A distribution network cable joint early fault detection system and method

Abstract

The present invention discloses a distribution network cable joint early fault detection system and method, belonging to the field of cable fault detection technology. The system includes: an early fault identification module for the inner surface of the joint and an early fault identification module for radial breakdown. The early fault identification module along the inner surface of the cable joint includes: a timing threshold detection unit, a waveform continuity detection unit, a harmonic sampling unit, a harmonic content comparison unit, and a first repeatability detection unit; the early fault identification module for radial breakdown includes: a current amplitude detection unit, a rate threshold detection unit, a duration detection unit, a start and end point detection unit, and a second repeatability detection unit. The present invention can provide technical support for the status inspection and fault warning of distribution network cable joints.

Description

Technical Field

[0001] The present invention relates to the technical field of cable fault detection, and more particularly to a distribution network cable joint early fault detection system and method. Background Art

[0002] Permanent failure of medium-voltage cable joints seriously affects the reliability of distribution networks. The radial breakdown caused by water ingress has several self-extinguishing arc discharges before forming a permanent failure, which is an early failure.

[0003] As a key connection component in distribution networks, the reliability of medium-voltage cable joints directly impacts the stability of the power supply system. However, water ingress into joints due to insulation aging, seal failure, or construction defects is a frequent problem during long-term operation. This water intrusion easily triggers partial discharge under the action of the electric field, gradually developing into an early-stage radial breakdown failure. This type of fault is typically accompanied by multiple short-cycle self-extinguishing arc discharges (single duration <10 ms) before forming a permanent short circuit. While the discharge energy is insufficient to trigger traditional protective devices, it continuously degrades the insulation material, ultimately causing permanent failure and widespread power outages. According to statistics, approximately 70% of power outages caused by cable joint failures in distribution networks are closely related to radial breakdown caused by water ingress, and the repair costs are high, with the average outage duration exceeding six hours.

[0004] Current monitoring methods for early-stage cable joint failures have the following limitations:

[0005] Insufficient detection sensitivity: Traditional overcurrent protection devices rely on steady-state current thresholds and cannot capture short-cycle discharge signals in the millisecond range.

[0006] High interference misjudgment rate: The waveform characteristics of high-frequency interference signals such as capacitor switching and resonant current in the line are similar to those of early faults, making it difficult for existing methods to effectively distinguish them.

[0007] Lack of fault feature research: There is a lack of quantitative models for the dynamic evolution of radial breakdown faults (such as the correlation between the arc start / end point and the voltage phase, and the change in harmonic energy distribution), resulting in low feature extraction accuracy;

[0008] Poor environmental adaptability: Underground cable joints are significantly affected by humidity and temperature fluctuations. Existing detection algorithms do not fully consider the randomness of interference signals and the cumulative effects of repetitive faults.

[0009] Furthermore, existing research has focused on monitoring creepage faults along the inner surface of cable connectors. These faults manifest as small current discharges lasting several seconds, with harmonic characteristics significantly different from interference signals. However, radial breakdown faults, due to their extremely short discharge duration, rapid current change rate (>10^3 A / s), and strong correlation with the voltage waveform phase, require the design of targeted, multi-dimensional criteria. Relying solely on a single parameter (such as current amplitude or harmonic content) can easily lead to missed or misjudgment due to environmental interference, failing to meet the high reliability requirements of distribution networks.

[0010] Therefore, how to provide a distribution network cable connector early fault detection system and method is a problem that those skilled in the art urgently need to solve. Summary of the Invention

[0011] In view of this, the present invention provides a distribution network cable connector early fault detection system and method to solve the technical problems existing in the above-mentioned prior art.

[0012] In order to achieve the above object, the present invention provides the following technical solutions:

[0013] A distribution network cable joint early fault detection system includes an early fault identification module along the inner surface of the cable joint and an early fault identification module for radial breakdown;

[0014] Among them, the early fault identification module along the inner surface of the cable joint includes: a timing threshold detection unit, a waveform continuity detection unit, a harmonic sampling unit, a harmonic content comparison unit and a first repeatability detection unit;

[0015] The radial breakdown early fault identification module includes: a current amplitude detection unit, a rate threshold detection unit, a duration detection unit, a start and end point detection unit and a second repeatability detection unit.

[0016] Furthermore, the timing threshold detection unit includes placing a detection device arranged on the grounding wire of the cable joint in a normally open state, starting recording when the amplitude of the grounding wire current signal is within a set threshold range, and recording the current signal in the initial stage.

[0017] Furthermore, the waveform continuity detection unit includes detecting the continuity of the current signal, wherein, during the signal acquisition process, if there is a continuous signal in the current signal and the peak point current amplitude of the continuous signal is always lower than the threshold, it is determined to be interference and the signal acquisition is stopped.

[0018] Furthermore, the harmonic sampling unit starts timing from the initial point of the signal that meets the timing threshold detection based on the signal output by the timing threshold detection unit and the waveform continuity detection unit, and further divides the recorded suspected fault signal in the middle of the signal. The division is at a first preset time point after the initial point. The value at the first preset time point is higher than the duration of the early fault start stage in the actual environment. After the division, the current signal with a time length of the second preset time point is recorded again as a waveform signal of the intermediate stage and output.

[0019] Furthermore, the harmonic content comparison unit introduces harmonic contribution as a supplementary criterion, and further calculates the harmonic contribution at the initial stage and the intermediate stage of the early fault based on the signal output from the harmonic sampling unit.

[0020] Furthermore, the current amplitude detection unit sets the detection device to be normally open, and starts recording when the ground wire current signal amplitude is within the threshold range, recording the current signal in the initial stage to distinguish short-cycle faults from other small / large pulse interference.

[0021] Furthermore, the rate threshold detection unit calculates the current change rate of adjacent sampling points based on the nonlinear change characteristics of the fault resistance. If the absolute value of the slope exceeds the slope corresponding to the 50 Hz sine wave constructed with the maximum current amplitude, it is determined that the rate threshold is met and output.

[0022] Furthermore, the start and end point detection unit sets the current amplitude to stop distorting for a period of time, and then sets the current amplitude change rate to drop sharply, and sets the first point showing non-conductive properties as the fault end point. The time between the fault start point and the fault end point is calculated, and whether the time is within the threshold range of the duration detection is evaluated and output.

[0023] Furthermore, the first repeatability detection unit and the second repeatability detection unit are respectively used to count the number of suspected faults of the same cable joint or multiple joints in the area. If the number exceeds a threshold within a cycle time, a high-level alarm is triggered to eliminate occasional interference.

[0024] A method for detecting early faults of distribution network cable joints, including identification of early faults along the inner surface of the cable joint and identification of early faults of radial breakdown;

[0025] Among them, the identification of early faults along the inner surface of the cable joint includes timing threshold detection, waveform continuity detection, harmonic sampling, harmonic content comparison and first repeatability detection;

[0026] The identification of early stage faults of radial breakdown includes: current amplitude detection, rate threshold detection, duration detection, start and end point detection and second repeatability detection.

[0027] It can be seen from the above technical solution that compared with the existing technology, the present invention discloses a distribution network cable joint early fault detection system and method, which can integrate multi-dimensional characteristics such as dynamic rate threshold, harmonic energy attenuation characteristics, voltage phase synchronization, etc., to achieve accurate identification of short-cycle early faults, and at the same time eliminate occasional interference through repetitive statistics, providing technical support for status inspection and fault warning of distribution network cable joints. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0029] Figure 1 Schematic diagram of the system structure of the present invention;

[0030] Figure 2 This is a schematic diagram of the specific location of the fault location in the distribution network line architecture of the present invention. DETAILED DESCRIPTION

[0031] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0032] See also Figure 1 , an embodiment of the present invention discloses a distribution network cable joint early fault detection system, including an early fault identification module along the inner surface of the cable joint and an early fault identification module for radial breakdown;

[0033] Among them, the early fault identification module along the inner surface of the cable joint includes: a timing threshold detection unit, a waveform continuity detection unit, a harmonic sampling unit, a harmonic content comparison unit and a first repeatability detection unit;

[0034] The radial breakdown early fault identification module includes: a current amplitude detection unit, a rate threshold detection unit, a duration detection unit, a start and end point detection unit and a second repeatability detection unit.

[0035] In one specific embodiment, when an incipient fault first occurs along the inner surface of a cable connector, there are no signs of ablation in the circuit, and the resistance in the circuit is primarily composed of water resistance. Experimental results show that when the number of discharges varies slightly, the initial value of the fault current does not differ significantly. Estimating the water resistance helps set the trigger threshold for the fault measurement device, while the incipient fault duration threshold is set based on an estimate of the heat generated during the evaporation process.

[0036] Typical characteristics of early stage failures along the inner surface of cable joints are as follows:

[0037] 1) A single fault lasts from several seconds to tens of seconds;

[0038] 2) The fault current amplitude is between tens and hundreds of mA;

[0039] 3) In the subsequent stage of the fault, the harmonic contribution is significantly smaller than that in the initial stage of the fault;

[0040] 4) The harmonic content above the fifth order is small and can be ignored;

[0041] 5) For the same sample, there are several to dozens of similar early failures with similar characteristics before the permanent failure occurs.

[0042] like Figure 2 The figure shows the specific location of the fault in the distribution network architecture. The front and back lines of the fault point include other cable lines and possible overhead lines.

[0043] Specifically, to distinguish fault signals from other interference signals, the logic of the early fault identification module along the inner surface of the cable connector can be divided into:

[0044] 1) Timing threshold detection unit:

[0045] The detection device set on the ground wire of the cable joint is in the normally open state. When the current signal flowing through the sample I f When the signal amplitude at the start stage is within the threshold range shown in the following formula, the recording of the suspected fault signal is started.

[0046] (5-2);

[0047] (5-3);

[0048] In the formula, [ I 1, I 2, …, I n ] array is the amplitude of the current signal at each peak point. From the experimental phenomenon, we know that nDetermined by the duration of the initial failure phase, it can be 5 to 20; I start is the current signal at the beginning stage of the fault signal; I set1 To determine the lower limit of the current threshold for fault recording, I set2 The upper limit of the current threshold for determining whether fault recording can continue.

[0049] 2) Waveform continuity detection unit:

[0050] Since the early fault along the inner surface of the cable joint is an uninterrupted small current discharge lasting from several seconds to tens of seconds, in order to further improve the monitoring accuracy, the current signal can be I f That is, during the signal acquisition process, if the signal I f There is a continuous signal I f1 If the peak current amplitude of the signal is always lower than the threshold, the signal acquisition is stopped. The specific detection method is shown in the following formula:

[0051] (5-4);

[0052] (5-5);

[0053] (5-6);

[0054] In the formula, [ I m , I m+1 ,…, I m+c ]for I f1 The amplitude of each peak point in the segment signal. I m for I f1 The current amplitude at the starting point of the signal; I set The lower limit of the current used to determine whether the signal is continuous; c is the threshold for signal continuity detection, I f1 The signal duration is 0.01 c .

[0055] The timing threshold detection unit and the waveform continuity detection unit can cooperate to perform preliminary screening of the waveform and eliminate high-frequency discharge signals with a duration shorter than that of early faults, such as capacitor switching and resonant current on the distribution network line.

[0056] 3) Harmonic sampling unit:

[0057] For signals that meet the conditions of the timing threshold detection unit and the waveform continuity detection unit, the initial point of the signal that meets the timing threshold detection is I 1, the recorded suspected fault signal is further divided in the middle of the signal, and the division is at the initial point I After 1 T 1 point in time, T The value of 1 should be slightly higher than the duration of the early fault in the actual environment. After the split, the time length is recorded again. T 2 is used as the waveform signal in the intermediate stage for subsequent comparison.

[0058] The two new signals are named I b , I s Calculate the harmonic energy of the two signals. The specific calculation method is as follows.

[0059] (5-7);

[0060] (5-8);

[0061] (5-9);

[0062] (5-10);

[0063] (5-11);

[0064] (5-12);

[0065] Where, j Represents the plural, i b (k) with i s (k) are the time domain signals I b and I s The frequency domain signal after discrete Fourier transform, m is the harmonic order, m =1 indicates fundamental wave. N is the number of points of discrete signals in the time domain and frequency domain, k is the number of the discrete signal in the time domain; E bm and Esm They are i b (k) with i s (k) m Energy on subharmonics; m =1, that is E b1 and E s1 Respectively i b (k) with i s (k) fundamental wave energy; PE bm and PE sm Signal i b (k) with i s (k) m The ratio of subharmonic energy to fundamental energy. The energy of the third harmonic can be considered as the total harmonic energy when calculating.

[0066] 4) Harmonic content comparison unit:

[0067] The harmonic energy method in the harmonic sampling unit can be used to evaluate the relative strength of different harmonics and the fundamental wave. In order to prevent the misjudgment problem that may exist in a single method, the harmonic contribution is introduced in the problem of comparing the harmonic content of the two stages. H As a supplementary criterion. i b (k) with i s (k) The harmonic contribution of the early fault initiation stage and the intermediate stage is further calculated. The calculation method is as follows:

[0068] (5-13);

[0069] (5-14);

[0070] Where, H bx and H sx They are time domain signals I b and I s The harmonic contribution of the xth harmonic, I bx is the root mean square of the xth harmonic current, I b1 is the root mean square current of the collected waveform in one stage. Isx is the root mean square of the corresponding xth harmonic current, I s1 is the RMS current of the fundamental wave of the collected second-stage waveform. When evaluating the harmonic contribution, only the third harmonic is calculated.

[0071] The purpose of the harmonic sampling unit and the harmonic content comparison unit is to remove long-lasting harmonic signals, such as three-phase imbalance and high-frequency components that exist in the line current for a long time. I f If the signal PE bm > PE sm ,and H bx > H sx . It can be preliminarily determined that the signal belongs to an early fault, and the identification device can issue a suspected early fault judgment signal.

[0072] 5) First repeatability test unit:

[0073] Since early faults along the inner surface of the cable connector are repetitive and cumulative, based on this characteristic, repeatability detection can be performed on the initially identified suspected early fault signals. The specific method is as follows:

[0074] For a sampling interval containing L cable joints C L A suspected fault signal from any cable connector increments the repeatability detection device's count by one. If the repeatability detection count exceeds a threshold within a certain period of time, a higher-level fault alarm is issued. Repeatability detection further eliminates occasional interference signals that resemble early-stage fault characteristics along the inner surface of the cable connector.

[0075] In a specific embodiment, although the early faults of radial breakdown and the early faults along the inner surface of the cable joint both occur inside the insulation of the cable joint and have similar characteristics such as insulation damage, evaporation of accumulated water and intermittent arcing, the differences in environmental characteristics such as hole length, diameter, and amount of accumulated water in the fault scenarios of the two make the electrical quantity characteristics such as current amplitude, single fault duration, and current amplitude evolution process essentially different. Therefore, another set of identification logic needs to be established for the early faults of radial breakdown. The early faults of radial breakdown also have the following typical characteristics of early faults:

[0076] 1) The fault starts near the voltage peak point;

[0077] 2) A single fault lasts slightly less than a quarter of a voltage cycle;

[0078] 3) For directly grounded circuits, the fault current amplitude can reach hundreds to thousands of amperes;

[0079] 4) The rate of increase and decrease of the fault current amplitude are significantly faster than those of a sine wave under the same environment, showing a pulse-like waveform;

[0080] 5) For the same sample, there are several to dozens of similar early failures with similar characteristics before the permanent failure occurs. This characteristic is the same as the long-period early failure.

[0081] In a specific embodiment, in order to distinguish the early fault signal from other possible interference signals, the early fault identification logic of radial breakdown based on the fault scenario can be divided into:

[0082] 1) Current amplitude detection unit:

[0083] The detection device is also set on the ground wire of the cable joint. The detection device is in the normally open state. When the current signal flowing through the sample I f When the signal amplitude at the start stage is within the threshold range shown in the following formula, the recording of the suspected fault signal is started.

[0084] (5-15);

[0085] (5-16);

[0086] In the formula, [ I 1, I 2, …, I n ] array is the amplitude of the current signal at each peak point. From the experimental phenomenon, we know that n is determined by the duration of the early fault starting stage. When the sampling rate is 10 K hour, n The value can be 5~10; I start is the current signal at the beginning stage of the fault signal; I set1 To determine the lower limit of the current threshold for fault recording, I set2 To determine the upper limit of the current threshold for continuing fault recording. This step is to distinguish short-term early faults from other small or large pulses.

[0087] 2) Rate threshold detection unit:

[0088] Since the fault resistance in the short-term early stage fault changes nonlinearly at high speed, the evaporation rate of the accumulated water accelerates as the current increases, making the rate of change of the current rise and fall faster than the sinusoidal waveform in the same period, so the current speed threshold detection can constitute a part of the fault detection index. I f Two adjacent current signal collection points in I S1 and I S2 , and their corresponding time points are t s1 and t s2 If the signal segment belongs to this type of early fault, the absolute value of the slope between the two is greater than the sine waveform with the maximum current in the signal segment as the amplitude and 50 Hz as the frequency. The specific detection method is shown in the following formula:

[0089] (5-17);

[0090] (5-18);

[0091] (5-19);

[0092] (5-20);

[0093] (5-21);

[0094] (5-22);

[0095] Where, represents the phase of the signal, t represents time, [ I m , I m+1 ,…, I m+c ]for I f1 The amplitude of each peak point in the segment signal. I m for I f The maximum current value in the signal; I m ’ For I m is a sine function constructed for the basis. I ms1 and I ms2 The constructed signals aret s1 and t s2 The corresponding value at the point. f is the voltage frequency. For AC distribution networks, this value is 50. If any two connected current signal acquisition points satisfy Equation (5-22), the signal is considered to have passed the rate threshold test. Otherwise, it fails the test and signal acquisition is stopped.

[0096] Rate threshold detection targets sinusoidal interference currents, such as short-term three-phase imbalance. In practice, to improve detection reliability and take into account environmental interference factors, the rate threshold detection pass rate can be set to a percentage threshold adjusted for the environment.

[0097] 3) Duration detection unit:

[0098] Since the water storage capacity of the internal holes of the insulation that form the early fault of radial breakdown is small, the duration of a single fault is slightly shorter than half a voltage cycle. Compared with insulators exposed to the air and easily affected by external rainfall, underground cable joints are affected by the humid environment to a certain extent. Therefore, the duration can be used as a stage characteristic to identify the fault. The specific principle is that for a continuously sampled signal sequence I f The first point in a continuous array that meets the current amplitude distortion threshold is set as the fault start point. Similarly, after the current amplitude stops distorting for a period of time, the first point where the current amplitude change rate drops sharply and becomes non-differentiable is set as the fault end point. The time between the fault start point and the fault end point is then calculated to determine whether this time falls within the duration detection threshold. The specific calculation method is shown below.

[0099] (5-23);

[0100] (5-24);

[0101] (5-25);

[0102] (5-26);

[0103] (5-27);

[0104] In the formula, [ I 1, I 2, …, I n ] array is the amplitude of the continuous current signal at each peak point, [ t1, t 2, …, t n ] The array is the corresponding time points when these current signals are collected. N tri To determine the current amplitude change rate threshold when the signal is in the short-cycle early fault rising stage, N etri To determine the current amplitude change rate threshold when the signal is in the short-cycle early fault falling stage, T sam is the duration of the fault signal calculated based on the identified fault initial point and fault end point. T sam If the value of is less than or equal to the preset short-period early fault single duration threshold, the signal is considered to have passed the duration detection; otherwise, the signal is considered not to be a short-period early fault.

[0105] 4) Start and end point detection unit:

[0106] Based on the current amplitude detection unit, the rate threshold detection unit and the duration detection unit, the interference waveforms with long duration, small amplitude and low current change rate can be preliminarily screened out. However, other pulses may interfere with the identification of this type of fault. For this reason, a one-step detection method is required to distinguish the interference between the fault signal and other pulses. Since the early fault of radial breakdown always occurs in the rising interval of the absolute value of the voltage amplitude and always ends in the interval where the absolute value of the voltage amplitude decreases. This characteristic is determined by the triggering principle of intermittent arcs and is a unique feature of the fault. Therefore, the start and end point detection can be used as a step in the identification method. The principle is to detect whether the starting point of the fault signal is in the rising interval of the absolute value of the voltage amplitude, and to detect whether the end point of the signal must be in the falling interval of the absolute value of the voltage amplitude. The calculation method is as follows:

[0107] (5-28);

[0108] (5-29);

[0109] (5-30);

[0110] (5-31);

[0111] (5-32);

[0112] (5-33);

[0113] Where, I u andI d They are time domain signals [ I 1, I 2, …, I n The current amplitude of any signal point in the first and second half of the segment. The dividing point of this segment is the maximum current in the signal I m The corresponding time node. I m (t) is the maximum value of the signal in this segment I m The constructed continuous sinusoidal current signal, I m ’ is the derivative of the continuous signal. t u and t d For the corresponding I u and I d Time nodes of two signal points. If the derivative of the sinusoidal signal at any time point in the first half of the signal is greater than 0, and the derivative of the sinusoidal signal at any time point in the second half of the signal is less than 0, then the signal can be considered to have passed the start and end point detection. Otherwise, the signal is considered not to be a short-cycle early fault. In actual situations, due to interference and fluctuations from environmental factors, there may be a certain current amplitude rebound in the signal. Therefore, in the start and end point detection steps, a pass rate threshold can be set in combination with the actual environment. When the ratio of the number of signal points that meet the detection to the total number of signal sampling points exceeds the threshold, it can be considered that the signal has met the detection conditions.

[0114] 5) Repeatability detection unit:

[0115] Since the two types of early faults have the same repetitive and cumulative manifestation characteristics and damage characteristics at the same fault point, the repeatability detection method can be used to screen out other pulses that may be similar to the characteristics of short-period early faults. The principle is the same as the above-mentioned repeatability detection and will not be elaborated here.

[0116] On the other hand, this embodiment discloses a method for detecting early faults of a distribution network cable joint, including identification of early faults along the inner surface of the cable joint and identification of early faults of radial breakdown;

[0117] Among them, the identification of early faults along the inner surface of the cable joint includes timing threshold detection, waveform continuity detection, harmonic sampling, harmonic content comparison and repeatability detection;

[0118] The identification of early stage faults of radial breakdown includes: current amplitude detection, rate threshold detection, duration detection, start and end point detection and repeatability detection.

[0119] Specifically, in the two identification methods, the previous and subsequent processes are in a logical series relationship, which means that a signal to be identified must simultaneously meet the timing threshold detection, waveform continuity detection, harmonic sampling, harmonic content comparison and repeatability detection, or simultaneously meet the detection steps of current amplitude detection, rate threshold detection, duration detection, start and end point detection and repeatability detection. If the signal does not pass any of the steps, it is not considered to be an early fault signal. In addition, if the signal has met all the steps except the repeatability detection, the measurement value of the repeatability detection counter will be increased by one. If a fault signal that meets the above conditions is still found at the detection point later, the measurement value will be increased by one again. When the measurement value reaches the calibration limit, an early fault alarm signal will be sent to the information node located at the substation.

[0120] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Reference can be made to the common and similar parts between the various embodiments. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the method description.

[0121] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A distribution network cable connector early fault detection system, characterized in that: It includes an early fault identification module along the inner surface of the cable joint and an early fault identification module for radial breakdown; Among them, the early fault identification module along the inner surface of the cable joint includes: a timing threshold detection unit, a waveform continuity detection unit, a harmonic sampling unit, a harmonic content comparison unit and a first repeatability detection unit; The radial breakdown early fault identification module includes: a current amplitude detection unit, a rate threshold detection unit, a duration detection unit, a start and end point detection unit and a second repeatability detection unit; The current amplitude detection unit sets the detection device to be normally open, and starts recording when the ground wire current signal amplitude is within the threshold range, recording the current signal in the initial stage, distinguishing short-cycle faults from other small / large pulse interferences, and the expression is: I set1 <|I start |<I set2 ; Where, [I1,I2,…,I n ] The array is the amplitude of the current signal at each peak point. From the experimental phenomenon, we know that n is determined by the duration of the early fault starting stage. When the sampling rate is 10K, n can be 5 to 10; I start is the current signal at the beginning of the fault signal; I set1 To determine the lower limit of the current threshold for fault recording, I set2 To determine the upper limit of the current threshold for continuing fault recording; The rate threshold detection unit calculates the current change rate of adjacent sampling points based on the nonlinear change characteristics of the fault resistance. If the absolute value of the slope exceeds the slope corresponding to the 50 Hz sine wave constructed with the maximum current amplitude, it is determined that the rate threshold is met and output. The expression is: I m =max[I m ,I m+1 ,..,I m+c ]; Where, represents the phase of the signal, t represents time, [I m ,I m+1 ,…,I m+c ] is I f1 The amplitude of each peak point in the segment signal; I m For I f Maximum current in the signal; I m 'For I m is the sine function of the basis structure; I ms1 with I ms2 The constructed signals are s1 With t s2 The corresponding value at the point, f is the voltage frequency; The start and end point detection unit sets the current amplitude to stop distorting for a period of time, then sets the first point where the current amplitude change rate drops sharply and exhibits non-conductive properties as the fault end point, calculates the time between the fault start point and the fault end point, and evaluates whether the time is within the threshold range of the duration detection and outputs the result. The expression is: I m ’(t u )>0 (5-32); I m ’(t d )<0 (5-33); Where, I u with I d They are the time domain signals [I1,I2,…,I n ] the current amplitude of any signal point in the first and second half; I m (t) is the maximum value I in this segment of the signal m Constructed continuous sinusoidal current signal, I m ' is the derivative of the continuous signal; t u With t d For the corresponding I u with I d Time nodes of two signal points; if the derivative of the sinusoidal signal at any time point in the first half of the signal is greater than 0, and the derivative of the sinusoidal signal at any time point in the second half of the signal is less than 0, then the signal passes the start and end point detection, otherwise it is considered that the signal does not belong to a short-period early fault.

2. A distribution network cable joint early fault detection system according to claim 1, characterized in that: The timing threshold detection unit includes placing a detection device arranged on the grounding wire of the cable joint in a normally open state, starting recording when the amplitude of the grounding wire current signal is within a set threshold range, and recording the current signal in the initial stage.

3. A distribution network cable joint early fault detection system according to claim 1, characterized in that: The waveform continuity detection unit includes detecting the continuity of the current signal. During the signal acquisition process, if there is a continuous signal in the current signal and the peak point current amplitude of the continuous signal is always lower than the threshold, it is determined to be interference and the signal acquisition is stopped.

4. A distribution network cable joint early fault detection system according to claim 1, characterized in that: The harmonic sampling unit starts timing from the initial point of the signal that meets the timing threshold detection based on the signal output by the timing threshold detection unit and the waveform continuity detection unit, and further divides the recorded suspected fault signal in the middle of the signal. The division is at a first preset time point after the initial point. The value at the first preset time point is higher than the duration of the early fault starting stage in an actual environment. After the division, the current signal with a time length of a second preset time point is recorded again as a waveform signal of the intermediate stage and outputted.

5. The distribution network cable joint early fault detection system according to claim 1, characterized in that: The harmonic content comparison unit introduces harmonic contribution as a supplementary criterion, and further calculates the harmonic contribution at the initial stage and the intermediate stage of the early fault based on the signal output from the harmonic sampling unit.

6. A distribution network cable joint early fault detection system according to claim 1, characterized in that: The first repeatability detection unit and the second repeatability detection unit are respectively used to count the number of suspected faults of the same cable joint or multiple joints in the area. If the number exceeds the threshold within the cycle time, a high-level alarm is triggered to eliminate occasional interference.

7. A method for detecting early faults of distribution network cable joints using the early fault detection system for distribution network cable joints according to any one of claims 1 to 6, characterized in that: Including the identification of early faults along the inner surface of the cable joint and the identification of early faults of radial breakdown; Among them, the identification of early faults along the inner surface of the cable joint includes timing threshold detection, waveform continuity detection, harmonic sampling, harmonic content comparison and first repeatability detection; The identification of early stage faults of radial breakdown includes: current amplitude detection, rate threshold detection, duration detection, start and end point detection and second repeatability detection.

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