Cable network grounding studying and judging system and method based on topological transient characteristics

Abstract

The invention provides a cable network grounding research and judgment system and method based on topological transient characteristics, and the system carries out the research and judgment of a cable network grounding fault based on the transient zero-sequence current characteristics of a cable network. Transient zero-sequence current direction characteristics of a ground fault are converted into topological transient characteristics based on topology and amplitude comparison, the research and judgment process of the research and judgment method is triggered by a zero-sequence voltage abrupt change event of a bus, high-speed, synchronous and short-window acquisition is performed in a sampling process, then the transient amplitude in a first half wave is calculated, and topological characteristics are analyzed; according to the technical scheme, the method is specially used for capturing the earth fault instant electromagnetic transient process, and the problem that the accuracy of a transient signal positioning method at the present stage is not high can be solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power grid operation and maintenance, and particularly relates to a cable network grounding judgment system and method based on topological transient characteristics. BACKGROUND

[0002] When a grounding fault occurs in a distribution network resonance grounding system, it may cause events such as breakdown of weak insulation point equipment, forest fire, and electric shock of personnel. The zero sequence current steady state method is not suitable for resonance grounding systems due to over-compensation of the arc suppression coil inductance, which results in poor effect. The transient signal positioning method is not affected by the arc suppression coil and is also widely used in distribution network equipment, but the overall judgment accuracy at the present stage cannot meet the target requirements.

[0003] The transient signal positioning method at the present stage is realized by a complete set of switches, and the grounding alarm is mainly judged by the first half wave characteristics, as shown in FIG. 1. The zero sequence voltage and the zero sequence current present an inverse polarity relationship in the first half wave of the transient signal, and the grounding alarm signal is sent when the inverse polarity requirement is met by collecting and analyzing the first half wave of the recorded wave data. Figure 1

[0004] Theoretically, the first half wave polarity method can completely distinguish whether a single-phase grounding fault occurs downstream of the installation point, but in fact, the collected waveforms all have noise interference and the like. The value of the noise is not large, but it is easy to cause misjudgment of the device, such as the inverse polarity characteristics of the non-fault line during a fault.

[0005] When a single-phase grounding fault occurs, the transient zero sequence current of the fault line has significant differences in amplitude, direction, and energy from the non-fault line, which is mainly caused by the transient zero sequence current dominated by the capacitance current. Since the cable network has a large capacitance, the amplitude and energy of the transient zero sequence current of the cable network are greater than those of the overhead network, which is more conducive to detection and analysis. The free oscillation frequency of the grounding transient process is inversely proportional to the equivalent inductance of the system and the ground capacitance of the system. Although the capacitance of the cable network is large, the equivalent inductance of the system is smaller than that of the overhead line. Related research shows that the free oscillation frequency of the general cable network is 1500Hz~3000Hz, which is higher than the free oscillation frequency range of 300Hz~1500Hz of the overhead network. That is, to obtain the transient current characteristics, the cable network needs a higher sampling frequency than the overhead line. The decay of the transient current is mainly determined by the resistance in the loop, and the line resistance of the cable network is small, so the energy dissipation is slower, and therefore the transient current decays more slowly and lasts longer. In summary, the cable network has the characteristics of large transient zero sequence current amplitude, large energy, high oscillation frequency, slow decay speed, and long duration.

[0006] ​The ground fault detection based on the transient method is a severe test when the fault phase voltage is zero, and the transient zero sequence current amplitude of the whole system is minimum and is easy to be submerged in noise. After the fault occurs, the voltage to ground of the non-fault phase becomes a line voltage, the voltage source on the capacitor of the non-fault phase changes, and the topology structure and excitation source of the circuit change. Even if the transient current of the fault phase itself is zero, the transient currents of the two non-fault phases are superimposed, and a transient zero sequence current is still formed. For the cable network, the transient zero sequence current generated by the non-fault phase cannot be ignored, and the direction characteristic of the transient zero sequence current of the fault line still exists.

[0007] Through the above analysis, the ground fault detection based on the transient method has better application effect in the cable network. The existing fusion switch realizes the ground fault detection by using the inverse polarity relationship between the zero sequence voltage and the zero sequence current in the first half wave of the transient. This method only uses the direction method and does not fully utilize the transient zero sequence current characteristics of the cable network, so that the small zero sequence current and the natural white noise jamming are superimposed to cause false alarms many times.

[0008] In view of the direction and amplitude characteristics of the transient zero sequence current of the cable network, the application provides a ground fault alarm method based on the topology transient characteristics.

[0009] In view of the resonance system of the cable network-based distribution network, the application provides a cable network ground fault detection system and method based on the topology transient characteristics, to solve the problem of low accuracy of the transient signal positioning method at the present stage. SUMMARY

[0010] The application provides a cable network ground fault detection system and method based on the topology transient characteristics, which is a technical solution specially for capturing the electromagnetic transient process at the moment of the ground fault, and can solve the problem of low accuracy of the transient signal positioning method at the present stage.

[0011] The application adopts the following technical solutions.

[0012] A cable network ground fault detection method based on topology transient characteristics, the detection method detects the ground fault of the cable network based on the transient zero sequence current characteristics of the cable network, captures the electromagnetic transient process at the moment of the ground fault, and converts the transient zero sequence current direction characteristics of the ground fault into topology transient characteristics based on topology and amplitude comparison. Figure 4 As shown in the figure, the detection process of the detection method is triggered by the zero sequence voltage mutation event of the bus, high-speed, synchronous and short window collection is performed in the sampling process, the transient amplitude in the first half wave is calculated, and topology characteristic analysis is performed.

[0013] The cable network is a distribution cable network, and when the ground fault occurs, the transient zero sequence current direction of the distribution cable network is as shown in the figure. Figure 2As shown, the transient zero-sequence current detected at the healthy line flows from the busbar to the line; while the transient zero-sequence current distribution in each section of the faulty line is divided by the fault point. The current flowing upstream of the fault point flows from the fault point to the busbar, and the amplitude increases with the distance from the busbar to the detection point; while the current flowing downstream of the fault point is the same as that of the healthy line, flowing from the busbar to the line, and its amplitude decreases with the distance from the busbar to the detection point. When the analysis method is applied to ring main units, a characteristic diagram of the transient zero-sequence current direction of the ring main unit is drawn based on the characteristics of the transient zero-sequence current direction, such as... Figure 3 As shown (assuming the cables between ring main units are grounded). When a ground fault occurs between ring main units, according to Kirchhoff's laws, assuming the busbar capacitance of the ring main unit is negligible, the transient zero-sequence current amplitude in the incoming bay of the healthy cable and the ring main unit downstream of the fault point is the largest, and its value is equal to the sum of the transient zero-sequence currents in the other bays. If the transient zero-sequence current amplitude of the incoming line bay of the ring main unit upstream of the fault point is not the maximum, and there exists a bay where the transient zero-sequence current amplitude is the sum of the amplitudes of the other bays, then the fault point is downstream of that bay. When the busbar of the ring main unit is grounded, the above conclusions do not affect the healthy phases and other ring main units upstream and downstream. If the fault point is on the busbar of the ring main unit, there is a large transient zero-sequence current in its incoming line interval, which is greater than the transient zero-sequence current collected by the upstream ring main unit, and satisfies the transient zero-sequence current direction characteristics.

[0014] When the judgment method is used to judge the grounding fault of the ring main unit, it includes the following steps; Step 1: Trigger the sampling process; Step 2: Perform high-speed synchronous sampling. Assuming the free oscillation frequency of the cable line is 1500Hz~3000Hz, sampling 256 points per cycle using the field terminal results in a sampling frequency of 12.8kHz. According to Shannon's sampling theorem, this meets the transient sampling requirements. Furthermore, all zero-sequence current channels at all intervals must be sampled synchronously to ensure amplitude comparison of data acquired at the same time. Step 3: Collect data within a specific time window; Step 4: Data Processing and Algorithm Execution; Step 5: Analyze topological features.

[0015] In step one, when the cable network system is running normally, the zero-sequence voltage U0 of the bus is continuously monitored. When the zero-sequence voltage suddenly rises and exceeds the set start threshold, it is determined that a grounding fault has occurred in the cable network system. The high-speed sampling of the zero-sequence current channels of all bays of the ring main unit is immediately started to ensure that the transient process of the fault moment can be captured.

[0016] In step three, a data window of fixed time length is collected, covering one power frequency cycle (20ms) after the fault. The time window includes the transient information of the first power frequency half-wave (10ms).

[0017] In step four, after acquiring the original transient data, the following processing is performed: Starting from the fault initiation point, the first half-wave zero-sequence current data within the first 10ms is extracted: i0 = [i0[0], i0[1], i0[2], ..., i0[N-1]], where each i0[n] represents the instantaneous value of the zero-sequence current at the nth sampling point (unit: A). Data preprocessing is performed, and calculations are performed according to the following formula 1. The data processing flow is as follows: Figure 5 As shown.

[0018] Formula 1.

[0019] In step five, based on the amplitude calculation results, identification is performed using the topological transient feature method; the topological feature process analysis is as follows: Figure 6 As shown. Under normal operating conditions, the incoming line bay is automatically determined by the power flow direction. During a ground fault, the zero-sequence transient current amplitude of each bay is extracted, and it is determined whether the amplitude of the incoming line bay is at its maximum. When the amplitude of the incoming line interval is the largest, two situations are analyzed: one is that it is a sound line or downstream of the fault point; the other is that it is suspected that the busbar of the ring network cabinet is grounded. If the topological transient characteristics are not met, it is suspected that the busbar of the ring network cabinet is grounded. If there is only one interval with an amplitude greater than the incoming line interval, it is judged to be suspected to be upstream of the fault point. When the topological transient characteristics are met at the same time, it is judged to be upstream of the fault point, and a grounding alarm signal should be sent.

[0020] The aforementioned judgment method uses Kirchhoff's laws to determine whether the interval current satisfies the topological transient characteristics. Specifically, it determines whether the maximum amplitude interval current is approximately equal to the sum of the amplitudes of the other interval currents. This involves comparing the amplitude of the maximum current interval with the sum of the amplitudes of the other interval currents to calculate the imbalance, as shown in Formula 2 below. Formula 2; In the formula, α represents the degree of imbalance, and I_ max I represents the maximum current interval amplitude. _other For other interval amplitude values, determine whether the imbalance is within the threshold range. If it is within the threshold range, the measurement result is determined to satisfy Kirchhoff's laws and the topological transient characteristics. The threshold data is derived using statistical methods. Based on a large amount of grounding amplitude data, with a threshold set at 10%, the accuracy rate of topological transient characteristic calculation reaches 98%, and the false alarm rate is 0%.

[0021] In the analysis method, when the topological characteristics between ring main units cannot be analyzed using a single ring main unit (i.e., when the ring main unit busbar is grounded), further analysis is performed through the main station. The main station topological transient characteristic analysis process is as follows: Figure 8 As shown. Specifically: When the ring main unit sends a grounding alarm signal, the main station further analyzes the alarm signal and determines whether the ring main unit bus grounding has occurred by judging whether the zero-sequence transient amplitude of the first ring main unit interval downstream of the initially judged fault section is greater than the amplitude of each interval upstream of the confirmed fault point. When the ring main unit does not send a grounding alarm signal, the signals downstream of the healthy lines and fault points identified by topology feature analysis are shielded, and the shielded ring main units are displayed. Unidentifiable signals are sorted by transient amplitude, and no results are derived. The shielded ring main units and other transient amplitudes are provided for further expert analysis, i.e., highly critical data is provided for expert analysis.

[0022] A cable network grounding assessment system based on topological transient characteristics is used in the aforementioned cable network grounding assessment method based on topological transient characteristics. The system is characterized by being used for ring main unit grounding alarms and includes a zero-sequence current transformer, an analog-to-digital converter, and a processor. For example... Figure 7 As shown; Zero-sequence current transformers (ZCTs) must have sufficient bandwidth to transmit high-frequency transient currents without distortion. They need good transient characteristics to prevent saturation under large transient currents. HFCTs are used, pre-inserted during new installation or cable pre-fabrication to ensure the captured signal is authentic and distortion-free. Twisted-pair shielded cables or coaxial cables are used as signal lines, grounded at the acquisition unit chassis to avoid ground loops and interference. Signal lines are kept away from power cables and high-frequency interference sources. The analog-to-digital converter (ADC) specifically requires high resolution and a high sampling rate to ensure accurate digitization of high-frequency signals. This proposal adopts the following approach: the ADC is placed on the signal conditioning board or main acquisition board, adjacent to the signal conditioning circuitry. Its front end receives the output signal from the signal conditioning circuitry. The front-end circuitry of the ADC includes an anti-aliasing filter and a programmable gain amplifier. The back end of the ADC is directly connected to the processor via a high-speed parallel or serial bus. It exists as a multi-channel synchronous sampling ADC, ensuring that the current signals and zero-sequence voltage signals of all lines are sampled simultaneously, which is crucial for subsequent analysis.

[0023] A powerful processor is required to perform a large amount of real-time data calculation and processing, run complex algorithms (such as wavelet transform and energy calculation), schedule and manage the entire system, and take into account heat sinks or chassis airflow design.

[0024] The present invention has the following advantages: 1. The transient grounding analysis method is unaffected by clutter interference. Specifically, while the transient current direction method has obvious characteristics, it is also easily affected by clutter interference, leading to false alarms. Based on the characteristics of transient current direction and large amplitude in cable network systems, this invention proposes a method to distinguish upstream of the fault point and non-upstream of the fault point by using the topological zero-sequence transient current characteristics. 2. Applicable to cable networks; Although the transient zero-sequence current amplitude and energy generated by cable networks are relatively large, ring main units cannot be directly grounded based on amplitude and energy. This invention realizes the application of transient zero-sequence current amplitude in ring main unit grounding analysis through topological features. 3. It can automatically locate fault sections; the topological transient feature method of the present invention classifies the ring main unit into three states: (1) ring main units downstream of the fault point and on the sound line, (2) ring main units upstream of the fault point, and (3) ring main units that cannot be identified. The method of the present invention can not only identify fault sections, but also distinguish between ring main units that cannot be identified and ring main units downstream of the fault point and on the sound line.

[0025] 4. When the faulty section cannot be identified, the present invention can provide highly critical data for expert judgment. That is, when the judgment result cannot be directly derived, the workload of expert judgment can be reduced by identifying the downstream of the fault point and the line of failure.

[0026] Compared with the traditional transient zero-sequence current direction, this invention is not affected by the direction misjudgment caused by the superposition of small zero-sequence current and natural white noise clutter. It makes full use of the large amplitude of transient zero-sequence current in cable networks and can be applied to resonant grounding systems with cable networks as the main component.

[0027] The advantages of this invention also include: 1. The zero-sequence transient direction method has been improved because the amplitude of the transient zero-sequence current is much larger than that of the noise interference, and it is no longer affected by the noise interference.

[0028] 2. The topological transient feature method can not only identify faulty sections, but also healthy lines and ring main units downstream of the fault point. When terminal abnormalities or other issues prevent the correct identification of faulty sections, the analysis of healthy lines and ring main units downstream of the fault point can assist experts in further analysis.

[0029] 3. The traditional zero-sequence transient direction method requires the installation polarity of the CT and PT, while the topological transient characteristic method realizes the direction calculation through amplitude characteristics, which has lower installation requirements.

[0030] 4. The transient zero-sequence current amplitude of each interval is calculated by the topological transient characteristic method. Theoretically, the transient zero-sequence current amplitude is the largest at the fault point, which can be used by experts for further analysis. Attached Figure Description

[0031] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments: Appendix Figure 1 This is a schematic diagram of the waveform of the complete set of switch grounding alarm principle in the transient signal location method in the existing technology; Appendix Figure 2 This is a schematic diagram of the transient zero-sequence current direction in a power distribution cable network according to an embodiment of the present invention; Appendix Figure 3 This is a schematic diagram illustrating the transient zero-sequence current direction characteristics of a ring main unit in an embodiment of the present invention; Appendix Figure 4 This is a schematic diagram of the technical process in an embodiment of the present invention; Appendix Figure 5 This is a schematic diagram of the data processing flow in an embodiment of the present invention; Appendix Figure 6 This is a schematic diagram of the topology feature analysis process in an embodiment of the present invention; Appendix Figure 7 This is a schematic diagram of the hardware requirements and topology in an embodiment of the present invention; Appendix Figure 8 This is a schematic diagram of the main station topology transient feature analysis process in an embodiment of the present invention. Detailed Implementation

[0032] As shown in the figure, a method for judging cable network grounding based on topological transient characteristics is presented. This method judges cable network grounding faults based on the transient zero-sequence current characteristics of the cable network. By capturing the instantaneous electromagnetic transient process of the grounding fault, the method transforms the transient zero-sequence current direction characteristics of the grounding fault into topological transient characteristics based on topological and amplitude comparisons. Figure 4 As shown, the judgment process of the judgment method is triggered by the zero-sequence voltage mutation event of the bus. First, high-speed, synchronous, short-window acquisition is carried out in the sampling process, then the transient amplitude in the first half-wave is calculated, and the topological characteristics are analyzed.

[0033] The cable network in question is a power distribution cable network. When a ground fault occurs, the direction of the transient zero-sequence current in the power distribution cable network is as follows: Figure 2 As shown, the transient zero-sequence current detected at the healthy line flows from the busbar to the line; while the transient zero-sequence current distribution in each section of the faulty line is divided by the fault point. The current flowing upstream of the fault point flows from the fault point to the busbar, and the amplitude increases with the distance from the busbar to the detection point; while the current flowing downstream of the fault point is the same as that of the healthy line, flowing from the busbar to the line, and its amplitude decreases with the distance from the busbar to the detection point. When the analysis method is applied to ring main units, a characteristic diagram of the transient zero-sequence current direction of the ring main unit is drawn based on the characteristics of the transient zero-sequence current direction, such as... Figure 3As shown (assuming the cables between ring main units are grounded). When a ground fault occurs between ring main units, according to Kirchhoff's laws, assuming the busbar capacitance of the ring main unit is negligible, the transient zero-sequence current amplitude in the incoming bay of the healthy cable and the ring main unit downstream of the fault point is the largest, and its value is equal to the sum of the transient zero-sequence currents in the other bays. If the transient zero-sequence current amplitude of the incoming line bay of the ring main unit upstream of the fault point is not the maximum, and there exists a bay where the transient zero-sequence current amplitude is the sum of the amplitudes of the other bays, then the fault point is downstream of that bay. When the busbar of the ring main unit is grounded, the above conclusions do not affect the healthy phases and other ring main units upstream and downstream. If the fault point is on the busbar of the ring main unit, there is a large transient zero-sequence current in its incoming line interval, which is greater than the transient zero-sequence current collected by the upstream ring main unit, and satisfies the transient zero-sequence current direction characteristics.

[0034] When the judgment method is used to judge the grounding fault of the ring main unit, it includes the following steps; Step 1: Trigger the sampling process; Step 2: Perform high-speed synchronous sampling. Assuming the free oscillation frequency of the cable line is 1500Hz~3000Hz, sampling 256 points per cycle using the field terminal results in a sampling frequency of 12.8kHz. According to Shannon's sampling theorem, this meets the transient sampling requirements. Furthermore, all zero-sequence current channels at all intervals must be sampled synchronously to ensure amplitude comparison of data acquired at the same time. Step 3: Collect data within a specific time window; Step 4: Data Processing and Algorithm Execution; Step 5: Analyze topological features.

[0035] In step one, when the cable network system is running normally, the zero-sequence voltage U0 of the bus is continuously monitored. When the zero-sequence voltage suddenly rises and exceeds the set start threshold, it is determined that a grounding fault has occurred in the cable network system. The high-speed sampling of the zero-sequence current channels of all bays of the ring main unit is immediately started to ensure that the transient process of the fault moment can be captured.

[0036] In step three, a data window of fixed time length is collected, covering one power frequency cycle (20ms) after the fault. The time window includes the transient information of the first power frequency half-wave (10ms).

[0037] In step four, after acquiring the original transient data, the following processing is performed: Starting from the fault initiation point, the first half-wave zero-sequence current data within the first 10ms is extracted: i0 = [i0[0], i0[1], i0[2], ..., i0[N-1]], where each i0[n] represents the instantaneous value of the zero-sequence current at the nth sampling point (unit: A). Data preprocessing is performed, and calculations are performed according to the following formula 1. The data processing flow is as follows:Figure 5 As shown.

[0038] Formula 1.

[0039] In step five, based on the amplitude calculation results, identification is performed using the topological transient feature method; the topological feature process analysis is as follows: Figure 6 As shown. Under normal operating conditions, the incoming line bay is automatically determined by the power flow direction. During a ground fault, the zero-sequence transient current amplitude of each bay is extracted, and it is determined whether the amplitude of the incoming line bay is at its maximum. When the amplitude of the incoming line interval is the largest, two situations are analyzed: one is that it is a sound line or downstream of the fault point; the other is that it is suspected that the busbar of the ring network cabinet is grounded. If the topological transient characteristics are not met, it is suspected that the busbar of the ring network cabinet is grounded. If there is only one interval with an amplitude greater than the incoming line interval, it is judged to be suspected to be upstream of the fault point. When the topological transient characteristics are met at the same time, it is judged to be upstream of the fault point, and a grounding alarm signal should be sent.

[0040] The aforementioned judgment method uses Kirchhoff's laws to determine whether the interval current satisfies the topological transient characteristics. Specifically, it determines whether the maximum amplitude interval current is approximately equal to the sum of the amplitudes of the other interval currents. This involves comparing the amplitude of the maximum current interval with the sum of the amplitudes of the other interval currents to calculate the imbalance, as shown in Formula 2 below. Formula 2; In the formula, α represents the degree of imbalance, and I_ max I represents the maximum current interval amplitude. _other For other interval amplitude values, determine whether the imbalance is within the threshold range. If it is within the threshold range, the measurement result is determined to satisfy Kirchhoff's laws and the topological transient characteristics. The threshold data is derived using statistical methods. Based on a large amount of grounding amplitude data, with a threshold set at 10%, the accuracy rate of topological transient characteristic calculation reaches 98%, and the false alarm rate is 0%.

[0041] In the analysis method, when the topological characteristics between ring main units cannot be analyzed using a single ring main unit (i.e., when the ring main unit busbar is grounded), further analysis is performed through the main station. The main station topological transient characteristic analysis process is as follows: Figure 8 As shown. Specifically: When the ring main unit sends a grounding alarm signal, the main station further analyzes the alarm signal and determines whether the ring main unit bus grounding has occurred by judging whether the zero-sequence transient amplitude of the first ring main unit interval downstream of the initially judged fault section is greater than the amplitude of each interval upstream of the confirmed fault point. When the ring main unit does not send a grounding alarm signal, the signals downstream of the healthy lines and fault points identified by topology feature analysis are shielded, and the shielded ring main units are displayed. Unidentifiable signals are sorted by transient amplitude, and no results are derived. The shielded ring main units and other transient amplitudes are provided for further expert analysis, i.e., highly critical data is provided for expert analysis.

[0042] A cable network grounding assessment system based on topological transient characteristics is used in the aforementioned cable network grounding assessment method based on topological transient characteristics. The system is characterized by being used for ring main unit grounding alarms and includes a zero-sequence current transformer, an analog-to-digital converter, and a processor. For example... Figure 7 As shown; Zero-sequence current transformers (ZCTs) must have sufficient bandwidth to transmit high-frequency transient currents without distortion. They need good transient characteristics to prevent saturation under large transient currents. HFCTs are used, pre-inserted during new installation or cable pre-fabrication to ensure the captured signal is authentic and distortion-free. Twisted-pair shielded cables or coaxial cables are used as signal lines, grounded at the acquisition unit chassis to avoid ground loops and interference. Signal lines are kept away from power cables and high-frequency interference sources. The analog-to-digital converter (ADC) specifically requires high resolution and a high sampling rate to ensure accurate digitization of high-frequency signals. This proposal adopts the following approach: the ADC is placed on the signal conditioning board or main acquisition board, adjacent to the signal conditioning circuitry. Its front end receives the output signal from the signal conditioning circuitry. The front-end circuitry of the ADC includes an anti-aliasing filter and a programmable gain amplifier. The back end of the ADC is directly connected to the processor via a high-speed parallel or serial bus. It exists as a multi-channel synchronous sampling ADC, ensuring that the current signals and zero-sequence voltage signals of all lines are sampled simultaneously, which is crucial for subsequent analysis.

[0043] A powerful processor is required to perform a large amount of real-time data calculation and processing, run complex algorithms (such as wavelet transform and energy calculation), schedule and manage the entire system, and take into account heat sinks or chassis airflow design.

Claims

1. A method for judging grounding of a cable network based on topological transient characteristics, characterized in that: The judgment method is based on the characteristics of the transient zero sequence current of the cable network to judge the grounding fault of the cable network, and the direction characteristics of the transient zero sequence current of the grounding fault are converted into the topological transient characteristics based on the comparison of the topology and the amplitude by capturing the electromagnetic transient process at the grounding fault moment. The judgment process of the judgment method is triggered by the sudden change of the zero sequence voltage of the bus, and the high-speed, synchronous and short window collection is performed in the sampling process, and then the transient amplitude in the first half wave is calculated, and the topological characteristics are analyzed.

2. The method of claim 1, wherein the method is characterized by: The cable network is a distribution cable network. When a grounding fault occurs, the transient zero sequence current detected at the healthy line is from the bus to the line. The transient zero sequence current of each section of the fault line is distributed with the fault point as the boundary. The current flowing through the upstream of the fault point is from the fault point to the bus, and the amplitude increases with the distance from the bus to the detection point. The current flowing through the downstream line of the fault point is the same as that of the healthy line, which is from the bus to the line, and the amplitude decreases with the distance from the bus to the detection point. When the judgment method is used for the ring network cabinet, when the current between the ring networks is grounded, the bus capacitance of the ring network cabinet is ignored, and the transient zero sequence current amplitude of the incoming line interval in the healthy cable line and the downstream ring network cabinet of the fault point is the largest, which is equal to the sum of the transient zero sequence currents of other intervals, If the transient zero sequence current amplitude of the incoming line interval of the upstream ring network cabinet is not the largest, there is an interval whose transient zero sequence current amplitude is the sum of the amplitudes of other intervals, then the fault point is downstream of the interval. When the bus of the ring network cabinet is grounded, if the fault point is in the ring network cabinet bus, there is a large transient zero sequence current in the incoming line interval, and the transient zero sequence current collected by the upstream ring network cabinet is greater than the transient zero sequence current, which meets the transient zero sequence current direction characteristic.

3. The method of claim 1, wherein: When the judgment method is used for the grounding fault judgment of the ring network cabinet, the following steps are included. Step one, trigger the sampling process; Step two, high-speed synchronous sampling; the free oscillation frequency of the cable line is 1500Hz-3000Hz, and the sampling of one cycle of the field terminal meets the transient sampling demand, and the zero sequence current channels of all intervals must be sampled synchronously to ensure that the data collected at the same time are compared in amplitude; Step three, collect data in a specific time window; Step four, data processing and algorithm execution; Step five, analyze the topological characteristics.

4. The method of claim 3, wherein the method is characterized by: In step one, when the cable network system is in normal operation, the zero sequence voltage U0 of the bus is continuously monitored. When the zero sequence voltage suddenly rises and exceeds the set starting threshold, it is determined that the cable network system has a grounding fault, and the high-speed sampling of the zero sequence current channels of all intervals of the ring network cabinet is immediately started to ensure that the transient process at the fault moment can be captured.

5. The method of claim 3, wherein the method further comprises: In step three, a fixed length of data window is collected, covering 1 power frequency cycle after the fault, and the time window contains the transient information of the first power frequency half wave.

6. The method of claim 3, wherein the method further comprises: In step four, after the original transient data is collected, the following processing is performed: from the fault starting point, the first half-wave zero sequence current data is extracted: i0= [i0[0], i0[1], i0[2],..., i0[N-1]], wherein each i0[n] represents the zero sequence current instantaneous value of the nth sampling point, data preprocessing is performed, and calculation is performed according to formula 1 as follows, Formula 1.

7. The method of claim 3, wherein: In step five, based on the amplitude calculation result, identification is performed through the topology transient characteristic method; The cable network automatically judges the incoming line interval through the power flow direction under the normal operation state, extracts the zero sequence transient current amplitude of each interval when the ground fault occurs, and judges whether the incoming line interval amplitude is the maximum; When the incoming line interval amplitude is the maximum, two cases are distinguished: one is a healthy line or a fault point downstream; one is a suspected ring network cabinet bus grounding; when the topology transient characteristic is not satisfied, it is suspected to be a ring network cabinet bus grounding; if there is only one interval whose amplitude is greater than the incoming line interval, it is judged that it is suspected to be upstream of the fault point, and when the topology transient characteristic is satisfied at the same time, it is judged to be upstream of the fault point.

8. The method of claim 3, wherein: When judging whether the interval current satisfies the topology transient characteristic, the Kirchhoff's law method is used, that is, whether the maximum amplitude interval current is equal to the sum of the amplitudes of the currents of other intervals is judged; specifically, the imbalance degree is calculated by comparing the amplitude of the maximum current interval collected with the sum of the amplitudes of the currents of other intervals, as shown in formula 2 as follows, Formula 2; In the formula, a is the unbalance degree, I max is the maximum current interval amplitude, I _other is the other interval amplitude; it is judged whether the unbalance degree is in the threshold range, and if it is in the threshold range, it is determined that the measurement result satisfies the Kirchhoff law and satisfies the topology transient state characteristic.

9. The method of claim 3, wherein: In the judgment method, when the topology characteristic between the ring network cabinets cannot be judged by a single ring network cabinet, that is, when the ring network cabinet bus is grounded, the master station further analyzes, and the master station topology transient characteristic judgment process is as follows: When the ring network cabinet sends a grounding alarm signal, the master station further analyzes based on the alarm signal, and distinguishes whether the ring network cabinet bus is grounded by judging whether the zero sequence transient amplitude of the first ring network cabinet interval downstream of the initial fault section is greater than the amplitudes of the intervals upstream of the confirmed fault point.

10. The cable network grounding judgment system based on topological transient characteristics, used for the cable network grounding judgment method based on topological transient characteristics as claimed in claim 3, characterized in that: The judgment system is used for ring network cabinet grounding alarm, and includes a zero sequence current transformer, an analog-digital converter and a processor. The zero sequence current transformer is specifically: an HFCT is used, which is pre-penetrated when newly installed or cable head is made, a twisted shielded wire or a coaxial cable is used as a signal line, the signal line is grounded at the collection box end to avoid forming a ground loop and introducing interference, and the signal line is away from the power cable and high-frequency interference sources; The analog-digital converter is specifically: the analog-digital converter ADC is arranged on a signal conditioning board or a main collection board, the front end of the analog-digital converter receives an output signal from a signal conditioning circuit, the front-end circuit of the analog-digital converter includes an anti-aliasing filter and a programmable gain amplifier, and the rear end of the analog-digital converter is directly connected to the processor through a high-speed parallel or serial bus.

Images