Distribution network cable joint early fault detection system and method

Abstract

The invention discloses a distribution network cable joint early fault detection system and method, and belongs to the technical field of cable fault detection. The system comprises an early fault identification module on 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 comprises 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 comprises a current amplitude detection unit, a rate threshold detection unit, a duration detection unit, a starting and ending point detection unit and a second repeatability detection unit. Technical support can be provided for state maintenance and fault early warning of the power distribution network cable joint.

Description

Technical Field

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

[0002] The permanent faults of medium-voltage cable joints seriously affect the reliability of the distribution network. Before the formation of permanent faults, there are several self-extinguishing arc discharges caused by water ingress, that is, early faults.

[0003] As a key connection component of the distribution network, the reliability of medium-voltage cable joints directly affects the stability of the power supply system. However, during long-term operation, problems such as joint water ingress caused by insulation aging, seal failure or construction defects occur frequently. After water intrusion, it is easy to cause partial discharge under the action of the electric field and gradually develop into a radial breakdown type of early fault. Before the formation of a permanent short circuit, such faults are usually accompanied by multiple short-cycle self-extinguishing arc discharges (single duration < 10 ms). Although the discharge energy is not enough to trigger the action of traditional protection devices, it will continuously deteriorate the insulating material and ultimately cause permanent faults and large-area power outages. According to statistics, about 70% of the power outage accidents in the distribution network caused by cable joint faults are closely related to the radial breakdown caused by water ingress, and the cost of fault repair is high, and the average power outage duration exceeds 6 hours.

[0004] At present, the monitoring means for early faults of cable joints have the following limitations: Insufficient detection sensitivity: Traditional overcurrent protection devices rely on steady-state current thresholds and cannot capture millisecond-level short-cycle discharge signals; 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, and existing methods are difficult to effectively distinguish them; Lack of research on fault characteristics: There is a lack of a quantitative model for the dynamic evolution law of radial breakdown faults (such as the correlation between the arc starting / ending point and the voltage phase, the change of harmonic energy distribution), resulting in low accuracy of feature extraction; Poor environmental adaptability: The moisture and temperature fluctuations of underground cable joints have a significant impact. Existing detection algorithms do not fully consider the randomness of interference signals and the cumulative effect of repeated faults.

[0005] In addition, existing research mainly focuses on the monitoring of creeping discharge faults along the inner surface of cable joints. Such faults are manifested as small current discharges lasting for several seconds, and their harmonic characteristics are significantly different from those of interference signals. However, for radial breakdown faults, due to the extremely short discharge time, fast current change rate (> 10^3 A / s), and strong correlation with the voltage waveform phase, it is necessary to design targeted multi-dimensional criteria. If only relying on a single parameter (such as current amplitude or harmonic content), it is very easy to cause missed judgment or misjudgment due to environmental interference and cannot meet the high reliability requirements of the distribution network.

[0006] Therefore, how to provide an early fault detection system and method for distribution network cable joints is an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0007] In view of this, the present invention provides an early fault detection system and method for distribution network cable joints to solve the technical problems existing in the above-mentioned prior art.

[0008] In order to achieve the above object, the present invention provides the following technical solutions: An early fault detection system for distribution network cable joints 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 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.

[0009] Further, the timing threshold detection unit includes keeping the detection device arranged on the grounding wire of the cable joint in an 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.

[0010] Further, the waveform continuity detection unit includes detecting the continuity of the current signal. Among them, 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 as interference and the signal acquisition is stopped.

[0011] Further, the harmonic sampling unit starts timing from the signal initial point that meets the timing threshold detection based on the signals output by the timing threshold detection unit and the waveform continuity detection unit, further divides the collected suspected fault signal in the middle of the signal. The division point is at a first preset time point after the initial point, and 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 point, the current signal with a time length of a second preset time point is recorded again as the waveform signal in the middle stage and output.

[0012] Further, the harmonic content comparison unit introduces the harmonic contribution degree as a supplementary criterion, and further calculates the harmonic contribution degrees of the early fault start stage and the middle stage based on the signals output by the harmonic sampling unit.

[0013] Further, the current amplitude detection unit sets the detection device to be normally open, starts recording when the amplitude of the grounding wire current signal is within the threshold range, records the current signal in the initial stage, and distinguishes short-cycle faults from other small / large pulse interferences.

[0014] Further, based on the non-linear change characteristic of the fault resistance, the rate threshold detection unit calculates the current change rate between adjacent sampling points. 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 satisfied and output.

[0015] Further, after the current amplitude stops distorting for a period of time, the start and end point detection unit sets the first point where the current amplitude change rate drops steeply and shows non-differentiable properties as the fault end point, calculates the time between the fault start point and the fault end point, and evaluates whether this time is within the threshold range of the continuous duration detection and outputs.

[0016] Further, the first repeatability detection unit and the second repeatability detection unit are respectively used to count the number of suspected faults of multiple joints in the same cable joint or area. If it exceeds the threshold within the cycle time, a high-level alarm is triggered to exclude accidental interferences.

[0017] A method for early fault detection of a distribution network cable joint, 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 faults of radial breakdown includes: current amplitude detection, rate threshold detection, continuous duration detection, start and end point detection, and second repeatability detection.

[0018] Through the above technical solutions, compared with the prior art, the present invention discloses an early fault detection system and method for a distribution network cable joint, which can integrate multi-dimensional features such as dynamic rate threshold, harmonic energy attenuation characteristics, and voltage phase synchronization to achieve accurate identification of short-cycle early faults. At the same time, accidental interferences are excluded through repeatability statistics, providing technical support for the condition-based maintenance and fault warning of distribution network cable joints. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only the embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained according to the provided drawings without creative efforts.

[0020] Figure 1 Schematic diagram of the system structure of the present invention; Figure 2 Schematic diagram of the specific location of the fault location of the present invention in the distribution network line structure. Specific embodiments

[0021] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0022] See Figure 1 , the embodiments of the present invention disclose an early fault detection system for a distribution network cable joint, including 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 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.

[0023] In a specific embodiment, when an early fault occurs along the inner surface of the cable joint for the first time, there is no ablation trace in the circuit, and the resistance in the circuit is mainly composed of the resistance of accumulated water. From the experimental results, when the number of discharge times is not much different, there is no obvious difference in the initial value of the fault current. Therefore, the estimation of the resistance of accumulated water helps to set the trigger threshold of the fault measurement device, and the setting of the early fault duration threshold is based on the estimation of the heat of the evaporation process.

[0024] The early faults along the inner surface of the cable joint have the following typical characteristics: 1) The single fault lasts from several seconds to dozens of seconds; 2) The fault current amplitude is in the range of dozens to hundreds of milliamperes; 3) In the subsequent stage of the fault, the harmonic contribution is significantly less than that in the starting stage of the fault; 4) The harmonic content of more than five times is small and can be ignored; 5) For the same sample, there are several to dozens of similar early faults with similar characteristics before the permanent fault occurs.

[0025] Such as Figure 2As shown in the figure, it is a schematic diagram of the specific location of the fault position in the distribution network line structure. The front and rear lines of the fault point include other cable lines and possible overhead lines.

[0026] Specifically, to distinguish the fault signal from other interference signals. The logic of the early fault identification module along the inner surface of the cable joint can be divided into: 1) Timing threshold detection unit: The detection device set on the grounding wire of the cable joint is in the normally open state. When the current signal flowing through the sample I f At the beginning stage, when the signal amplitude is within the threshold range shown in the following formula, the recording of the suspected fault signal is started.

[0027] (5-2); (5-3); In the formula, I 1, I 2, …, I n The array is the amplitude of the current signal at each peak point. It can be seen from the experimental phenomenon that n It is determined by the duration of the early fault start stage and can take 5-20; I start Is the current signal at the start stage of the fault signal; I set1 Is the lower bound of the current threshold for determining that fault recording can be performed, I set2 Is the upper bound of the current threshold for determining that fault recording can continue.

[0028] 2) Waveform continuity detection unit: Since the early fault along the inner surface of the cable joint is a continuous small current discharge for several seconds to dozens of seconds, to further improve the monitoring accuracy. The continuity of the current signal I f Can be detected. That is, during the signal acquisition process, if the signal I f There is a continuous signal I f1 In it, and the peak point current amplitude of this signal is always lower than the threshold, the signal acquisition is stopped. The specific detection method is shown in the following formula: (5-4); (5-5); (5-6); In the formula, I m, I m+1 ,…, I m+c is I f1 the amplitude of each peak point within the segment signal. I m is I f1 the current amplitude at the start point of the signal; I set is the lower current threshold value for determining whether the signal is continuous; c is the threshold value for signal continuity detection, I f1 The signal duration is 0.01 c .

[0029] The timing threshold detection unit and the waveform continuity detection unit cooperate to preliminarily screen the waveform, excluding high-frequency discharge signals with short durations such as capacitor switching and resonance current on the distribution network line, which are shorter than the early fault.

[0030] 3) Harmonic sampling unit: For signals that meet the conditions of the timing threshold detection unit and the waveform continuity detection unit, start timing from the initial point of the signal that meets the timing threshold detection I 1, and further divide the collected suspected fault signal in the middle of the signal. The division point is at the I after the initial point T 1 time point, T The value of 1 should be slightly higher than the duration of the early fault start stage in the actual environment. After the division point, record the current signal with a time length of T 2 again as the waveform signal in the intermediate stage for subsequent comparison.

[0031] For the two newly formed signals, name them respectively I b , I s . Calculate the harmonic energy of the two signals. The specific calculation method is as follows.

[0032] (5-7); (5-8); (5-9); (5-10); (5-11); (5-12); In the formula,j represents a complex number, i b (k) and i s (k) are time-domain signals I b and I s frequency-domain signals after discrete Fourier transform, m is the harmonic order, m When = 1, it represents the fundamental wave. N is the number of points of the time-domain and frequency-domain discrete signals, k is the number of the time-domain discrete signal; E bm and E sm are respectively i b (k) and i s (k) on the m th harmonic energy; m When = 1, that is E b1 and E s1 respectively represent i b (k) and i s (k) fundamental wave energy; PE bm and PE sm are respectively the signal i b (k) and i s (k) m ratio of the th harmonic energy to the fundamental wave energy. When calculating, the energy of the third harmonic can be regarded as the total harmonic energy.

[0033] 4) Harmonic content comparison unit: The harmonic energy method in the harmonic sampling unit can be used to evaluate the relative intensities of different harmonics and the fundamental wave. To prevent misjudgment problems that may exist in a single method, in the problem of comparing the harmonic contents in two stages, the harmonic contribution degree H is introduced as a supplementary criterion. Using the i b (k) and i s (k) calculated in 3) to further calculate the harmonic contribution degrees in the early fault start stage and the intermediate stage. The calculation method is as follows: (5 - 13); (5 - 14); In the formula, H bx and H sx are respectively the harmonic contribution degrees of the x - th harmonics of the time - domain signals I b and I s . I bx is the root - mean - square value of the x - th harmonic current, I b1 is the root - mean - square value of the fundamental - wave current of the first - stage waveform collected. I sx is the root - mean - square value of the corresponding x - th harmonic current, I s1 is the root - mean - square value of the fundamental - wave current of the second - stage waveform collected. When evaluating the harmonic contribution degree, only the third - harmonic is calculated in the same way.

[0034] The purpose of the two units of the harmonic sampling unit and the harmonic content comparison unit is to remove the harmonic signals that exist for a long time, such as three - phase imbalance and high - frequency components that exist in the line current for a long time. For the signal after steps (1) - (4) I f , if the PE bm > PE sm , and H bx > H sx . Then it can be preliminarily determined that the signal belongs to an early fault, and the identification device can issue a suspected early - fault determination signal accordingly.

[0035] 5) The first repeatability detection unit: Due to the repeatability and cumulativeness of the early faults along the inner surface of the cable joint, according to this feature, the repeatability detection of the preliminarily identified suspected early - fault signals can be carried out. The specific method is as follows: For a sampling interval containing L cable joints C L , the suspected fault signal of any one of the cable joints can increase the count of the repeatability detection device by one. When the repeatability detection count exceeds the threshold within a certain time, a fault warning signal with a higher level can be issued. The use of repeatability detection can further exclude the occasional interference signals similar to the characteristics of the early faults along the inner surface of the cable joint.

[0036] In a specific embodiment, although the early faults of radial breakdown and the early faults along the inner surface of the cable joint occur inside the insulation of the cable joint and have similar characteristics such as insulation damage, water accumulation evaporation, and intermittent arcs, the differences in environmental characteristics such as the hole length, diameter, and water accumulation volume in the fault scenarios of the two make there be essential differences in electrical quantity characteristics such as current amplitude, single-fault duration, and current amplitude evolution process. Therefore, another set of identification logics 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: 1) The fault starts near the peak point of the voltage; 2) The single-fault duration is slightly shorter than one-quarter of the voltage cycle; 3) For a directly grounded circuit, the fault current amplitude can reach hundreds to thousands of amperes; 4) The increasing speed and decreasing rate of the fault current amplitude are both significantly faster than the sine wave under the same environment, showing a pulse-like waveform; 5) For the same sample, there are several to dozens of similar early faults with similar characteristics before the permanent fault occurs, and this feature is the same as that of the long-cycle early faults.

[0037] In a specific embodiment, to distinguish the early fault signal from other possible interference signals. The identification logic of the early faults of radial breakdown based on the fault scenario can be divided into: 1) Current amplitude detection unit: The detection device is also set on the grounding wire of the cable joint. The detection device is in the normally open state. When the current signal flowing through the sample I f At the beginning stage, when the signal amplitude is within the threshold range shown in the following formula, the recording of the suspected fault signal is started.

[0038] (5-15); (5-16); In the formula, I 1, I 2, …, I n The array is the amplitude of the current signal at each peak point. From the experimental phenomena, it can be seen that n is determined by the duration of the beginning stage of the early fault. When the sampling rate is 10 K , n It can take 5 to 10; I start Is the current signal at the beginning stage of the fault signal; I set1 Is the lower bound of the current threshold for determining that the fault recording can be carried out, I set2To determine the upper bound of the current threshold for continuing fault recording. This step aims to distinguish short-cycle early faults from other small or large pulses.

[0039] 2) Rate threshold detection unit: Since the fault resistance in short-cycle early faults changes at a high speed non-linearly, and the evaporation rate of accumulated water accelerates with the increase of current, the change rates of both the rising and falling processes of the current are faster than the sine waveform in the same period, making the current speed threshold detection an indicator for fault detection. During the signal acquisition process, for a continuous signal I f the adjacent two current signal acquisition points I S1 and I S2 in it, the corresponding time points are t s1 and t s2 . If this section of the signal belongs to such early faults, the absolute value of the slope between the two is greater than the sine waveform with the maximum current in this section of the signal as the amplitude and 50 Hz as the frequency. The specific detection method is shown as follows: (5-17); (5-18); (5-19); (5-20); (5-21); (5-22); In the formula, 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 within the I m section of the signal. I f I m ’ m is the maximum current in the signal; I m is the sine function constructed with I ms1 and I ms2Are the corresponding values of the constructed signals at t s1 and t s2 points. f is the voltage frequency. For an AC distribution network, this value is 50. If Equation (5-22) is satisfied for any two connected current signal acquisition points, it can be considered that the signal has passed the rate threshold detection. Otherwise, the detection fails and the signal acquisition stops.

[0040] The interference quantity targeted by the rate threshold detection is the interference current in the form of a sine wave, such as short-term three-phase imbalance. In actual situations, considering the existence of environmental interference factors, to improve the detection reliability, the passing rate of the rate threshold detection can be set as the percentage threshold adjusted according to the environment.

[0041] 3) Duration detection unit: Since the water storage capacity of the internal holes of the insulation for the early fault of forming radial breakdown is small, the duration of a single fault is slightly shorter than half a voltage cycle. Compared with the insulator exposed to the air and vulnerable to continuous influence of external rainfall, the cable joints located underground have a certain discontinuity in being affected by the humid environment. Therefore, the duration can be used as a characteristic of the fault stage for identification. The specific principle is for a continuously sampled signal sequence I f , the first point in a continuous array that satisfies the distortion threshold of the current amplitude is set as the fault start point. Similarly, after setting that the current amplitude stops distorting for a period of time, the first point where the rate of change of the current amplitude drops steeply and shows non-differentiable properties is set as the fault end point. Then calculate the time between the fault start point and the fault end point, and evaluate whether this time is within the threshold range of the duration detection. The specific calculation method is as follows.

[0042] (5-23); (5-24); (5-25); (5-26); (5-27); In the formula, I 1, I 2, …, I n array is the amplitude of the continuous current signal at each peak point, t 1, t 2, …, t n array is the corresponding time points for collecting these current signals.N tri The current amplitude change rate threshold for determining that the signal is in the rising stage of the short - cycle early - stage fault N etri The current amplitude change rate threshold for determining that the signal is in the falling stage of the short - cycle early - stage fault T sam Is the duration of the fault signal calculated based on the identified fault starting point and fault ending point. If T sam The value is less than or equal to the preset short - cycle early - stage fault single - duration threshold, then it is considered that the signal passes the duration detection; otherwise, it is considered that the signal does not belong to the short - cycle early - stage fault

[0043] 4) Starting and ending point detection unit: Based on the current amplitude detection unit, rate threshold detection unit, and duration detection unit, interference waveforms with long duration, small amplitude, and low current change rate can be preliminarily filtered out. However, since other pulses may interfere with the identification of such faults. Therefore, a further detection method needs to be set up to distinguish the fault signal from the interference of other pulses. Since the early - stage 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 belongs to the unique feature of the fault. Therefore, the starting and ending point detection can be used as one step of the identification method. Its 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 ending point of the signal must be in the falling interval of the absolute value of the voltage amplitude. The calculation methods are as follows: (5 - 28); (5 - 29); (5 - 30); (5 - 31); (5 - 32); (5 - 33); In the formula, I u and I d Are the current amplitudes of any signal points in the first half and the second half of the time - domain signal I 1, I 2, …, I n , respectively. The demarcation point of this segmentation is the time node corresponding to the maximum current I m in the signalI m (t) is the maximum value in this segment of the signal I m Constructed continuous sinusoidal current signal I m ’ is the derivative of this continuous signal. t u and t d correspond to I u and I d The time nodes of two signal points respectively. If the derivative of any time point in the first half of the signal on the sine signal is greater than 0, and the derivative of any time point in the second half of the signal on the sine signal is less than 0, then it can be considered that the signal has passed the start and end point detection. Otherwise, it is considered that the signal does not belong to the short-cycle early fault. In actual situations, due to environmental factor interference and fluctuations, there may be a certain current amplitude recovery in the signal. Therefore, a passing rate threshold can be set in combination with the actual environment in the start and end point detection step. When the proportion of the number of signal points that meet the detection in the total number of signal sampling points exceeds this threshold, it can be considered that the signal has met the detection conditions.

[0044] 5) Repeatability detection unit: Since the two types of early faults also have the characteristics of repeatability, cumulative manifestation, 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 short-cycle early fault characteristics. The principle is the same as the above repeatability detection and will not be elaborated here.

[0045] On the other hand, this embodiment discloses a method for detecting early faults of distribution network cable joints, 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 repeatability detection. The identification of early faults of radial breakdown includes: current amplitude detection, rate threshold detection, duration detection, start and end point detection, and repeatability detection.

[0046] Specifically, in the two recognition methods, the front and back processes are in a logically serial relationship, meaning that a signal to be recognized needs to simultaneously meet the detection steps of 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 fails any of these steps, it is not considered an early fault signal. In addition, if the signal has met all steps except repeatability detection, the count value of the repeatability detection counter will be incremented by one. If a fault signal meeting the above conditions is still found at this detection point subsequently, the count value will be incremented by one again. When the count value reaches the calibration limit, an early fault alarm signal will be sent to the information node located in the substation.

[0047] The various embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. The same or similar parts among the embodiments can be referred to each other. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple. For the relevant parts, refer to the description in the method section.

[0048] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An early fault detection system for distribution network cable joints, 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 early fault identification module for the 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.

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

3. The early fault detection system for a distribution network cable joint according to claim 1, characterized in that, The waveform continuity detection unit includes detecting the continuity of the current signal. Among them, 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 as interference and the signal acquisition is stopped.

4. The early fault detection system for a distribution network cable joint according to claim 1, characterized in that, Based on the signals output by the timing threshold detection unit and the waveform continuity detection unit, the harmonic sampling unit starts timing from the initial point of the signal that meets the timing threshold detection, further divides the suspected fault signal collected in the middle of the signal. The division point is at the first preset time point after the initial point, and 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 point, the current signal with a time length of the second preset time point is recorded again as the waveform signal in the middle stage and output.

5. The early fault detection system for a distribution network cable joint according to claim 1, characterized in that, The harmonic content comparison unit introduces the harmonic contribution degree as a supplementary criterion, and further calculates the harmonic contribution degrees of the early fault start stage and the middle stage based on the signals output in the harmonic sampling unit.

6. The early fault detection system for a distribution network cable joint according to claim 1, characterized in that, The current amplitude detection unit sets the detection device to be normally open, starts recording when the amplitude of the ground wire current signal is within the threshold range, records the current signal in the initial stage, and distinguishes short-cycle faults from other small / large pulse interferences.

7. The early fault detection system for a distribution network cable joint according to claim 1, wherein Based on the non-linear change characteristic of the fault resistance, the rate threshold detection unit calculates the current change rate of adjacent sampling points. 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.

8. An early fault detection system for a distribution network cable joint according to claim 1, characterized in that, After the current amplitude stops distorting for a period of time, the start and end point detection unit sets the first point where the current amplitude change rate drops steeply and shows non-differentiable properties as the fault end point, calculates the time between the fault start point and the fault end point, and evaluates whether this time is within the threshold range of the duration detection and outputs.

9. The early fault detection system for a distribution network cable joint 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 multiple joints in the same cable joint or area. If it exceeds the threshold within the cycle time, a high-level alarm is triggered to exclude occasional interferences.

10. A method for early fault detection of a distribution network cable joint using the early fault detection system for a distribution network cable joint according to any one of claims 1 to 9, characterized in that, It includes 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 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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