CN113311288B - A method and system for finding and locating small current ground faults - Google Patents

Abstract

The invention discloses a small-current grounding fault finding and locating method and system, comprising the steps of: a fault monitoring device detects a three-phase voltage change, performs load data call measurement and stores it; extracts historical load data that meets the load data confidence requirements; calculates The zero-sequence sudden change current of each fault monitoring device marks the fault monitoring device whose zero-sequence sudden change current exceeds the threshold; the fault line and fault section are judged according to the marked fault monitoring device. The invention realizes the line selection of small current grounding fault lines and the location of fault sections through the changes of zero-sequence voltage and current amplitudes and phase fault characteristics before and after the fault, as well as the installation position of the fault monitoring device in the distribution network topology structure. , and preprocess the inaccurate historical data of the fault monitoring device, calculate the zero-sequence current mutation value and perform secondary processing of the inaccurate data according to the distribution network topology, which can accurately find the ground fault interval and improve the reliability of power supply.

Description

A method and system for finding and locating small current ground faults

technical field

The invention relates to the technical field of power systems, in particular to a method and system for finding and locating a small current grounding fault.

Background technique

After the permanent single-phase grounding occurs in the low-current grounding system, no short-circuit loop is formed, the grounding current is small, and the fault signal is weak, which makes it difficult to locate the grounding fault interval. The fault indicator in the existing power distribution automation system does not have the function of locating the small current ground fault interval, and the manual line inspection method is still widely used in the field to determine the fault location. The distribution network has many branches and long distances, and it is very difficult to locate the fault location manually by patrolling the line. Moreover, most of the faults are invisible faults, and it is very inconvenient to find the fault point, which not only consumes a lot of manpower and material resources, but also causes large economic losses to users due to short-term power outages caused by pulling the road. The fault judgment function of the fault monitoring device in the distribution automation system is generally used for line short-circuit faults. Because the short-circuit current can reach several to ten times the load current, it is easy to distinguish it from the load current. However, when the line is single-phase grounded, the grounding current will only increase by a few amperes to tens of amperes. At the same time, because the zero-sequence current transformer of the fault monitoring device is generally less accurate, and due to product quality, installation and maintenance and other issues, it is easy to generate ground fault interval errors. The robustness is poor, and due to the outdoor wind and rain and the quality of its own products, some fault monitoring devices collect load data wrongly, and the data quality has a certain impact on the ground fault judgment. Therefore, when the fault characteristic current is not obvious enough, in general, the fault monitoring device cannot automatically judge and distinguish it from the load current.

For example, Chinese patent CN111650474A, published on September 11, 2020, is a method for locating interphase short-circuit fault sections of distribution network based on multi-system integration, with distribution automation system as the core, GIS system, PMS system, EMS system, The power consumption information acquisition system is the source of fault data, and the internal data of each system is integrated according to certain standards, and through the analysis and processing of these data, the section location of the phase-to-phase short-circuit fault in the distribution network is realized, including identifying the faulty line and determining the faulty line. The number of the distribution transformer and the line segment where the transformer is located, the information of the distribution transformer is determined, the information of the distribution transformer in the fault line is collected, and the fault interval is determined. This method can quickly locate the short-circuit fault of the distribution network, but it cannot accurately determine the fault-occurring section for a small-current grounding fault such as single-phase grounding of the line.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is that the current method for locating the fault section of the distribution network line cannot accurately locate the fault section of the single-phase grounding fault. A method and system for finding and locating a small-current grounding fault that can quickly determine the single-phase grounding fault section are proposed.

In order to solve the above-mentioned technical problems, the technical scheme adopted by the present invention is: a method for finding and locating a small current grounding fault, comprising the following steps:

S1: The fault monitoring device detects the three-phase voltage change, and the load data is called and stored;

S2: extract historical load data that meets the load data confidence requirements;

S3: Calculate the zero-sequence mutation current of each fault monitoring device, and mark the fault monitoring device whose zero-sequence mutation current exceeds the threshold;

S4: Determine the faulty line and the faulty section according to the marked fault monitoring device. Through the fusion and judgment of multi-source grounding fault feature information, that is, through the change of zero-sequence voltage and current amplitude and phase fault characteristics before and after the fault, as well as the installation position of the fault monitoring device in the distribution network topology, the small current grounding fault can be realized. Line selection and faulty section location, so as to quickly restore the power supply of non-faulty sections. And for the faulty historical data existing in the fault monitoring device, preprocess the inaccurate historical data of the fault monitoring device, calculate the zero-sequence current mutation value and perform secondary processing of the inaccurate data according to the distribution network topology, and increase the ground fault line and interval judgment. robustness.

Preferably, the step S2 includes the following steps:

S21: Read the historical load data of each fault monitoring device of the distribution line;

S22: Determine whether the historical load data of each fault monitoring device meets the load data confidence requirement, if so, go to step S24, if not, go to step S23;

S23: Ignore fault monitoring devices that do not meet the load data confidence requirements;

S24: Retain and extract the historical load data of the fault monitoring device that meets the load data confidence requirement. All distribution line fault monitoring devices under the substation conduct historical data analysis, determine that the confidence level of the historical load data results of the selected fault monitoring device meets specific requirements, and then retain participation in fault location determination.

Preferably, the determination process of the load data confidence level requirement in the step S22 includes the following steps:

A1: Read the historical load data of the fault monitoring device;

A2: Determine whether the historical ABC three-phase current balance in the historical load data is less than the set threshold a, if so, go to step A3, if not, go to step A6;

A3: Determine whether the ratio of the zero-sequence current amplitude to the maximum phase current in the historical load data is less than the set threshold b, if so, go to step A4, if not, go to step A6;

A4: Determine whether the ABC three-phase currents in the historical load data are all greater than the set threshold c, if so, go to step A5, if not, go to step A6;

A5: The historical load data of the fault monitoring device meets the load data confidence requirements;

A6: The historical load data of the fault monitoring device does not meet the load data confidence requirements;

A7: Read the historical load data of the next fault monitoring device, and return to step A2. The historical load data of a fault monitoring device can be read randomly, and after it is judged according to the load data confidence requirements, the historical load data of the new fault monitoring device that has not been read can be selected for reading after reading the next one. . The confidence level of the historical load data results is based on the historical ABC three-phase current balance less than the set threshold a, the zero sequence current amplitude/maximum phase current less than the set threshold b, and the ABC three-phase current greater than the set threshold c. Because the three-phase imbalance of the distribution network with different structures and different loads is different, the set threshold a, the set threshold b and the set threshold c need to be adaptively adjusted according to the situation of different power distribution networks. The above three conditions are satisfied Then the confidence level of the historical load data result of the fault monitoring device is satisfied.

Preferably, the step S3 includes the following steps:

S31: Extract the amplitude and phase data of the zero-sequence current of the fault monitoring device that meets the confidence requirement;

S32: Calculate the zero-sequence current amplitude variation of each fault monitoring device before and when the fault occurs;

S33: Sort the fault monitoring devices according to the corresponding zero-sequence current amplitude changes from large to small;

S34: Set the zero-sequence current mutation threshold y;

S35: compare the zero-sequence current amplitude variation of the fault monitoring device with the zero-sequence current mutation threshold y in sequence;

S36: Determine whether the zero-sequence current amplitude variation of the fault monitoring device is greater than the zero-sequence current mutation threshold y, if so, go to step S37, if not, go to step S38;

S37: mark the fault monitoring device for comparison, select the next fault monitoring device in order for comparison, and return to step S36;

S38: Record and save the marked fault monitoring device information. Extract the data of all fault monitoring devices whose confidence levels meet the requirements for zero-sequence current amplitude and phase comparison. Compare the magnitude and phase of the zero-sequence current before and when the fault occurs, and sort according to the magnitude of the zero-sequence current change from large to small. If the zero-sequence mutation current of a fault monitoring device is greater than the threshold value, the fault monitoring device is marked. Calculate the zero-sequence current amplitude change of each fault monitoring device before and when the fault occurs, that is, calculate the sudden change current of each fault monitoring device when the fault occurs, and change the fault monitoring device according to the corresponding zero-sequence current amplitude. It is easy to mark by sorting the small ones. When marking in order, the change of the zero-sequence current amplitude of the previous fault monitoring devices is greater than the mutation threshold y. When it is detected that the zero-sequence current amplitude change of a fault monitoring device is less than the mutation threshold y , indicating that the zero-sequence current amplitude changes of the following fault monitoring devices are all smaller than the sudden change threshold y, so there is no need to search and mark the following fault monitoring devices.

Preferably, the step S4 includes the following steps:

S41: Count the distribution lines to which the marked fault monitoring devices belong;

S42: Determine whether the distribution lines to which each fault monitoring device belongs is the same line, if so, go to step S46, if not, go to step S43;

S43: Determine whether the abrupt zero-sequence current of each marked fault monitoring device flowing from the grounding point to the busbar has connectivity, if yes, go to step S44, if not, go to step S45;

S44: Retain the flag that the mutation zero-sequence current has a connectivity fault monitoring device, and return to step S41;

S45 : remove the flag that the abrupt zero-sequence current does not have a connectivity fault monitoring device, and return to step S41 .

S46: select the distribution line as the ground fault line;

S47: Calculate the distance from each marked fault monitoring device to the substation;

S48: Determine the rear-stage distribution line of the marked fault monitoring device farthest from the substation as the ground fault section. Under the normal operation of the low-current grounding system, the zero-sequence current and voltage will not change abruptly, but fluctuate within a certain range. In the event of a permanent single-phase grounding fault, the zero-sequence current direction of the grounding line is the zero-sequence current, voltage amplitude and phase flow from the grounding point along the line to the busbar, and the zero-sequence current, voltage amplitude and phase of the fault monitoring device between the substation busbar and the line fault point. If a sudden change occurs, the zero-sequence current and voltage in the non-fault area will not change suddenly. Judge whether the distribution line to which the marked fault monitoring device belongs is the same line. If it is not the same line, judge according to the logic of the distribution network topology structure, and check whether the zero-sequence current of the fault monitoring device flowing from the grounding point along the line to the bus has connectivity, and remove the non-compliant fault monitoring device. Judge the distribution line to which the marked fault monitoring device belongs, until the distribution line to which the fault monitoring device belongs is judged to be the same line; , judging that the rear section of the marked fault monitoring device farthest from the substation is the ground fault section, that is, the position of the marked fault monitoring device farthest from the substation in the distribution line is set as the edge node of the fault section, and the distribution line is set as the edge node of the fault section. The section of the distribution line located at the node away from the substation is determined as the ground fault section.

Preferably, the step S1 includes the following steps:

S11: The fault monitoring device detects and calculates the variation of the voltage of each phase of the distribution line;

S12: Set the preset value x of the voltage variation of each phase of the distribution line;

S13: Determine whether the variation of the voltage of each phase of the distribution line exceeds the preset value x, if so, go to step S14, if not, return to step S11;

S14: Call and store the load data for the fault monitoring devices of all distribution lines under the substation. A single-phase ground fault occurs in the distribution line, and the fault monitoring device detects the three-phase voltage change, such as one-phase voltage rise, two-phase voltage drop, and the change exceeds the set threshold, and the fault monitoring device of all distribution lines under the substation is automatically detected. Load data is called and stored.

A small-current grounding fault finding and locating system, using the above method, includes a fault information collection module, the fault information collection module transmits data to a fault area locating module, and the fault information collection module includes a three-phase detection module for power distribution lines. A voltage change detection unit and a data collection and transmission unit for transmitting data to a fault area location module, the fault area location module includes a confidence analysis unit, a zero-sequence sudden change current analysis unit and a fault section determination unit, the The confidence analysis unit is sequentially connected with the zero-sequence sudden change current analysis unit and the fault section determination unit, and the data collection and transmission unit is respectively connected with the detection unit and the confidence analysis unit. A small-current grounding fault finding and locating system automatically detects three-phase voltage changes through the fault information collection module, and conducts fault monitoring device load data for all distribution lines under the substation and stores it. The analysis unit and the zero-sequence sudden change current analysis unit successively perform historical data confidence determination and zero-sequence current change amplitude determination on the collected load data of the fault monitoring device, and complete the preprocessing of the historical data. When the faulty section cannot be locked, the inaccurate data can be re-processed according to the distribution network topology, so as to determine the faulty line and the faulty section.

Preferably, the fault information collection module includes several fault monitoring devices located on the power distribution line. Based on the function of the current fault monitoring device, this patent preprocesses the data of the inaccurate fault monitoring device, and realizes the low-current grounding fault through the fusion judgment of multi-source grounding fault feature information and the installation position of the fault monitoring device in the distribution network topology. Line selection and fault section location solve the defect that the fault monitoring device cannot locate the ground fault.

The substantial effect of the present invention is: the present invention realizes the small current grounding fault through the change of the zero-sequence voltage and current amplitude before the fault and the change of the phase fault characteristic, and the installation position of the fault monitoring device in the distribution network topology structure. Line selection and fault section location, and preprocess inaccurate historical data of the fault monitoring device, calculate the zero-sequence current mutation value, and perform secondary processing of the inaccurate data according to the distribution network topology. The screening method uses effective data to improve the accuracy of judgment, increases the robustness of ground fault line and interval judgment, and can accurately find the ground fault zone, which is conducive to quickly restoring the power supply of non-faulty sections and improving the reliability of power supply.

Description of drawings

Fig. 1 is the flow chart of the main implementation steps of this embodiment;

FIG. 2 is a flowchart of step S22 of this embodiment;

3 is a flowchart of step S3 of this embodiment;

FIG. 4 is a schematic diagram of the composition of this embodiment.

Among them: 1. Fault information collection module, 2. Fault area location module, 3. Detection unit, 4. Data collection and transmission unit, 5. Confidence analysis unit, 6. Zero-sequence mutation current analysis unit, 7. Fault segment determination unit.

Detailed ways

The specific embodiments of the present invention will be further described in detail below through specific embodiments and in conjunction with the accompanying drawings.

A method for finding and locating a small current ground fault, as shown in Figure 1, includes the following steps:

S1: The fault monitoring device detects the three-phase voltage change, performs load data call measurement and stores it; step S1 includes the following steps:

S11: The fault monitoring device detects and calculates the variation of the voltage of each phase of the distribution line;

S12: Set the preset value x of the voltage variation of each phase of the distribution line;

S13: Determine whether the variation of the voltage of each phase of the distribution line exceeds the preset value x, if so, go to step S14, if not, return to step S11;

S14: Call and store the load data for the fault monitoring devices of all distribution lines under the substation. A single-phase ground fault occurs in the distribution line, and the fault monitoring device detects the three-phase voltage change, such as one-phase voltage rise, two-phase voltage drop, and the change exceeds the set threshold, and the fault monitoring device of all distribution lines under the substation is automatically detected. Load data is called and stored.

S2: extract historical load data that meets the load data confidence requirements; step S2 includes the following steps:

S21: Read the historical load data of each fault monitoring device of the distribution line;

S22: Determine whether the historical load data of each fault monitoring device meets the load data confidence requirement, if so, go to step S24, if not, go to step S23;

S23: Ignore fault monitoring devices that do not meet the load data confidence requirements;

S24: Retain and extract the historical load data of the fault monitoring device that meets the load data confidence requirement. All distribution line fault monitoring devices under the substation conduct historical data analysis, determine that the confidence level of the historical load data results of the selected fault monitoring device meets specific requirements, and then retain participation in fault location determination. The determination process of the load data confidence requirement in step S22 includes the following steps, as shown in Figure 2:

A1: Read the historical load data of the fault monitoring device;

A2: Determine whether the historical ABC three-phase current balance in the historical load data is less than the set threshold a, if so, go to step A3, if not, go to step A6;

A3: Determine whether the ratio of the zero-sequence current amplitude to the maximum phase current in the historical load data is less than the set threshold b, if so, go to step A4, if not, go to step A6;

A4: Determine whether the ABC three-phase currents in the historical load data are all greater than the set threshold c, if so, go to step A5, if not, go to step A6;

A5: The historical load data of the fault monitoring device meets the load data confidence requirements;

A6: The historical load data of the fault monitoring device does not meet the load data confidence requirements;

A7: Read the historical load data of the next fault monitoring device, and return to step A2. The historical load data of a fault monitoring device can be read randomly, and after it is judged according to the load data confidence requirements, the historical load data of the new fault monitoring device that has not been read can be selected for reading after reading the next one. . The confidence level of the historical load data results is based on the historical ABC three-phase current balance less than the set threshold a, the zero sequence current amplitude/maximum phase current less than the set threshold b, and the ABC three-phase current greater than the set threshold c. Because the three-phase imbalance of the distribution network with different structures and different loads is different, the set threshold a, the set threshold b and the set threshold c need to be adaptively adjusted according to the situation of different power distribution networks. The above three conditions are satisfied Then the confidence level of the historical load data result of the fault monitoring device is satisfied.

S3: Calculate the zero-sequence sudden change current of each fault monitoring device, and mark the fault monitoring device whose zero-sequence sudden change current exceeds the threshold; as shown in FIG. 3, step S3 includes the following steps:

S31: Extract the amplitude and phase data of the zero-sequence current of the fault monitoring device that meets the confidence requirement;

S32: Calculate the zero-sequence current amplitude variation of each fault monitoring device before and when the fault occurs;

S33: Sort the fault monitoring devices according to the corresponding zero-sequence current amplitude changes from large to small;

S34: Set the zero-sequence current mutation threshold y;

S35: compare the zero-sequence current amplitude variation of the fault monitoring device with the zero-sequence current mutation threshold y in sequence;

S36: Determine whether the zero-sequence current amplitude variation of the fault monitoring device is greater than the zero-sequence current mutation threshold y, if so, go to step S37, if not, go to step S38;

S37: mark the fault monitoring device for comparison, select the next fault monitoring device in order for comparison, and return to step S36;

S38: Record and save the marked fault monitoring device information. Extract the data of all fault monitoring devices whose confidence levels meet the requirements for zero-sequence current amplitude and phase comparison. Compare the magnitude and phase of the zero-sequence current before and when the fault occurs, and sort according to the magnitude of the zero-sequence current change from large to small. If the zero-sequence mutation current of a fault monitoring device is greater than the threshold value, the fault monitoring device is marked. Calculate the zero-sequence current amplitude change of each fault monitoring device before and when the fault occurs, that is, calculate the sudden change current of each fault monitoring device when the fault occurs, and change the fault monitoring device according to the corresponding zero-sequence current amplitude. It is easy to mark by sorting the small ones. When marking in order, the change of the zero-sequence current amplitude of the previous fault monitoring devices is greater than the mutation threshold y. When it is detected that the zero-sequence current amplitude change of a fault monitoring device is less than the mutation threshold y , indicating that the zero-sequence current amplitude changes of the following fault monitoring devices are all smaller than the sudden change threshold y, so there is no need to search and mark the following fault monitoring devices.

S4: Determine the faulty line and the faulty section according to the marked fault monitoring device. Step S4 includes the following steps:

S41: Count the distribution lines to which the marked fault monitoring devices belong;

S42: Determine whether the distribution lines to which each fault monitoring device belongs is the same line, if so, go to step S46, if not, go to step S43;

S43: Determine whether the abrupt zero-sequence current of each marked fault monitoring device flowing from the grounding point to the busbar has connectivity, if yes, go to step S44, if not, go to step S45;

S44: Retain the flag that the mutation zero-sequence current has a connectivity fault monitoring device, and return to step S41;

S45 : remove the flag that the abrupt zero-sequence current does not have a connectivity fault monitoring device, and return to step S41 .

S46: select the distribution line as the ground fault line;

S47: Calculate the distance from each marked fault monitoring device to the substation;

S48: Determine the rear-stage distribution line of the marked fault monitoring device farthest from the substation as the ground fault section. Under the normal operation of the low-current grounding system, the zero-sequence current and voltage will not change abruptly, but fluctuate within a certain range. In the event of a permanent single-phase grounding fault, the zero-sequence current direction of the grounding line is the zero-sequence current, voltage amplitude and phase flow from the grounding point along the line to the busbar, and the zero-sequence current, voltage amplitude and phase of the fault monitoring device between the substation busbar and the line fault point. If a sudden change occurs, the zero-sequence current and voltage in the non-fault area will not change suddenly. Judge whether the distribution line to which the marked fault monitoring device belongs is the same line. If it is not the same line, judge according to the logic of the distribution network topology structure, and check whether the zero-sequence current of the fault monitoring device flowing from the grounding point along the line to the bus has connectivity, and remove the non-compliant fault monitoring device. Judge the distribution lines to which the marked fault monitoring device belongs, until the distribution line to which the fault monitoring device belongs is judged whether it is the same line; , judging that the rear section of the marked fault monitoring device farthest from the substation is the ground fault section, that is, the position of the marked fault monitoring device farthest from the substation in the distribution line is set as the edge node of the fault section, and the distribution line is set as the edge node of the fault section. The section of the distribution line located at the node away from the substation is determined as the ground fault section.

A small-current grounding fault finding and locating system, as shown in FIG. 4, using the above method, includes a fault information collection module 1, the fault information collection module 1 transmits data to the fault area locating module 2, and the fault information collection module 1 includes a A detection unit 3 for detecting changes in three-phase voltage of a distribution line and a data collection and transmission unit 4 for transmitting data to a fault area location module 2, the fault area location module 2 includes a confidence analysis unit 5 and a zero-sequence mutation current analysis unit 6 and the fault section determination unit 7, the confidence analysis unit 5 is sequentially connected with the zero-sequence sudden change current analysis unit 6 and the fault section determination unit 7, and the data collection and transmission unit 4 is respectively connected with the detection unit 3 and the confidence analysis unit 5. The fault information collection module 1 includes several fault monitoring devices located on the distribution line. Based on the current fault monitoring device function, this patent preprocesses the inaccurate data of the fault monitoring device, and realizes the low-current grounding fault through the fusion judgment of multi-source grounding fault feature information and the installation position of the fault monitoring device in the distribution network topology. Line selection and fault section location solve the defect that the fault monitoring device cannot locate the ground fault. The three-phase voltage change is automatically detected through the fault information collection module 1, and the load data of the fault monitoring device is called and stored for all distribution lines under the substation. The analysis unit 6 sequentially performs historical data confidence determination and zero-sequence current change amplitude determination on the collected fault monitoring device load data, completes the preprocessing of the historical data, and locks the fault zone through the fault zone determination unit 7. When When the faulty section cannot be locked, the inaccurate data can be reprocessed according to the distribution network topology, so as to determine the faulty line and the faulty section.

In order to overcome the low accuracy of the zero-sequence current transformer in the fault monitoring device, misjudgment of the ground fault interval is likely to occur due to problems such as product quality, installation and maintenance, and the robustness is poor; the fault monitoring device is exposed to outdoor wind and rain and its own product quality. The reason is that some fault monitoring devices collect the load data by mistake in missed collection and wrong transmission, and the data quality has a certain impact on the ground fault judgment. sex.

In this embodiment, the multi-source grounding fault feature information fusion judgment is used, that is, the zero-sequence voltage and current amplitude changes before and after the fault, and the phase fault characteristics, as well as the installation position of the fault monitoring device in the distribution network topology. Current ground fault line selection and fault section location, so as to quickly restore the power supply of non-fault sections. And for the faulty historical data existing in the fault monitoring device, preprocess the inaccurate historical data of the fault monitoring device, calculate the zero-sequence current mutation value and perform secondary processing of the inaccurate data according to the distribution network topology, and increase the ground fault line and interval judgment. Robustness, it can accurately find the ground fault area, which is conducive to quickly recovering the power supply of the non-faulty area and improving the reliability of the power supply.

The above examples only represent several embodiments of the present invention, and the descriptions thereof are specific and detailed, but should not be construed as a limitation on the scope of the invention patent. It should be pointed out that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can also be made, which all belong to the protection scope of the present invention.

Claims (6)

1. a small current ground fault finding and locating method, is characterized in that, comprises the steps: S1: The fault monitoring device detects the three-phase voltage change, and the load data is called and stored; S2: extract historical load data that meets the load data confidence requirements; S3: Calculate the zero-sequence mutation current of each fault monitoring device, and mark the fault monitoring device whose zero-sequence mutation current exceeds the threshold; S4: Determine the faulty line and faulty section according to the marked fault monitoring device; The step S2 includes the following steps: S21: Read the historical load data of each fault monitoring device of the distribution line; S22: Determine whether the historical load data of each fault monitoring device meets the load data confidence requirement, if so, go to step S24, if not, go to step S23; S23: Ignore fault monitoring devices that do not meet the load data confidence requirements; S24: Retain and extract the historical load data of the fault monitoring device that meets the load data confidence requirement; The determination process of the load data confidence level requirement in the step S22 includes the following steps: A1: Read the historical load data of the fault monitoring device; A2: Determine whether the historical ABC three-phase current balance in the historical load data is less than the set threshold a, if so, go to step A3, if not, go to step A6; A3: Determine whether the ratio of the zero-sequence current amplitude to the maximum phase current in the historical load data is less than the set threshold b, if so, go to step A4, if not, go to step A6; A4: Determine whether the ABC three-phase currents in the historical load data are all greater than the set threshold c, if so, go to step A5, if not, go to step A6; A5: The historical load data of the fault monitoring device meets the load data confidence requirements; A6: The historical load data of the fault monitoring device does not meet the load data confidence requirements; A7: Read the historical load data of the next fault monitoring device, and return to step A2. 2. The method for finding and locating a small current ground fault according to claim 1, wherein the step S3 comprises the following steps: S31: Extract the amplitude and phase data of the zero-sequence current of the fault monitoring device that meets the confidence requirement; S32: Calculate the zero-sequence current amplitude variation of each fault monitoring device before and when the fault occurs; S33: Sort the fault monitoring devices according to the corresponding zero-sequence current amplitude changes from large to small; S34: Set the zero-sequence current mutation threshold y; S35: compare the zero-sequence current amplitude variation of the fault monitoring device with the zero-sequence current mutation threshold y in sequence; S36: Determine whether the zero-sequence current amplitude variation of the fault monitoring device is greater than the zero-sequence current mutation threshold y, if so, go to step S37, if not, go to step S38; S37: mark the fault monitoring device for comparison, select the next fault monitoring device in order for comparison, and return to step S36; S38: Record and save the marked fault monitoring device information. 3. The method for finding and locating a small current ground fault according to claim 1 or 2, wherein the step S4 comprises the following steps: S41: Count the distribution lines to which the marked fault monitoring devices belong; S42: Determine whether the distribution lines to which each fault monitoring device belongs is the same line, if so, go to step S46, if not, go to step S43; S43: Determine whether the abrupt zero-sequence current of each marked fault monitoring device flowing from the grounding point to the busbar has connectivity, if yes, go to step S44, if not, go to step S45; S44: Retain the flag that the mutation zero-sequence current has a connectivity fault monitoring device, and return to step S41; S45: remove the mark of the abrupt zero-sequence current that does not have a connectivity fault monitoring device, and return to step S41; S46: select the distribution line as the ground fault line; S47: Calculate the distance from each marked fault monitoring device to the substation; S48: Determine the rear-stage distribution line of the marked fault monitoring device farthest from the substation as the ground fault section. 4. The method for finding and locating a small current ground fault according to claim 3, wherein the step S1 comprises the following steps: S11: The fault monitoring device detects and calculates the variation of the voltage of each phase of the distribution line; S12: Set the preset value x of the voltage variation of each phase of the distribution line; S13: Determine whether the variation of the voltage of each phase of the distribution line exceeds the preset value x, if so, go to step S14, if not, return to step S11; S14: Call and store the load data for the fault monitoring devices of all distribution lines under the substation. 5. A small-current grounding fault finding and locating system, using any one of the low-current grounding fault finding and locating methods according to claims 1-4, characterized in that it comprises a fault information collection module (1), the fault information The collection module (1) transmits the data to the fault area location module (2), the fault information collection module (1) includes a detection unit (3) for detecting the three-phase voltage change of the distribution line and a detection unit (3) for transmitting the data A data collection and transmission unit (4) for a fault area location module (2), the fault area location module (2) comprising a confidence analysis unit (5), a zero-sequence mutation current analysis unit (6) and a fault segment determination unit (7), the confidence analysis unit (5) is sequentially connected to the zero-sequence sudden change current analysis unit (6) and the fault section determination unit (7), and the data collection and transmission unit (4) is respectively connected to the The detection unit (3) is connected with the confidence analysis unit (5). 6 . The system for finding and locating a small current grounding fault according to claim 5 , wherein the fault information collection module ( 1 ) comprises a plurality of fault monitoring devices located on the power distribution line. 7 .

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