Excitation controller

An excitation controller controls excitation of a synchronous machine, which is connected to a power transmission system through a transformer, so that a high-side voltage of the transformer is maintained at a target voltage with high accuracy. An output terminal target voltage of the synchronous machine is set to precisely compensate for a voltage drop in the transformer, corresponding to the transformer phase angle variation. To achieve this result, the excitation controller detects an output terminal voltage and an output current of the synchronous machine and calculates active and reactive currents of the output current, sets the output terminal target voltage of the synchronous machine from the active and reactive currents, the high-side voltage of the transformer, and the reactance of the transformer, and controls excitation of the synchronous machine to compensate for the voltage drop in the transformer corresponding to phase angle variation of the transformer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an excitation controller for stabilizing voltage in an electric power system.

2. Description of the Background Art

An example of a conventional excitation controller for controlling excitation of a synchronous machine connected to a power transmission system through a transformer is disclosed in Japanese Laid-open Patent Publication No. 2000-308397 (corresponding to U.S. Pat. No. 6,265,852). The excitation controller of the Publication detects a voltage VGat an output terminal of the synchronous machine and a reactive current IQoutput from the synchronous machine, causes a voltage setter to set an output terminal target voltage VGrefof the synchronous machine based on the reactive current IQand a high-side target voltage VHrefof the transformer such that a relationship expressed by VGref=VHref+Xt·IQis satisfied (where Xtis the reactance of the transformer), and controls an excitation system of the synchronous machine based on a deviation of the detective output terminal voltage VGfrom the output terminal target voltage VGrefof the synchronous machine.

More specifically, the aforementioned conventional excitation controller estimates a high-side voltage VHof the transformer from the output terminal voltage VG, the reactive current IQof the synchronous machine and the reactance Xtof the transformer by using a relationship VH=VG−Xt·IQ, from which the output terminal voltage VGof the synchronous machine is expressed by the following equation:
VG=VH+Xt·IQ(1)

Then, the excitation controller sets the output terminal target voltage VGrefas indicated by the following equation to compensate for a voltage drop occurring in the transformer from its high-side target voltage VHref:
VGref=VHref+Xt·IQ(2)

However, since the amount of a voltage change in the transformer varies also with phase angle variations Δ δ occurring in the transformer, the output terminal voltage VGof the synchronous machine is actually given by the following equation:
VG=VH·cos Δδ+Xt·IQ(3)
which is different from the value given by equation (1).

It is therefore impossible to exactly set the output terminal target voltage VGrefof the synchronous machine, because the aforementioned phase angle variations Δ δ are not taken into account in the output terminal target voltage VGrefcalculated by equation (2) above. This calculation error becomes more significant as the phase angle variation Δ δ in the transformer increases. For this reason, it has been difficult to keep the high-side voltage VHof the transformer, that is, the voltage applied to a transmission bus, at the target voltage VHrefwith high reliability.

SUMMARY OF THE INVENTION

The present invention is intended to provide a solution to the aforementioned problem of the prior art. Accordingly, it is an object of the invention to provide an excitation controller of a synchronous machine which can improve voltage stability of an entire power transmission system by setting an accurate output terminal target voltage VGrefof the synchronous machine taking into account phase angle variations occurring in a transformer and thereby maintaining a high-side voltage VHof the transformer, or the voltage applied to a transmission bus, at a desired level with high reliability.

According to the invention, an excitation controller includes a voltage detector for detecting an output terminal voltage of a synchronous machine connected to a power transmission system through a transformer, a current detector for detecting an output current of the synchronous machine, and a voltage setter for setting an output terminal target voltage of the synchronous machine based on the output current of the synchronous machine detected by the current detector, the reactance of the transformer, and a high-side target voltage of the transformer. The excitation controller of the invention controls an excitation system of the synchronous machine based on a deviation of the output terminal voltage of the synchronous machine detected by the voltage detector from the output terminal target voltage set by the voltage setter, wherein active current and reactive current of the output current of the synchronous machine are calculated from the output current detected by the current detector and the output terminal voltage detected by the voltage detector, and the voltage setter calculates and sets the output terminal target voltage of the synchronous machine to compensate for a voltage drop in the transformer corresponding to a phase angle variation which is a voltage phase difference between high-voltage and low-voltage sides of the transformer.

The excitation controller thus constructed makes it possible to maintain the high-side voltage of the transformer at its high-side target voltage with high reliability and improve voltage stability of the entire power transmission system.

These and other objects, features and advantages of the invention will become more apparent upon reading the following detailed description along with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Specific embodiments of the invention is now described with reference to the drawings.

First Embodiment

FIG. 1is a general configuration diagram of an excitation controller according to a first embodiment of the invention. A synchronous machine21is connected to a power transmission system through a transformer22. The excitation controller controls an exciter31which supplies a field current to a field winding32of the synchronous machine21. As depicted inFIG. 1, the excitation controller includes a potential transformer (hereinafter referred to as PT)26serving as a voltage detector for detecting an output terminal voltage VGof the synchronous machine21, a current transformer (hereinafter referred to as CT)27serving as a current detector for detecting a current IGoutput from the synchronous machine21, a voltage setter28for setting an output terminal target voltage VGrefof the synchronous machine21, a subtracter29, and an automatic voltage regulator (hereinafter referred to as AVR)30for controlling rectification timing of the exciter31. Referring also toFIG. 1, designated by the reference numeral23is a circuit breaker, designated by the reference numeral24is a transmission line, and designated by the reference numeral25is a transmission bus of a power plant.

Operation of the excitation controller thus constructed is described in the following referring to a flowchart shown in FIG.2.

First, the PT26detects the output terminal voltage VGof the synchronous machine21(step ST11), and the CT27detects the output current IGof the synchronous machine21(step ST12).

Then, the voltage setter28calculates an active current IPand a reactive current IQof the output current IGfrom the output terminal voltage VGand the output current IGof the synchronous machine21detected by the PT26and the CT27, respectively, and determines and sets an output terminal target voltage VGrefof the synchronous machine21from the active current IPand the reactive current IQso obtained as well as a preset high-side target voltage VHrefand a known reactance Xtof the transformer22using a specific calculation process which will be later described (step ST13).

Next, the subtracter29subtracts the output terminal voltage VGof the synchronous machine21detected by the PT26from the target voltage VGrefset by the voltage setter28and outputs a deviation signal indicating the result of subtraction (step ST14). The deviation signal output from the subtracter29is delivered to the AVR30, and the AVR30produces a timing signal for controlling the rectification timing of the exciter31using the deviation signal as an input condition (step ST15). The exciter31supplies the field current to the field winding32of the synchronous machine21according to the timing signal fed from the exciter31(step ST16).

As a result, the output terminal voltage VGof the synchronous machine21is controlled such that it coincides with the target voltage VGref, and a high-side voltage VHof the transformer22is controlled such that it coincides with the high-side target voltage VHref.

The output terminal target voltage VGrefof the synchronous machine21set by the voltage setter28in step ST13above is now described in detail below.

Taking into account a phase angle variation Δ δ, which is a voltage phase difference between high-voltage and low-voltage sides of the transformer22, the relationship between the output terminal voltage VGof the synchronous machine21and the high-side voltage VHof the transformer22is expressed by the earlier-mentioned equation (3) by using the reactive current IQof the synchronous machine21and the reactance Xtof the transformer22. The relationship between the high-side voltage VHand the reactive current IQis expressed by equation (4) below:
VH·sin Δδ=Xt·IQ(4)

From equations (3) and (4), the high-side voltage VHof the transformer22is given by equation (5) below:
VH=√{square root over ((Xt·IP)2+(VG−Xt·IQ)2)}{square root over ((Xt·IP)2+(VG−Xt·IQ)2)}  (5)

Also, the output terminal voltage VGof the synchronous machine21is given by equation (6) below:
VG=√{square root over (VH2)}
−(Xt·IP)2
+Xt·IQ(6)

Using equation (6) above, the output terminal target voltage VGrefof the synchronous machine21can be calculated from the active current IP, the reactive current IQ, the high-side target voltage VHrefof the transformer22and the reactance Xtof the transformer22as shown by equation (7) below:
VGref=√{square root over (VHref2)}
−(Xt·IP)2
+Xt·IQ(7)

According to the present embodiment, the active current IPand the reactive current IQof the output current IGare calculated from the output terminal voltage VGof the synchronous machine21detected by the PT26and the output current IGOf the synchronous machine21detected by the CT27, and the output terminal target voltage VGrefof the synchronous machine21is set by using the active current IPand the reactive current IQso obtained as well as the preset high-side target voltage VHrefand the known reactance Xtof the transformer22to compensate for a voltage drop in the transformer22corresponding to the phase angle variation Δ δ occurring therein. This arrangement of the embodiment makes it possible to maintain the high-side voltage VHof the transformer22, or the voltage applied to the transmission bus25, at the high-side target voltage VHrefwith high reliability and improve voltage stability of the entire power transmission system.

Second Embodiment

In the aforementioned first embodiment, voltage changes caused by the reactance Xtof the transformer22are fully (100%) compensated for on the assumption that only one synchronous machine21is connected to the power transmission system as shown in FIG.1. If two synchronous machines21,41or more are connected to the power transmission system as shown in FIG.4and the reactance Xtof each transformer22is fully compensated for, however, the reactance between the two synchronous machines21,41becomes nearly zero, so that a cross current flows between the synchronous machines21,41due to a difference in their output terminal voltages VGand a difference in their responses to voltage changes. This would destroy a load balance between the two synchronous machines21,41, potentially overloading one of them. InFIGS. 3 and 4, XLdesignates the reactance of the transmission line24.

A second embodiment of the invention is directed toward the solution of this problem. Specifically, the output terminal target voltage VGrefof the synchronous machine21set by the voltage setter28is calculated by using a value obtained by subtracting a reactance XDRcorresponding to a suppressed component of the cross current of the reactance Xtof the transformer22from the reactance Xtas shown in equation (8) below:
VGref=√{square root over (VHref2−{(Xt−XDR)·IP}2)}+(Xt−XDR)·IQ(8)
where the reactance XDRis determined empirically based on such conditions as the characteristics of the synchronous machines21,41and the power transmission system. For example, it is set to a value corresponding to a few percent based on the capacity of the synchronous machine21(41).

The high-side voltage VHof the transformer22becomes lower than the high-side target voltage VHrefdue to the influence of the reactance XDRas individual components (active current IP, reactive current IQ) of the output current IGof the synchronous machine21increase as shown in FIG.5. This does not pose any practical problem in this embodiment, however, because the reactance XDRhas the value corresponding to a few percent and the high-side voltage VHof the transformer22is so controlled as to become approximately match the target voltage VHref.

In this embodiment, the output terminal target voltage VGrefof the synchronous machine21set by the voltage setter28is calculated by equation (8) shown above. This makes it possible to reliably maintain the voltage applied to the transmission bus25by compensating for voltage changes occurring in the transformer22due to phase angle variations therein as in the first embodiment, avoid the occurrence of the cross current between the synchronous machines21,41connected to the power transmission system, and prevent overloading the synchronous machines21,41, thereby improving overall system reliability.

If the reactance XDRcorresponding to the suppressed component of the cross current of the reactance Xtis set to a common value for all the synchronous machines connected to the power transmission system, a situation equivalent to what would occur when the transformers of the same reactance (i.e., the reactance XDRto be set) are connected to the multiple synchronous machines connected to the power transmission system would take place. Accordingly, the embodiment obviates the need for taking into account the difference between the reactances of the multiple transformers in operating the transmission system, effectively facilitating system operation.

Third Embodiment

While the reactance XDRcorresponding to the suppressed component of the cross current is subtracted from the reactance Xtof the transformer22in the aforementioned second embodiment, the cross current is caused only by the reactive current IQof the output current IG(active current IP, reactive current IQ) of the synchronous machine21(41).

Taking this into consideration, a third embodiment of the invention employs an arrangement for calculating the output terminal target voltage VGrefof the synchronous machine21set by the voltage setter28using equation (9) below:
VGref=√{square root over (VHref2−(Xt·IP)2)}+(Xt−XDR)·IQ(9)

As indicated in the above equation, the reactance Xtof the transformer22is used directly as an active current which does not cause the cross current and only the reactive current which causes the cross current, or the reactance XDRfor suppressing the cross current, is subtracted from the reactance Xtof the transformer22. As a result, the present embodiment makes it possible to control the high-side voltage VHof the transformer22to match the target voltage VHrefin an improved fashion while effectively suppressing the cross current.

Although the high-side voltage VHof the transformer22becomes progressively lower than the target voltage VHrefas the reactive current IQof the output current IGof the synchronous machine21increases as shown inFIG. 6in this embodiment, the amount of the active current IPdoes not have a marked influence on the high-side voltage VH.

Fourth Embodiment

In the aforementioned second embodiment, the reactance XDRcorresponding to the suppressed component of the cross current of the reactance Xtis used so that the high-side voltage VHof the transformer22matches the target voltage VHrefwhen the individual components (active current IP, reactive current IQ) of the output current IGof the synchronous machine21are zero, and becomes lower than the target voltage VHrefas the individual components of the output current IGincrease as shown in FIG.5.

A fourth embodiment of the invention employs an arrangement for correcting the high-side voltage VHof the transformer22such that it matches the target voltage VHrefwhen the output current IG(active current IP, reactive current IQ) of the synchronous machine21coincides with a reference current value I0(active current IP0, reactive current IQ0), such as a value effective under rated operating conditions, as shown in FIG.7.

Specifically, the output terminal target voltage VGrefof the synchronous machine21set by the voltage setter28is calculated by using equation (10) below in this embodiment:
VGref=√{square root over (VHref2−{(Xt−XDR)·IP+XDR·IP0}2)}+(Xt−XDR)·IQ+XDR·IQ0(10)

In this embodiment, the high-side voltage VHof the transformer22is controlled such that it matches the target voltage VHrefwhen the synchronous machine21outputs the reference current value I0(active current IP0, reactive current IQ0). According to this arrangement, the high-side voltage VHof the transformer22can be controlled such that it matches the target voltage VHrefmore accurately than in the second embodiment. It is therefore possible to maintain the voltage applied to the transmission bus25at the high-side target voltage VHrefwith high reliability while preventing the occurrence of a cross current between the synchronous machines connected to the power transmission system. This serves to further improve voltage stability of the entire power transmission system.

Fifth Embodiment

In the aforementioned third embodiment, only the reactive current which causes the cross current, or the reactance XDRfor, suppressing the cross current, is subtracted from the reactance Xtof the transformer22so that the high-side voltage VHof the transformer22matches the target voltage VHrefwhen the reactive current IQof the output current IGof the synchronous machine21is zero, and becomes lower than the target voltage VHrefas the reactive current IQincreases as shown in FIG.6.

A fifth embodiment of the invention employs an arrangement for correcting the high-side voltage VHof the transformer22such that it matches the target voltage VHrefwhen the reactive current IQof the output current IGof the synchronous machine21matches a reference reactive current value IQ0, such as a value effective under rated operating conditions, as shown in FIG.8.

Specifically, the output terminal target voltage VGrefof the synchronous machine21set by the voltage setter28is calculated by using equation (11) below in this embodiment:
VGref=√{square root over (VHref2−(Xt·IP)2)}+(X−XDR)·IQ+XDR·IQ0(11)

In this embodiment, the high-side voltage VHof the transformer22is controlled such that it matches the target voltage VHrefwhen the synchronous machine21outputs the reference reactive current value IQ0. According to this arrangement, the high-side voltage VHof the transformer22can be controlled such that it matches the target voltage VHrefmore accurately than in the third embodiment. It is therefore possible to maintain the voltage applied to the transmission bus25at the high-side target voltage VHrefwith high reliability while preventing the occurrence of the cross current between the synchronous machines connected to the power transmission system. This serves to further improve voltage stability of the entire power transmission system.

Sixth Embodiment

In the aforementioned fourth and fifth embodiments, the high-side voltage VHof the transformer22is controlled such that it matches the target voltage VHrefwhen the synchronous machine21outputs the reference current value I0(active current IP0, reactive current IQ0) and the reference reactive current value IQ0, respectively. In these embodiments, the active current IPvaries depending on operating conditions of the synchronous machine21and the reactive current IQvaries when the target voltage VHrefis altered.

Taking this into consideration, a sixth embodiment of the invention employs an arrangement for setting the reference active current value IP0and the reference reactive current value IQ0according to the operating conditions of the synchronous machine21and the target voltage VHrefof the transformer22. For example, the reference current value I0(active current IP0, reactive current IQ0) is set for a high-side target voltage VHref0of the transformer22and a reference current value I1(active current IP1, reactive current IQ1) is set for a high-side target voltage VHref1of the transformer22as shown in FIG.9. As a result, it becomes possible to control the high-side voltage VHof the transformer22such that it matches the target voltage VHrefeven when the high-side target voltage VHrefof the transformer22is changed.

This arrangement of the embodiment makes it possible to further improve the reliability of control for maintaining the voltage applied to the transmission bus25and achieve an effect of maintaining a higher voltage on the power transmission system and its voltage stability.

Seventh Embodiment

While the value obtained by subtracting the suppressed component of the cross current from the reactance Xtof the transformer22is used in the calculation performed by the voltage setter28in the foregoing first to sixth embodiments, a seventh embodiment of the invention employs an arrangement for setting the output terminal target voltage VGrefof the synchronous machine21such that the high-side voltage VHof the transformer22varies with changes in the reactive current IQonly, regardless of changes in the active current IP.

Specifically, the high-side voltage VHof the transformer22is expressed by equation (12) below, using a voltage droop rate XDset to a specific value representing the influence of the reactive current IQon the target voltage VHrefof the high-side voltage VHof the transformer22:
VH=VHref−XD·IQ(12)

Using equation (12) above and the earlier-mentioned equation (5) which gives the high-side voltage VHof the transformer22as a function of the output terminal voltage VGof the synchronous machine21, the active current IPand the reactive current IQ, the output terminal target voltage VGrefof the synchronous machine21set by the voltage setter28is calculated by equation (13) below:
VGref=√{square root over ((VHref)}
−XD·IQ)2
−(Xt·IP)2
+Xt·IQ(13)
where the voltage droop rate XDis determined empirically based on such conditions as the characteristics of the synchronous machine21and the power transmission system. For example, the voltage droop rate XDis set to a value corresponding to a few percent based on the capacity of the synchronous machine21(41).

Although the high-side voltage VHof the transformer22becomes lower than the high-side target voltage VHrefas the reactive current IQof the output current IGof the synchronous machine21increases in this embodiment, the high-side voltage VHof the transformer22may be regarded as being practically controlled by the target voltage VHref, because the voltage droop rate XDis set to the value corresponding to a few percent.

In this embodiment, the output terminal target voltage VGrefof the synchronous machine21is set such that the high-side voltage VHof the transformer22varies with changes in the reactive current IQonly, regardless of changes in the active current IP. Therefore, the high-side voltage VHof the transformer22does not vary as a result of load variations, or variations in active power, under normal operating conditions. This makes it possible to easily operate the synchronous machine21(41) in a controlled fashion with high reliability, effectively avoid the occurrence of the cross current between the synchronous machines connected to the power transmission system, and prevent overloading the synchronous machines. Furthermore, because the output terminal target voltage VGrefof the synchronous machine21is calculated by using equation (13) above derived from the earlier-mentioned equation (5) expressing the high-side voltage VHof the transformer22by the output terminal voltage VGof the synchronous machine21, the active current IPand the reactive current IQ, it is possible to reliably maintain the voltage applied to the transmission bus25by compensating for voltage changes occurring in the transformer22due to phase angle variations therein as in the first embodiment.

Eighth Embodiment

In the foregoing first to seventh embodiments, the output terminal target voltage VGrefof the synchronous machine21is set by the voltage setter28to compensate for the voltage drop occurring in the transformer22. It is to be noted that there exists a resistance33between the synchronous machine21through the transformer22and the transmission bus25as shown inFIG. 10, so that it is necessary to take this resistance33into consideration when the transmission line length between the synchronous machine21and the transmission bus25is large.

An eighth embodiment of the invention employs an arrangement for setting the target voltage VGrefby the voltage setter28to compensate for not only a voltage drop corresponding to the reactance Xtof the transformer22but also a voltage drop caused by the active current IPof the output current IGof the synchronous machine21and the resistance33. This arrangement serves to further improve the reliability of control for maintaining the voltage applied to the transmission bus25and achieve an effect of maintaining a higher voltage on the power transmission system and its voltage stability.

Ninth Embodiment

While the reactance Xtof the transformer22is assumed to have a fixed value in the foregoing first to eighth embodiments, a transformer22A having a function of controlling tap switching operation may be used instead of the transformer22as shown in FIG.11.

In this ninth embodiment of the invention, the output terminal target voltage VGrefof the synchronous machine21is set by the voltage setter28according to a “tap ratio” selected when tap connection of the transformer22A is changed. The tap switching operation alters the point of connection to a high-voltage winding of the transformer22A. When the tap ratio is n, the number of turns of the high-voltage winding is 1/n of the rated number of turns of the high-voltage winding.

Given the tap ratio n, the output terminal voltage VGof the synchronous machine21shown by equation (6) of the first embodiment is expressed by equation (14) below:
VG=√{square root over ((VH/n)2)}
−(n·Xt·IP)2
+n·Xt·IQ(14)
and the output terminal target voltage VGrefof the synchronous machine21shown by equation (7) is expressed by equation (15) below:
VGref=√{square root over ((VHref/n)2)}
−(n·Xt·IP)2
+n·Xt·IQ(15)

The output terminal target voltage VGrefof the synchronous machine21is calculated and set by using the active current IP, the reactive current IQ, the high-side target voltage VHrefof the transformer22A, the tap ratio n of the transformer22A and its reactance Xtas shown by equation (15) above. This arrangement makes it possible to maintain the high-side voltage VHof the transformer22A, or the voltage applied to the transmission bus25, at the high-side target voltage VHrefwith high reliability and improve voltage stability of the entire power transmission system, regardless of the point of tap connection of the transformer22A.

While the transformer22A having the tap switching control function of this embodiment is applied to the earlier-described control operation of the first embodiment, the transformer22A is applicable in a similar fashion to the control operation of the foregoing second to eighth embodiments as well.

Tenth Embodiment

While the aforementioned ninth embodiment uses the tap ratio n of the transformer22A for calculating the target voltage VGref, a voltage ratio ngconcerning voltage conversion and a reactance ratio nrconcerning reactance conversion do not necessarily coincide with each other in an actual transformer.

Taking this into consideration, a tenth embodiment of the invention uses a target voltage VGrefobtained by substituting the voltage ratio ngand the reactance ratio nrfor the tap ratio n in equation (15) as shown by equation (16) below:

The terminal target voltage VGrefof the synchronous machine21is calculated and set by using the voltage ratio ngand the reactance ratio nrcorresponding to the tap ratio n which varies when the point of tap connection to the high-voltage winding of the transformer22A is switched. This arrangement makes it possible to maintain the high-side voltage VHof the transformer22A, or the voltage applied to the transmission bus25, at the high-side target voltage VHrefwith higher accuracy and improve voltage stability of the entire power transmission system, regardless of the point of tap connection of the transformer22A.