JP2002262597A - Excitation control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an excitation control device which can improve the operability of a plant by carrying out a limit control corresponding to an under- excitation state of a synchronous machine whenever necessary. SOLUTION: In the excitation control device, which controls the excitation of a synchronous machine 1 according to the output voltage of the synchronous machine 1, there are provided power detection means 14 and 15, which detect active and reactive powers of the synchronous machine, are provided; a plurality of under-excitation limiters 17, 18, and 19, which detect the under-excitation states according to a active power value, reactive power value, and an under- excitation limit curve set beforehand and output under-excitation signals according to the under-excitation state, are provided in parallel with each other; and further, a control signal calculator 20, which summarizes respective under- excitation limit signals from the under-excitation limiters, and a control device 10, which controls the excitation value of the synchronous machine according to the output signal of the control signal calculator 20.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an excitation limiting device for a synchronous machine.

[0002]

2. Description of the Related Art A conventional excitation control device will be described with reference to FIG. In FIG. 13, the synchronous machine 1 includes a field winding 2, and an output terminal thereof is connected to a power system 5 via a main transformer 3 and a system parallel breaker 4. The thyristor rectifier 6 is provided in an exciting circuit that excites the field winding 2 of the synchronous machine 1 via a field breaker 7, and the AC input terminal of the thyristor rectifier 6 is connected to the synchronous machine 1 via an exciting power supply transformer 8. Connected to the output end.

The automatic voltage regulator 9 controls the excitation of the synchronous machine 1. The controller 10 has an input terminal connected to an output terminal of the synchronous machine 1 via a fuse 11 and an instrument transformer 12. The thyristor of the thyristor rectifier 6 is gate-controlled so that the output voltage of the thyristor 6 becomes equal to the voltage set by the voltage setting device 16.

The underexcitation limiting device 17 converts the synchronous machine voltage input through the fuse 11 and the instrument transformer 12 and the synchronous machine current inputted through the instrument current transformer 13 by the converter 14. The synchronous machine active power and the synchronous machine reactive power converted by the converter 15 are input, and the underexcitation state of the synchronous machine 1 is detected. The limiting device 10 outputs a limiting signal in accordance with an output signal from the underexcitation limiting device 17 and performs a limiting operation in the underexcitation state.

FIG. 14 illustrates an underexcitation limiting curve of the underexcitation limiting device 17. The vertical axis shows the synchronous machine reactive power Q
Is shown with the synchronous machine active power P on the horizontal axis. Therefore, the underexcitation limiting device 17 outputs the underexcitation limit signal so that when the synchronous machine 1 is in the leading phase operation during the load operation and exceeds the underexcitation limit line 100, the operating point is returned to the retard side by the excess value. Output.

As described above, when the synchronous machine shifts to the underexcitation state due to factors such as system load reduction and plant operating conditions, pullback control to the late phase is performed by underexcitation restriction control.

[0007]

However, the underexcitation limiting control described above always controls pulling back to the retard side at a constant speed regardless of the amount by which the operating point of the synchronous machine exceeds the underexcitation limiting line. Is going. For this reason, it takes more time to control the retraction to the retard side as the operation becomes more advanced,
Therefore, the iron core of the synchronous machine armature is overheated, causing damage. Such damage can lead to plant shutdowns and must be avoided.

The present invention has been made in view of the above, and when the synchronous machine shifts to the underexcitation state, the operability of the plant can be improved by performing limit control according to the underexcitation state as needed. It is an object to provide a control device.

[0009]

According to the first aspect of the present invention, there is provided an excitation control device for controlling excitation of a synchronous machine based on an output voltage of the synchronous machine. Power detection means for detecting reactive power,
An under-excitation limiting device that detects an under-excitation state based on the active power value and the reactive power value detected by the power detection means and a preset under-excitation limit curve, and outputs an under-excitation limit signal according to the under-excitation state. A control signal calculator for adding the respective underexcitation limiting signals from these underexcitation control devices, and a control device for controlling the excitation amount of the synchronous machine according to an output signal from the control signal calculator. It is characterized by having.

According to a second aspect of the present invention, there is provided an excitation control device for performing excitation control of a synchronous machine based on an output voltage of the synchronous machine, comprising: power detection means for detecting active power and reactive power of the synchronous machine; An underexcitation limiting device that detects an underexcitation state based on the active power value and the reactive power value detected by the power detection unit and a preset underexcitation limit curve, and outputs an underexcitation limit signal in accordance with the underexcitation state. A plurality of under-excitation limit control detectors that are provided in parallel, detect under-excitation control states of these under-excitation control devices, and output under-excitation control state signals, and based on the under-excitation control state signals from the under-excitation control devices. An underexcitation limit control signal selector for selecting an output signal to be used for control from a plurality of underexcitation limit signals from a plurality of underexcitation limit devices; Characterized by comprising a control device for controlling the amount of excitation of the synchronous machine in response to the output signal from the signal selector.

According to a third aspect of the present invention, there is provided an excitation control device for performing excitation control of a synchronous machine based on an output voltage of the synchronous machine, comprising: power detection means for detecting active power and reactive power of the synchronous machine; An under-excitation limiting device that detects an under-excitation state based on the active power value and the reactive power value detected by the power detection unit and a preset under-excitation limit curve, and outputs an under-excitation limit signal according to the under-excitation state. Are provided in parallel, a high value selector for selecting a high value among the respective underexcitation limiting signals from these underexcitation limiting devices, and an excitation amount of the synchronous machine is controlled according to an output signal from the high value selector. And a control device.

According to the fourth aspect of the present invention, there is provided an excitation control device for performing excitation control of a synchronous machine based on an output voltage and a field current of the synchronous machine, wherein power detection for detecting active power and reactive power of the synchronous machine. Means, a field current detecting means for detecting a field current of the synchronous machine, and an active power value and a reactive power value detected by the power detecting means. A first under-excitation limiting device for outputting an under-excitation limiting signal and a field current value detected by the field current detection means to detect an under-excitation state;
A high value is selected from a second underexcitation limiting device that outputs an underexcitation limiting signal according to the underexcitation state, and the underexcitation limiting signals from the first underexcitation limiting device and the second underexcitation limiting device. It is characterized by comprising a high value selector and a control device for controlling the excitation amount of the synchronous machine in accordance with an output signal from the high value selector.

According to a fifth aspect of the present invention, in the excitation control device according to any one of the first to fourth aspects, the control device controls the amount of excitation of the synchronous machine based on the underexcitation limited phase control signal corresponding to the output signal. Is controlled.

According to a sixth aspect of the present invention, there is provided an excitation control device for performing excitation control of a synchronous machine based on an output voltage and a field current of the synchronous machine. Means for detecting an underexcitation state by inputting a field current value detected by the field current detection means, and outputting an underexcitation limit signal in accordance with the underexcitation state; A control device for controlling the excitation amount of the synchronous machine in accordance with the underexcitation limiting signal output by the excitation limiting device.

According to the seventh aspect of the present invention, there is provided an excitation control device for performing excitation control of a synchronous machine based on an output voltage and a field current of the synchronous machine, wherein power detection for detecting active power and reactive power of the synchronous machine. Means, a field current detecting means for detecting a field current of the synchronous machine, and an active power value and a reactive power value detected by the power detecting means, to detect an underexcitation state, and to respond to the underexcitation state. A first under-excitation limiting device for outputting an under-excitation limiting signal and a field current value detected by the field current detection means to detect an under-excitation state;
A second underexcitation limiting device that outputs an underexcitation limiting signal in accordance with the underexcitation state, and a high value among the respective underexcitation limiting signals from the first underexcitation limiting device and the second underexcitation limiting device. A high value selector to be selected and a control device for controlling the excitation amount of the synchronous machine in accordance with an output signal from the high value selector are provided.

According to an eighth aspect of the present invention, in the excitation control apparatus according to any one of the first to seventh aspects, the active current value and the reactive current value are calculated from the active power value and the reactive power value detected by the power detection means. A computing unit for calculating, wherein the underexcitation limiting device uses the active current signal and the reactive current signal from the computing unit as input signals instead of the active power value and the reactive power value detected by the power detection means. .

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a configuration diagram of an excitation control device according to a first embodiment of the present invention. It should be noted that a plurality of underexcitation limiting devices 17, 18, and 19 are provided in parallel with the automatic voltage adjusting device 9, and a control signal calculator 20 that adds the underexcitation limiting signals from the underexcitation limiting devices 17, 18, and 19. 13 is the same as the conventional excitation control device shown in FIG. 13, and the same parts as those in FIG. 13 are denoted by the same reference numerals and description thereof is omitted.

Next, the operation of the excitation control device according to this embodiment will be described.

The converter 14 and the converter 15 are connected to the fuse 1
1 and the synchronous machine voltage detected via the instrument transformer 12 and the synchronous machine current detected via the instrument current transformer 13 are input to output active power and reactive power, respectively.
The active power and the reactive power are supplied to the underexcitation limiting devices 17 and 1.
8 and 19, respectively, to monitor the underexcitation state. Underexcitation limiting devices 17, 18, and 19 detect an underexcitation state based on a preset underexcitation limit curve. This underexcitation limiting curve can be set individually for each underexcitation limiting device as shown in the underexcitation limiting curve diagram of FIG. In the figure, underexcitation limiting curve 101,
102 and 103 are underexcitation limiting devices 17 and 1 respectively.
8 and 9 show underexcitation limiting curves.

First, when the synchronous machine 1 is in the leading phase operation and the operating point exceeds the underexcitation limit curve 101, the underexcitation limiter 17 causes the underexcitation limit value to return to the retard side by a value exceeding the underexcitation limit curve 101. Outputs the excitation limit signal. The underexcitation limit signal is input to the control device 10 via the control signal calculator 20, and the control device 10 controls the excitation amount of the synchronous machine 1 according to the underexcitation limit signal.

Next, if the underexcitation state continues even though the underexcitation limiting device 17 is operated and the operating point exceeds the underexcitation limit curve 102, the underexcitation limiting device 18 An underexcitation limiting signal is output so as to return to the late side by a value exceeding 102. This underexcitation limiting signal is input to the control signal calculator 20 and is added to other control signals. In this case, the signal is added to the underexcitation limiting signal from the underexcitation limiting device 17, and the added output signal is input to the control device 10. The control device 10 controls the amount of excitation of the synchronous machine 1 according to the output signal.

Further, if the underexcitation state continues and the operating point exceeds the underexcitation limit curve 103 despite the operation of the underexcitation limit devices 17 and 18, the underexcitation limit device 19 causes the underexcitation limit curve 103 to operate. An underexcitation limit signal is output so as to return to the delay side by a value exceeding. This underexcitation limiting signal is input to the control signal calculator 20 and added to other control signals. In this case, the signals are added to the respective underexcitation limiting signals from the underexcitation limiting devices 17 and 18, and the added output signal is input to the control device 10. Control device 1
0 controls the amount of excitation of the synchronous machine 1 according to this output signal.

That is, the underexcitation state is limited stepwise by adding the underexcitation limiting signal from each underexcitation limiting device as needed according to the underexcitation state of the synchronous machine 1.

As described above, by providing a plurality of underexcitation limiting devices and setting different underexcitation limiting curves, it is possible to perform limit control according to the underexcitation state of the synchronous machine at any time. Thus, the operability of the plant can be improved by making the most of the underexcitation control.

Further, as shown in FIG.
7 can also be provided. The phase adjustment circuit 27 receives the output signal of the control signal calculator 20, calculates the firing phase angle α of the thyristor rectifier 6 according to this signal, and outputs the underexcitation limited phase control signal, thereby underexcitation. It limits the state.

As described above, by adjusting the phase of the thyristor rectifier, the underexcitation state can be suppressed, and the same effect can be obtained.

FIG. 4 shows a configuration diagram of an excitation control device according to a second embodiment of the present invention. The control signal calculator 2
Except that an underexcitation limit control detector 21 and an underexcitation limit control signal selector 22 are provided instead of 0, it is the same as the first embodiment shown in FIG. The same reference numerals are given and the description is omitted.

The underexcitation limiting control detector 21 is a synchronous machine 1
At which the operating point reaches the underexcitation limit curves 101, 102, and 103, and outputs an underexcitation control state signal. Underexcitation limit control signal selector 22
Is to select one of the underexcitation limiting signals from the underexcitation limiting devices 17, 18, and 19 based on the underexcitation control state signal. The control device 10 controls the excitation amount of the synchronous machine 1 according to the selected signal.

That is, the underexcitation state is limited by selecting an appropriate underexcitation limit signal in accordance with the underexcitation state of the synchronous machine 1 and the timing of reaching the underexcitation limit curve.

As described above, a plurality of under-excitation limiters having different under-excitation limit curves are provided, and by detecting the arrival timing of the operating point, the limit control according to the under-excitation state of the synchronous machine can be appropriately controlled. It can be performed at the timing. Thus, the operability of the plant can be improved by making the most of the underexcitation control.

As in the first embodiment, the same effect can be obtained by providing a phase adjusting circuit and adjusting the phase of the thyristor rectifier.

FIG. 5 shows a configuration diagram of an excitation control device according to a third embodiment of the present invention. It should be noted that an underexcitation limiting device 23, 24 is provided instead of the underexcitation limiting device 17, and a high value selection circuit 26 for comparing the underexcitation limiting signals from the respective underexcitation limiting devices 23, 24 and selecting a high value is provided. The other points are the same as those of the conventional excitation control device shown in FIG. 13, and the same parts as those in FIG. 13 are denoted by the same reference numerals and description thereof will be omitted.

As shown in the underexcitation limiting curve diagram of FIG.
Underexcitation limiting devices 23 and 24 output underexcitation limiting signals based on underexcitation limiting curves 104 and 105 having different characteristics, respectively. The high value selector 26 selects a high value from these underexcitation limiting signals and outputs it to the control device 10. The control device 10 controls the excitation amount of the synchronous machine 1 according to the selected signal.

That is, by selecting the high value of the underexcitation limiting signal, the operation can be performed in the maximum operable range according to the tolerance curve of the device.

Thus, the operability of the plant can be improved by making the best use of the underexcitation control.

FIG. 7 shows a modification of the excitation control device shown in FIG. 5 as another embodiment. It should be noted that the excitation control shown in FIG. 5 is provided except that an underexcitation limiting device 24a which receives a synchronous machine field current detected via the auxiliary current transformer 30 as an input instead of the underexcitation limiting device 24 is provided. The configuration is the same as that of the apparatus, and the same parts as those in FIG. 5 are denoted by the same reference numerals and description thereof will be omitted.

As shown in the underexcitation limiting curve diagram of FIG.
Underexcitation limiting devices 23 and 24a output underexcitation limiting signals with underexcitation limiting curves 104 and 106 having different characteristics, respectively. The high value selector 26 selects a high value from these underexcitation limiting signals and outputs it to the control device 10. The control device 10 controls the excitation amount of the synchronous machine 1 according to the selected signal.

As described above, by selecting the high value of the underexcitation limiting signal, the same effect can be obtained.

FIG. 9 shows a configuration diagram of an excitation control device according to a fourth embodiment of the present invention. It should be noted that a constant excitation control device 31 is provided instead of the automatic voltage adjustment device 9, an underexcitation limiting device 25 and a current setting device 32 are provided in the constant excitation control device 31, and an auxiliary current transformer 30 is provided. The other points are the same as those of the conventional excitation control device shown in FIG. 13, and the same parts as those in FIG. 13 are denoted by the same reference numerals and description thereof is omitted.

The constant excitation control device 31 controls the excitation based on the field current of the synchronous machine 1. The field current of the synchronous machine 1 input via the auxiliary current transformer 30 is set by the current setting device 32. The control device 10 performs the gate control of the thyristor rectifier 6 so that the obtained value is obtained.

The underexcitation limiting device 25 receives the field current of the synchronous machine 1 as input and monitors the underexcitation state. Underexcitation limiting device 25 detects an underexcitation state based on a preset underexcitation limit curve. That is, when the synchronous machine is in the phase leading operation and exceeds the underexcitation limit curve 106 shown in the underexcitation limit curve diagram of FIG. 10, the underexcitation limit device 25 outputs the underexcitation limit signal. The underexcitation limit signal is input to the control device 10, and the control device 10 controls the excitation amount of the synchronous machine 1 according to the underexcitation limit signal.

As described above, even when the constant excitation control device is used, the excitation control can be performed in the same manner as when the automatic voltage regulator is used. Operability can be improved.

As another embodiment, an example in which the excitation control device shown in FIG. 9 is applied to the excitation control device shown in FIG. 7 is shown in FIG.
It is shown in FIG. 9 except that the constant excitation control device 31 shown in FIG. 9 is provided instead of the automatic voltage regulator 9 and the underexcitation limiting device 25 shown in FIG. 9 is provided instead of the underexcitation limiting device 24a. 7 is the same as the configuration of the excitation control device shown in FIG. 7, and the same parts as those in FIG.

The underexcitation limiting device 23 includes converters 14 and 15
And the synchronous machine active power and reactive power from
Underexcitation limiting device 25 receives a synchronous machine field current detected via auxiliary current transformer 30 as an input, detects an underexcitation state, and outputs an underexcitation limiting signal, respectively. These underexcitation limiting signals are input to the high value selector 26, and a high value is selected. The control device 10 controls the excitation amount of the synchronous machine 1 according to the selected signal.

As described above, even when the constant excitation control device is used, the excitation control can be performed in the same manner as when the automatic voltage adjustment device is used, and the same effect can be obtained.

In all the embodiments described above, the underexcitation limiting device can use the synchronous machine active current and the reactive current as input signals. In this case, an arithmetic unit is provided to calculate a synchronous machine active current and a reactive current from the synchronous machine active power and the reactive power. For example, as shown in FIG. 12, an arithmetic unit 2 is added to the configuration of the first embodiment shown in FIG.
8 and 29, and the output thereof is reduced by the underexcitation limiting device 17, 1
It is configured to input to 8 and 19.

Arithmetic units 28 and 29 calculate synchronous machine active current and reactive current from synchronous machine active power and reactive power from converters 14 and 15, respectively. Underexcitation limiter 1
7, 18, and 19, using the synchronous machine active current and the reactive current as input signals, detect an underexcitation state based on a preset underexcitation limit curve. As described above, the excitation control can be performed in the same manner as in the first embodiment in which the synchronous machine active power and the reactive power are input signals, and the same effect can be obtained.

[0049]

As described above, in the excitation control apparatus according to the present invention, it is possible to perform the limit control according to the underexcitation state of the synchronous machine, making the most of the underexcitation limit control. The operability of the plant can be improved. .

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an excitation control device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an underexcitation limiting curve of the underexcitation limiting device according to the first embodiment of the present invention.

FIG. 3 is a configuration diagram of another example of the first embodiment of the present invention.

FIG. 4 is a configuration diagram of an excitation control device according to a second embodiment of the present invention.

FIG. 5 is a configuration diagram of an excitation control device according to a third embodiment of the present invention.

FIG. 6 is a diagram showing an underexcitation limiting curve of an underexcitation limiting device according to a third embodiment of the present invention.

FIG. 7 is a configuration diagram of another example of the third embodiment of the present invention.

8 is a diagram showing an underexcitation limiting curve of the underexcitation limiting device shown in FIG. 7;

FIG. 9 is a configuration diagram of an excitation control device according to a fourth embodiment of the present invention.

FIG. 10 is a diagram showing an underexcitation limiting curve of an underexcitation limiting device according to a fourth embodiment of the present invention.

FIG. 11 is a configuration diagram of another example of the fourth embodiment of the present invention.

FIG. 12 is a diagram showing a modification of the first embodiment of the present invention.

FIG. 13 is a configuration diagram of a conventional excitation control device.

FIG. 14 is a diagram showing an underexcitation limiting curve of an underexcitation limiting device in a conventional excitation control device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Synchronous machine, 2 ... Field winding, 3 ... Main transformer, 4 ... Circuit breaker for system parallelism, 5 ... Power system, 6 ... Thyristor rectifier, 7
... Field circuit breaker, 8 ... Exciting power supply transformer, 9 ... Automatic voltage regulator, 10 ... Control device, 11 ... Fuse, 12 ... Instrument transformer, 13 ... Instrument current transformer, 14 ... Power converter 1 , 1
5 Power converters 2, 16 Voltage setting devices 17, 18, 1
9, 23, 24, 25: underexcitation limiting device, 20: control signal calculator, 21: underexcitation limit control detector, 22: underexcitation limit control signal selector, 26: high value selection circuit, 27
... Phase adjustment circuit, 28 ... Calculators 1 and 29 ... Calculators 2 and 3
0: auxiliary current transformer, 31: constant excitation control device, 32: current setting device, 100, 101, 102, 103, 104, 10
5,106: Underexcitation limit curve

Continued on the front page (72) Inventor Hidekatsu Takeda 2-chome, Harumi-cho 2-chome, Fuchu-shi, Tokyo Toshiba System Technology Corporation F-term (reference) 5H590 AA01 AA30 AB09 CC01 CE01 DD24 DD64 DD69 EB02 EB21 FA06 FB05 FC15 GA01 GA05 GA06 GA07 HA02 HA04 HA05 HA06 HA07 HB02 HB03 KK04

Claims (8)

[Claims] 1. An excitation control device for performing excitation control of a synchronous machine based on an output voltage of the synchronous machine, comprising: power detection means for detecting active power and reactive power of the synchronous machine; and active power detected by the power detection means. Under-excitation state is detected by the value and reactive power value and a preset under-excitation limit curve,
A plurality of under-excitation limiting devices that output under-excitation limiting signals in accordance with the under-excitation state, a control signal calculator that adds the respective under-excitation limiting signals from these under-excitation control devices, An excitation control device comprising: a control device that controls an excitation amount of a synchronous machine according to an output signal from a computing unit. 2. An excitation control device for performing excitation control of a synchronous machine based on an output voltage of the synchronous machine, comprising: power detection means for detecting active power and reactive power of the synchronous machine; and active power detected by the power detection means. Under-excitation state is detected by the value and reactive power value and a preset under-excitation limit curve,
A plurality of under-excitation limiting devices that output an under-excitation limiting signal in accordance with the under-excitation state are provided in parallel, an under-excitation limit that detects an under-excitation control state of these under-excitation control devices and outputs an under-excitation control status signal. A control detector, and an underexcitation limit for selecting an output signal used for control from among the respective underexcitation limit signals from the plurality of underexcitation control devices based on the underexcitation control state signal from the underexcitation control device. An excitation control device comprising: a control signal selector; and a control device that controls an excitation amount of the synchronous machine in accordance with an output signal from the underexcitation restriction control signal selector. 3. An excitation control device for performing excitation control of a synchronous machine based on an output voltage of the synchronous machine, comprising: power detection means for detecting active power and reactive power of the synchronous machine; and active power detected by the power detection means. Under-excitation state is detected by the value and reactive power value and a preset under-excitation limit curve,
A plurality of under-excitation limiting devices that respectively output under-excitation limiting signals according to the under-excitation state are provided in parallel, and a high value selector that selects a high value among the respective under-excitation limiting signals from these under-excitation limiting devices, A controller for controlling the amount of excitation of the synchronous machine in accordance with the output signal from the high value selector. 4. An excitation control device for performing excitation control of a synchronous machine based on an output voltage and a field current of the synchronous machine, wherein power detection means for detecting active power and reactive power of the synchronous machine; A field current detecting means for detecting a current, an active power value and a reactive power value detected by the power detecting means are inputted, an underexcitation state is detected, and an underexcitation limit signal is output according to the underexcitation state. A first underexcitation limiting device for detecting the underexcitation state by inputting the field current value detected by the field current detection means, and outputting an underexcitation limit signal according to the underexcitation state. Two underexcitation limiting devices, a high value selector for selecting a high value among the underexcitation limiting signals from the first underexcitation limiting device and the second underexcitation limiting device, and an output signal from the high value selector Excitation of synchronous machine according to An excitation control device comprising a control device for controlling the amount. 5. The excitation control device according to claim 1, wherein the control device controls an excitation amount of the synchronous machine based on an underexcitation limited phase control signal corresponding to the output signal. Excitation control device. 6. An excitation control device for performing excitation control of a synchronous machine based on an output voltage and a field current of the synchronous machine, a field current detecting means for detecting a field current of the synchronous machine, and a field current detecting means. An underexcitation limiter that detects the underexcitation state by inputting the field current value detected in step 2, and outputs an underexcitation limit signal according to the underexcitation state; A controller for controlling the excitation amount of the synchronous machine in accordance with the excitation limit signal. 7. An excitation control device for controlling excitation of a synchronous machine based on an output voltage and a field current of the synchronous machine, wherein an electric power detecting means for detecting active power and reactive power of the synchronous machine; A field current detecting means for detecting a current, an active power value and a reactive power value detected by the power detecting means are inputted, an underexcitation state is detected, and an underexcitation limit signal is output according to the underexcitation state. A first under-excitation limiting device for detecting the under-excitation state by inputting the field current value detected by the field current detection means, and outputting an under-excitation restriction signal according to the under-excitation state. 2 under-excitation limiting devices, a high-value selector for selecting a high value among the respective under-excitation limiting signals from the first under-excitation limiting device and the second under-excitation limiting device, Same according to the output signal An excitation control device comprising: a control device that controls the amount of excitation of the starting machine. 8. An excitation control device according to claim 1, further comprising a computing unit for calculating an active current value and a reactive current value from the active power value and the reactive power value detected by said power detection means, and An excitation control device, wherein an excitation limiting device uses the active current signal and the reactive current signal from the computing unit as input signals instead of the active power value and the reactive power value detected by the power detection means.