A method for early fault identification of medium-voltage cables based on shield layer current

Abstract

The present invention discloses a method for early fault identification of medium-voltage cables based on the current of the shielding layer, comprising the following steps: S1, obtaining the monitoring data of the grounding current of the end shielding layer; S2, calculating the effective value and the instantaneous value of the grounding current of the end shielding layer; S3, detecting whether a fault warning is triggered, and if so, obtaining the fault waveform; S4, calculating the fault duration and the number of fault continuous sampling points to determine whether the fault is an early fault; S5, judging whether the early fault section is the downstream section of the fault or the non-main trunk fault section; S6, calculating the cross-correlation function between the current of the end shielding layer and the zero-sequence current; S7, distinguishing the fault section and the upstream section of the fault according to the optimal delay. The present invention improves the accuracy and efficiency of early fault identification of medium-voltage cables, and promotes the transformation of protection from "post-fault protection" to "pre-warning".

Description

Technical Field

[0001] The present invention relates to the technical field of power supply reliability, and particularly to a method for early fault identification of medium-voltage cables based on shield current. Background Art

[0002] Due to characteristics such as large transmission capacity, small floor area, and less maintenance work, medium-voltage cables are widely used in high-reliability power transmission and distribution sites such as cities, mines, and offshore wind farms. A large number of measured data show that early faults of medium-voltage cables are manifested as repetitive transient power disturbances (kHz level) before permanent faults in electrical characteristics. Although the disturbance amplitude is small and the duration is short, and it will not trigger protection actions, such disturbances can be recorded by devices with waveform monitoring capabilities in the distribution network.

[0003] Currently, efficient identification of early fault states can be achieved through monitoring and analysis based on power disturbances. Power disturbances contain a large amount of early fault state information and can propagate over long distances along the cable line. And with the continuous development of distribution network informatization, there are already 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 the monitoring of power disturbances (kHz), providing important data support for the state perception of medium-voltage cables based on power disturbance monitoring, which is of great significance for practical applications.

[0004] The monitoring of cable abnormal states mainly relies on the understanding of limited characteristic quantities, including dielectric loss, insulation resistance, partial discharge, temperature, etc. These characteristic quantities can all reflect the abnormal states of cables to a certain extent. However, the existing research on the whole process of the development of cable abnormal states does not cover the entire life cycle of the cable, and the characteristic quantities cannot cover the whole process of cable abnormal states. More importantly, medium-voltage cables are usually three-core cables, and the shielding layers of the three-phase cables are directly grounded at both ends of the cable, increasing the difficulty of monitoring cable abnormal states because it leads to problems such as complex shielding layer current distribution, multi-path current interference, and electromagnetic interference, making it more difficult to identify and locate fault signals. Summary of the Invention

[0005] In order to solve the problem of difficult fault identification and location of medium-voltage cables, the present invention proposes a method for early fault identification of medium-voltage cables based on shield current to solve the above problems.

[0006] The present application discloses a method for early fault identification of medium-voltage cables based on shield current, including the following steps:

[0007] S1. Obtain the monitoring data of the grounding current of the end shield layer;

[0008] S2. Calculate the effective value and instantaneous value of the grounding current of the end shield layer according to the data obtained in S1, and establish a trigger detection criterion;

[0009] S3. Detect whether a fault warning is triggered by triggering a detection criterion. If a fault warning is triggered, start fault recording and obtain the fault waveform;

[0010] S4. Calculate the fault duration and the number of fault continuous sampling points based on 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 continuous sampling points;

[0011] S5. Determine whether the early fault section is a downstream fault section or a non-main trunk fault section by comparing the magnitude of the effective value of the grounding current of the end shielding layer with a specific threshold;

[0012] S6. If the fault section is not a downstream fault section or a non-main trunk fault section, calculate the cross-correlation function between the end shielding layer current and the zero-sequence current, and determine the optimal delay between the end shielding layer current and the zero-sequence current;

[0013] S7. Distinguish the fault section and the upstream fault section according to the optimal delay obtained in S6.

[0014] Preferably, the calculation formula for the effective value of the grounding current of the end shielding layer is:

[0015]

[0016] Wherein, is the total number of sampling points of the half-wave detection signal, is the grounding current sampling signal of the end shielding layer in section is the sampling sequence, is the current sampling point;

[0017]

[0017] Trigger detection criterion 1:

[0018] The magnitude of the effective value of the grounding current of the end shielding layer exceeds the limit, that is:

[0019]

[0020] Wherein, is the triggering threshold of the magnitude of the effective value of the grounding current of the end shielding layer, , is the effective value of the rated current, is the rated current effective value coefficient;

[0021] Trigger detection criterion 2:

[0022] The mutation amount of the effective value of the grounding current of the end shielding layer exceeds the limit, that is:

[0023]

[0024] Wherein, is the trigger threshold of the sudden change in the effective value of the grounding current of the terminal shielding layer, , is the coefficient of the effective value of the rated current.

[0025] Preferably, the calculation formula for the sudden change in the instantaneous value of the grounding current of the terminal shielding layer is:

[0026]

[0027] wherein, is the number of sampling points in one cycle of the signal, is the sampling signal of the grounding current of the terminal shielding layer;

[0028] Trigger detection criterion 3:

[0029] The sudden change in the instantaneous value of the single - end grounding current of the shielding layer exceeds the limit, that is:

[0030]

[0031] wherein, is the trigger threshold of the sudden change in the instantaneous value of the grounding current of the terminal shielding layer, , is the coefficient of the effective value of the rated current.

[0032] Preferably, S3 includes the following steps:

[0033] When the grounding current of the terminal shielding layer meets any one of trigger detection criterion 1, trigger detection criterion 2, and trigger detection criterion 3, a fault warning is triggered, and at this time, fault recording is started to obtain the fault waveform.

[0034] Preferably, S4 includes the following steps:

[0035] The criterion for the fault starting sampling point is:

[0036]

[0037] wherein, is the fault starting sampling point, is the effective value of the grounding current of the shielding layer, and the moment corresponding to the fault starting sampling point is recorded as the fault starting moment ;

[0038] The criterion for the fault ending sampling point is:

[0039]

[0040] wherein, is the fault ending sampling point, and the moment corresponding to the fault ending sampling point is recorded as the fault ending moment ;

[0041] The fault duration is:

[0042]

[0043] The number of sampling points for the continuous fault is:

[0044]

[0045] The early fault identification criterion is:

[0046] When the number of sampling points within the fault duration of the shielding layer grounding current is less than the fault identification threshold, it indicates that the fault is an early fault, that is:

[0047]

[0048] Wherein, is the fault identification threshold, and , is the sampling frequency of the monitoring device, is the fundamental frequency.

[0049] Preferably, the S5 includes the following steps:

[0050] The magnitudes of the shielding layer grounding currents in the downstream section of the fault and the non-main fault section are much smaller than those in the fault section and the upstream section of the fault. By setting a specific threshold, when the effective value magnitude of the shielding layer grounding current is less than this specific threshold, it indicates that the fault section is the downstream section of the fault or the non-main fault section, that is:

[0051]

[0052] Wherein, is the section discrimination threshold based on the effective value magnitude of the shielding layer grounding current, , is the half-wave effective value of the zero-sequence current of the section, is the zero-sequence current effective value coefficient.

[0053] Preferably, the cross-correlation function is as follows:

[0054]

[0055] Wherein, represents the zero-sequence current of this section and the grounding current of the end shielding layer at the delay is the cross-correlation value, is after the delay value;

[0056] When takes the maximum value, the corresponding is the optimal delay between the end shielding layer current and the zero-sequence current.

[0057] Preferably, the S7 includes the following steps:

[0058] If the optimal delay between the end shielding layer current and the zero-sequence current satisfies the following formula:

[0059]

[0060] then this early fault section is the fault section, otherwise it is the fault upstream section;

[0061] wherein, is the cross-correlation section discrimination threshold based on the end shielding layer current and the zero-sequence current, and , is the sampling frequency of the monitoring device, is the fundamental frequency.

[0062] Advantages of the present invention:

[0063] (1) Based on the characteristic analysis of the fault waveform of the shielding layer grounding current, the present invention proposes a trigger detection method for the effective value or sudden change of the amplitude of the end shielding layer grounding current, and further proposes an early fault identification method based on the signal characteristics of the end shielding layer grounding current, improving the accuracy and efficiency of early fault identification of medium-voltage cables.

[0064] (2) Through the implementation of the present invention, a technical route for "active protection" of medium-voltage cables will be formed, providing important theoretical and practical support for promoting the transformation of protection from "post-fault protection" to "pre-warning", and for 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. Description of the Drawings

[0065] Figure 1 is the flowchart of the method for early fault identification of medium-voltage cables based on shielding layer current according to the embodiment of the present invention;

[0066] Figure 2 is the schematic diagram of the fault current path according to the embodiment of the present invention;

[0067] Figure 3 is the half-cycle effective value of the shielding layer grounding current in the case of early faults of half-cycle and multi-cycle according to the embodiment of the present invention;

[0068] Figure 4 is the schematic diagram of the detection of the start and end times of early faults according to the embodiment of the present invention;

[0069] Figure 5 Schematic diagram of the grounding current direction characteristics at the head and end of different sections in the embodiment of the present invention;

[0070] Figure 6 Topological schematic diagram of the 10kV distribution network early fault simulation model in the embodiment of the present invention;

[0071] Figure 7 Schematic diagram of the fault simulation module for the independent shielded three-core cable in the embodiment of the present invention;

[0072] Figure 8 Distribution diagram of the fault duration characteristics of the grounding current of the shield layer in each section of Case 1 - Case 4 in the embodiment of the present invention;

[0073] Figure 9 Distribution diagram of the periodic effective value characteristics of the grounding current of the shield layer in each section of Case 1 - Case 4 in the embodiment of the present invention;

[0074] Figure 10 Distribution diagram of the cross-correlation characteristics of the grounding current of the shield layer in each section of Case 1 - Case 4 in the embodiment of the present invention. Detailed implementation manners

[0075] To make the objectives, technical solutions and advantages of the present application clearer and more understandable, the following gives examples with reference to the accompanying drawings and further elaborates on the present application in detail.

[0076] The embodiment of the present application discloses a method for identifying early faults of medium-voltage cables based on shield layer current, as Figure 1 shown, including the following steps:

[0077] S1. Obtain the monitoring data of the grounding current of the end shield layer through a recording device such as a high-frequency current transformer.

[0078] In the embodiment of the present application, the identification of the fault section is achieved only through the grounding current of the single-end shield layer. Therefore, it becomes a problem to be solved whether to select the grounding current of the head or the end shield layer as the identification object.

[0079] As Figure 2The figure shows a schematic diagram of the fault current path considering the cable structure and the grounding of the shielding layer. Here, the direction in which the current in the shielding layer flows from the cable to the ground is defined as the positive direction. When a fault occurs, the three-phase currents are no longer balanced, and the fault current has the same direction as the zero-sequence current. In the upstream section of the fault, the current in the shielding layer is mainly the induced current of the fault current, and its magnitude is much larger than the equivalent capacitance current. Therefore, the direction of the comprehensive current in the shielding layer is the same as that of 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 tail end. In the fault section, the fault breakdown current flows from the cable conductor to the ground through the shielding layer. Therefore, the current directions at both ends of the shielding layer are from the cable to the ground, and both the head and tail ends are in the positive direction. In the downstream section of the fault and the non-fault section, the current in the shielding layer is the equivalent capacitance current. Therefore, the current directions at both ends of the shielding layer are also from the cable to the ground, and both the head and tail ends are in the positive direction. Moreover, the magnitude of the equivalent capacitance current is also very small compared to the fault current.

[0080] Through the comparative analysis of the grounding currents of the shielding layer at both ends, it is found that the grounding current at the tail end of the shielding layer has a directional characteristic. The current direction of the grounding current at the head end of the shielding layer is the positive direction whether in the healthy section or the fault section, while the current direction of the grounding current at the tail end of the shielding layer is the negative direction only in the upstream section of the fault, and the current directions in other sections are the positive direction. Therefore, in the embodiments of the present application, the grounding current at the tail end is selected as the input of the identification signal.

[0081] S2. Calculate the effective value and instantaneous value of the grounding current at the tail end of the shielding layer according to the data obtained in S1, and establish a trigger detection criterion.

[0082] Under normal operating conditions, the current in the shielding layer is almost zero. When an early fault occurs in the cable, the cable core will have an instantaneous recoverable breakdown with the steel armor layer of the shielding layer, 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, the present application detects early faults through two trigger algorithms of the effective value and the instantaneous value.

[0083] Trigger detection algorithm based on the effective value of the grounding current of the shielding layer:

[0084] The formula for calculating the effective value of the grounding current at the tail end of the shielding layer is:

[0085]

[0086] Among them, is the total number of sampling points of the half-wave detection signal, is the section sampling signal of the grounding current at the tail end of the shielding layer, is the sampling sequence, is the current sampling point. The sliding step for calculating the effective value is calculated point by point.

[0087] When the cable is operating normally, the grounding current at one end of the shielding layer is very small, and the effective value per cycle is very small. When an early fault occurs in the cable, the fault current flows through the shielding layer to the grounding point at the fault location. Due to the sudden change in current, three-phase imbalance will occur in the cable downstream of the fault section, but this three-phase imbalance is not very large. What is reflected in the shielding layer grounding current is the capacitive current caused by the three-phase imbalance. At this time, the effective value of the single-end grounding current of the shielding layer will increase. As Figure 3 shown, where Figure 3 Figure (a) is a schematic diagram of the half-cycle effective value of the shielding layer grounding current for a half-cycle early fault, Figure 3 and figure (b) is the half-cycle effective value of the shielding layer grounding current in the case of a multi-cycle early fault. When an early fault occurs, the effective value of the single-end grounding current of the shielding layer increases within this cycle, and the effective values of the current in the fault section and the upstream of the fault section are significantly higher than those in the downstream section. When it is detected that the single-end grounding current of the shielding layer increases, an early warning will be triggered and the waveform will be recorded, indicating that a fault has occurred. During this period, the calculation and recording of the effective value of the current for 1 / 2 cycle will be carried out.

[0088] Trigger detection criterion 1:

[0089] The amplitude of the effective value of the grounding current at the end of the shielding layer exceeds the limit, that is:

[0090]

[0091] where is the trigger threshold for the amplitude of the effective value of the grounding current at the end of the shielding layer, , is the effective value of the rated current, is the coefficient of the effective value of the rated current. In this embodiment, is taken as 1.05.

[0092] Trigger detection criterion 2:

[0093] The sudden change in the effective value of the grounding current at the end of the shielding layer exceeds the limit, that is:

[0094]

[0095] where is the trigger threshold for the sudden change in the effective value of the grounding current at the end of the shielding layer, , is the coefficient of the effective value of the rated current. In this embodiment, is taken as 0.5.

[0096] Trigger detection algorithm based on the instantaneous value of the shielding layer grounding current:

[0097] The difference between the sampling points corresponding to the adjacent cycles of the shielding layer grounding current is the instantaneous value mutation amount. The calculation formula for the instantaneous value mutation amount of the grounding current of the terminal shielding layer is:

[0098]

[0099] Among them, is the number of sampling points in one cycle of the signal, is the sampling signal of the grounding current of the terminal shielding layer.

[0100] Trigger detection criterion 3:

[0101] The instantaneous value mutation amount of the single-end grounding current of the shielding layer exceeds the limit, that is:

[0102]

[0103] Among them, is the trigger threshold of the instantaneous value mutation amount of the grounding current of the terminal shielding layer, , is the effective current value coefficient of the rated current. In this embodiment takes the value of 0.3.

[0104] S3. Detect whether to trigger a fault warning through the trigger detection criterion. When the grounding current of the terminal shielding layer meets any one of the trigger detection criterion 1, trigger detection criterion 2, and trigger detection criterion 3, trigger a fault warning, and at this time, start the fault recording to obtain the fault waveform.

[0105] S4. Calculate the fault duration and the number of fault continuous sampling points according to the fault waveform obtained in S3, and judge whether the fault is an early fault according to the fault duration and the number of fault continuous sampling points.

[0106] Since early faults are usually single-phase breakdown faults of 1 / 4 to 4 cycles, by calculating the entire fault duration, and then using a threshold to judge 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 operating state will return to normal. Therefore, in addition to the starting time of the fault, there must be a fault end moment.

[0107] The criterion for the fault starting sampling point is:

[0108]

[0109] Among them, is the fault starting sampling point, is the effective value of the shielding layer grounding current. The moment corresponding to the fault starting sampling point is recorded as the fault starting moment ;

[0110] The criterion for the fault ending sampling point is:

[0111]

[0112] Among them, is the fault end sampling point, and the moment corresponding to the fault end sampling point is recorded as the fault end time . The schematic diagram of the detection of the start and end times of the early fault is as Figure 4 shown.

[0113] The fault duration is:

[0114]

[0115] The number of fault continuous sampling points is:

[0116]

[0117] The early fault identification criterion is:

[0118] When the number of sampling points within the fault duration of the shield layer grounding current is less than the fault identification threshold, it indicates that the fault is an early fault, that is:

[0119]

[0120] Otherwise, it indicates that the fault is a permanent breakdown fault.

[0121] Among them, is the fault identification threshold. The early fault duration is within 1 / 4 - 4 cycles. In this implementation, is set as the number of sampling points within 4 cycles, that is:

[0122]

[0123] is the sampling frequency of the monitoring device, is the fundamental frequency.

[0124] S5. By comparing the effective value amplitude of the shield layer grounding current at the end with the size of a specific threshold, determine whether the early fault section is the downstream section of the fault or a non-main trunk fault section.

[0125] It can be known from Figure 5 that in terms of amplitude, the shield layer grounding current in the downstream section of the fault is much smaller than that in the fault section and the upstream section of the fault. Therefore, the downstream section of the fault can be distinguished from other sections through this feature. By setting a specific threshold, when the effective value amplitude of the shield layer grounding current is less than the specific threshold, it indicates that the fault section is the downstream section of the fault or a non-main trunk fault section, that is:

[0126]

[0127] Among them, is the section discrimination threshold based on the effective value amplitude of the shielding layer grounding current, , is the half-wave effective value of the zero-sequence current in the section, is the zero-sequence current effective value coefficient. In this embodiment, takes a value of 8%.

[0128] S6. If the faulty section is not the downstream section of the fault or the non-main trunk fault section, then calculate the cross-correlation function between the end shielding layer current and the zero-sequence current, and determine the optimal delay between the end shielding layer current and the zero-sequence current.

[0129] As Figure 5 can be seen, in the current direction, the direction of the end shielding layer grounding current of the faulty section is basically the same as that of the zero-sequence current, while the direction of the end current of the upstream section of the fault is opposite to the direction of the zero-sequence current. This characteristic will be reflected in the phase of the fault grounding current waveform. Therefore, this characteristic can be used to distinguish the upstream section of the fault from the faulty section. The premise of the identification method proposed in the embodiment of this application is that communication is not possible between sections, and fault identification can only be completed on the local ring main unit or waveform monitoring device. Therefore, a same reference object needs to be found to determine the direction characteristic of the end shielding layer grounding current. In this embodiment, the zero-sequence current of this section of current is used as the direction reference object. When an early fault occurs, the direction of the end shielding layer grounding current of the faulty section is almost the same as that of the zero-sequence current, that is, the phase difference between the two waveforms is 0; while the direction of the end shielding layer grounding current of the upstream section of the fault is almost opposite to that of the zero-sequence current, that is, the phase difference between the two waveforms is π.

[0130] By analyzing the cross-correlation between the end current of the shielding layer and the zero-sequence current, the time relationship and phase difference between them can be quantified, and then the relative position and possible direction of the fault can be inferred. If the cross-correlation of 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 synchronous changes in these two currents, and thus can be used to determine the fault direction. Calculate the cross-correlation function (CCF) between the two signals:

[0131]

[0132] Among them, represents the zero-sequence current of this section and the end shielding layer grounding current at a delay of , is the value of after a delay of . When takes the maximum value, the corresponding Namely, it is the optimal delay between the current of the terminal shielding layer and the zero-sequence current.

[0133] S7. Distinguish the fault section and the upstream fault section according to the optimal delay obtained in S6.

[0134] Set a reasonable threshold to distinguish the fault section and the upstream fault section. If the optimal delay between the current of the terminal shielding layer and the zero-sequence current satisfies the following formula:

[0135]

[0136] Then this early fault section is the fault section, otherwise it is the upstream fault section.

[0137] Wherein, is the threshold for distinguishing sections based on the cross-correlation between the current of the terminal shielding layer and the zero-sequence current. In this embodiment, the threshold for distinguishing sections based on the cross-correlation between the current of the terminal shielding layer and the zero-sequence current is:

[0138]

[0139] is the sampling frequency of the monitoring device, is the fundamental frequency.

[0140] In a specific embodiment, in order to simulate and verify the method for early fault identification of medium-voltage cables based on the shielding layer current proposed in the above embodiment, a simulation model of early faults of a 10 kV three-core armored cable in a distribution network was established in the PSCAD / EMTDC software, as Figure 6 shown. An 110 kV power supply was selected, with a frequency of 50 Hz. The transformer was connected in Y / Y, with a turns ratio of 110 kV / 10.5 kV and a rated capacity of 45 MV·A. The neutral point was grounded through a small resistor with a resistance value of 6 Ω. According to the data requirements of the method proposed in the embodiment of the present application, measuring points were arranged at the ends of the grounding currents of the shielding layers in each cable section. There are 5 feeders on the 10 kV bus. Feeder 1 is relatively complex, with 4 sections, and the other 4 feeders have the same parameters except for different lengths and load sizes.

[0141] Figure 7In it, due to the fact that the cables in PSCAD do 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 respectively; Encompassing Pipe is the containing pipe of the cable; the two ends of the Pipe interface and each shielding interface are grounded through a small resistor, serving as the "ground wire" for the grounding of the shielding layers at both ends of the cable. To facilitate adding faults to the cable, the two ends of the cable are selected 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.

[0142] Based on the above model, in a specific embodiment, four cases are presented. Through these cases, an empirical analysis is conducted on the method for early fault identification of medium-voltage cables based on shielding layer current proposed in this application. Each case details the specific section where the fault occurs, 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.

[0143] Table 1 Fault Simulation Schemes for Each Case

[0144]

[0145] 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 these two events are both 6 kilometers, but the durations of the faults are different, 0.02 seconds and 0.10 seconds respectively. The comparison between 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. The fault distance is 5 kilometers and the duration is 0.01 seconds. This case is used to demonstrate the ability to identify early faults with different fault distances and different durations in different sections. Case number 4 involves a fault that 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 on different feeders.

[0146] Table 2 shows the fault identification results of different cases. It can be seen from Table 2 that the method for early fault identification of medium-voltage cables based on shielding layer current proposed in this application has correct identification results in the four typical cases.

[0147] Table 2 Fault Identification Results of Different Cases

[0148]

[0149] Figures 8 to 10 shows the distribution of the extracted features of the shielding layer grounding current in each section for Cases 1 - 4 by the algorithm proposed in this chapter, including the distribution of the fault duration feature, the periodic effective value feature, and the cross - correlation feature, and marks the fault sections with boxes. First, analyze Figure 8 the distribution of the fault duration feature of the shielding layer grounding current in each section. It can be clearly seen that the fault duration features of Cases 1, 3, and 4 are all about 50, while the feature value of Case 2 reaches more than 100. The distribution of this feature value 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 early faults. Secondly, analyze Figure 9 the distribution of the periodic effective value feature. It can be seen that the effective value features of the fault section and the upstream section are much larger than those of other sections. Therefore, it is relatively easy to distinguish other sections from the upstream and fault sections. Then, analyze Figure 10 the distribution of the cross - correlation feature. It can be clearly seen that only the cross - correlation feature value of the section upstream of the fault is negative, while the feature values of the fault section and other sections are positive. Through the analysis of the distribution of the three features, the effectiveness and feasibility of the proposed method for early fault identification of medium - voltage cables based on shielding layer current in this application can be obtained.

[0150] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above - mentioned embodiments. What is described in the above - mentioned embodiments and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for early fault identification of medium-voltage cables based on the current in the shielding layer, characterized in that, It includes the following steps: S1. Obtain the monitoring data of the grounding current of the terminal shielding layer; S2. Calculate the effective value and instantaneous value of the grounding current of the terminal shielding layer according to the data obtained in S1, and establish a trigger detection criterion; S3. Detect whether a fault warning is triggered through the trigger detection criterion. If a fault warning is triggered, start the fault recording wave to obtain the fault waveform; S4. Calculate the fault duration and the number of sampling points for the fault duration according to the fault waveform obtained in S3, and judge whether the fault is an early fault based on the fault duration and the number of sampling points for the fault duration; S5. Judge whether the early fault section is the fault downstream section or the non-main fault section by comparing the magnitude of the effective value of the grounding current of the terminal shielding layer with a specific threshold; S6. If the fault section is not the fault downstream section or the non-main fault section, calculate the cross-correlation function between the terminal shielding layer current and the zero-sequence current, and determine the optimal delay between the terminal shielding layer current and the zero-sequence current; 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 the current of the shielding layer according to claim 1, wherein The formula for calculating the effective value of the grounding current of the terminal shielding layer is: Among them, is the total number of sampling points of the half-wave detection signal, is the section end shielding layer grounding current sampling signal, is the sampling sequence, is the current sampling point; Trigger detection criterion 1: The magnitude of the effective value of the grounding current of the terminal shielding layer exceeds the limit, that is: Among them, is the triggered threshold value of the effective value amplitude of the end shielding layer grounding current, , is the effective value of the rated current, is the rated current effective value coefficient; Trigger detection criterion 2: The sudden change of the effective value of the grounding current of the terminal shielding layer exceeds the limit, that is: Among them, is the trigger threshold of the sudden change of the effective value of the end shielding layer grounding current, , is the rated current effective value coefficient.

3. The method for early fault identification of medium-voltage cables based on shield layer current according to claim 2, wherein, The formula for calculating the sudden change of the instantaneous value of the grounding current of the terminal shielding layer is: Among them, is the number of sampling points in one period of the signal, is the sampling signal of the grounding current of the terminal shielding layer; Trigger detection criterion 3: The sudden change of the instantaneous value of the single-ended grounding current of the shielding layer exceeds the limit, that is: wherein, is the triggering threshold of the instantaneous change amount of the grounding current of the end shielding layer, , is the effective value coefficient of the rated current.

4. The method for early fault identification of medium-voltage cables based on the current of the shielding layer according to claim 3, wherein S3 includes the following steps: When the grounding current of the terminal shielding layer meets any one of trigger detection criterion 1, trigger detection criterion 2, and trigger detection criterion 3, trigger a fault warning, and at this time start the fault recording wave to obtain the fault waveform.

5. The method for early fault identification of medium-voltage cables based on the current of the shielding layer according to claim 4, wherein, S4 includes the following steps: The criterion for the starting sampling point of the fault is: Among them, is the starting sampling point of the fault, is the effective value of the shielding layer grounding current, and the moment corresponding to the starting sampling point of the fault is recorded as the starting time of the fault ; The criterion for the ending sampling point of the fault is: Among them, is the fault end sampling point, and the fault end sampling point corresponds to the moment denoted as the fault end moment ; The fault duration is: The number of sampling points for the fault duration is: The criterion for early fault identification is: When the number of sampling points within the fault duration of the grounding current of the shielding layer is less than the fault identification threshold, it indicates that the fault is an early fault, that is: Among them, is the fault recognition threshold, and , is the sampling frequency of the monitoring device, is the fundamental frequency.

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

7. The method for early fault identification of medium voltage cables based on the current of the shielding layer according to claim 6, wherein The cross-correlation function is as follows: Among them, represents the zero-sequence current of this section and the grounding current of the terminal shielding layer at the cross-correlation value at the delay time, is the value after the delay time; When reaches the maximum value, the corresponding is the optimal delay between the end shielding layer current and the zero-sequence current.

8. The method for early fault identification of medium-voltage cables based on the current of the shielding layer according to claim 7, characterized in that, S7 includes the following steps: If the optimal delay between the terminal shielding layer current and the zero-sequence current satisfies the following formula: Then this early fault section is the fault section, otherwise it is the fault upstream section; Among them, is the cross-correlation section discrimination threshold based on the end shielding layer current and the zero-sequence current, and , is the sampling frequency of the monitoring device, is the fundamental frequency.

Images