Intermittent ground fault detection method based on waveform numeric value sudden change direction

Abstract

The invention provides an intermittent ground fault detection method based on the sudden change direction of a waveform bare value, and relates to the technical field of ground fault detection.The method comprises the steps that voltage and current signals are sampled through an AD sampler, and a sampling timestamp is designed based on the Nyquist sampling theorem; and enough sampling points can be obtained in the pulse width range. And by calculating a reference sampling bare value and setting threshold values of voltage and current, the waveform conforming to the intermittent grounding characteristic is further screened. A pulse width counter and a pulse interval counter are also introduced, and waveforms which do not conform to intermittent grounding characteristics are filtered out through adaptive judgment of pulse width and interval. And finally, through a voltage and current curvature direction judging device and fault distance calculation, the position and phase of the fault are determined. The objective of the invention is to solve the problem that intermittent grounding faults cannot be accurately identified in the prior art.

Description

Technical Field

[0001] The present invention relates to the technical field of ground fault detection, and in particular to a method for detecting intermittent ground faults based on the sudden change direction of waveform naked value. Background Art

[0002] The detection and diagnosis of ground faults remain a critical component of stable power system operation. Intermittent ground faults, in particular, present numerous challenges to the safe operation of power systems due to their complex and variable characteristics. These faults frequently occur in distribution and secondary transmission networks, encompassing voltage levels from low to medium and high voltage, such as in motor control and 400V-35kV distribution. Different countries and regions employ varying grounding methods in these systems to suit their respective grid structures and operational requirements. These methods can be broadly categorized into two main types: high-current grounding and low-current grounding. Different grounding methods exhibit distinct grounding phenomena. Among high-current grounding methods, direct neutral point grounding and low-resistance grounding are the most common. These methods rapidly generate a high short-circuit current when a ground fault occurs, facilitating fault detection and protective action. The low-current grounding methods include ungrounded neutral point, high-resistance grounding of the neutral point, and grounding through arc suppression coils. These grounding methods have a relatively high insulation level to the ground during normal operation, that is, conventional zero-sequence current protection can detect most grounding faults in time; but when a grounding fault occurs, the fault characteristics are relatively hidden, especially intermittent grounding faults, the voltage and current change characteristics are not obvious, the characteristics of the voltage and current cannot be detected in time by conventional methods, and are difficult to be detected by conventional protection methods. Long-term intermittent grounding will affect the insulation performance of the distribution equipment, and then create safety hazards.

[0003] Currently, ground fault detection methods in the power industry face several limitations. Traditional detection methods primarily rely on manual observation of recorded waveforms to identify faults. This approach is not only inefficient but also susceptible to human error, resulting in inaccurate fault diagnosis and a delay in timely detection. Furthermore, some methods attempt to identify faults by counting the number of abnormal voltage and current events within a fixed period (e.g., 20ms). However, this approach is overly simplistic and inadequate for various types of intermittent ground faults, prone to misjudgment or omission. Further complicating matters, traditional detection methods based on zero-sequence voltage and current require stable fault data. While theoretically capable of detecting ground faults, due to the sporadic and unstable nature of intermittent faults, these methods often rely on complex Fourier transforms and phase analysis to extract fault signatures, making them incapable of accurately identifying intermittent ground faults of varying widths and intervals. This not only increases computational complexity and places high demands on hardware performance, but in practice, the instability of fault signatures makes it difficult to accurately identify intermittent ground faults of varying widths and intervals. The existence of these problems means that existing ground fault detection technology is often unable to detect intermittent ground faults in a timely and accurate manner, resulting in the continued existence of the fault, which may in turn cause more serious equipment damage and safety hazards.

[0004] In view of this, this application is filed. Summary of the Invention

[0005] The present invention provides a method for detecting intermittent ground faults based on the sudden change direction of waveform naked value, which can at least partially improve the above-mentioned problem.

[0006] To achieve the above object, the present invention adopts the following technical solutions: A method for detecting intermittent ground faults based on the direction of waveform naked value mutation, comprising: Designing an AD sampling sampler according to the preset waveform characteristics of intermittent grounding, and obtaining AD sampling values ​​collected by the AD sampling sampler; Determine the relationship between the AD sampling value and the preset rated values ​​of voltage and current. When the counting timer is triggered, perform intermittent ground fault judgment preprocessing based on the waveform characteristics, the relationship between the AD sampling value and the preset rated values ​​of voltage and current to determine whether an intermittent ground fault occurs. When it is determined that the number of intermittent ground faults exceeds a preset value, a voltage and current curvature direction determiner is used to perform direction determination on the preprocessed waveform data, and a phase determination process is performed to generate a determination result.

[0007] In summary, this invention proposes a method for detecting intermittent ground faults based on the direction of waveform null value mutations. This method can quickly identify different types of intermittent ground faults, enabling timely fault detection and preventative measures. This method aims to address existing issues such as inaccurate intermittent ground fault detection, reliance on complex calculations, and hardware performance issues. By analyzing the mutation direction, pulse width, and interval of the voltage and current waveforms and combining them with an adaptive algorithm, this method can quickly and accurately identify different types of intermittent ground faults. Specifically, this method first samples the voltage and current signals using an AD sampler. Based on the Nyquist sampling theorem, sampling timestamps are designed to ensure sufficient sampling points within the pulse width range. By calculating baseline sample null values ​​and setting voltage and current thresholds, waveforms that meet intermittent ground fault characteristics are further screened. Furthermore, a pulse width counter and pulse interval counter are introduced to adaptively determine pulse width and interval to filter out waveforms that do not meet intermittent ground fault characteristics. Finally, a voltage and current curvature direction detector and fault distance calculation are used to determine the fault location and phase.

[0008] In simple terms, this method achieves rapid and accurate detection of intermittent grounding faults by calculating the baseline value of the waveform's bare value and analyzing the equivalence of the AD value with the rated value. It does not require complex Fourier operations or related phase analysis, but uses a unique method to analyze the pulse width and interval of the original data, distinguishes the pulse width and pulse interval through adaptive calculation, and filters out fault waveforms that do not meet the requirements. By analyzing the waveform's curvature characteristic algorithm, the waveform trend is predicted to accurately identify intermittent grounding faults. Combined with fault example calculations, the upstream and downstream of the fault and the distance to the fault are accurately calculated, and the fault phase is accurately output, assisting on-site personnel in discovering early potential faults and avoiding fault evolution and expansion. The scheme and algorithm of the present invention can be used in small single-chip microcomputer processing systems and are suitable for distribution network automation control equipment such as FTU, DTU, and TTU. It does not rely on complex data calculations, greatly expanding its scope of application. It effectively improves the detection efficiency and accuracy of intermittent grounding faults, reduces the impact of faults on the insulation performance of equipment, and provides a strong guarantee for the safe operation of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 1 is a flow chart of a method for detecting intermittent ground faults based on a sudden change direction of a waveform bare value provided by a first embodiment of the present invention; Figure 2 1 is a schematic diagram of the overall process framework of the intermittent grounding fault detection method based on the sudden change direction of the waveform naked value provided by an embodiment of the present invention; Figure 3 This is an intermittent grounding waveform diagram provided by an embodiment of the present invention; Figure 4Schematic diagram of AD sampling and sampling provided by an embodiment of the present invention; Figure 5 This is an intermittent ground fault equivalent circuit provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0010] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0011] refer to Figure 1 、 Figure 2 As shown, the first embodiment of the present invention discloses a method for detecting intermittent ground faults based on the direction of waveform raw value mutations. The method can be performed by an intermittent ground fault detection device based on the direction of waveform raw value mutations (hereinafter referred to as the detection device), and in particular, by one or more processors within the detection device to implement the following method: S1, designing an AD sampling sampler according to a preset waveform characteristic of intermittent grounding, and obtaining AD sampling values ​​collected by the AD sampling sampler; Preferably, the waveform characteristics include: when the waveform is intermittently grounded, the voltage and current suddenly change, the peak pulse width is maintained at 400-800us, the pulse interval is maintained at 5-300ms, and the voltage fluctuation amplitude U o At 0.01U n to 1.0U n The current fluctuation range is between 0.1A and 5KA.

[0012] Specifically, step S1 further includes: setting the sampling timestamp of the AD sampling sampler based on the Nyquist sampling theorem and the 50 Hz waveform, and the formula is: ,in, is the unit impulse function, is the impulse train signal, is the sampling interval, is a preset integer; The AD sampling device is used every time, generates an impulse, which is used to Take samples.

[0013] In this embodiment, according to the waveform characteristics of the intermittent ground fault (such as Figure 3An AD sampling device (as shown) is designed and the AD sampled values ​​collected by the sampler are acquired. These waveform characteristics include: when an intermittent ground fault occurs, the voltage and current experience sudden changes, the peak pulse width (Pulse Width) is maintained between 400-800 microseconds, the pulse interval (Pulse Interval) is maintained between 5-300 milliseconds, the voltage fluctuation amplitude (Uo) is between 0.01Un and 1.0Un, and the current fluctuation amplitude is between 0.1A and 5kA. These characteristics are typical of intermittent ground faults and provide the basic data features for the detection method of the present invention.

[0014] In order to ensure that these waveform features can be accurately captured, this method sets the sampling timestamp of the AD sampling sampler based on the Nyquist sampling theorem and the 50Hz waveform. Where p(t) in the formula is the impulse train signal. Figure 4 The waveform is represented by a series of equally spaced impulses (upward arrows). According to the Nyquist sampling theorem, the sampling frequency should be at least twice the highest frequency of the signal. The sampling timer n is set to a fixed value of 256 points per cycle. For a 50Hz waveform, the timer interval is T = 20ms / 256 = 78.125µs. Furthermore, the minimum pulse width is 400µs, meaning that at least five points can be sampled within this timeframe, greatly increasing data accuracy. Based on the waveform characteristics of intermittent grounding, strict pulse width, pulse interval, and AD raw value changes are used to screen for intermittent grounding characteristics.

[0015] In practical applications, an AD sampler generates an impulse every T time interval to sample a continuous signal. For example, a 50Hz AC signal has a period of 20ms, so the AD sampler samples the signal every 20ms to ensure that the signal waveform changes are fully captured. This method can obtain sufficiently accurate AD sampling values, providing reliable data support for further analysis and judgment.

[0016] This sampling setup, based on the Nyquist sampling theorem, not only ensures the integrity and accuracy of the sampled data but also effectively avoids signal aliasing, improving the reliability and stability of the detection system. Furthermore, by properly setting the sampling frequency and sampling interval, this method can achieve efficient detection of intermittent ground faults without excessively increasing hardware costs, demonstrating high practicality and cost-effectiveness.

[0017] S2, determining the relationship between the AD sampling value and the preset rated values ​​of voltage and current. When the counting timer is triggered, performing intermittent ground fault judgment preprocessing based on the waveform characteristics, the relationship between the AD sampling value and the preset rated values ​​of voltage and current to determine whether an intermittent ground fault occurs; Specifically, step S2 further includes: according to the hardware characteristics of the power distribution equipment, taking the reference voltage value during sampling as the reference sampling raw value ADref, and respectively recording the positive half-cycle maximum values UAD n under the condition of rated voltage U n and the positive half-cycle maximum values IAD max under the condition of rated current I max , and the negative half-cycle minimum values UAD min , IAD min of the AD sampling, to obtain the relationship between the AD sampling values and the preset rated values of voltage and current; Calculating the voltage reference voltage raw value ADref1 and the current reference voltage raw value ADref2 according to the symmetry of the steady-state waveform, and storing the voltage reference voltage raw value ADref1 and the current reference voltage raw value ADref2 into the preset storage module flash. The formula is: ADref1 = (UAD max + UAD min ) / 2, ADref2 = (IAD max + IAD min ) / 2; When it is judged that intermittent grounding occurs according to the intermittent grounding judgment condition, record the voltage value and current value at the starting moment, and control the pulse interval counter Pulse_inteval_cnt to increment by 1. Among them, the intermittent grounding judgment condition is: |I0V| > |UAD max [3]|, and |I0j| > |IAD max [4]|, where I0V is the zero-sequence voltage raw value and I0j is the zero-sequence current raw value; ​​​​​​​​​​ Repeat the above steps until all waveforms are compared and judged, and the waveforms that do not meet the intermittent grounding characteristics are filtered out.

[0019] Preferably, the first preset interval is 400us-800us, and the second preset interval is 5000us-300000us.

[0020] In this embodiment, the core of this step is to perform pre-processing to determine intermittent ground faults by analyzing the relationship between the AD sampling values ​​and the preset voltage and current ratings, combined with waveform characteristics. This process not only effectively identifies intermittent ground faults but also filters out waveforms that do not meet the characteristics, ensuring the accuracy and reliability of the detection results.

[0021] Specifically, first, according to the hardware characteristics of the power distribution equipment, the reference voltage value at the time of sampling is used as the reference sampling bare value ADref, corresponding to Figure 3 The relative coordinates of the 0 point in f(t). At rated voltage U n and rated current I n In the case of the positive half cycle, record the maximum value UAD of the AD sampling value. max and IAD max , and the negative half cycle minimum value UAD min and IAD min Through these sampling values, the relationship between the AD sampling value and the preset rated values ​​of voltage and current can be obtained. The establishment of this relationship provides an important reference basis for subsequent fault diagnosis.

[0022] Next, based on the symmetry of the steady-state waveform, calculate the voltage reference bare value ADref1 and the current reference bare value ADref2. According to the formula UAD max -ADref1=ADref1-UAD min , the specific formula for the voltage reference ADref1 can be derived; similarly, the specific formula for the current reference ADref2 can be further derived. The calculated ADref1 and ADref2 are saved to the preset flash memory module for easy access during subsequent fault detection. This reference value calculation method, based on steady-state waveform symmetry, effectively reduces errors caused by hardware characteristic variations, improving the adaptability and accuracy of the detection system.

[0023] In this embodiment, the threshold value UAD of the voltage and current AD raw data is further set. n , IAD n[5]; Set the number of times of AD raw value mutation to be 2 - 20 times. This number of times conforms to the number of times of intermittent grounding faults, and different numbers of times can be set according to different field applications. Set the 1us timer as a counting timer to record the comparison of AD raw values of intermittent grounding waveforms and the adaptive calculation of pulse width.

[0024] When the counting timer is triggered, according to the judgment condition of intermittent grounding, judge whether an intermittent grounding fault occurs. When this condition is met, it indicates that an intermittent grounding fault may occur. At this time, record the voltage value and current value at the start moment, and control the pulse interval counter Pulse_interval_cnt to increment by 1 for timing; at this time, Pulse_inteval_cnt = 0, with the unit of us, and set the pulse width timing start flag Pulse_flag = 1. The beneficial effect of this process is that by real-time monitoring the changes in zero-sequence voltage and current, the initial characteristics of intermittent grounding faults can be quickly captured, providing an accurate time reference for subsequent further judgment.

[0025] Subsequently, design the pulse interval timing. When it is judged that |I0j| < IADn, it indicates that the peak current of intermittent grounding decreases, and enters the pulse falling edge timing mode. At this time, continue to control the pulse interval counter Pulse_interval_cnt to increment by 1, and at the same time set the pulse width timing flag Pulse_flag to 0, and judge whether the value of the pulse interval counter Pulse_interval_cnt is within the first preset interval (400us - 800us). If it is within this interval, it indicates that it conforms to the characteristics of intermittent grounding. At this time, control the preset pulse count value Pulse_num to increment by 1, and set the pulse interval timing start flag Pulse_preflag to 1. The beneficial effect of this process is that through precise pulse interval timing and judgment, intermittent grounding faults can be effectively distinguished from steady-state fault currents, avoiding misjudgment and missed judgment.

[0026] If the value of the pulse interval counter Pulse_interval_cnt is not within the first preset interval, it indicates that the current is a steady-state fault current. In this case, use conventional sensitive grounding protection for judgment. This strategy combining conventional protection methods not only improves the reliability of the detection system but also can accurately distinguish between intermittent grounding faults and steady-state fault currents, ensuring the safe operation of the power system.

[0027] If an intermittent ground fault condition is detected again based on the intermittent ground fault condition determination criteria, the method further checks whether the value of the pulse interval counter, Pulse_interval_cnt, is within a second preset range (5000µs - 300,000µs). If so, the pulse width timing start flag, Pulse_flag, is set to 1. This method continuously monitors the characteristic changes of intermittent ground faults and accurately determines the condition within the preset range. This process has the beneficial effect of further improving the accuracy of detection results through multiple determinations and verifications, reducing the errors caused by a single determination condition.

[0028] Repeat the above steps, comparing the waveform bare values ​​and accumulating the pulse count until all waveforms have been compared and evaluated. Based on the aforementioned pulse width adaptive judgment, waveforms that do not meet the intermittent ground fault characteristics are filtered out. By individually evaluating and screening all waveforms, noise and interference signals can be effectively removed, ensuring that the final detected fault waveform has clear intermittent ground fault characteristics. This waveform-based screening method not only improves the detection system's anti-interference capabilities but also accurately identifies intermittent ground faults in complex power system environments, providing strong technical support for power system fault diagnosis and maintenance.

[0029] Through the above steps, efficient detection and accurate judgment of intermittent grounding faults are achieved. This method does not rely on complex Fourier calculations and phase analysis, can be implemented in a small single-chip computer system, and has low hardware requirements and high computing efficiency. At the same time, through reasonable preset intervals and judgment conditions, the present invention can effectively distinguish intermittent grounding faults from steady-state fault currents, avoid the occurrence of misjudgments and missed judgments, and provide reliable protection for the safe operation of the power system. In short. The intermittent grounding fault detection method based on the direction of waveform naked value mutation does not require a complex hardware system, has low requirements on hardware performance, does not require complex Fourier calculations, does not require phase analysis, and can achieve accurate judgment of intermittent grounding faults through related software processing. It is easy to use in distribution terminals and secondary distribution networks, accurately identify intermittent grounding faults, discover potential insulation risks in advance, and avoid the expansion of faults.

[0030] S3, when it is determined that the number of intermittent ground faults exceeds a preset value, a voltage and current curvature direction determiner is used to perform direction determination on the preprocessed waveform data, and a phase determination process is performed to generate a determination result.

[0031] Specifically, step S3 further includes: using a voltage and current curvature direction determiner to determine each sampling point of the preprocessed waveform data, specifically: The curvature trend judgment is carried out in the sampling timer, and the formula is: fault_I1 = fault_I2, fault_I2 = If(t), fault_U1 = fault_U2; fault_U2 = Uf(t), where fault_I1 is the first real-time change value of the current AD, fault_I2 is the second real-time change value of the current AD, If(t) is the real-time value function of the current sampling, fault_U1 is the first real-time change value of the voltage AD, fault_U2 is the second real-time change value of the voltage AD, and Uf(t) is the real-time value function of the voltage sampling; When it is judged that fault_I2 > fault_I1 and fault_U2 > fault_U1 or fault_I2 < fault_I1 and fault_U2 < fault_U1, it is determined that the voltage and current directions are the same, the power direction is positive, and the fault occurs downstream of the device; When it is judged that the situation is other than the above two cases, it is determined that the power direction is negative and the fault is upstream of the device; The phase judgment process is carried out to generate a judgment result, specifically: Compare the voltages of the three phases at this time, and the lowest voltage phase among the three phases is the intermittently grounded voltage phase. According to the Thevenin equivalent theorem, a fault equivalent circuit is generated; According to the fault equivalent circuit and the formula , it is deduced to obtain , , to obtain the value of the fault distance m and generate a fault result, where , is the zero-sequence impedance, is the positive-sequence impedance, is the total impedance, is the fault-phase voltage, is the fault-phase current, is the zero-sequence current at the fault moment, is the fault distance, is the voltage at the fault point, is the resistance at the fault point, is the current at the fault point.

[0032] In this embodiment, when the number of detected intermittent grounding faults exceeds the preset value, the voltage-current curvature direction discriminator is used to perform direction judgment on the preprocessed waveform data and perform phase judgment processing to generate a final judgment result. This process can not only accurately judge the direction and location of the fault, but also further determine the specific phase and distance of the fault through phase judgment, providing strong support for the rapid location and handling of the fault.

[0033] First, use a voltage-current curvature direction discriminator (in the 78.125 us timer) to judge each sampling point of the preprocessed waveform data. Specifically, judge the curvature trend in the sampling timer, and its formula is: fault_I1 = fault_I2, fault_I2 = If(t), fault_U1 = fault_U2, fault_U2 = Uf(t). Through this real-time update mechanism, the changing trends of voltage and current can be dynamically tracked, providing an accurate data basis for subsequent direction judgment.

[0034] When it is judged that fault_I2 > fault_I1 and fault_U2 > fault_U1, or fault_I2 < fault_I1 and fault_U2 < fault_U1, it is determined that the voltage and current directions are the same, the power direction is positive, and the fault occurs downstream of the device. This direction judgment method is based on the changing trends of voltage and current, and can quickly and accurately judge the fault direction, avoiding the errors and delays caused by complex calculations and phase analysis in traditional methods. When it is judged to be other situations except the above two situations, it is determined that the power direction is negative and the fault is upstream of the device. This reverse judgment mechanism further improves the fault direction judgment logic, ensuring that the fault direction can be accurately judged under various complex working conditions, and improving the robustness and reliability of the detection system.

[0035] After the direction judgment is completed, further phase judgment processing is carried out, that is, to judge which phase in the three phases has an intermittent grounding fault and generate the final judgment result. Specifically, compare the voltages of the three phases at this time, and determine the phase with the lowest voltage among the three phases as the intermittent grounding voltage phase. This process is based on the relative magnitude relationship of voltages, and can quickly identify the fault phase, avoiding misjudgment caused by voltage fluctuations. Taking the fault phase in phase A as an example, according to the Thevenin equivalent theorem, a fault equivalent circuit is generated, as Figure 5 shown. Through the fault equivalent circuit and combined with relevant formulas, the value of the fault distance m is deduced. Through this calculation method based on the equivalent circuit, the fault distance can be accurately calculated. According to whether the fault is upstream or downstream, it is convenient for the line patrol personnel to quickly determine the location of the intermittent grounding fault, providing strong support for the rapid location of the fault.

[0036] Briefly speaking, the intermittent grounding fault detection method based on the waveform raw value mutation direction does not rely on manual waveform recognition; does not rely on big data waveform analysis, can be applied in a small single-chip microcomputer system, reduces the complexity of calculation, can quickly identify different types of intermittent grounding faults, and can detect potential intermittent grounding faults before the fault range expands.

[0037] In summary, this method targets industrial frequency AC signals. It calculates the reference value of the waveform bare value and performs an equivalence analysis of the AD value and the rated value. It does not require complex Fourier operations or related phase analysis. It performs pulse width and interval analysis of the original data in a unique way, distinguishes the pulse width and pulse interval through adaptive calculation, and filters out fault waveforms that do not meet the requirements. It uses an algorithm to analyze the curvature characteristics of the waveform to predict the waveform trend and accurately identify intermittent grounding faults. Combined with fault example calculations, it accurately calculates the upstream and downstream of the fault and the distance of the fault, accurately outputs the fault phase, and assists on-site personnel in discovering early potential faults and avoiding the evolution and expansion of faults. The scheme and algorithm of the present invention can be used with a small single-chip microcomputer processing system. The method includes: using waveform bare value and reference value analysis, AD value and rated value equivalence analysis, adaptive recognition analysis of pulse width and interval, waveform curvature trend analysis, upstream and downstream fault calculation method, and fault phase location analysis method.

[0038] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A method for detecting intermittent ground faults based on the direction of waveform naked value mutation, characterized in that: Comprising: Design an AD sampling sampler according to the waveform characteristics of preset intermittent grounding, and obtain the AD sampling values collected by the AD sampling sampler; Determine the relationship between the AD sampling value and the preset rated values of voltage and current. When the counting timer is triggered, perform preprocessing for intermittent grounding fault judgment according to the waveform characteristics and the relationship between the AD sampling value and the preset rated values of voltage and current, so as to judge whether an intermittent grounding fault occurs; When it is judged that the number of intermittent grounding faults exceeds the preset value, use a voltage-current curvature direction discriminator to perform direction judgment on the preprocessed waveform data, and perform phase judgment processing to generate a judgment result.

2. The intermittent ground fault detection method based on the sudden change direction of the waveform naked value according to claim 1 is characterized in that: The waveform characteristics include: when the waveform is intermittently grounded, the voltage and current suddenly change, the peak pulse width is maintained at 400-800us, the pulse interval is maintained at 5-300ms, and the voltage fluctuation amplitude U o At 0.01U n to 1.0U n The current fluctuation range is between 0.1A and 5KA.

3. The intermittent ground fault detection method based on the sudden change direction of the waveform bare value according to claim 1 is characterized in that: Design an AD sampling sampler according to the waveform characteristics of preset intermittent grounding, specifically: Based on the Nyquist sampling theorem and the 50 Hz waveform, the sampling timestamp of the AD sampling sampler is set. The formula is: ,in, is the unit impulse function, is the impulse train signal, is the sampling interval, is a preset integer; The AD sampling device is used every time, generates an impulse, which is used to Take samples.

4. The intermittent ground fault detection method based on the sudden change direction of waveform naked value according to claim 1 is characterized in that: Determine the relationship between the AD sampling value and the preset rated values of voltage and current. When the counting timer is triggered, perform preprocessing for intermittent grounding fault judgment according to the waveform characteristics and the relationship between the AD sampling value and the preset rated values of voltage and current, so as to judge whether an intermittent grounding fault occurs, specifically: According to the hardware characteristics of the power distribution equipment, the reference voltage value at the time of sampling is used as the reference sampling bare value ADref, and the rated voltage U n Case and rated current I n In this case, the corresponding AD sampling positive half cycle maximum value UAD max 、IAD max , and the minimum value of the negative half cycle of AD sampling should be UAD min , IAD min , obtain the relationship between the AD sampling value and the preset rated values ​​of voltage and current; The voltage reference voltage bare value ADref1 and the current reference voltage bare value ADref2 are calculated based on the symmetry of the steady-state waveform, and the voltage reference voltage bare value ADref1 and the current reference voltage bare value ADref2 are saved to the preset storage module flash. The formula is: ADref1=(UAD max +UAD min ) / 2, ADref2=(IAD max +IAD min ) / 2; When intermittent grounding is detected according to the intermittent grounding judgment condition, the voltage and current values ​​at the start time are recorded, and the pulse interval counter Pulse_interval_cnt is controlled to increase by 1. The intermittent grounding judgment condition is: |I0V|>|UAD max [1]|, and |I0j|>|IAD max [2]|, I0V is the bare value of zero-sequence voltage, I0j is the bare value of zero-sequence current; When it is judged that |I0j| < IADn, the peak current of intermittent grounding decreases, enter the pulse falling edge timing mode, control the pulse interval counter Pulse_inteval_cnt to increment by 1. At this time, the pulse width timing flag Pulse_flag = 0, and judge whether the value of the pulse interval counter Pulse_inteval_cnt is within the first preset interval; If so, it indicates that the characteristics of intermittent grounding are met, control the preset pulse count value Pulse_num to increment by 1, and set the pulse interval timing start flag Pulse_preflag = 1; If not, it indicates that the current is a steady-state fault current, and use conventional sensitive grounding protection for judgment.

5. The intermittent ground fault detection method based on the sudden change direction of waveform naked value according to claim 4 is characterized in that: Also comprising: When it is judged again that intermittent grounding occurs according to the intermittent grounding judgment condition, judge whether the value of the pulse interval counter Pulse_inteval_cnt is within the second preset interval; If so, set the pulse width timing start flag Pulse_flag = 1; Repeat the above steps until all waveforms are compared and judged, and filter out waveforms that do not conform to the characteristics of intermittent grounding.

6. The intermittent ground fault detection method based on the sudden change direction of the waveform naked value according to claim 5 is characterized in that: The first preset interval is 400us - 800us, and the second preset interval is 5000us - 300000us.

7. The intermittent ground fault detection method based on the sudden change direction of waveform naked value according to claim 1 is characterized in that: When it is judged that the number of intermittent grounding faults exceeds the preset value, use a voltage-current curvature direction discriminator to perform direction judgment on the preprocessed waveform data, specifically: Use the voltage-current curvature direction discriminator to judge each sampling point of the preprocessed waveform data, specifically: The curvature trend judgment is carried out in the sampling timer, and its formula is: fault_I1 = fault_I2, fault_I2 = If(t), fault_U1 = fault_U2; fault_U2 = Uf(t), where fault_I1 is the first real-time change value of the current AD, fault_I2 is the second real-time change value of the current AD, If(t) is the real-time value function of the current sampling, fault_U1 is the first real-time change value of the voltage AD, fault_U2 is the second real-time change value of the voltage AD, and Uf(t) is the real-time value function of the voltage sampling; When it is judged that fault_I2 > fault_I1 and fault_U2 > fault_U1 or fault_I2 < fault_I1 and fault_U2 < fault_U1, it is determined that the voltage and current directions are the same, the power direction is positive, and the fault occurs downstream of the device; When it is judged that the situation is other than the above two cases, it is determined that the power direction is negative and the fault is upstream of the device.

8. The intermittent ground fault detection method based on the sudden change direction of waveform naked value according to claim 1 is characterized in that: The phase judgment process is carried out to generate a judgment result, specifically: Compare the voltages of the three phases at this time, and the phase with the lowest voltage among the three phases is the intermittent grounding voltage phase, and according to the Thevenin equivalent theorem, a fault equivalent circuit is generated; According to the fault equivalent circuit and formula , calculated , , get the value of the fault distance m, generate the fault result, where the fault point , is the zero-sequence impedance, is the positive sequence impedance, is the total impedance, is the fault phase voltage, is the fault phase current, is the zero-sequence current at the moment of fault, is the fault distance, is the fault point voltage, is the resistance at the fault point, is the fault point current.

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