Early fault identification method for medium-voltage cable based on shielding layer current

Abstract

The invention discloses a shielding layer current-based medium voltage cable early fault identification method. The method comprises the following steps of S1, obtaining monitoring data of a tail end shielding layer grounding current; s2, calculating an effective value and an instantaneous value of the grounding current of the tail end shielding layer; s3, whether fault early warning is triggered or not is detected, and if yes, a fault waveform is obtained; s4, calculating the fault duration and the number of fault continuous sampling points, and judging whether the fault is an early fault or not; s5, judging whether the early fault section is a fault downstream section or a non-trunk fault section; s6, calculating a cross correlation function of the tail end shielding layer current and the zero sequence current; and S7, distinguishing a fault section and a fault upstream section according to the optimal delay. According to the invention, the accuracy and efficiency of early fault identification of the medium-voltage cable are improved, and the transformation of protection from post protection to pre-warning is promoted.

Description

Technical Field

[0001] The present invention relates to the technical field of power supply reliability, and in particular to a method for identifying early faults of a medium-voltage cable based on shielding layer current. Background Art

[0002] Medium voltage cables are widely used in high-reliability power transmission and distribution sites such as cities, mines, and offshore wind power plants due to their large transmission capacity, small footprint, and low maintenance workload. A large amount of measured data shows that the electrical characteristics of early faults in medium voltage cables are repetitive instantaneous power disturbances (kHz level) before permanent faults. Although the disturbance amplitude is small and the duration is short, it will not trigger protection actions, but this type of disturbance can be recorded by devices with waveform monitoring capabilities in the distribution network.

[0003] At present, efficient identification of early fault status can be achieved through monitoring and analysis based on power disturbances. Power disturbances contain a large amount of early fault status information and can be transmitted over long distances along cable lines. And with the continuous development of distribution network informatization, there are a large number of disturbance waveform monitoring devices (including fault recorders, power quality monitoring devices, fault indicators, etc.) in the distribution network that can realize power disturbance (kHz) monitoring, which provides important data support for medium-voltage cable status perception based on power disturbance monitoring, which is of great significance for practical applications.

[0004] The monitoring of cable abnormal conditions mainly relies on the recognition of limited characteristic quantities, including dielectric loss, insulation resistance, partial discharge, temperature, etc. These characteristic quantities can reflect the abnormal conditions of the cable to a certain extent. However, the existing research on the entire process of cable abnormal conditions does not cover the entire life cycle of the cable, and the characteristic quantities cannot cover the entire process of cable abnormal conditions. More importantly, medium-voltage cables are usually three-core cables, and the shielding layer of the three-phase cable is directly grounded at both ends of the cable, which increases the difficulty of monitoring the abnormal conditions of the cable, because it leads to problems such as complicated shielding layer current distribution, multi-path current interference and electromagnetic interference, making the identification and location of fault signals more difficult. Summary of the invention In order to solve the problem of difficulty in identifying and locating medium voltage cable faults, the present invention proposes a medium voltage cable early fault identification method based on shielding layer current to solve the above problem.

[0005] The present application discloses a method for early fault identification of a medium voltage cable based on shielding layer current, comprising the following steps: S1, obtaining monitoring data of the grounding current of the terminal shielding layer; S2, calculate the effective value and instantaneous value of the terminal shielding layer grounding current according to the data obtained in S1, and establish the trigger detection criterion; S3. Detect whether a fault warning is triggered by triggering a detection criterion. If a fault warning is triggered, start the fault recording to obtain the fault waveform; S4, calculating the fault duration and the number of fault continuous sampling points according to the fault waveform obtained in S3, and judging whether the fault is an early fault according to the fault duration and the number of fault continuous sampling points; S5, by comparing the effective value amplitude of the terminal shielding layer grounding current with the specific threshold, determining whether the early fault section is a fault downstream section or a non-main fault section; S6. If the fault section is not a fault downstream section or a non-main fault section, a cross-correlation function between the terminal shield layer current and the zero-sequence current is calculated, and an optimal delay between the terminal shield layer current and the zero-sequence current is determined; S7. Distinguish the fault section and the fault upstream section according to the optimal delay obtained in S6.

[0006] Preferably, the calculation formula for the effective value of the terminal shielding layer grounding current is:

[0007] in, is the total number of sampling points of the half-wave detection signal, For segment The terminal shielding layer grounding current sampling signal, is the sampling sequence, is the current sampling point; Trigger detection criterion 1: The effective value of the grounding current of the terminal shield layer exceeds the limit, that is:

[0008] in, is the trigger threshold of the effective value amplitude of the terminal shielding layer grounding current, , is the rated current effective value, is the rated current effective value coefficient; Trigger detection criterion 2: The effective value of the grounding current of the terminal shielding layer exceeds the limit, that is:

[0009] in, The trigger threshold of the effective value mutation of the terminal shielding layer grounding current, , is the rated current effective value coefficient.

[0010] Preferably, the calculation formula for the instantaneous value mutation of the terminal shielding layer grounding current is:

[0011] in, is the number of sampling points in one cycle of the signal, It is the terminal shielding layer grounding current sampling signal; Trigger detection criterion 3: The instantaneous value of the single-end grounding current of the shield layer exceeds the limit, that is:

[0012] in, is the trigger threshold of the instantaneous value mutation of the terminal shielding layer grounding current, , is the rated current effective value coefficient.

[0013] Preferably, S3 comprises the following steps: When the grounding current of the terminal shielding layer meets any one of the trigger detection criteria 1, trigger detection criteria 2 and trigger detection criteria 3, a fault warning is triggered, and the fault recording is started at this time to obtain the fault waveform.

[0014] Preferably, S4 comprises the following steps: The fault initial sampling point judgment criterion is:

[0015] in, is the fault starting sampling point, is the effective value of the shield layer grounding current, the fault starting sampling point The corresponding time is recorded as the fault start time ; The fault end sampling point judgment criteria are:

[0016] in, is the fault end sampling point, the fault end sampling point The corresponding time is recorded as the fault end time ; The fault duration is:

[0017] The number of fault continuous sampling points is:

[0018] The early fault identification criteria are: When the number of sampling points of the shield layer grounding current during the fault duration is less than the fault identification threshold, it indicates that the fault is an early fault, that is:

[0019] in, is the fault identification threshold, and , is the sampling frequency of the monitoring device, is the base frequency.

[0020] Preferably, S5 comprises the following steps: The shielding layer grounding current amplitude of the fault downstream section and the non-trunk fault section is much smaller than the shielding layer grounding current amplitude of the fault section and the fault upstream section. By setting a specific threshold, when the shielding layer grounding current effective value amplitude is less than the specific threshold, it indicates that the fault section is the fault downstream section or the non-trunk fault section, that is:

[0021] in, is the segment differentiation threshold based on the effective value amplitude of the shield layer grounding current, , for The half-wave effective value of the zero-sequence current in the section, is the zero-sequence current effective value coefficient.

[0022] Preferably, the cross-correlation function is as follows:

[0023] in, Indicates the zero sequence current of this section and the terminal shield ground current In Delay The cross-correlation value at for After delay The value of when When the maximum value is taken, the corresponding This is the optimal delay between the terminal shield current and the zero-sequence current.

[0024] Preferably, the S7 comprises the following steps: If the optimal delay between the terminal shield current and the zero-sequence current satisfies the following equation:

[0025] Then the early fault section is the fault section, otherwise it is the fault upstream section; in, is a threshold for distinguishing sections based on the cross-correlation between the terminal shield layer current and the zero-sequence current, and , is the sampling frequency of the monitoring device, is the base frequency.

[0026] Beneficial effects of the present invention: (1) Based on the characteristic analysis of the shielding layer grounding current fault waveform, the present invention proposes a trigger detection method for the effective value or mutation amount of the end shielding layer grounding current amplitude, and further proposes an early fault identification method based on the signal characteristics of the end shielding layer grounding current, thereby improving the accuracy and efficiency of early fault identification of medium-voltage cables.

[0027] (2) Through the implementation of the present invention, a technical route for "active protection" of medium-voltage cables will be formed, which will provide important theoretical and practical support for promoting the transformation of protection from "after-the-fact protection" to "pre-warning" and realizing "preventive protection" of medium-voltage cables. At the same time, it has important theoretical value and engineering significance for improving power supply reliability and avoiding major accidents caused by cable faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a flow chart of a method for identifying early faults of medium voltage cables based on shielding layer current according to an embodiment of the present invention; Figure 2 A schematic diagram of a fault current path according to an embodiment of the present invention; Figure 3 is the half-cycle effective value of the shielding layer grounding current under half-cycle and multi-cycle early fault conditions of the embodiment of the present invention; Figure 4 A schematic diagram of early fault start and end time detection according to an embodiment of the present invention; Figure 5 It is a schematic diagram of the grounding current direction characteristics of the shielding layer at the head end and the end end of different sections of an embodiment of the present invention; Figure 6 This is a topological schematic diagram of a 10kV distribution network early fault simulation model according to an embodiment of the present invention; Figure 7 This is a schematic diagram of an independent shielded three-core cable fault simulation module according to an embodiment of the present invention; Figure 8 It is a characteristic distribution diagram of the shielding layer grounding current fault duration in each section of Case 1 to Case 4 of the embodiment of the present invention; Fig. 9 It is a characteristic distribution diagram of the periodic effective value of the grounding current of the shielding layer in each section of Case 1 to Case 4 of the embodiment of the present invention; Fig.10 This is a distribution diagram of the cross-correlation characteristics of the shielding layer grounding current in each section of Case 1 to Case 4 of the embodiments of the present invention. DETAILED DESCRIPTION

[0029] In order to make the objectives, technical solutions and advantages of the present application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and examples.

[0030] The present application embodiment discloses a method for early fault identification of a medium voltage cable based on shielding layer current, such as Figure 1 As shown, the following steps are included: S1. Obtain monitoring data of the terminal shielding layer grounding current through a wave recording device such as a high-frequency current transformer.

[0031] The embodiment of the present application realizes the identification of the fault section only through the shielding layer grounding current at a single end, so it becomes a problem to be solved whether to select the shielding layer grounding current at the head end or the tail end as the identification object.

[0032] like Figure 2 The figure shows a schematic diagram of the fault current path considering the cable structure and the grounding of the shield layer. Here, the direction of the current in the shield layer flowing from the cable to the earth is named the positive direction. When a fault occurs, the three-phase current is no longer balanced, and the fault current is in the same direction as the zero-sequence current. In the upstream section of the fault, the current in the shield layer is mainly the induced current of the fault current, which is much larger than the equivalent capacitive current. Therefore, the direction of the comprehensive current in the shield layer is the same as the induced current, and the directions of the grounding currents at both ends are opposite, with the positive direction at the head end and the negative direction at the end end. In the fault section, the fault breakdown current flows from the cable conductor to the earth through the shield layer. Therefore, the current direction at both ends of the shield layer is from the cable to the earth, and both ends are in the positive direction. In the downstream section of the fault and the non-fault section, the current in the shield layer is the equivalent capacitive current, so the current direction at both ends of the shield layer is also from the cable to the earth, and both ends are in the positive direction. Moreover, the amplitude of the equivalent capacitive current is also very small compared with the fault current.

[0033] By comparing and analyzing the shielding layer grounding currents at both ends, it is found that the end shielding layer grounding current has a directional feature. The current direction of the head end shielding layer grounding current is positive in both the healthy section and the fault section, while the end shielding layer grounding current is negative only in the upstream section of the fault, and the current direction of other sections is positive. Therefore, the end grounding current is selected as the input of the identification signal in the embodiment of the present application.

[0034] S2. Calculate the effective value and instantaneous value of the terminal shielding layer grounding current based on the data obtained in S1, and establish a trigger detection criterion.

[0035] Under normal operating conditions, the current in the shielding layer is almost zero. When an early fault occurs in the cable, the cable core and the steel armor layer of the shielding layer will undergo an instantaneous and recoverable breakdown, causing the current in the shielding layer to increase instantaneously; secondly, since the early fault is a "self-clearing" fault, the voltage / current operating state will return to normal after the fault is eliminated. Based on the above two characteristics, this application detects early faults through two trigger algorithms: effective value and instantaneous value.

[0036] Trigger detection algorithm based on the effective value of the shield grounding current: The calculation formula of the effective value of the terminal shielding layer grounding current is:

[0037] in, is the total number of sampling points of the half-wave detection signal, For segment The terminal shielding layer grounding current sampling signal, is the sampling sequence, is the current sampling point. The effective value calculation sliding step is point by point calculation.

[0038] When the cable is operating normally, the grounding current at one end of the shield layer is very small, and the effective value of each cycle is very small. When an early fault occurs in the cable, the fault current flows through the shield layer at the fault point to the grounding point. The cable downstream of the fault section will have a three-phase imbalance due to the sudden change in current. This three-phase imbalance is not very large, and the shield layer grounding current is reflected as a capacitive current caused by the three-phase imbalance. At this time, the effective value of the grounding current at one end of the shield layer will increase. Figure 3 As shown, Figure 3 (a) is a schematic diagram of the half-cycle effective value of the shielding layer grounding current in the early fault of the half-cycle. Figure 3 (b) is the effective value of the shielding layer grounding current in the half cycle under the condition of multi-cycle early fault. When an early fault occurs, the effective value of the shielding layer single-end grounding current increases in this cycle, and the effective value of the current in the fault section and upstream of the fault section is significantly higher than that in the downstream section. When the increase of the single-end grounding current of the shielding layer is detected, an early warning will be triggered and the waveform will be recorded, and it is considered that a fault has occurred. The effective value of the 1 / 2 cycle current is calculated and recorded during this period.

[0039] Trigger detection criterion 1: The effective value of the grounding current of the terminal shield layer exceeds the limit, that is:

[0040] in, is the trigger threshold of the effective value amplitude of the terminal shielding layer grounding current, , is the rated current effective value, is the rated current effective value coefficient. In this embodiment The value is 1.05.

[0041] Trigger detection criterion 2: The effective value of the grounding current of the terminal shielding layer exceeds the limit, that is:

[0042] in, The trigger threshold of the effective value mutation of the terminal shielding layer grounding current, , is the rated current effective value coefficient. In this embodiment The value is 0.5.

[0043] Trigger detection algorithm based on the instantaneous value of the shield grounding current: The difference between the sampling points at the corresponding positions of the adjacent cycles of the shielding layer grounding current is the instantaneous value mutation. The calculation formula for the instantaneous value mutation of the terminal shielding layer grounding current is:

[0044] in, is the number of sampling points in one cycle of the signal, It is the terminal shielding layer ground current sampling signal.

[0045] Trigger detection criterion 3: The instantaneous value of the single-end grounding current of the shield layer exceeds the limit, that is:

[0046] in, is the trigger threshold of the instantaneous value mutation of the terminal shielding layer grounding current, , is the rated current effective value coefficient. In this embodiment The value is 0.3.

[0047] S3. Detect whether a fault warning is triggered by triggering the detection criteria. When the grounding current of the terminal shielding layer meets any one of the trigger detection criteria 1, trigger detection criteria 2 and trigger detection criteria 3, the fault warning is triggered. At this time, the fault recording is started to obtain the fault waveform.

[0048] S4. Calculate the fault duration and the number of fault duration sampling points according to the fault waveform obtained in S3, and determine whether the fault is an early fault according to the fault duration and the number of fault duration sampling points.

[0049] Since early faults are usually single-phase breakdown faults of 1 / 4 to 4 cycles, the duration of the entire fault is calculated, and then the threshold is used to determine whether it is an early fault. When the fault is an early fault, there will be a "self-clearing" situation, that is, after the fault is eliminated, the current operation state will return to normal. Therefore, in addition to the start time, the fault must have a fault end time.

[0050] The fault initial sampling point judgment criterion is:

[0051] in, is the fault starting sampling point, is the effective value of the shield layer grounding current, the fault starting sampling point The corresponding time is recorded as the fault start time ; The fault end sampling point judgment criteria are:

[0052] in, is the fault end sampling point, the fault end sampling point The corresponding time is recorded as the fault end time The schematic diagram of early fault start and end time detection is as follows: Figure 4 shown.

[0053] The fault duration is:

[0054] The number of fault duration sampling points is:

[0055] The early fault identification criteria are: When the number of sampling points of the shield layer grounding current during the fault duration is less than the fault identification threshold, it indicates that the fault is an early fault, that is:

[0056] Otherwise it indicates that the fault is a permanent breakdown fault.

[0057] in, is the fault identification threshold, the duration of early fault is between 1 / 4 and 4 cycles. is the number of sampling points within 4 cycles, that is:

[0058] is the sampling frequency of the monitoring device, is the base frequency.

[0059] S5. By comparing the effective value amplitude of the terminal shielding layer grounding current with the specific threshold, it is determined whether the early fault section is a fault downstream section or a non-main fault section.

[0060] Depend on Figure 5 It can be seen that in terms of amplitude, the shielding layer grounding current in the downstream section of the fault is much smaller than the amplitude of the fault section and the upstream section of the fault section. Therefore, this feature can be used to distinguish the downstream section of the fault from other sections. By setting a specific threshold, when the shielding layer grounding current effective value amplitude is less than the specific threshold, it indicates that the fault section is the downstream section of the fault or the non-mainline fault section, that is:

[0061] in, is the segment differentiation threshold based on the effective value amplitude of the shield layer grounding current, , for The half-wave effective value of the zero-sequence current in the section, is the zero-sequence current effective value coefficient. The value is 8%.

[0062] S6. If the fault section is not a fault downstream section or a non-main fault section, the cross-correlation function between the terminal shielding layer current and the zero-sequence current is calculated, and the optimal delay between the terminal shielding layer current and the zero-sequence current is determined.

[0063] Depend on Figure 5 It can be seen that in terms of current direction, the end shielding layer grounding current of the fault section is basically the same as the direction of the zero-sequence current, while the end current direction of the fault upstream section is opposite to the current direction of the zero-sequence current. This feature will be reflected in the phase of the fault grounding current waveform, so this feature can be used to distinguish the fault upstream section from the fault section. The premise of the identification method proposed in the embodiment of the present application is that the sections cannot communicate with each other, and fault identification can only be completed on the local ring network cabinet or waveform monitoring device. Therefore, it is necessary to find an identical reference object to determine the directional characteristics of the end shielding layer grounding current. In this embodiment, the zero-sequence current of this section of current is used as the directional reference object. When an early fault occurs, the end shielding layer grounding current of the fault section is almost the same as the zero-sequence current, that is, the phase difference between the two waveforms is 0; and the end shielding layer grounding current of the fault upstream section is almost opposite to the zero-sequence current, that is, the phase difference between the two waveforms is π.

[0064] By analyzing the cross-correlation between the shield end current and the zero-sequence current, the time relationship and phase difference between them can be quantified, and the relative position and possible direction of the fault can be inferred. If the cross-correlation of the two signals shows that they are closely related in time, that is, they have significant peaks and short time delays, this indicates that the fault may have caused the synchronous changes of the two currents, which can be used to determine the fault direction. Calculate the cross-correlation function (CCF) between the two signals:

[0065] in, Indicates the zero sequence current of this section and the terminal shield ground current In Delay The cross-correlation value at for After delay When When the maximum value is taken, the corresponding This is the optimal delay between the terminal shield current and the zero-sequence current.

[0066] S7. Distinguish the fault section and the fault upstream section according to the optimal delay obtained in S6.

[0067] Set a reasonable threshold to distinguish the fault section from the fault upstream section. If the optimal delay between the terminal shield layer current and the zero-sequence current satisfies the following formula:

[0068] Then the early fault section is the fault section, otherwise it is the fault upstream section.

[0069] in, The threshold for distinguishing the cross-correlation segment between the terminal shielding layer current and the zero-sequence current is based on the threshold. In this embodiment, the threshold for distinguishing the cross-correlation segment between the terminal shielding layer current and the zero-sequence current is:

[0070] is the sampling frequency of the monitoring device, is the base frequency.

[0071] In a specific embodiment, in order to simulate and verify the medium voltage cable early fault identification method based on shield layer current proposed in the above embodiment, a simulation model of early fault of 10kV three-core armored cable in distribution network is established in PSCAD / EMTDC software, such as Figure 6 As shown. A 110kV power supply with a frequency of 50Hz is selected, the transformer adopts Y / Y connection, the transformation ratio is 110kV / 10.5kV, the rated capacity is 45MV·A, and the neutral point is grounded with a small resistor with a resistance value of 6Ω. According to the data requirements required by the method proposed in the embodiment of the present application, measuring points are arranged at the end of the shielding layer grounding current of each cable section. The 10kV bus has 5 feeders. Feeder 1 is relatively complex with 4 sections. The other 4 feeders have the same parameters except for the length and load size.

[0072] Figure 7 In PSCAD, because the cable does not support direct connection, the remote termination mode is selected. Cable_2 and Cable_4 are cables, and both ends are cable interfaces. C1, C2, and C3 are the three cores of the cable; S1, S2, and S3 are the shielding layers corresponding to the three cores; Encompassing Pipe is the encompassing pipe of the cable; the pipe interfaces at both ends and each shielding interface are grounded through a small resistor, which serves as the "grounding wire" for the grounding of the shielding layer at both ends of the cable. To facilitate adding faults to the cable, select the cables at both ends to be connected. BRKA is the fault switch, Rarc is the fault resistor, and Timed Bteaker logic open@t0 is the fault simulation control module, which is responsible for controlling the fault switch.

[0073] According to the above model, four cases are demonstrated in a specific embodiment. Through these cases, the medium voltage cable early fault identification method based on shielding layer current proposed in this application is empirically analyzed. Each case records in detail the specific section where the fault occurred, the fault distance and the duration of the fault, providing basic data for further analysis. The fault simulation schemes for each case are shown in Table 1.

[0074] Table 1 Fault simulation schemes for each case

[0075] Case numbers 1 and 2 record two fault events that occurred in the same section (section 2) of the same cable feeder (cable feeder 1). The fault distances of both events were 6 kilometers, but there were differences in the duration of the faults, which were 0.02 seconds and 0.10 seconds respectively. The comparison of these two cases shows the ability of the method proposed in this application to distinguish between early faults and permanent faults. Case number 3 records a fault that occurred in another section (section 3) of cable feeder 1, with a fault distance of 5 kilometers and a duration of 0.01 seconds. This case demonstrates the ability to identify early faults of different fault distances and durations in different sections. The fault involved in case number 4 occurred on a different cable feeder (cable feeder 4), with a fault distance of 7 kilometers and a duration of 0.04 seconds. Compared with the first three cases, this case demonstrates the ability of the method proposed in this application to identify early faults of different feeders.

[0076] Table 2 shows the fault identification results of different cases. From Table 2, it can be seen that the identification results of the medium voltage cable early fault identification method based on shield layer current proposed in this application are all correct results in four typical cases.

[0077] Table 2 Fault identification results of different cases

[0078] Figures 8 to 10 The distribution of the shielding layer grounding current extraction features of each section using the algorithm proposed in this chapter in Case 1-Case 4 is shown, including the distribution of fault duration features, periodic effective value features and cross-correlation features, and the fault section is marked with a box. Figure 8 The characteristic distribution of the fault duration of the shielding layer grounding current in each section clearly shows that the fault duration characteristics of Case 1, Case 3 and Case 4 are all around 50, while the characteristic value of Case 2 is over 100. The characteristic value distribution is consistent with the fault setting, because only the fault type of Case 2 is a permanent fault, and the fault types of the other three sections are all early faults. Fig. 9From the distribution of periodic effective value characteristics, it can be seen that the effective value characteristics of the fault section and the upstream section are much greater than those of other sections, so it is relatively easy to distinguish other sections from the upstream and fault sections. Fig.10 From the distribution of the cross-correlation characteristics, it can be clearly seen that only the cross-correlation characteristic value of the upstream section of the fault is negative, while the characteristic values ​​of the fault section and other sections are all positive. Through the analysis of the three characteristic distributions, it can be concluded that the effectiveness and feasibility of the medium-voltage cable early fault identification method based on shield layer current proposed in this application can be obtained.

[0079] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for early fault identification of medium voltage cables based on shielding layer current, characterized in that: The following steps are involved: S1, obtaining monitoring data of the grounding current of the terminal shielding layer; S2, calculate the effective value and instantaneous value of the terminal shielding layer grounding current according to the data obtained in S1, and establish the trigger detection criterion; S3. Detect whether a fault warning is triggered by triggering a detection criterion. If a fault warning is triggered, start the fault recording to obtain the fault waveform; S4, calculating the fault duration and the number of fault continuous sampling points according to the fault waveform obtained in S3, and judging whether the fault is an early fault according to the fault duration and the number of fault continuous sampling points; S5, by comparing the effective value amplitude of the terminal shielding layer grounding current with the specific threshold, determining whether the early fault section is a fault downstream section or a non-main fault section; S6. If the fault section is not a fault downstream section or a non-main fault section, a cross-correlation function between the terminal shield layer current and the zero-sequence current is calculated, and an optimal delay between the terminal shield layer current and the zero-sequence current is determined; S7. Distinguish the fault section and the fault upstream section according to the optimal delay obtained in S6.

2. The method for early fault identification of medium voltage cables based on shielding layer current according to claim 1 is characterized in that: The calculation formula of the effective value of the terminal shielding layer grounding current is: in, is the total number of sampling points of the half-wave detection signal, For segment The terminal shielding layer grounding current sampling signal, is the sampling sequence, is the current sampling point; Trigger detection criterion 1: The effective value of the grounding current of the terminal shield layer exceeds the limit, that is: in, is the trigger threshold of the effective value amplitude of the terminal shielding layer grounding current, , is the rated current effective value, is the rated current effective value coefficient; Trigger detection criterion 2: The effective value of the grounding current of the terminal shielding layer exceeds the limit, that is: in, The trigger threshold of the effective value mutation of the terminal shielding layer grounding current, , is the rated current effective value coefficient.

3. The method for early fault identification of medium voltage cables based on shielding layer current according to claim 2 is characterized in that: The calculation formula for the instantaneous value mutation of the terminal shielding layer grounding current is: in, is the number of sampling points in one cycle of the signal, It is the terminal shielding layer grounding current sampling signal; Trigger detection criterion 3: The instantaneous value of the single-end grounding current of the shield layer exceeds the limit, that is: in, is the trigger threshold of the instantaneous value mutation of the terminal shielding layer grounding current, , is the rated current effective value coefficient.

4. The method for early fault identification of medium voltage cables based on shielding layer current according to claim 3 is characterized in that: The S3 comprises the following steps: When the grounding current of the terminal shielding layer meets any one of the trigger detection criteria 1, trigger detection criteria 2 and trigger detection criteria 3, a fault warning is triggered, and the fault recording is started at this time to obtain the fault waveform.

5. The method for identifying early faults of medium voltage cables based on shielding layer current according to claim 4 is characterized in that: The S4 comprises the following steps: The fault initial sampling point judgment criterion is: in, is the fault starting sampling point, is the effective value of the shield layer grounding current, the fault starting sampling point The corresponding time is recorded as the fault start time ; The fault end sampling point judgment criteria are: in, is the fault end sampling point, the fault end sampling point The corresponding time is recorded as the fault end time ; The fault duration is: The number of fault continuous sampling points is: The early fault identification criteria are: When the number of sampling points of the shield layer grounding current during the fault duration is less than the fault identification threshold, it indicates that the fault is an early fault, that is: in, is the fault identification threshold, and , is the sampling frequency of the monitoring device, is the base frequency.

6. The method for early fault identification of medium voltage cables based on shielding layer current according to claim 5 is characterized in that: The S5 comprises the following steps: The shielding layer grounding current amplitude of the fault downstream section and the non-trunk fault section is much smaller than the shielding layer grounding current amplitude of the fault section and the fault upstream section. By setting a specific threshold, when the shielding layer grounding current effective value amplitude is less than the specific threshold, it indicates that the fault section is the fault downstream section or the non-trunk fault section, that is: in, is the segment differentiation threshold based on the effective value amplitude of the shield layer grounding current, , for The half-wave effective value of the zero-sequence current in the section, is the zero-sequence current effective value coefficient.

7. The method for identifying early faults of medium voltage cables based on shielding layer current according to claim 6 is characterized in that: The cross-correlation function is as follows: in, Indicates the zero sequence current of this section and the terminal shield ground current In Delay The cross-correlation value at for After delay The value of when When the maximum value is taken, the corresponding That is the optimal delay between the terminal shield current and the zero-sequence current.

8. The method for identifying early faults of medium voltage cables based on shielding layer current according to claim 7 is characterized in that: The S7 comprises the following steps: If the optimal delay between the terminal shield current and the zero-sequence current satisfies the following equation: Then the early fault section is the fault section, otherwise it is the fault upstream section; in, is a threshold for distinguishing sections based on the cross-correlation between the terminal shield layer current and the zero-sequence current, and , is the sampling frequency of the monitoring device, is the base frequency.

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