A cable defect detection method and system based on transient disturbance characteristics

Abstract

The present invention discloses a method and system for detecting cable defects based on transient disturbance characteristics. The present invention obtains a voltage disturbance signal generated by a local insulation defect of a cable through a local insulation defect test of the cable; analyzes the main characteristics of the voltage disturbance signal to obtain the change ranges of different characteristic parameters, and performs amplitude conversion on the voltage disturbance signal after denoising to extract the characteristic parameters; uses the characteristic parameters of the voltage disturbance signal for detecting the local insulation defect of the cable, first detects the threshold value of the voltage disturbance signal to determine whether it exceeds the normal threshold value, then determines whether the characteristic parameters of the voltage disturbance signal meet the requirements, and repeats the judgment for the next cycle signal to evaluate the severity of the local insulation defect of the cable. The present invention uses the voltage disturbance signal during the operation of the cable to judge the local insulation defect of the cable, can give early warning in time for easy maintenance, and avoid the development of local insulation defects into permanent faults.

Description

Technical Field

[0001] The present invention relates to the technical field of cable insulation detection, and in particular to a cable defect detection method and system based on transient disturbance characteristics. Background Art

[0002] During the production, installation and operation of power cables and joints, local insulation defects will inevitably occur. Such defects often start from partial discharge, go through the process of partial discharge from mild to severe, gradually develop into incipient faults, and finally form permanent insulation faults after a certain period of incipient fault stage, resulting in the tripping of power cable protection and power outage. Since power cables are usually laid underground, it is difficult to find and repair faults, and the power outage recovery time caused by faults is generally long, affecting the operation reliability of the power system.

[0003] The incipient faults formed when the local insulation defects of power cables develop to a certain extent are manifested as a certain degree of distortion of the voltage waveform. This distorted waveform has a short duration, a small amplitude, and can recover by itself, without triggering the action of relay protection equipment. If the faults can be detected in time at the incipient fault stage of power cable insulation, early warnings can be issued and maintenance can be arranged, thus avoiding unplanned power outages caused by the tripping of permanent insulation faults of cables. Summary of the Invention

[0004] The present invention provides a cable defect detection method and system based on transient disturbance characteristics to solve the problem of detecting the characteristics of local insulation defects of cables.

[0005] For this purpose, a technical solution adopted by the present invention is: a cable defect detection method based on transient disturbance characteristics, which includes:

[0006] Obtaining the voltage disturbance signal generated by the local insulation defect of the cable through the local insulation defect test of the cable, and performing statistical analysis on the voltage disturbance signal;

[0007] Analyzing the main characteristics of the voltage disturbance signal to obtain the change ranges of different characteristic parameters, and performing amplitude conversion on the voltage disturbance signal after denoising to extract the characteristic parameters; wherein, the change ranges of the different characteristic parameters are used to detect the local insulation defects of the cable;

[0008] Using the characteristic parameters of the voltage disturbance signal for local insulation defect detection of the cable, first detecting the threshold of the voltage disturbance signal to judge whether it exceeds the normal threshold, then judging whether the characteristic parameters of the voltage disturbance signal meet the requirements, and repeating the judgment for the next cycle signal to evaluate the severity of the local insulation defect of the cable.

[0009] By the above method, the problem that the existing protection cannot detect the voltage disturbance generated by the local insulation defect of the cable insulation is effectively solved.

[0010] In one embodiment of the present application, the voltage disturbance waveform is obtained through experiments. The main process of the experiment is as follows:

[0011] Build a test platform for local insulation defects of cables, create artificial local insulation defects on the test cables, measure the relative ground voltage of the cables using a high-voltage probe, and collect and store the voltage data through an oscilloscope;

[0012] By gradually increasing the voltage, observe the change of the voltage waveform in the oscilloscope until the voltage disturbance waveform generated by the local insulation defect of the cable is observed; repeat the experiment to obtain the voltage disturbance waveform data when there is a local insulation defect in the cable.

[0013] In one embodiment of the present application, statistical analysis is performed on the observed voltage disturbance waveform. The distortion disturbances formed by local insulation defects of the cable on the voltage waveform are summarized as follows:

[0014] 1) The distortion starting point occurs before and after the peak of the voltage waveform, and the distance from the peak is ΔT (cycles), ΔT = -0.1 to +0.05; a negative starting point indicates that it occurs before the peak, and a positive starting point indicates that it occurs after the peak;

[0015] 2) The distortion waveform presents an asymmetric U shape;

[0016] 3) The depth range of the distortion waveform drop ΔU (voltage) is 0.1 to 0.9 times the normal voltage peak;

[0017] 4) High-frequency oscillation components with different frequencies and amplitudes are superimposed on the U-shaped distortion;

[0018] 5) The duration of the U-shaped distortion Δt (cycles) is 1 / 8 to 1 / 4 cycle;

[0019] 6) If the U-shaped distortion appears in consecutive cycles, it is a serious local insulation defect; otherwise, it is determined as a general local insulation defect.

[0020] In one embodiment of the present application, the voltage disturbance signal needs to be preprocessed to extract the characteristics of the disturbance signal, specifically including:

[0021] According to the measured relative ground voltage of the cable, perform low-pass filtering on the sampling signal to remove the interference of high-frequency signals, convert the instantaneous voltage to instantaneous amplitude change through Kalman filtering, and extract the characteristics of the voltage disturbance signal based on the instantaneous amplitude change: the depth range of the distortion waveform drop ΔU and the duration of the U-shaped distortion Δt.

[0022] In one embodiment of the present application, before performing feature detection on the voltage disturbance signal, the cable defect detection method further includes:

[0023] Compare the voltage disturbance signal in a single power frequency cycle with the voltage disturbance signal in the previous power frequency cycle, and calculate the sum of the squares of the differences between the two signals as an index; when the calculation result exceeds the set threshold, perform the cable partial insulation defect feature detection process.

[0024] In an embodiment of the present application, based on the voltage disturbance waveform characteristics generated by the cable partial insulation defect, the voltage disturbance signal is detected, specifically including:

[0025] First, determine whether the voltage distortion waveform drop depth range ΔU of the voltage disturbance signal is between 0.1 and 0.9 times the normal voltage peak value. When the distortion signal meets the condition, perform the next judgment; then determine whether the voltage U-shaped distortion duration Δt is between 1 / 8 and 1 / 4. Meeting the condition indicates that the single power frequency cycle signal is the distortion signal generated by the cable partial insulation defect;

[0026] Judge the data in the next sampling cycle. If consecutive cycles meet the determination conditions, it is judged as a serious partial insulation defect, otherwise it is a general partial insulation defect.

[0027] Another technical solution adopted by the present invention is: a cable defect detection system based on transient disturbance characteristics, which includes:

[0028] A voltage disturbance signal acquisition unit, which obtains the voltage disturbance signal generated by the cable partial insulation defect through the cable partial insulation defect test, and performs statistical analysis on the voltage disturbance signal;

[0029] A characteristic parameter extraction unit analyzes the main characteristics of the voltage disturbance signal to obtain the change ranges of different characteristic parameters, and performs amplitude conversion on the voltage disturbance signal after denoising to extract characteristic parameters; wherein, the change ranges of the different characteristic parameters are used to detect the cable partial insulation defect;

[0030] A cable partial insulation defect evaluation unit uses the characteristic parameters of the voltage disturbance signal for cable partial insulation defect detection. First, detect the threshold of the voltage disturbance signal to determine whether it exceeds the normal threshold, then determine whether the characteristic parameters of the voltage disturbance signal meet the requirements, and repeat the judgment for the next cycle signal to evaluate the severity of the cable partial insulation defect.

[0031] The beneficial effects of the present invention: The present invention uses the voltage signal during the operation of the cable to judge the cable partial insulation defect, which can give early warnings in time for easy maintenance and avoid the development of partial insulation defects into permanent faults. The present invention effectively solves the problem that the existing protection cannot detect the voltage disturbance generated by the cable partial insulation defect. Description of the Drawings

[0032] The accompanying drawings described herein are used to provide a further understanding of the present invention and form a part of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0033] Figure 1 is the schematic diagram of the test for obtaining the voltage distortion waveform of the local insulation defect of the power cable provided by the present invention;

[0034] Figure 2 is the characteristic diagram of the voltage distortion waveform of the local insulation defect of the power cable provided by the present invention;

[0035] Figure 3(a) is the distribution diagram of the drop depth ΔU and the duration Δt of the U-shaped distortion of the voltage of the local insulation defect of the power cable provided by the present invention;

[0036] Figure 3(b) is the distribution diagram of the starting time ΔT of the U-shaped distortion of the voltage of the local insulation defect of the power cable provided by the present invention;

[0037] Figure 4 is the overall detection flow chart of the local insulation defect characteristics of the power cable provided by the present invention. Detailed Embodiments

[0038] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention and the corresponding drawings. 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 making creative efforts fall within the scope of protection of the present invention.

[0039] When the local insulation defect of the power cable develops to a certain extent, the initial fault is manifested as a certain degree of distortion of the voltage waveform. This distorted waveform has a short duration, a small amplitude, and can recover by itself, without triggering the action of the relay protection equipment, so the disturbance signal generated by the local insulation defect of the cable cannot be detected, and the change in the insulation state of the cable cannot be detected.

[0040] Embodiment 1

[0041] This embodiment is a cable defect detection method based on transient disturbance characteristics, which is used to solve the problem of detecting the characteristics of such local insulation defects of the cable.

[0042] Figure 1 is a schematic diagram of the test for obtaining the voltage distortion waveform of the local insulation defect of the power cable provided by the present invention. As Figure 1 shown, the main steps are as follows:

[0043] Taking a 10 kV cable as an example, a cable sample for testing is made. The length of each sample is about 1.3 m. The external insulation and shielding layer are stripped, and only the cable core, inner semiconductor layer, main insulation, and outer semiconductor layer are retained. The outer semiconductor layer at both ends of the cable is stripped by 20 cm to prevent surface discharge, and the main insulation at both ends is stripped by 10 cm for connecting to the voltage source. The circuit is connected according to the test schematic diagram.

[0044] In the present invention, artificial defects are made in the middle of the cable sample, such as cutting the main insulation to different depths with a knife and adding water, brine, etc. at the defect, driving steel nails into the cable or adding iron wires at the defect, etc.

[0045] It should be noted that in the test circuit, the voltage between the cable core and the ground is measured by a high-voltage probe, and the voltage is increased step by step to observe whether there is a voltage disturbance waveform. The voltage data is displayed and stored by an oscilloscope.

[0046] In the present invention, the voltage disturbance signal caused by the local insulation defect of the cable is monitored by an oscilloscope, and the disturbance waveform is statistically analyzed to obtain its statistical characteristic law. The disturbance formed by the local insulation defect of the power cable on the voltage waveform is as Figure 2 shown.

[0047] In the present invention, the distance ΔT between the voltage drop moment of the voltage distortion waveform and the voltage peak value, the voltage drop depth ΔU of the distortion waveform, and the duration Δt of the U-shaped distortion are mainly analyzed, and ΔU and Δt are used as the basis to detect the initial fault of the local insulation of the power cable. The voltage waveform characteristics of the above-mentioned initial fault of the local insulation of the cable are obtained through the test of the high-voltage cross-linked polyethylene single-core power cable, and the distribution of each characteristic quantity is shown in Figure 3.

[0048] It should be noted that the voltage drop depth ΔU of the distortion waveform and the duration Δt of the U-shaped distortion are calculated through the amplitude change of a single power frequency cycle signal; among them, the duration Δt needs to be the continuous voltage drop time.

[0049] The overall detection process of the voltage disturbance signal is as Figure 4 shown.

[0050] First, the preprocessing process of the voltage signal is carried out:

[0051] Step 101: Collect the relative ground voltage of the single-core power cable and monitor the voltage disturbance waveform generated in the relative ground voltage.

[0052] In an embodiment of the present invention, the relative ground voltage of the cable is measured by a high-voltage probe, and the oscilloscope collects the voltage signal at a sampling frequency of 20 kHz to retain most of the characteristic frequency bands of the signal as much as possible.

[0053] Step 102: Eliminate the noise interference by filtering the collected voltage data.

[0054] In one embodiment of the present invention, the main frequency band width of the voltage disturbance signal generated by the local insulation defect of the cable in the collected voltage signal is analyzed, and the signal higher than the frequency band width of the disturbance signal is processed by a low-pass filter to reduce the noise interference in the voltage disturbance signal.

[0055] Step 103: Extract the characteristics of the filtered voltage disturbance signal for subsequent detection of the characteristics of the local insulation defect of the cable.

[0056] In one embodiment of the present invention, the Kalman filtering method is used to calculate the instantaneous amplitude change of the distorted voltage, and then the voltage drop depth ΔU and the duration Δt of the U-shaped distortion waveform are calculated.

[0057] It should be noted that when there is a disturbance in the high-frequency oscillation part of the distorted signal, the overall change trend of the signal needs to be extracted, and then the characteristic parameters ΔU and Δt of the signal are extracted.

[0058] Then, the fault detection process is carried out:

[0059] Step 104: First, judge the degree of disturbance of the signal to be detected to determine whether it exceeds the preset disturbance degree threshold.

[0060] Specifically, calculate the sum of the squares of the differences between the N voltage sampling values in each power frequency cycle and the N voltage sampling values in the previous cycle, which is represented by the following formula:

[0061]

[0062] where u i 、 are the i-th voltage sampling values of the current cycle and the previous cycle respectively, N is the total number of sampling points in each cycle, and ε 阈值 is the set distortion degree threshold, and ε is the distortion degree of the current cycle.

[0063] When the calculation result exceeds the set threshold, it indicates a distorted waveform, and other characteristics are further judged. The characteristic parameters ΔU and Δt of the voltage distortion waveform are used for the next judgment.

[0064] Step 105: First, judge whether the characteristic parameter ΔU exceeds the preset voltage drop depth ΔU range.

[0065] In one embodiment of the present invention, the voltage amplitude normally fluctuates near 1 p.u. When the voltage waveform drops between 0.1 and 0.9 p.u., it conforms to the characteristics of the voltage distortion signal caused by the local insulation defect of the cable, and the next judgment is carried out.

[0066] Step 106: Then, determine whether the characteristic parameter Δt exceeds the preset variation range of the distortion waveform duration Δt.

[0067] In an embodiment of the present invention, when the characteristic parameter duration Δt satisfies the range of 1 / 8 to 1 / 4 cycle, the distortion signal satisfies two characteristic parameter ranges simultaneously, and it is determined that the single-cycle voltage signal is a voltage distortion signal caused by a local insulation defect of the cable.

[0068] Then, judge the next waveform. When it still meets the above conditions and is determined to be a voltage distortion signal caused by a local insulation defect of the cable, it is determined to be a serious local insulation defect (i.e., a serious initial fault), otherwise it is a general local insulation defect (i.e., a general initial fault).

[0069] Embodiment 2

[0070] This embodiment is a cable defect detection system based on transient disturbance characteristics, which includes:

[0071] A voltage disturbance signal acquisition unit, which acquires the voltage disturbance signal generated by the local insulation defect of the cable through the local insulation defect test of the cable, and conducts statistical analysis on the voltage disturbance signal;

[0072] A characteristic parameter extraction unit, which analyzes the main characteristics of the voltage disturbance signal, obtains the variation ranges of different characteristic parameters, and performs amplitude conversion on the voltage disturbance signal after denoising to extract characteristic parameters; wherein, the variation ranges of the different characteristic parameters are used to detect local insulation defects of the cable;

[0073] A cable local insulation defect evaluation unit, which uses the characteristic parameters of the voltage disturbance signal for cable local insulation defect detection. First, detect the threshold of the voltage disturbance signal to determine whether it exceeds the normal threshold, then determine whether the characteristic parameters of the voltage disturbance signal meet the requirements, and repeat the judgment for the next cycle signal to evaluate the severity of the cable local insulation defect.

[0074] In the voltage disturbance signal acquisition unit, the voltage disturbance waveform is obtained through experiments, and the main process of the experiment is as follows:

[0075] Build a test platform for local insulation defects of the cable, create artificial local insulation defects on the test cable, measure the relative ground voltage of the cable using a high-voltage probe, and collect and store voltage data through an oscilloscope;

[0076] By gradually increasing the voltage, observe the change of the voltage waveform in the oscilloscope until the voltage disturbance waveform generated by the local insulation defect of the cable is observed; repeat the experiment to obtain the voltage disturbance waveform data when there is a local insulation defect of the cable.

[0077] Statistical analysis is carried out on the observed voltage disturbance waveform. The distortion disturbances formed by local insulation defects of the cable on the voltage waveform are characterized as follows:

[0078] 1) The starting point of the distortion occurs before and after the peak of the voltage waveform, and the distance from the peak is ΔT, where ΔT = -0.1 to +0.05; a negative starting point indicates it occurs before the peak, and a positive starting point indicates it occurs after the peak;

[0079] 2) The distortion waveform presents an asymmetrical U shape;

[0080] 3) The depth range of the distortion waveform drop ΔU is 0.1 to 0.9 times the normal voltage peak;

[0081] 4) High-frequency oscillation components with different frequencies and amplitudes are superimposed on the U-shaped distortion;

[0082] 5) The duration of the U-shaped distortion Δt is 1 / 8 to 1 / 4 cycle;

[0083] 6) If the U-shaped distortion appears in consecutive cycles, it indicates a serious local insulation defect; otherwise, it is determined as a general local insulation defect.

[0084] The voltage disturbance signal needs to be preprocessed to extract the characteristics of the disturbance signal, which specifically includes:

[0085] According to the measured relative ground voltage of the cable, the sampling signal is low-pass filtered to remove the interference of high-frequency signals, the instantaneous voltage is converted into an instantaneous amplitude change through Kalman filtering, and the characteristics of the voltage disturbance signal are extracted based on the instantaneous amplitude change: the depth range of the distortion waveform drop ΔU and the duration of the U-shaped distortion Δt.

[0086] Before the feature detection of the voltage disturbance signal, it also includes: comparing the voltage disturbance signal of a single power frequency cycle with that of the previous power frequency cycle, and calculating the sum of the squares of the differences between the two signals as an index; when the calculation result exceeds the set threshold, the feature detection process of the local insulation defect of the cable is carried out.

[0087] In the cable local insulation defect evaluation unit described above, based on the characteristics of the voltage disturbance waveform generated by the cable local insulation defect, the voltage disturbance signal is detected, which specifically includes:

[0088] First, it is judged whether the depth range of the voltage distortion waveform drop ΔU of the voltage disturbance signal is between 0.1 and 0.9 times the normal voltage peak. When the distortion signal meets the condition, the next judgment is made; then it is judged whether the duration of the voltage U-shaped distortion Δt is between 1 / 8 and 1 / 4. Meeting the condition indicates that the signal of a single power frequency cycle is the distortion signal generated by the local insulation defect of the cable;

[0089] Judge the data for the next sampling period. If the judgment conditions are met for consecutive periods, it is judged as a serious local insulation defect; otherwise, it is a general local insulation defect.

[0090] The above are only the embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the scope of the claims of the present invention.

Claims

1. A cable defect detection method based on transient disturbance characteristics, characterized in that The method includes: Obtaining the voltage disturbance signal generated by the cable partial insulation defect through the cable partial insulation defect test, and performing statistical analysis on the voltage disturbance signal; Analyzing the main features of the voltage disturbance signal to obtain the change ranges of different characteristic parameters, and performing amplitude conversion on the voltage disturbance signal after denoising to extract the characteristic parameters; wherein, the change ranges of the different characteristic parameters are used to detect the cable partial insulation defect; Using the characteristic parameters of the voltage disturbance signal for cable partial insulation defect detection, first detecting the threshold of the voltage disturbance signal to determine whether it exceeds the normal threshold, then determining whether the characteristic parameters of the voltage disturbance signal meet the requirements, and repeating the determination for the next cycle signal to evaluate the severity of the cable partial insulation defect; Performing statistical analysis on the observed voltage disturbance waveform. The distortion disturbance formed by the cable partial insulation defect on the voltage waveform has the following characteristics: 1) The distortion starting point occurs before and after the peak of the voltage waveform, and the distance from the peak is ΔT, ΔT = -0.1 p.u. to +0.05 p.u.; a negative starting point indicates it occurs before the peak, and a positive starting point indicates it occurs after the peak; 2) The distortion waveform presents an asymmetric U shape; 3) The depth range of the distortion waveform drop ΔU is 0.1 to 0.9 times the normal voltage peak; 4) High-frequency oscillation components with different frequencies and amplitudes are superimposed on the U-shaped distortion; 5) The duration of the U-shaped distortion Δt is 1 / 8 to 1 / 4 cycle; 6) If the U-shaped distortion appears in consecutive cycles, it is a severe partial insulation defect; otherwise, it is determined as a general partial insulation defect.

2. The cable defect detection method based on transient disturbance characteristics according to claim 1, wherein The voltage disturbance waveform is obtained through experiments. The main process of the experiment is as follows: Building a cable partial insulation defect test platform, creating artificial partial insulation defects on the test cable, measuring the relative ground voltage of the cable using a high-voltage probe, and collecting and storing voltage data through an oscilloscope; By gradually increasing the voltage, observing the change of the voltage waveform in the oscilloscope until the voltage disturbance waveform generated by the cable partial insulation defect is observed; repeating the experiment to obtain the voltage disturbance waveform data when there is a cable partial insulation defect.

3. The cable defect detection method based on transient disturbance characteristics according to claim 2, characterized in that, The voltage disturbance signal needs to be preprocessed to extract the disturbance signal features, specifically including: According to the measured relative ground voltage of the cable, performing low-pass filtering on the sampling signal to remove the interference of high-frequency signals, converting the instantaneous voltage to the instantaneous amplitude change through Kalman filtering, and extracting the features of the voltage disturbance signal based on the instantaneous amplitude change: the depth range of the distortion waveform drop ΔU and the duration of the U-shaped distortion Δt.

4. A cable defect detection method based on transient disturbance characteristics according to claim 3, characterized in that Before performing feature detection on the voltage disturbance signal, the method further includes: Comparing the voltage disturbance signal of a single power frequency cycle with the voltage disturbance signal of the previous power frequency cycle, and calculating the sum of the squares of the differences between the two signals as an index; when the calculation result exceeds the set threshold, performing the cable partial insulation defect feature detection process.

5. A cable defect detection method based on transient disturbance characteristics according to any one of claims 1 to 4, characterized in that, Using the characteristics of the voltage disturbance waveform generated by the cable partial insulation defect as a basis to detect the voltage disturbance signal, specifically including: First, judge whether the voltage distortion waveform drop depth range ΔU of the voltage disturbance signal is between 0.1 and 0.9 times the normal voltage peak value. When the distortion signal meets the condition, proceed to the next judgment; then judge whether the voltage U-shaped distortion duration Δt is between 1 / 8 and 1 / 4. Meeting the condition indicates that the single power frequency cycle signal is the distortion signal generated by the local insulation defect of the cable; Judge the data of the next sampling cycle. If consecutive cycles meet the judgment conditions, it is judged as a serious local insulation defect, otherwise it is a general local insulation defect.

6. A cable defect detection system based on transient disturbance characteristics, characterized in that, Including: A voltage disturbance signal acquisition unit, which acquires the voltage disturbance signal generated by the local insulation defect of the cable through the local insulation defect test of the cable, and conducts statistical analysis on the voltage disturbance signal; A characteristic parameter extraction unit, which analyzes the main characteristics of the voltage disturbance signal, obtains the change ranges of different characteristic parameters, and performs amplitude conversion on the voltage disturbance signal after denoising to extract characteristic parameters; among them, the change ranges of the different characteristic parameters are used to detect the local insulation defect of the cable; A cable local insulation defect evaluation unit, which uses the characteristic parameters of the voltage disturbance signal for cable local insulation defect detection. First, detect the threshold of the voltage disturbance signal to judge whether it exceeds the normal threshold, then judge whether the characteristic parameters of the voltage disturbance signal meet the requirements, and repeat the judgment for the next cycle signal to evaluate the severity of the cable local insulation defect; Conduct statistical analysis on the observed voltage disturbance waveform. The distortion disturbances formed by the local insulation defect of the cable on the voltage waveform are characterized as follows: 1) The distortion starting point occurs before and after the voltage waveform peak, and the distance from the peak is ΔT, ΔT = -0.1 p.u. to +0.05 p.u.; a negative starting point means it occurs before the peak, and a positive starting point means it occurs after the peak; 2) The distortion waveform presents an asymmetric U shape; 3) The distortion waveform drop depth range ΔU is 0.1 to 0.9 times the normal voltage peak; 4) High-frequency oscillation components with different frequencies and amplitudes are superimposed on the U-shaped distortion; 5) The U-shaped distortion duration Δt is 1 / 8 to 1 / 4 cycle; 6) If the U-shaped distortion appears in consecutive cycles, it is a serious local insulation defect; otherwise, it is judged as a general local insulation defect.

7. A cable defect detection system based on transient disturbance characteristics according to claim 6, characterized in that In the voltage disturbance signal acquisition unit, the voltage disturbance waveform is obtained through experiments. The main process of the experiment is as follows: Build a cable local insulation defect test platform, create an artificial local insulation defect on the test cable, measure the relative ground voltage of the cable using a high-voltage probe, and collect and store the voltage data through an oscilloscope; By gradually increasing the voltage, observe the change of the voltage waveform in the oscilloscope until the voltage disturbance waveform generated by the local insulation defect of the cable is observed; repeat the experiment to obtain the voltage disturbance waveform data when there is a local insulation defect in the cable.

8. A cable defect detection system based on transient disturbance characteristics according to claim 7, characterized in that, The voltage disturbance signal needs to be preprocessed to extract the disturbance signal characteristics, specifically including: According to the measured relative ground voltage of the cable, the sampled signal is low-pass filtered to remove the interference of high-frequency signals. The instantaneous voltage is converted into the change of instantaneous amplitude through Kalman filtering, and the characteristics of the voltage disturbance signal are extracted based on the change of instantaneous amplitude: the distortion waveform drop depth range ΔU and the U-shaped distortion duration Δt.

9. A cable defect detection system based on transient disturbance characteristics according to any one of claims 6-8, characterized in that, In the cable partial insulation defect evaluation unit described above, based on the characteristics of the voltage disturbance waveform generated by the cable partial insulation defect, the voltage disturbance signal is detected, specifically including: First, it is judged whether the voltage distortion waveform drop depth range ΔU of the voltage disturbance signal is between 0.1 and 0.9 times the normal voltage peak value. When the distortion signal meets the condition, the next judgment is carried out; then it is judged whether the voltage U-shaped distortion duration Δt is between 1 / 8 and 1 / 4. Meeting the condition indicates that the single power frequency cycle signal is the distortion signal generated by the cable partial insulation defect; The data of the next sampling period is judged. If the judgment conditions are met in consecutive cycles, it is judged as a serious partial insulation defect, otherwise it is a general partial insulation defect.

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