage V_Grefof each synchronous machine from the new reactive current reference value I_Q0.

[0182] An electric power system including the excitation controller according to the tenth embodiment has the same structure as that of the aforementioned eighth embodiment as shown in FIG. 19, and therefore the description of the structure will be omitted hereinafter.

[0183] Next, a description will be made as to the operation of the excitation controller of the tenth embodiment with reference to FIG. 22, a flowchart of an excitation control method in accordance with the tenth embodiment of the present invention. All steps of FIG. 22 except step ST104, which takes the place of step ST44 of FIG. 13, are already shown in FIG. 13, and therefore the description of the same steps will be omitted hereinafter.

[0184] The voltage setter 10, in step ST104, calculates and sets the output terminal reference voltage V_Grefof the first synchronous machine 1 from the reactive current I_Qoutput from the synchronous machine 1, the reactive current reference value I_Q0given by equation (9), the step-up ratio n_vand reactance change ratio n_r(=n_r1) of the first tap-changer-equipped transformer 2b,and the new high-side reference voltage V_Href2of the first tap-changer-equipped transformer 2b,according to the following equation (35):

[0185] V_Gref=V_Href2/n_v+n_r/n_v*X_t*I_Q−X_DR/n_v*(I_Q−I_Q0)  (35)

[0186] Similarly, another voltage setter (not shown) intended for the second synchronous machine 1acalculates and sets the output terminal reference voltage V_Grefof the second synchronous machine 1afrom the reactive current I_Qoutput from the second synchronous machine 1a,the reactive current reference value I_Q0given by equation (9), the step-up ratio n_vand reactance change ratio n_r(=_nr2) of the second tap-changer-equipped transformer 2c,and the new high-side reference voltage V_Href2of the second tap-changer-equipped transformer 2c,according to equation (35).

[0187] Thus the output terminal voltage V_Gof each of the first synchronous machines 1 and 1aand the high-side voltage V_Hof each of the first and second tap-changer-equipped transformers 2band 2ccan be given by the following equations:

[0188] V_G=V_Href2/n_v+n_r/n_v*X_t*I_Q−X_DR/n_v*[I_Q−{I_Q01+(V_Href2−V_Href1)/X_L}]  (36)

[0189] V_H=V_Href2−X_DR*[I_Q−{I_QO1+(V_Href2−V_Href1)/X_L}]  (37)

[0190] As previously mentioned, the excitation controller in accordance with the tenth embodiment of the present invention can set the output terminal reference voltage V_Grefof each of the plurality of synchronous machines using the new reactive current reference value I_Q0corresponding to the high-side reference voltage V_Href, even when the step-up ratio n_vand reactance change ratio n_rof each of the plurality of tap-changer-equipped transformers differ from each other. Accordingly, the excitation controller according to the present embodiment can keep the voltage on the transmission system constant even if the reference voltage V_Hrefis changed. Furthermore, the excitation controller according to the present embodiment can maintain the actual output terminal voltage V_Gof each of the plurality of synchronous machines at about a nominal value. In addition, the present embodiment offers an advantage of being able to offer a more equivalent sharing of the total reactive current among the plurality of generators.

[0191] Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.