JP2923572B2 - Change width detector - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a change width detection device, and more particularly to a method for detecting a ground fault accident with high sensitivity even in a system in which a residual zero-phase voltage or a residual zero-phase current exists in a distribution line. And a change width detecting device suitable for detecting the change width.

[Conventional technology]

In general, the step-down distribution system is wired in a tree shape, and the high-voltage distribution system, which is composed of three-phase distribution lines, has a geometrically asymmetric part with respect to the ground. The capacitance may become unbalanced. Therefore, a residual zero-sequence voltage and a residual zero-sequence current always exist in such a system.

By the way, in order to detect a ground fault in such a system, a ground fault direction relay is generally used, and the detection sensitivity of this relay is desired to be high in terms of security.

[Problems to be solved by the invention]

However, in the related art, even in a system in which a residual voltage or a residual current always exists, a ground fault is detected using a ground fault direction relay. For this reason, when a ground fault occurs in the system, phase discrimination is performed using the vector synthesis amount of the residual electricity amount and the zero-phase component electricity amount generated by the ground fault accident as inputs, and particularly high power near the operation limit of the relay. Phase discrimination may be erroneous in the event of a resistance ground fault. In order to prevent such errors in phase discrimination, a method has been adopted in which the detection sensitivity is reduced and the phase discrimination performance is not affected by the residual voltage or residual current. Because the zero-sequence voltage or zero-sequence current is superimposed on the residual zero-sequence voltage or current, depending on the value of the residual zero-sequence voltage or current, the relay may operate even with the same fault resistance. May not be performed, and a ground fault may not be reliably detected.

SUMMARY OF THE INVENTION An object of the present invention is to provide a change that can reliably detect a ground fault without being affected by the magnitude of these residual electric quantities even if a residual zero-sequence voltage and a residual zero-phase current always exist in the distribution system. An object of the present invention is to provide a width detecting device.

[Means for solving the problem]

In order to achieve the above object, the present invention provides, as a first device, a ground voltage detecting means for detecting a ground voltage of each phase of a distribution line, and a line voltage of each phase from a detection output of the ground voltage detecting means. Line voltage calculation means for calculating, phase current detection means for detecting a phase current of each phase of the distribution line, zero-phase detection means for detecting a zero-phase current from a detection output of the phase current detection means, and line voltage calculation Multiplying means for multiplying the output of the means by the output of the zero-phase current detecting means; filter means for extracting a DC component from the output of the multiplying means; delay means for delaying the output of the filter means for a predetermined time; Comprising a deviation calculating means for calculating a deviation between the output of the filter means and a fault signal generating means for generating a fault signal when the output of the deviation calculating means exceeds a set level. It is.

As a second device, a ground voltage detecting means for detecting a ground voltage of each phase of the distribution line, a line voltage calculating means for calculating a line voltage of each phase from a detection output of the ground voltage detecting means, A phase current detecting means for detecting a phase current of each phase; a zero-phase detecting means for detecting a zero-phase current from a detection output of the phase current detecting means; an output of the line voltage calculating means and an output of the zero-phase current detecting means; Multiplying means, a filter means for extracting a DC component from the output of the multiplying means, a delay means for delaying the output of the filter means for a predetermined time, and a hold means for holding and outputting a delay means output when a hold command is input by a hold command. Then, the holding means for outputting the output of the delay means as it is in response to the hold release command, the deviation calculating means for outputting the difference between the output of the delay means and the output of the holding means, and the output of the deviation calculating means exceeding the set level. When an accident signal is generated, an accident signal generating means for generating an accident signal, and receiving the accident signal, outputting a hold command while the accident signal is being generated, and when the generation of the accident signal is stopped, continuously generating the hold command for a certain period of time Thereafter, a change width detecting device having a hold command means for outputting a hold release command is configured.

As a third device, a ground voltage detecting means for detecting a ground voltage of each phase of the distribution line, a line voltage calculating means for calculating a line voltage of each phase from a detection output of the ground voltage detecting means, and a ground voltage detecting means A zero-phase voltage detection means for detecting a zero-phase voltage of the distribution line from an output of the means, a multiplication means for multiplying an output of the line voltage detection means and an output of the zero-phase voltage detection means, and a DC component from an output of the multiplication means. Filter means, a delay means for delaying the output of the filter means for a predetermined time, a deviation calculation means for calculating a deviation between the output of the delay means and the output of the filter means, and an output of the deviation calculation means exceeding a set level. And a fault signal generating means for generating a fault signal when the vehicle is running.

As a fourth device, a ground voltage detecting means for detecting a ground voltage of each phase of the distribution line, a line voltage calculating means for calculating a line voltage of each phase from a detection output of the ground voltage detecting means, and a ground voltage detecting means A zero-phase voltage detection means for detecting a zero-phase voltage of the distribution line from an output of the means, a multiplication means for multiplying an output of the line voltage detection means and an output of the zero-phase voltage detection means, and a DC component from an output of the multiplication means. , A delay means for delaying the output of the filter means for a predetermined time, a hold command for holding and outputting a delay means output when a hold command is input, and a hold for outputting a delay means output as it is by a hold release command Means, a deviation calculating means for outputting a deviation between the output of the delay means and the output of the holding means, and an accident signal for generating an accident signal when the output of the deviation calculating means exceeds a set level. Generating means for receiving a fault signal, outputting a hold command during the generation of the fault signal, and when the generation of the fault signal is stopped, continuously generating the hold command for a fixed time and thereafter outputting a hold release command; and Is a variation width detecting device having the following.

As a fifth device including the second or fourth device, between the hold command means of each phase and the hold command means, there is a gate means for outputting a signal according to the logical sum of the input signals to the hold means, The output of the hold command means of each phase is connected to the input of its own gate means and the input of the gate means of another phase.

As a sixth device including the first, second, third or fourth device, a variation width detecting device using a band-pass filter as delay means is configured.

As an eighth device including the second, fourth, fifth or sixth device, the occurrence of an accident signal is monitored, and when the occurrence of the accident signal continues for a set time or longer, a hold release command is issued to the hold command means. This configuration constitutes a change width detection device having hold release instruction means for forcibly outputting.

As an eighth device including any one of the first to seventh devices, the fault signal generating means includes a level comparator for generating a fault signal when an output of the deviation calculating means exceeds a plus set level. And a level comparator which generates an accident signal when the output of the deviation calculating means exceeds a negative set level.

[Action]

Find the line voltage of each phase from the ground voltage of each phase of the distribution line,
Using the line voltage as a reference signal, the zero-phase current is multiplied by the line voltage, a DC component is extracted from the multiplied value, the extracted signal is delayed for a certain time, and the deviation between the delayed signal and the undelayed signal is calculated. When the deviation exceeds a set level, an accident signal is generated. That is, even when the residual zero-sequence voltage and the residual zero-sequence current are present in the wiring system, the amount of change from the reference value is detected using the residual amount of electricity as a steady-state reference value, and the change exceeds the set level. Sometimes an accident signal is generated as a ground fault. For this reason, even when a residual zero-sequence voltage and a residual zero-sequence current exist in the distribution system, a ground fault in the distribution system can be reliably detected without being affected by the level of the residual quantity of electricity. Such detection can be similarly performed by multiplying the reference signal by the zero-phase voltage and detecting a ground fault from the multiplied value.

When having a hold means and a hold command means, the output of the delay means is held by a hold command,
A deviation between the held signal and the output of the delay means is determined, and an accident signal is generated when the deviation exceeds a set level. That is, when the residual zero-sequence voltage and the residual zero-sequence current exist in the distribution system, these levels are stored as the steady-state levels, and when a ground fault occurs, a change from the stored levels is detected. When the change exceeds a set level, an accident signal is generated as a ground fault. When the ground fault has recovered,
Even after the occurrence of the ground fault has been stopped, the state in which the level at the time of the ground fault occurrence is stored for a certain period of time is continued, the stored level is erased, and the newly input signal is set as the reference level. Detection is being performed. Therefore, the subsequent ground fault can be detected at a steady level without setting the reference level after the accident recovery to a high level.
Also, in this case, if the gate means is provided, all phases can simultaneously hold and release the input signal of the phase in which the fault is detected, thereby preventing the phases other than the fault phase from malfunctioning. Can be.

〔Example〕

 Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

In FIG. 1, the R-phase, S-phase, and T-phase of the three-phase distribution line 10 have voltage dividing capacitors CT, CS,
CR, CTG, CSG, and CRG are connected, and the line voltages VR, VS, and VT divided by the respective capacitors are input to the ground fault relay 12 as a ground voltage. The three-phase distribution line 10 is provided with current transformers CTR, CTS, CTT constituting a phase current detecting means, and outputs of the respective current transformers are input to the relay 12 as phase currents IR, IS, IT. . The ground fault directional relay 12 includes an operation unit 14 shown in FIG. 2 and detection units 16, 18, and 20 shown in FIG.

The operation unit 14 includes vector operation phase shifters 22, 24, 26 and vector operation units 28, 30.
Reference numerals 2, 24, and 26 constitute line voltage calculating means for calculating line voltages VRS, VST, and VTR as reference signals for each phase from the line voltages VR, VS, and VT. That is, in the present embodiment, when a ground fault occurs, the phase voltage changes, but the line voltage does not change. Therefore, the line voltage is determined as the reference signal of each phase. When obtaining the line voltage, the line voltage between the phases is obtained by vector operation from the detected values of the line voltages VR, VS, and VT of each phase, and the phase of the line voltage between the phases is shifted by 30 °. Find the line voltage
Line voltages VRS, VST, and VTR having the same phase as the line voltage are output.

On the other hand, the vector calculator 28 is configured as a zero-sequence voltage detecting means for detecting the zero-sequence voltage Vo from the vector values of the line voltages VR, VS, VT, and the vector calculator 30 is a vector of the phase currents IR, IS, IT. It is configured as zero-phase current detection means for detecting the zero-phase current Io from the value. The outputs of the operation unit 14 are supplied to the detection units 16, 18, and 20 of each phase. R phase detector
16 is a multiplier 32, a low-pass filter 34, a level memory 36,
It comprises a computing unit 38 and a level comparator 40, and the S-phase detector 18 and the T-phase detector 20 are the same as the R-phase detector 16.

The multiplying unit 32 is configured as a multiplying unit that multiplies the line voltage VRS of the R phase and the S phase as the reference signal of the R phase by the zero-phase current Io and outputs the multiplied signal to the low-pass filter 34. Reference numeral 34 denotes a filter for extracting a DC component from an input signal. That is, since the low-pass filter 34 includes a double harmonic in the multiplied value of the multiplier 32, the harmonic is removed and only the DC component is extracted. The output of the low-pass filter 34 is input to a level storage 36 and a calculator 38.

The level memory 36 includes a delay circuit 42 having a resistor R and a capacitor C, a sample and hold 44, an OR gate 46, and a time limit recovery timer 48. The output of the sample hold 44 is input to the minus input terminal of the arithmetic unit 38, and the output of the level comparator 40 is input to the sample hold 44 via the timer 48 and the OR gate 46.
The signal from the low-pass filter 34 is input to the sample / hold 44 after being delayed by a predetermined time by the delay circuit 42. When the input level of the sample / hold terminal S / H is "0", that is, the hold release command is issued. When input, the input signal is output as it is to the arithmetic unit 38,
When the input level of the sample hold pin S / H is “1”,
That is, when a hold command is input, the holding unit is configured to hold the level of the input signal at the time of inputting the hold command and output the held signal to the computing unit 38. The arithmetic unit 38 is configured as a deviation calculating unit that outputs a signal corresponding to the deviation between the output signal of the low-pass filter 34 and the output signal of the sample and hold 44, and the level comparator 40 outputs the output of the arithmetic unit 38 and the set level Ve. And a fault signal generating means for outputting a signal of "1" to the terminal 1A and the timer 48 as a fault signal when the comparison signal of the arithmetic unit 38 exceeds the set level Ve. Timed recovery timer
Reference numeral 48 denotes when an accident signal is input, that is, the level comparator 40
When the output of “1” is “1”, the instantaneous hold command
A signal of "1" is output to the OR gate 46 and the OR gate of the S phase and the T phase. When the generation of the accident signal is stopped, the generation of the hold command is continued for a certain period of time. It is configured as a hold command means for outputting a signal.

In the above configuration, when the residual zero-sequence voltage and the residual zero-sequence current exist in the distribution line 10, the output of the low-pass filter 34 as a multiplication result of the reference signal of each phase and the zero-sequence current Io is shown in FIG. A signal having a waveform as shown is output.
For example, the level of the R phase is offset in the positive direction, the level of the T phase is offset in the negative direction, and the level of the S phase is substantially zero V.
Signal is generated. As shown in FIG. 6, such an offset occurs because the residual zero-sequence current Ioe exists in the distribution line 10, and even in the steady state, VR × Ioe, VS
This is because the calculated values of × Ioe and VT × Ioe are generated in the output of the low-pass filter 34.

When an R-phase ground fault of the distribution line 10 occurs in such a state, as shown in FIG. 6, the fault current Ios is added to the residual zero-phase current Ioe, and the low-pass filter 34 is turned off.
Generates a signal in which the R-phase level is high and the S-phase and T-phase levels are low. At this time, as shown in FIG. 7, the output level of the low-pass filter 34 sharply increases, and this signal is directly input to the arithmetic unit 38. on the other hand,
As shown by a broken line, a signal delayed for a predetermined time by the delay circuit 42 is input to the sample hold 44. Then, the deviation at this time is calculated by the calculator 38. When the calculated value exceeds the set level Ve, the level comparator 40 generates an accident signal of "1". As a result, the signal of “1” is
The signal is input to the sample / hold terminal S / H via the OR gate 46, and the level SHo at the time when the ground fault occurs is held as the reference level. Therefore, while the ground fault occurs, the signal of “1” is continuously output from the level comparator 40. At this time, the signal at the terminal 1A is input to a controller for controlling a circuit breaker or the like, and control for shutting off the circuit breaker or the like is performed when an accident signal is generated. Further, at this time, the output of the timer 48 is input to the OR gate of the S phase and the T phase, and the sample and hold of the S phase and the T phase are also held with the level at the time T1 when the ground fault occurs as the reference level.

When the ground fault is recovered, the output level of the delay circuit 42 gradually decreases from time T2, but the output of the timer 48 remains "1" until the output level of the delay circuit 42 returns to the reference level SHo before the occurrence of the ground fault. Will be kept as is. Then, at time T3 at which the fixed time T has elapsed from time T2, the output of the timer 48 is inverted to "0", and the release of the hold is commanded to the sample hold 44. Thereafter, the output of the delay circuit 42 remains unchanged Entered in 38.

As described above, according to the present embodiment, even when the residual amount of electricity is present in the distribution line 10, the residual component is used as the reference level, a change from the reference level is detected, and the detection of the change is detected. Since it is determined that the ground fault has occurred when the value exceeds the set level Ve, the ground fault can be reliably detected without being affected by the level of the residual electric quantity of the wiring line 10. Furthermore, since the ground fault is detected from the multiplied value of the zero-phase current Io and the line voltage, the direction in which the ground fault has occurred can be determined. For example, relay 12
Are installed on the distribution line 10, and the direction of occurrence of the ground fault can be determined based on which relay 12 detects the ground fault when a ground fault occurs.

Further, according to the present embodiment, the timer is maintained even after the ground fault has occurred.
Since the output of 48 is held at “1”, after the ground fault has been recovered, the ground fault has continued after the ground fault has been recovered without maintaining the high level as the reference level of the sample hold 44. Ground fault can be reliably detected. Furthermore, erroneous detection that a ground fault has occurred again when the ground fault has recovered can be prevented.
That is, when the timer 48 is not provided, as shown in FIG. 8, when the ground fault is recovered after the ground fault has occurred in the R phase, the level of the T phase changes in the positive direction. If the level exceeds the set level Ve, it is determined that a ground fault has occurred in the T phase due to this signal, and a fault signal is generated from the level comparator of the T phase. Therefore, if the timer 48 is provided, the low-pass filter
Even if the output of 34 changes in the plus direction, it is possible to prevent erroneous detection of a ground fault.

Also, in the above embodiment, the output of the timer 48 of each phase is the OR of the other phase.
Since the signal is input to the gate 46, when a ground fault is detected in any of the phases, the sample and hold 34 of each phase performs the hold and hold release operations simultaneously.
Therefore, when a ground fault is detected in either phase,
Malfunction of other phases can be prevented.

Next, another embodiment of the present invention will be described with reference to FIGS. 9 and 10. FIG.

In this embodiment, the band-pass filter 50, the level comparators 40 and 52, and the flip-flop 5
4 is provided so that the output of the flip-flop 54 is output to the timer 48 and the terminal 1A, and the other configuration is the same as that of the above embodiment.

The band-pass filter 50 is a non-vibration type band-pass filter having no vibration attenuation, and is configured to have a Q of 0.7 or less. Bandpass filter 50
When a signal as shown in FIG. 10A is input from the low-pass filter 34 to cut a DC component,
A signal is generated only when the signal level changes, and when the signal level is constant, the DC component is cut off.
Are output as shown in FIG. When the output level of the band-pass filter 50 exceeds the plus set level + Ve, a signal of “1” is output from the level comparator 40 as a fault signal, and the flip-flop 54
Is set. As a result, a signal of "1" is output from the flip-flop 54 as an accident signal.

On the other hand, when the ground fault is recovered, the output level of the band-pass filter 50 changes to the negative side, and when the output level falls below the negative set level −Ve of the level comparator 52, the flip-flop 54 is reset, and the flip-flop 54 is reset. 54 outputs a signal of “0”. Note that timer 48
After that, when a certain period of time has elapsed after the level of the flip-flop 54 has become "0", that level changes to "0".

Also in the present embodiment, a ground fault can be reliably detected without being affected by the level of the amount of residual electricity in the distribution line 10, as in the above-described embodiment.

Further, in each of the above embodiments, the detection of the ground fault has been described based on the multiplied value of the zero-phase current Io and the line voltage, but the multiplication value of the zero-phase voltage Vo and the line voltage has been described. It is also possible to detect a ground fault accident. Furthermore, in order to detect the zero-sequence current Io, the detection of the zero-sequence current has been described. Instead of detecting the zero-sequence current from the line current, the zero-sequence current is directly detected from the ZCT using the zero-sequence current detector ZCT. Can also be detected.

In each of the above embodiments, when the output of the level comparator 40 or the flip-flop 54 is monitored and the occurrence of the accident signal continues for a certain period of time or longer, the hold release command for forcibly setting the output of the timer 48 to "0" If the means is provided, even if the level of residual electricity increases from the level before the occurrence of the ground fault due to the state of the system of the distribution line 10 after the ground fault is recovered, the increased level is used as a reference level. The hold 44 can be returned to the original state.

For example, the level comparator 4 is connected to the output side of the arithmetic unit 38 in FIG.
0, 52, the output of each level comparator 40, 52 is connected to the output terminal, respectively, and connected to the timer 48 via an OR gate, and the output level of the low-pass filter 34 is connected to each level comparator 40, 52. If the determination is made, the direction in which the ground fault occurs can be determined even if the value of the zero-phase current Io becomes positive or negative depending on the place where the ground fault occurs. That is, when the output of the low-pass filter 34 changes to the plus side due to the occurrence of the ground fault, the ground fault can be detected by the level comparator 40, and when it changes to the minus side, the ground fault is detected by the output of the level comparator 52. Detection can be performed reliably, and the direction can be determined based on the outputs of the level comparators 40 and 52.

〔The invention's effect〕

As described above, according to the present invention, the residual amount of electricity in the distribution system is used as a reference level, a change from this reference level is detected, and when this detection level exceeds a set level, a ground fault is detected. Therefore, even if residual electric energy is present in the distribution line, a ground fault can be reliably detected without being affected by the level of the residual electric energy.

Also, when the ground fault has been recovered, the hold command continues to be generated for a certain period of time, and then the hold is released.This prevents the phases other than the accident phase from malfunctioning when the ground fault is recovered. it can.

When the occurrence of an accident signal has continued for a certain period of time or more, the hold state is forcibly released, so even if the level of residual electricity changes when the ground fault recovers, the changed level is used as the reference level. It can return to the original state.

As an accident signal generating means, a level comparator for judging whether the output of the deviation calculating means has exceeded a positive set level and a level comparison for judging whether the output of the deviation calculating means has exceeded a negative setting level If a device is provided, the direction of occurrence of a ground fault can be easily determined.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing an embodiment of the present invention, FIG. 2 is a configuration diagram of an operation unit, FIG. 3 is a configuration diagram of a detection unit, FIG. FIG. 6 is a waveform diagram showing the output waveform of the low-pass filter, FIG. 6 is a vector diagram showing the amount of residual electricity in the distribution system, FIG. 7 is a waveform diagram of each part in FIG. 4, and FIG. FIG. 9 is a waveform diagram of each part, FIG. 9 is a main part configuration diagram showing another embodiment of the present invention, and FIG. 10 is a waveform diagram for explaining the operation of FIG. 10 ... distribution line, 12 ... ground fault direction relay, 14 ... arithmetic unit, 16, 18,
20: detection unit, 34: low-pass filter, 36: level memory, 38: arithmetic unit, 40: level comparator, 42: delay circuit, 44
… Sample hold, 46… OR gate, 48… Time limit recovery timer, 50… Band pass filter, 52… Level comparator, 54…
flip flop.

Claims (8)

(57) [Claims] 1. A ground voltage detecting means for detecting a ground voltage of each phase of a distribution line, a line voltage calculating means for calculating a line voltage of each phase from a detection output of the ground voltage detecting means, Phase current detection means for detecting a phase current of a phase, zero-phase current detection means for detecting a zero-phase current from a detection output of the phase current detection means, an output of the line voltage calculation means and an output of the zero-phase current detection means Multiplying means, a filter means for extracting a DC component from the output of the multiplying means, a delay means for delaying the output of the filter means for a predetermined time, and a deviation for calculating a deviation between the output of the delay means and the output of the filter means. A variation width detecting device comprising: a calculating means; and an accident signal generating means for generating an accident signal when an output of the deviation calculating means exceeds a set level. 2. A ground voltage detecting means for detecting a ground voltage of each phase of a distribution line; a line voltage calculating means for calculating a line voltage of each phase from a detection output of the ground voltage detecting means; Phase current detection means for detecting a phase current of a phase, zero-phase current detection means for detecting a zero-phase current from a detection output of the phase current detection means, an output of the line voltage calculation means and an output of the zero-phase current detection means Multiplying means, a filter means for extracting a DC component from the output of the multiplying means, a delay means for delaying the output of the filter means for a predetermined time, and a hold means for holding and outputting a delay means output when a hold command is input by a hold command. A holding means for directly outputting the output of the delay means in response to the hold release command, a deviation calculating means for outputting a deviation between the output of the delay means and the output of the holding means, and an output of the deviation calculating means exceeding a set level. An accident signal generating means for generating an accident signal when an accident signal is received, and outputting a hold command while the accident signal is being generated. And a hold command means for outputting a hold release command. 3. A ground voltage detecting means for detecting a ground voltage of each phase of a distribution line, a line voltage calculating means for calculating a line voltage of each phase from a detection output of the ground voltage detecting means, and a ground voltage detecting means. Zero-phase voltage detection means for detecting the zero-phase voltage of the distribution line from the output of the multiplication means for multiplying the output of the line voltage detection means and the output of the zero-phase voltage detection means, and the DC component from the output of the multiplication means Filter means for extracting, delay means for delaying the output of the filter means for a predetermined time, deviation calculating means for calculating a deviation between the output of the delay means and the output of the filter means, and an output of the deviation calculating means exceeding a set level. And a fault signal generating means for generating a fault signal when the vehicle is in the off state. 4. A ground voltage detecting means for detecting a ground voltage of each phase of a distribution line, a line voltage calculating means for calculating a line voltage of each phase from a detection output of the ground voltage detecting means, and a ground voltage detecting means. Zero-phase voltage detection means for detecting the zero-phase voltage of the distribution line from the output of the multiplication means for multiplying the output of the line voltage detection means and the output of the zero-phase voltage detection means, and the DC component from the output of the multiplication means Filter means for extracting, delay means for delaying the output of the filter means for a fixed time, hold means for holding and outputting the delay means output when a hold command is input by a hold command, and outputting the delay means output as it is by a hold release command A deviation calculating means for outputting a deviation between the output of the delay means and the output of the holding means; and an accident signal generating means for generating an accident signal when the output of the deviation calculating means exceeds a set level. And a hold command means for receiving a fault signal, outputting a hold command during the generation of the fault signal, and continuing to generate the hold command for a fixed time when the generation of the fault signal is stopped, and thereafter outputting a hold release command. Change width detecting device. 5. A gate means for outputting a signal in accordance with the logical sum of input signals to the hold means between the hold command means of each phase and the hold command means, wherein the output of the hold command means of each phase is self-contained. 5. The change width detecting device according to claim 2, wherein the change width detecting device is connected to the inputs of the gate means of the second phase and the gate means of another phase. 6. The change width detecting device according to claim 1, wherein a band-pass filter is used as the delay means. 7. Hold release command means for monitoring the occurrence of an accident signal and forcibly outputting a hold release command to the hold command means when the occurrence of the accident signal has continued for a set time or more. , 5 or 6. 8. The fault signal generating means includes a level comparator for generating a fault signal when the output of the deviation calculating means exceeds a positive set level, and a fault comparator for generating an error signal when the output of the deviation calculating means exceeds a negative set level. 8. The change width detecting device according to claim 1, further comprising a level comparator for generating an accident signal.