KR940010663B1 - High resistance grounding fault detecting apparatus and the method - Google Patents

Abstract

The device has an advantage of total protection relay system against the fault and control of the feeder line, and identifies the fault type by analysis of current. The device includes the data collection means, and the fault signal detection means which has a counter means, and a control and arithmetic means (11). The method employs an odd power (Podd) detection means (272), a calculation means (273) which computes the ratio of even power (Peven), a ratio variation(Pvar) calculation means (274), 1st target value compare and increment means (275a)(275b), 2nd target value compare and increment means (276a)(276b), 3rd target value compare and increment means (277a)(277b), and 4th target value compare and increment means(278).

Description

High resistance ground fault detection device and method

1 is a circuit diagram of a high resistance ground fault detection apparatus according to the present invention.

2 is a flowchart showing a procedure of controlling the overall function of the central processing unit in FIG.

3 is a detailed flowchart of the fault detection operation routine in FIG.

* Explanation of symbols for main parts of the drawings

10: input conversion means 20: band pass filter

30: multiplexer 40: sampling holder

50: A / D conversion means 60: DMA request signal generating means

70: timing pulse generator 80: buffer

90: DMA controller 100: memory device

110: central processing unit 120: input and output device

The present invention relates to a high resistance ground fault detection apparatus and method for stably detecting a high resistance ground fault of a distribution line of a multi-ground or direct ground system.

In particular, the high resistance ground fault detection apparatus and method of the present invention because it is difficult to distinguish between the image current (O-Sequence Current) caused by load imbalance in the multi-ground or direct ground distribution line and the current during high resistance ground fault An apparatus and method capable of stably and with high reliability detecting a high resistance ground fault which is very difficult to detect.

In addition, the high resistance ground fault detection apparatus of the present invention is implemented as a digital protective relay device using a microprocessor, and excellent after performing a frequency analysis after sampling (wave) of the waveform of arc current generated during high resistance ground fault If harmonic power, ratio of good harmonic power to odd harmonic power, and ratio of good harmonic power to odd harmonic power of each of two cycles of successive arc waveforms exceeds a predetermined value, a high-resistance ground fault occurs. It is a device that determines and transmits a fault trip signal to a circuit breaker system or a reclosing system.

In general, the power supply of the power distribution line, when the three-phase four-wire distribution system and three-phase three-wire direct ground method The protection relay method uses an overcurrent relay having a function of detecting an accident due to a short circuit of a distribution line and an overcurrent ground relay having a function of detecting an accident due to a ground fault of the distribution line.

Therefore, when the above accidents occur, the relays cut off the power supply to the power line or other power equipment to prevent electric shock or fire accidents, and also to prevent the possibility of spreading the accident to other healthy equipment and to prevent unnecessary leakage power. It has been used to

Specifically, the overcurrent relay method compares the difference between the fault current generated by a short circuit accident during power supply and a predetermined setting current set to about 1.5 to 2 times the load current, and wherein the fault current is the set current. When it is larger, the occurrence of a short circuit accident is determined, which cuts off the power supply.

In addition, the ground fault and current relaying method is a method of determining the ground fault by sensing the magnitude of the image current unbalanced when the fault current caused by the ground fault is returned during power supply.

In the three-phase three-wire or three-phase four-wire direct-grounded distribution system using the relay method, since the power load fluctuates frequently during normal operation, the current balance in each phase cannot be achieved. As a result, residual current is generated in the image current line, that is, the neutral line (N phase).

The residual current IN of the video current line (neutral line) at this time is represented by the following equation.

IN = IA + IB + IC = 3Io, where IA, IB, IC and Io represent A-phase, B-phase, C-phase and N-phase currents, respectively.

At this time, when the magnitude of the image current IN in the steady state is larger than the operating tap (Settingtap-value) of the overcurrent ground fault relay operating in a ground fault, the operation tap is adjusted upward by an appropriate size.

In this state, when a high resistance ground fault occurs on the distribution line, that is, when the distribution line is disconnected and contacts the ground, a large resistance component due to arc is present or a large resistance component (sand, asphalt, etc.) is touched. When the fault current does not increase and maintains the load current, the relay cannot detect the line fault and supply incomplete power.

That is, such a high resistance ground fault occurs when the power line is disconnected and the contact surface of the wire is incompletely grounded with trees (wood, gravel, sand, asphalt, etc.), and thus the ground fault caused by any protective relay including a conventional overcurrent relay. Was not possible.

Therefore, the above ground fault has caused economic loss by causing unexpected electric shock, fire, and equipment accident. In addition, in recent years, since the use of insulated wires has gradually increased in distribution lines, almost all occurrences of ground faults due to disconnection of the insulated wires are incompletely grounded due to wire coating, resulting in the occurrence of high resistance ground faults. This is even higher, and thus the incidence of human and economic losses is increasing.

To solve these problems, a technique developed by the present inventors and many others is disclosed in patent publication 89-4955.

According to this technique, after sampling the arc current waveform, frequency analysis is performed to extract even harmonic components caused by asymmetry within one period of the arc current waveform, and when the amount of change exceeds a predetermined value, By determining the ground fault, it is possible to more stably detect a high resistance ground fault of the cutting line.

However, this method has the following disadvantages, although it has great advantages over the prior art.

First, since the ground fault is detected only by the amount of power of the wave, if there is a load generating the wave in the loads connected to the line, the device is more likely to malfunction due to the wave component generated by the load. However, the reliability of the detection function is still low.

Secondly, according to this method, since the A, B, C phase voltage unit, input conversion means, etc. are required, the structure of the device is not only complicated, but also expensive to manufacture.

The present invention has been made to solve the conventional problems as described above, to simplify the structure of the device, to increase the reliability of the accident detection function to be able to detect the accident more stably.

In order to achieve the above object, the apparatus of the present invention samples an arc current waveform generated during a high resistance ground fault, converts it into a digital signal, and then performs a frequency analysis of the converted signal, thereby performing an analysis of the arc current waveform. Extracts even order harmonics due to asymmetry in the period, and when the information related to the above even harmonics exceeds a predetermined value, determines a high resistance ground fault and outputs a signal for controlling the protection relay. Has a configuration.

More specifically, the apparatus of the present invention includes a digital protective relay device using a microprocessor to sample a current signal of each phase provided from a power distribution line, convert the digital signal into a digital signal, and then use the digital signal to generate an even wave power amount or odd number. Calculate the amount of change in the ratio of the amount of good wave power to the amount of wave power, and the ratio of the amount of good wave power to the amount of odd wave power between two consecutive periods. In comparison, high-resistance ground faults occurring in multi-ground and direct-ground distribution lines are detected stably and with high reliability.

The power line fluctuations occur because the energy of the power line is distributed with extra high voltage during the ground fault of the line, where the amount of good harmonics in each period is distributed randomly and the radiated wave power of each of the two consecutive cycles Since a large deviation of the ratios of the even wave power is generated, it is possible to detect an accident with the magnitude of this variation amount (change amount between ratios of each of two cycles).

In the present invention, the detection method by the existing wave power in which an accident is detected by comparing the amount of change of the wave harmonic component and a predetermined set value within one period of the arc current waveform, Add the ratio detection method by the ratio and the ratio deviation detection method using the deviation between the ratios of the even wave power to the odd wave power of each of two consecutive cycles, and count the number of accident detection cycles by the counters, When the number of counted periods exceeds the set value, it is determined that an accident has occurred. As a result, the reliability of the detection function can be improved.

In an embodiment; The apparatus of the present invention comprises: first to fourth counter means for performing a counting function; Calculate the odd wave power from the collected data along with the even wave power, calculate the ratio of the good wave power to the odd wave power from the calculated good wave power and the odd wave power, and then each of two consecutive periods. Calculates a change amount between the ratios of the even wave power to the odd wave power, and calculates the ratio of the wave power and the wave power to the odd wave power and the change amount of the ratio to a predetermined value for fault detection. Comparing the first set value with the second set value and the third set value, respectively, and increase the count value of each of the first to third counter means by one when the comparison values are larger than the first to third set values. And when the at least one of the first to third counter means increases by one, the count value of the fourth counter means is increased by one, and then the count of the fourth counter means is increased. Control operation means for diagnosing whether a failure occurs outside the distribution line by analyzing the count values of the first to fourth counter means when the count value of the fourth counter means is larger than the predetermined fourth set value. Include.

In an embodiment; The method of the present invention divides the data corresponding to one period of the input signal into half periods, and adds the data corresponding to each position corresponding to the rear half periods from the beginning of the data corresponding to the previous half period among the divided data. Obtaining the wave power and subtracting the data corresponding to the latter half period from the data corresponding to the previous half period to obtain the odd wave power; Calculating a ratio of the even wave power to the odd wave power; Calculating a rate change amount between the ratio of the preceding period and the ratio of the next period in two successive periods; Comparing the even wave power with a predetermined first set value for fault detection, if the even wave power is greater than the first set value, the count value of the first counter is increased by one, and the count value of the first counter is increased. Comparing the ratio of the even wave power to the odd wave power with a second predetermined value after completion of the step or if the even wave power is not greater than the first set value; If the ratio is greater than the second set value, increase the count value of the second counter by one; Comparing the ratio change amount with a predetermined third set value after completion of the count value increase of the second counter or if the ratio is not greater than the second set value; When the ratio change amount is greater than the third set value, the count value of the third counter is increased by one, and after the count value increase of the third counter is completed or the ratio change amount is not greater than the third set value, Incrementing by one the count value of the fourth counter for counting an accident occurrence period of the input signal when the count value of at least one of the third counters is increased by one; The count value of the fourth counter is compared with a fourth set value which is a predetermined periodic value for fault detection, and if the count value of the fourth counter is not greater than the fourth set value, Repeatedly performing the above steps, and if the count value of the fourth counter is greater than the fourth set value, comparing the count values of the first to fourth counters with predetermined set values for fault detection determination. And determining that a high resistance ground fault has occurred on the distribution line if the count values of the first to fourth counters are larger than the predetermined set values.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a circuit diagram of a high resistance ground fault detection apparatus of the present invention, the present invention is a data collector for converting and collecting the current of each phase, which is an analog signal provided from a distribution line to a digital signal, and the collected data failure It consists of an accident signal detector which processes by a detection algorithm and detects that a high resistance ground fault has occurred in the distribution line.

The data collection unit includes an input conversion means 10 for converting currents A, B, C, and N of the distribution line into voltage signals having an appropriate level, and filtering each signal provided from the input conversion means 10 to perform the necessary filtering. A band pass filter 20 for outputting only signals of a predetermined band in order to easily detect even wave power components, an analog multiplexer 30 for sequentially selecting and outputting the band signals for current signals of each phase, and the multiplexer A sampling holder 40 which temporarily holds the output of 30 to output a signal of a predetermined level by an output pulse of a timing pulse generator which will be described later, and the sampling holder 40 Analog-to-digital converting means 50 for sampling the output of < RTI ID = 0.0 > 64 point < / RTI > every cycle and converting it into a 12-bit digital signal, DMA request signal generating means 50 for outputting a direct memory access (DMA) request signal DMAREQ by the conversion end signal output from the return means 50, the multiplexer 30, the sampling holder 40, and the conversion means ( And a timing pulse generator 70 for providing an output pulse to 50).

The accident signal detection unit generates a DMA recognition signal DMAACK in response to the buffer 80 temporarily storing the output of the analog-digital conversion means 50 and the DMA request signal DMAREQ. In addition, a DMA controller 90 for controlling data transfer so that data is directly supplied from the buffer 80 to the storage device, and a memory for storing output data of the buffer 80 and data and a fault detection algorithm as a result of the arithmetic processing. The apparatus 100, the central processing unit 110, which is a control operation means for controlling all functions of the data collecting unit and the accident signal detection unit and performing a failure detection algorithm, and the state of the digital relay. Data to allow the operator of the display keyboard to perform the display function, input / output function of data through the keyboard, and an operator using an external system to check the apparatus of the present invention. It is composed of input and output means 120 for outputting through a common communication port or input and output data through a common communication port at a long distance.

The operation of the detection apparatus of the present invention having the above configuration will be described in detail with reference to the flowchart of FIG.

The input conversion means (10) is a current transducer (Current transducer) for converting the current signal Ia, Ib, Ic and In on the A, B, C, N on the secondary side of the distribution line to a voltage signal of a predetermined level Consists of

The four lines of signals (step 200) provided from the input converting means 10 are each filtered by a bandpass filter 20 having a number of filters corresponding to the number of signals, so that even good harmonic components (360 Hz to 1920 Hz) are required. ) Is converted into a signal of a predetermined band pass for easy detection (step 210).

The four line bandpass signals are applied to the multiplexer 30 and are sequentially selected (step 220). The signal output from the multiplexer 30 is provided to the analog-digital converting means 50 through the sampling holder 40.

The sampling holder 40 prevents a large error from occurring when a new analog signal is input to the conversion means 50 during the data conversion operation of the analog-digital conversion means 50 for converting the analog signal into a digital signal. In order to accomplish this, the circuit having the function of temporarily holding the output signal of the multiplexer 30 until the analog signal is completed is converted.

The output signal of the sampling holder 40 is converted into a digital signal by the analog-digital converting means 50 (step 230), and the converted digital signal is stored in a buffer 80 connected to a data bus or the like.

The converting means 50 is an analog-to-digital converter having a function of converting an analog signal into a digital signal and a digital filter having a function of filtering out the converted digital signal to remove noise. The analog-to-digital converter is configured to include an analog current waveform having an even wave power component is converted to a digital signal by sampling at 64 points per cycle.

The digital filter includes a photo coupler. On the other hand, when the conversion operation in the conversion means 50 is completed, the conversion end signal STS is output from the conversion means 50 and is provided to the DMA request signal generation means 60.

At this time, the DMA request signal generating means 60 outputs the DMA request signal DMAREQ to the DMA controller 90 (step 240).

As soon as the signal DMAREQ is input, the DMA controller 90 outputs a DMA recognition signal DMAACK to the output control terminal OE of the buffer 80. As a result, data temporarily stored from the above-enabled buffer 80 is output and stored in the storage device 100 via the bus (step 250).

The data stored in the storage device 100 is a digital signal containing even harmonic components obtained from the current signals of the A, B, C and N phases.

The memory device 100 stores RAM stored in the buffer 80 and data calculated by the data processing process of the central processing unit 110. ROM (read only memory) that stores the function control algorithm and the failure detection algorithm of the central processing unit 110 to control.

As described above, the data collector performs a function of converting a current signal, which is an analog signal, into digital data.

Next, a process of performing the function of detecting the occurrence of a ground fault by the central processing unit 110 in the accident signal detection unit from the above-described digital signals stored in the memory device 100 in the accident signal detection unit is shown in FIG. It will be described in detail with reference to the flow chart shown in.

Returning to the first road, the DMA controller 90 performs the function of controlling data transfer as described above, but directly from the buffer 80 to the storage device 100 or the storage device without passing through the central processing unit 110. Control to transmit directly to the external system at (100).

The accident signal detection unit analyzes the data stored in the memory device 100 by a failure detection algorithm performed by the central processing unit 110 and detects a high resistance ground fault (step 270 of FIG. 2), and turns the blocking signal into a breaker or the like. The transmitting function is performed (step 280).

The means for detecting the ground fault includes a fault detection operation routine, which is a fault detection algorithm performed by the central processing unit 110.

The fault detection arithmetic routine is stored in the ROM of the memory device 100, and the data is analyzed to calculate even wave power and odd wave power, and from these powers, the ratio of the good wave power to the odd wave power is shown. And calculate the deviation of the ratios of the even-wave power to the odd-power in the following subsequent cycles, and compare each calculated data with the already set data, that is, the ground fault detection setting. It is a means for determining.

Meanwhile, the process of calculating the even-wave power and the odd-wave power by calling the data stored at the address designated by the DMA controller 90 by the central processing unit 110 will be described.

The current in each phase flowing through the distribution line is converted into a digital signal with a frequency of 50 Hz or 60 Hz and stored in the storage device 100.

The CPU 110 sequentially calls the stored data to divide data corresponding to one cycle into half cycles, and each position corresponding to a half cycle later from the start point of the data corresponding to the previous half cycle among the divided data. The superior power is obtained by adding the corresponding data, that is, the time difference between the previous half cycle and the next half cycle, and the odd wave power is obtained by subtracting the data corresponding to the second half cycle from the data corresponding to the previous half cycle.

In other words, the formula for calculating the power is as follows.

Here, i is the sequential number of the preceding half cycle of the data sampled at 64 points in one cycle, and (i + 31) is the sequential number of the later half cycle.

In step 270a of FIG. 3, the DMA controller 90 is initialized to clear the following counter to " 0 " and to prepare to recall the information stored in the storage device 100 sequentially.

In step 271, information corresponding to one cycle is called from the storage device 100 described above, and the flow proceeds to step 272. In step 272, the odd wave power Podd of the even-wave power Peven is calculated as described above, the result of the calculation is stored in the predetermined area of the storage device 100, and the flow proceeds to step 273.

In step 273, the ratio of the even-wave power Peven to the above-mentioned odd-wave power Podd, that is, Calculate and proceed to step 274.

In step 274, the ratio Pratio of the even-wave power in the previous cycle to the odd-wave power and the Pratio deviation in the subsequent period are calculated, and then the flow proceeds to step 275a. In step 575a, the even wave power is compared with the first set value for the even wave power of the fault detection.

If the even wave power is larger than the first set value, the process proceeds to step 275b or the counter CNT1 is increased by one, and then the process proceeds to step 276a. When the even wave power is smaller than the first set value, Proceed directly to step 276a. In step 276a, the ratio Pratio and the second set value of the even wave power to the odd wave power calculated in step 273 are compared.

The second set value is set to Pref to detect a failure. If Pratio is greater than the second set value Pref, the process proceeds to step 276b to increase the counter CNT2 by one, and then proceeds to step 277a. When Pratio is less than the second set value Pref, the process proceeds directly to step 277a. do.

In step 277a, if the ratio change amount Pvar with respect to the odd wave power of the continuous wave power of the continuous cycle calculated in step 274 above is larger than the third set value Pvref, which is the reference value of the fault detection, the procedure proceeds to step 277b, where the counter CNT3 Increases by 1 and proceeds to step 278, and if otherwise, proceeds directly to step 278. In step 278, the counter CNT4 is incremented by one each time it is detected by at least one of the three failure detection schemes described above, and then proceeds to step 279.

One increase of the counter CNT4 means that one or more counters of the counters CNT1 to CNT3 are increased by one, and an even wave component associated with a high resistance ground fault is detected.

That is, when the counter CNT4 initially increases by one, it indicates that the above even wave component is first detected from the current input signal. Therefore, whenever any one of the counters CNT1 to CNT3 is counted, the counter CNT4 is counted up to the count value for the setting period.

The counter CNT4 is cleared again after a predetermined period of the input current signal passes, and is counted each time information on the even wave component from the period of the new current signal to the predetermined period is detected.

Therefore, when information corresponding to one cycle of the current signal is analyzed and information about the even wave component related to the high resistance ground fault is detected, the counter CNT4 increases by one. On the other hand, in step 279, if the count value of the counter CNT4 is larger than the fourth set value for which the predetermined period required for the determination of the ground fault accident detection is set, the process proceeds to step 279a to perform a trip routine, and conversely, the counter CNT4 When the count value is smaller than the fourth set value, the control jumps to step 270a.

For example, when the fourth set value is 30, the process proceeds to step 279a only when the counter CNT4 is 30 or more.

In the triproutine (step 279a), the contents of the counters CNT1 to CNT4 are analyzed to determine fault detection for a high resistance ground fault. That is, the triproutine analyzes the current signal for 30 cycles as in the above example, analyzes the counter values of the counters CNT1 to CNT3, and compares them with the set values determined as the fault detection, and the counter values of the counters Through the process of judging that a high resistance ground fault has occurred on the distribution line when it is larger than the set value of, the signal controlling the breaker or the recloser is transmitted through the input / output device, and vice versa When it is smaller than this set value, it jumps to step 270 to detect a high resistance ground fault. As described above, in the above embodiment, the failure detection algorithm is executed by the central processing unit 110.

However, in the present invention, in order to increase the performance of the fault detection algorithm, a signal processing processor capable of correctly processing a multiplication operation may be added. In this case, the memory device 100 may include the central processing unit 110 and the processor. It is configured to share the signal processing processor of the processor and to process the same data in real time when calling the same data in the shared storage device is configured not to call the same data at the same time.

In the present invention, the counters CNT1 to CNT4 can be configured in hardware. Preferably, the counters CNT1 to CNT4 are configured in software so as to perform a function of the counter by allocating a predetermined area of the storage device 100 as a buffer.

That is, the signal processing processor is a device that performs only multiplication operations in order to enable rapid real-time processing of such a calculation process because a multiplication operation requiring a lot of calculation time is required in an accident detection operation process. ROM for storing data, a RAM for storing data for storing data for operation, and a RAM for temporarily storing calculation results for temporarily storing data generated during arithmetic processing.

The signals for controlling the signal processor are provided by the central processing unit 110, and the data for performing the multiplication operation are provided from the RAM of the memory device 100 to the data storage RAM of the signal processor. The data stored in the data storage RAM is processed at a high speed by the signal processor for performing a multiplication operation program.

The data of the result calculated by the processor is stored in the RAM for storing the calculation result, and the central processing unit 110 receives the data from the RAM for storage to perform an accident detection operation of a high resistance ground fault in real time. It is to detect the occurrence of ground fault.

The present invention is not limited to the three-phase four-wire distribution system as described above, but can be applied to the three-phase three-wire distribution system by changing the number of input current signals of the input converter. It will be obvious to man.

As described above, according to the present invention, it is possible to simultaneously perform arithmetic processing and data collection in a central processing unit by sending input data directly to a memory by a DMA controller, and also to perform high-speed calculations using a signal processing processor. Because it can be performed, the incident detection operation can be processed in real time.

In addition, even in the case of a high resistance ground fault in which the line is incompletely grounded, the accident can be stably detected, thereby preventing an accident caused by a high resistance ground fault in the distribution line.

Even if there are loads that generate storm waves among the loads connected to the line, the malfunction of the equipment can be prevented from falling due to the malfunction of the equipment due to the storm wave components generated by the loads. .

In the conventional apparatus, part of circuits such as A, B, C, N-phase voltage input means and input conversion means can be omitted, thereby reducing the channels of the multiplexer, sampling hold and A / D converter in half. The production cost can be reduced.

In addition, by outputting a control signal to a circuit breaker or a recloser system as well as ground fault detection through the input and output device, there is an advantage that can implement a comprehensive protective relay system for accidents and control.

In addition, by analyzing the currents of the A, B, C, and N phases provided from the distribution lines, a high-resistance ground fault can be determined, so that failures of other feeders can be identified and the phases of each phase can be identified. Accurately determine which track is grounded.

Claims (2)

Data collection means for converting the analog current signals of A, B, C, and N provided from the distribution line into digital signals, respectively, and calculating the even wave power from the collected data corresponding to one period to set a predetermined value. A high resistance ground fault detection apparatus comprising an accident signal detection means for detecting a ground fault in the distribution line by comparing with a value; The accident signal detecting means includes first to fourth counter means for performing a counting function; Calculate the odd wave power with the even wave power from the collected data, calculate the ratio of the good wave power to the odd wave power from the calculated good wave power and the odd wave power, and then The amount of change between the ratios of the even wave power to the odd wave power in each period is calculated, and the ratio of the ratio of the even wave power to the odd wave power and the odd wave power and the amount of change of the ratio are determined by fault detection. Comparing the predetermined first set value with the second set value and the third set value, respectively, and when the comparison values are greater than the first to third set values, the count value of each of the first to third counter means is 1; Increasing the count value of the fourth counter means by one when at least one of the first to third counter means is increased by one, and then A control for determining whether a failure occurs on the distribution line by analyzing the count values of the first to fourth counter means when the count value of the fourth counter means is large by comparing a count value with a predetermined fourth set value. High resistance ground fault detection apparatus comprising a calculation means (110). By calculating the even wave power from the data corresponding to one period of the data obtained by converting the analog current signals of A, B, C, and N provided from the power distribution line into digital data signals, and comparing them with a predetermined set value. What is claimed is: 1. A method for detecting ground faults in a distribution line; Divide the data corresponding to the one period into half periods, and add the data corresponding to each position corresponding to the half periods from the beginning of the data corresponding to the previous half period among the divided data to obtain the even wave power (Peven). And subtracting data corresponding to the second half period from data corresponding to the previous half period to obtain the odd wave power Podd (272); Calculating a ratio (Pratio) of the even wave power Peven to the odd wave power Pddd; Calculating (274) a rate change amount (Pvar) between the ratio (Pratio) of the previous period and the ratio (Pratio) of the next period in two consecutive periods; The power wave power Peven is compared with a predetermined first set value for fault detection (275a). If the power wave power is larger than the first set value, the coefficient value CNT1 of the first counter is increased by one. 275b, after the count value increase of the first counter is completed or the even wave power is not greater than the first set value, the ratio of the even wave power to the odd wave power and the predetermined second Comparing the set value (276a); If the ratio is greater than the second set value, the count value CNT2 of the second counter is increased by 1 (276b), and after the count value increase of the second counter is completed, or the ratio is set to the value. Comparing the ratio change amount Pvar with a predetermined third set value if it is not larger than a second set value (277a); If the ratio change amount Pvar is greater than the third set value, the count value CNT3 of the third counter is increased by 1 (277b), and after the count value increase of the third counter is completed or the ratio change amount Pvar is increased. Increases the count value CNT4 of the fourth counter for counting the number of cycles of failure of the input signal when the count value of at least one of the first to third counters is increased by one without being greater than the third set value by one. Step 278; The count value CNT4 of the fourth counter is compared with a fourth set value which is a predetermined periodic value for fault detection (279), and if the count value CNT4 is not greater than the fourth set value, Repeat the above steps for the next period, and if the count value CNT4 is greater than the fourth set value, the count values CNT1 to CNT4 of the first to fourth counters and for fault detection determination Comparing predetermined values and determining that a high resistance ground fault has occurred on the distribution line if the count values of the first to fourth counters are larger than the predetermined values. High resistance ground fault detection method.