Nondestructive positioning method for cable sheath grounding loop resistance defects based on low-voltage pulse traveling wave ranging

Abstract

The application discloses a kind of cable sheath ground loop resistance defect nondestructive positioning methods based on low-voltage pulse traveling wave ranging, belong to high-voltage cable operation and maintenance field.The method comprises the following steps: step one, the loop resistance of cable three-phase sheath ground loop is detected, and the qualified phase is distinguished from the abnormal phase;Step two, head detection, when single-phase anomaly, with the qualified phase as reference, the waveform of abnormal phase and another qualified phase is collected for comparison;When two-phase or three-phase anomaly, with the qualified phase or auxiliary cable as reference, the waveform is collected and compared with the typical waveform library;The distance of defect distance test point is calculated;Step three, after interchanging head and tail wiring, retest at tail end, output positioning result after checking the results of two times.The application does not need to excavate or damage cable sheath, is suitable for single-phase, two-phase, three-phase anomaly three kinds of working conditions, avoids missed detection and multiple point defect misjudgment through two-way checking, greatly reduces construction period and engineering cost.

Description

Technical Field

[0001] This invention relates to the field of high-voltage cable operation and maintenance, and in particular to a non-destructive method for locating cable sheath grounding circuit resistance defects based on low-voltage pulse traveling wave ranging. Background Technology

[0002] The low-voltage pulse reflection method is a common technique in cable fault detection. Its basic principle is as follows: a low-voltage pulse signal is injected into the cable under test. When this signal propagates along the cable to a point of impedance discontinuity (such as a fault point or joint), it is reflected. By measuring the time difference between the transmitted and reflected pulses and combining this with the known wave velocity, the location of the impedance discontinuity can be calculated. This method has been widely used for locating faults such as open circuits, short circuits, and low-resistance grounding in cables.

[0003] Based on this, Chinese invention patent CN110244189A proposes a method for locating ground faults in power lines. The method's steps include: first, fully discharging each phase of the cable to ground; then connecting the two clamps of a low-voltage pulse test lead to the two faulty phases respectively; injecting a low-voltage pulse into the cable and recording the reflected pulse generated at the impedance mismatch point; calculating the fault distance based on the time difference between the transmitted and reflected pulses; furthermore, comparing the reflected waveform of the faulty line with that of the intact cable, extracting the differences between the two waveforms, calculating the distance from the point of difference to the cable's starting point using a vector algorithm, obtaining the secondary calculated fault distance, and averaging the results of multiple measurements to further improve the location accuracy. This method, through waveform comparison and averaging of multiple measurements, improves the accuracy of fault location to a certain extent.

[0004] However, the technical solution described in CN110244189A is mainly aimed at locating phase-to-ground faults in power lines, and its test leads have two clamps directly connected to the faulty phase line. When this method is directly applied to the detection of resistance defects in the grounding circuit of a cable with a metal sheath, the following shortcomings exist: First, this method does not propose a corresponding wiring scheme and testing procedure for the special structure of the metal sheath grounding circuit, and cannot effectively distinguish between the gradual impedance anomalies caused by corrosion, loose connections, etc., inside the grounding circuit and the impedance abrupt changes of the phase-to-ground fault. Second, this method relies on an intact cable as a reference for waveform comparison. When multiple phase grounding circuits are abnormal at the same time and there is no intact circuit available for comparison, this method cannot be implemented. Third, this method does not involve the technical means of bidirectional testing at both ends of the cable line for mutual verification. Single-direction testing is easily affected by on-site electromagnetic interference, resulting in insufficient positioning reliability, and when there are multiple defects in the line, it is easy to miss distant defects. Summary of the Invention

[0005] The purpose of this invention is to provide a non-destructive method for locating cable sheath grounding circuit resistance defects based on low-voltage pulse traveling wave ranging, so as to solve the problems existing in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a non-destructive method for locating cable sheath grounding loop resistance defects based on low-voltage pulse traveling wave ranging, comprising the following steps: Step 1: In the event of a power outage on the high-voltage cable, adjust the cable grounding system to a single-end grounding method, and use a loop resistance tester to test the loop resistance of the three-phase sheath grounding loop of the cable to distinguish between phases with qualified resistance and phases with abnormal resistance. Step 2: First, perform testing at the cable end; if only one phase of the cable's sheath grounding circuit resistance is abnormal, execute the single-phase abnormality detection procedure; if two phases of the cable's sheath grounding circuit resistance is abnormal, execute the two-phase abnormality detection procedure; if all three phases of the cable's sheath grounding circuit resistance are abnormal, execute the three-phase abnormality detection procedure. The single-phase anomaly detection process is as follows: short-circuit and ground the three-phase grounding wires at the cable tail end, and leave the three-phase grounding wires at the cable head end suspended; connect a pulse traveling wave tester to the cable head end, connect the signal terminal of the pulse traveling wave tester to the grounding wire of the phase with abnormal resistance, and connect the reference terminal to the grounding wire of the first phase with qualified resistance, and acquire the first waveform; keep the reference terminal unchanged, switch the signal terminal to the grounding wire of the second phase with qualified resistance, and acquire the second waveform; compare the location and time delay of the distortion points of the first waveform and the second waveform, and calculate the distance L from the defect to the test point. x =v*Δt / 2, where v is the traveling wave velocity and Δt is the time delay at the waveform distortion point; The two-phase anomaly detection process is as follows: short-circuit and ground the three-phase grounding wires at the cable tail end, and leave the three-phase grounding wires at the cable head end suspended; connect a pulse traveling wave tester to the cable head end, connect the reference terminal of the pulse traveling wave tester to the grounding wire of the phase with acceptable resistance, and connect the signal terminal to the grounding wires of the two phases with abnormal resistance in sequence, and collect waveforms respectively; compare the measured waveforms with the pre-established typical waveform library to determine the location and time delay of the waveform distortion point, and calculate the distance L from the defect to the test point. x =v*Δt / 2, where v is the traveling wave velocity and Δt is the time delay at the waveform distortion point; The three-phase anomaly detection process is as follows: An independent auxiliary cable is laid along the cable laying path as an external reference ground wire; the three-phase ground wire at the cable tail end is short-circuited and grounded to the auxiliary cable, while the three-phase ground wire at the cable head end and the auxiliary cable are both suspended; a pulse traveling wave tester is connected to the cable head end, the reference end of the pulse traveling wave tester is connected to the auxiliary cable, and the signal end is sequentially connected to the A, B, and C phase ground wires to collect the three-phase waveforms; the measured waveforms are compared with a pre-established typical waveform library to determine the location and time delay of the waveform distortion point, and the distance L from the defect to the test point is calculated. x=v*Δt / 2, where v is the traveling wave velocity and Δt is the time delay at the waveform distortion point; Step 3: After swapping the wiring status of the cable tail end and head end, repeat the test at the tail end according to the corresponding abnormal working condition test procedure, cross-check the two test results, and output the positioning result.

[0007] This invention relies entirely on injecting low-voltage pulse traveling wave signals into the cable grounding wire for detection, without damaging the cable's metal sheath throughout the process. This fundamentally eliminates the risk of secondary defects such as water ingress and moisture absorption after sheath damage, significantly reducing construction time and project costs. The detection method is applicable to single-phase, two-phase, and three-phase abnormalities, adapting to various on-site operating conditions. Simultaneously, bidirectional testing at both ends of the cable is used for mutual verification, effectively suppressing stray electromagnetic interference at the power frequency, significantly improving positioning reliability, and avoiding missed detection of multiple defects.

[0008] Preferably, in step two, the waveform acquisition adopts a multi-level pulse comprehensive detection strategy. The pulse amplitude is fixed at 20V, and the pulse width is adjustable from 20ns to 1000ns, with the pulse width increasing sequentially from low to high. The waveform that clearly reflects the defect reflection is detected and recorded. Among them, the narrow pulse has a small near-end detection blind zone and is suitable for detecting near-end defects; the wide pulse has high energy and improves the signal-to-noise ratio of weak impedance reflection signals at the far end, making it suitable for detecting far-end defects. The multi-level pulse comprehensive detection strategy can ensure that defects at the near end, middle, and far end of the cable are all effectively detected. If ultra-narrow pulse hardware is not available, the near-end connector can be moved out of the detection blind zone by adding an equivalent external test endpoint to the test end of the extension cable.

[0009] The combination of narrow and wide pulses can simultaneously meet the requirements of small near-end detection blind zone and high signal-to-noise ratio of far-end reflected signal, ensuring that defects along the entire length of the cable can be effectively detected; adding extension cables can effectively eliminate the near-end detection blind zone without replacing hardware equipment, reducing equipment cost requirements.

[0010] Preferably, in step two, after acquiring the waveform, preprocessing and noise reduction are performed: under each pulse parameter, ≥10 sets of waveforms are repeatedly acquired at a single point for mean filtering, combined with a power frequency bandpass digital filter to filter out 50Hz electromagnetic interference from the field. This eliminates false distortion points caused by clutter.

[0011] By combining mean filtering with power frequency bandpass digital filtering, 50Hz power frequency interference and random noise in complex electromagnetic environments can be effectively suppressed, avoiding false distortion points caused by noise and improving the accuracy of defect identification and the reliability of detection results.

[0012] Preferably, before step two, the electrical connection between the cable sheath grounding wire and the relay protection device and online monitoring device is disconnected.

[0013] It can effectively prevent low-voltage pulse signal-induced overvoltage from entering the secondary weak current circuit during the testing process, avoid equipment damage, and ensure the safety of testing operations.

[0014] Preferably, the quantification standard for judging waveform distortion in step two is as follows: at the same time delay position, if the abnormal loop waveform has a bulge and the difference in reflection amplitude between it and the reference loop waveform is greater than a preset threshold ΔU, it is determined that there is a loop resistance defect at that position, where ΔU = 10mV. The defect types corresponding to the impedance abrupt change point include loose grounding conductor connection, corrosion of connection point, poor crimping of grounding terminal, and cracking of lead seal.

[0015] By setting the amplitude difference quantization threshold ΔU=10mV as the criterion for waveform distortion, a clear and operable quantitative basis is provided for defect identification, avoiding the uncertainty brought about by human experience judgment and improving the consistency and repeatability of detection results.

[0016] Preferably, the quantitative criterion for cross-checking the two test results in step three is as follows: set the defect distance L1 measured at the tail end, the defect distance L2 measured at the head end, the total length L of the cable under test, and the allowable error threshold ΔL; if |L1+L2-L|≤ΔL, it is determined that the two test results are the same defect; if the difference exceeds ΔL, the wiring is re-checked and retested; if the difference still exceeds the standard after retesting, it is determined that there are multiple impedance defects in the line. At this time, there is one defect at the waveform distortion point of the two tests at the head end and tail end, respectively, and ΔL=L×10%.

[0017] By using two-way verification and quantitative criteria, a clear judgment standard is provided for distinguishing between single defects and multiple defects, effectively avoiding misjudgments caused by on-site interference during single-direction testing; when there are multiple defects in the line, the defects near their respective ends can be located by two tests at the beginning and end, avoiding missed detections.

[0018] Preferably, the conductor of the auxiliary cable is a copper or aluminum conductor with a cross-section of not less than 6 square millimeters, and is a bare wire or a wire with insulation, laid on the ground along the path of the cable segment to be tested.

[0019] The auxiliary cable uses copper or aluminum conductors with a cross-section of not less than 6 square millimeters, which can ensure that the signal circuit has a sufficiently low impedance and reduce signal attenuation; it can be laid on the ground without burial, making on-site operation simple and quick, without additional construction, further reducing testing costs.

[0020] Preferably, the typical waveform library detects and stores the waveform characteristics of typical defect locations by arranging typical defect locations on a simulated line, and includes at least four types of defects: near-end high-resistance defects, near-end low-resistance defects, far-end high-resistance defects, and far-end low-resistance defects. Near-end defects refer to defects within 20 meters of the current test end, and far-end defects refer to defects within 20 meters of the opposite end of the cable. When testing for the first time at the beginning end, 'near end' refers to the beginning end, and 'far end' refers to the end end. When testing again at the end end, 'near end' refers to the end end, and 'far end' refers to the beginning end.

[0021] By comparing the measured waveform with a typical waveform library, it is possible to quickly determine whether the defect is located near or far, and whether it is a high-resistance or low-resistance defect. This provides a basis for maintenance personnel to prepare corresponding maintenance plans in advance, and greatly improves the efficiency of on-site inspection.

[0022] Preferably, the calculation of the distance between the defect and the test point in step two is divided into two modes: Mode 1 (qualitative coarse judgment): Since the cable segment for locating the loop resistance defect generally only involves two cable joints (or terminals), and the defect location is concentrated near the joints (or terminals), there is no need for a particularly precise wave velocity. A fixed traveling wave velocity of 190m / μs is uniformly used to coarsely calculate the distance between the defect and the test point, qualitatively determine whether the defect is located at the near end or the far end, and lock the joint or terminal to which the defect belongs; Mode 2 (precise quantitative calculation): When there is an intact grounding loop and the cable length of the intact loop is known, the traveling wave propagation velocity is precisely calibrated on the intact loop. During detection, the distance between the defect point and the test point is precisely calculated based on the time difference between the transmitted pulse and the reflected pulse and the calibrated wave velocity.

[0023] Two calculation modes can be flexibly selected. In the rapid on-site investigation stage, the qualitative coarse judgment mode is used to quickly locate the joint to which the defect belongs, without the need for precise calibration of wave velocity, which greatly improves the detection efficiency. When precise positioning is required, mode two is used for accurate quantitative calculation. The two modes complement each other and take into account the different needs of detection efficiency and positioning accuracy.

[0024] Preferably, after the test is completed, the residual charge in the circuit is discharged, and after the voltage drops to 0, all wiring and secondary equipment connections are restored.

[0025] By fully discharging the residual charge in the circuit and waiting for the voltage to drop to 0 before reconnecting the wiring, it is possible to effectively avoid electric shock to the testing personnel caused by the residual charge. At the same time, the standardized operation of the secondary equipment connection restoration sequence ensures the safe commissioning of the equipment.

[0026] The beneficial effects of this invention are as follows: This invention abandons the traditional operation mode of locating cable grounding circuit resistance defects by excavating direct-buried trenches and breaking open the cable outer sheath. It relies entirely on injecting low-voltage pulse traveling wave signals into the cable grounding wire for detection, without damaging the cable's metal sheath throughout the process. This fundamentally eliminates the risk of secondary defects such as water ingress and moisture after sheath damage, significantly reducing the construction cycle and project costs. The detection method is applicable to the three common grounding circuit abnormalities: single-phase abnormality, two-phase abnormality, and three-phase abnormality, and its adaptability covers various on-site operating conditions. For single-phase abnormalities in grounding circuits, impedance mutation points are accurately identified by comparing with intact circuits. For two-phase and three-phase abnormalities in grounding circuits, impedance mutation points are accurately identified by rapid comparison with a pre-established standard waveform feature library. At the same time, bidirectional testing at both ends of the cable is used for mutual verification, effectively suppressing stray electromagnetic interference at the power frequency on site, significantly improving the reliability of location, and avoiding missed detection of multiple defects. It provides clear guidance for on-site maintenance decisions, further improving the efficiency of investigation and the practicality of the project. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the process of this invention; Figure 2 This is a schematic diagram of the wiring for detecting single-phase abnormalities in this invention; Figure 3 This is a schematic diagram of the detection wiring for two-phase abnormalities in this invention; Figure 4 This is a schematic diagram of the wiring for detecting three-phase abnormalities in this invention; Figure 5 This is a comparison diagram of typical traveling wave reflection waveforms in this invention. Detailed Implementation

[0028] like Figure 1 As shown, the detection object of the present invention is the grounding circuit of the metal sheath of high-voltage power cable, and it is especially applicable to cable lines with voltage levels of 66kV and above that are directly buried, laid in pipes or tunnels.

[0029] Before testing, a loop resistance tester must be used to conduct a general survey of the three-phase sheath grounding circuit of the cable to identify phases with acceptable resistance and those with abnormal resistance. For cable lines with cross-interconnected grounding, the cross-interconnection plate must be opened before testing, and the test should be conducted according to the single-end grounding method. According to relevant power industry regulations, a loop resistance of 200mΩ or higher in a high-voltage cable grounding system is considered abnormal and requires power outage inspection. This testing method is specifically designed for power outage inspection of lines with defects in loop resistance exceeding 200mΩ. Based on the survey results, select a single-phase abnormality testing procedure, a two-phase abnormality testing procedure, or a three-phase abnormality testing procedure.

[0030] I. Specific Implementation Methods for Abnormal Resistance in Single-Phase Grounding Circuits like Figure 2As shown, when the DC resistance of the sheath grounding circuit of only one phase (e.g., phase A) exceeds the standard, while the resistance values ​​of phases B and C are within acceptable limits, follow these steps: First, disconnect the electrical connection between the cable sheath grounding wire and the relay protection device and online monitoring device to prevent low-voltage pulse induced overvoltage from entering the secondary weak current circuit and causing equipment damage during the detection process.

[0031] The wiring method is as follows: Short-circuit and reliably ground the three-phase grounding wires at the cable tail end. Leave all three-phase grounding wires at the cable head end unconnected to any grounding electrode. Connect a pulse traveling wave tester to the cable head end. Connect the signal terminal of the tester to the grounding wire of phase A (abnormal phase), and the reference terminal to the grounding wire of phase B (qualified phase). Start the tester to emit a low-voltage pulse, injecting a low-voltage pulse into the grounding circuit. The pulse amplitude is fixed at 20V, and the pulse width is 20ns-1000ns, increasing the pulse width from low to high. Acquire the first waveform.

[0032] Keeping the connection between the reference terminal of the tester and the grounding wire of phase B unchanged, disconnect the signal terminal from the grounding wire of phase A and switch it to the grounding wire of phase C (another qualified phase). Keeping the pulse parameters unchanged, transmit the low-voltage pulse again and collect the second waveform.

[0033] The first and second waveforms are compared and analyzed on the same time axis. If the reflection distortion point appears at the same time position in both waveforms, it is determined to be a normal abrupt change in the loop impedance and is not considered a defect point, so it is excluded. If a distortion point appears only in the first waveform and not in the second waveform, it is determined to be an impedance abrupt change point on the A-phase grounding loop, i.e., the location of the A-phase loop resistance defect. The distance L from the defect to the test point is calculated. x =v*Δt / 2, where v is the traveling wave velocity and Δt is the time delay at the waveform distortion point.

[0034] To ensure accurate positioning, the wiring configuration is reversed: the three-phase grounding wires at the beginning of the cable are short-circuited and grounded, while all three-phase grounding wires at the end are left unconnected. A pulse traveling wave tester is connected to the end of the cable, and phase B is used as the reference terminal, with phases A and C as the signal terminals. Two sets of waveforms are collected and compared to calculate the distance from the defect point to the detection terminal at the end. The distance measurement results at both ends are cross-checked. If |L1+L2-L|≤L×10%, the positioning result is verified to be accurate and reliable. If the difference exceeds the allowable range, the wiring is checked for reliability and the grounding busbar contact is checked before retesting. If the difference still exceeds the standard after retesting, it is determined that there are multiple impedance defects in the line. In this case, there is one defect at the waveform distortion point of each of the two tests at the beginning and end.

[0035] II. Specific Implementation Methods for Abnormal Resistance in Both-Phase Grounding Circuits like Figure 3As shown, when the DC resistance of both phase sheath grounding circuits exceeds the standard, it is impossible to construct a complete circuit inside the cable as a reference. At this time, the location of the defect is determined by comparing the detected waveform with the typical waveform library.

[0036] The wiring method is as follows: Short-circuit and reliably ground the three-phase grounding wires at the cable tail end, and leave all three-phase grounding wires at the cable head end unconnected to any grounding electrode. Connect the reference terminal of the pulse traveling wave tester to the grounding wire of the phase with acceptable resistance, and connect the signal terminal sequentially to the grounding wires of the two phases with abnormal resistance. Set the pulse amplitude to a fixed 20V and the pulse width to 20ns-1000ns, increasing the pulse width from low to high, and acquire waveforms accordingly. Compare the measured waveforms with a pre-established typical waveform library to identify the location and delay of waveform distortion points, and calculate the distance L from the defect to the test point. x =v*Δt / 2, where v is the traveling wave velocity and Δt is the time delay at the waveform distortion point.

[0037] The method of bidirectional testing and mutual verification at both ends of the cable is still adopted. The test is performed once at the end and once at the beginning. The location of the defect is finally determined after comparing and verifying the positioning results at both ends.

[0038] III. Specific Implementation Methods for All Three-Phase Grounding Circuit Resistances Being Abnormal like Figure 4 As shown, when the DC resistance of the three-phase sheath grounding circuit exceeds the standard simultaneously, there are no qualified and intact phases inside the cable that can be used as a reference. In this case, an independent auxiliary cable is laid along the cable's surface laying path as an external reference ground wire. The defect location is then determined by comparing the detected waveform with a typical waveform library. The conductor of this auxiliary cable is a copper or aluminum conductor with a cross-section of not less than 6 square millimeters. It can be a bare conductor or a conductor with insulation. It can be laid on the ground along the path of the cable section to be tested.

[0039] The wiring method is as follows: Short-circuit and ground the three-phase grounding wire at the cable tail end to the auxiliary cable, while leaving both the three-phase grounding wire at the cable head end and the auxiliary cable suspended. Fix the reference end of the pulse traveling wave tester to the head end of the auxiliary cable, and sequentially connect the signal end to the A-phase, B-phase, and C-phase sheath grounding wires at the cable head end. Acquire the three-phase waveforms under the same pulse parameters. Compare the measured waveforms with a pre-established typical waveform library to identify the location and delay of waveform distortion points, and calculate the distance L from the defect to the test point. x =v*Δt / 2, where v is the traveling wave velocity and Δt is the time delay at the waveform distortion point.

[0040] The method of bidirectional testing and mutual verification at both ends of the cable is still adopted. The test is performed once at the end and once at the beginning. The location of the defect is finally determined after comparing and verifying the positioning results at both ends.

[0041] IV. Specific Implementation Methods for Rapid Judgment Using Typical Waveform Libraries The waveform acquisition employs a multi-level pulse comprehensive detection strategy. The pulse amplitude is fixed at 20V, and the pulse width is adjustable from 20ns to 1000ns, with the pulse width increasing sequentially from low to high. The system detects and records waveforms that clearly reflect defect reflections. Narrow pulses have a small near-end detection blind zone, suitable for detecting near-end defects; wide pulses have high energy, improving the signal-to-noise ratio of weak impedance reflection signals at the far end, suitable for detecting far-end defects. This multi-level pulse comprehensive detection strategy ensures effective detection of defects at the near, middle, and far ends of the cable. If ultra-narrow pulse hardware is unavailable, an equivalent external test endpoint can be added to the test end to move the near-end connector out of the detection blind zone.

[0042] After acquiring the waveform, preprocessing and noise reduction are performed: Under each pulse parameter, ≥10 sets of waveforms are repeatedly acquired at a single point for mean filtering, and combined with power frequency bandpass digital filtering to filter out 50Hz electromagnetic interference on site and eliminate false distortion points caused by noise.

[0043] The quantitative standard for judging waveform distortion is: at the same time delay position, if the abnormal loop waveform has a bulge and the difference between the reflection amplitude of the abnormal loop waveform and the reference loop waveform is greater than the preset threshold ΔU, it is determined that there is a loop resistance defect at that position, ΔU=10mV.

[0044] There are two modes for calculating the distance between the defect and the test point: Mode 1 uses a fixed traveling wave velocity of 190m / μs to roughly calculate the distance between the defect and the test point, qualitatively determine whether the defect is located at the near end or the far end, and locate the joint or terminal to which the defect belongs; Mode 2, when there is an intact grounding loop and the cable length of the intact loop is known, the traveling wave propagation velocity is accurately calibrated on the intact loop, and the distance between the defect point and the test point is accurately calculated based on the time difference between the transmitted pulse and the reflected pulse and the calibrated wave velocity during the test.

[0045] Statistical analysis of extensive field testing data reveals that cable grounding circuit resistance defects mostly occur at cable joint locations. For lines where a single test simultaneously covers both near-end and far-end cable joints, typical traveling wave reflection waveforms of normal grounding circuits of the same specification are pre-collected to establish a typical waveform library. This library is established by simulating typical defect locations on a line, detecting and storing the waveform characteristics of these locations, such as near-end high-resistance defects, near-end low-resistance defects, far-end high-resistance defects, and far-end low-resistance defects. During testing, the measured waveform of the abnormal circuit is compared with the corresponding waveform in the standard waveform feature library. The comparison criteria are: comparing the test results from step two with the standard waveform library to identify the location and time delay of distortion points, and calculating the distance L from the defect to the test point. x=v*Δt / 2, where v is the traveling wave velocity and Δt is the waveform distortion point delay. Near-end defects refer to defects within 20 meters of the current test end, and far-end defects refer to defects within 20 meters of the opposite end of the cable. When testing for the first time at the beginning end, 'near-end' refers to the beginning end, and 'far-end' refers to the end end; when testing again at the end end, 'near-end' refers to the end end, and 'far-end' refers to the beginning end.

[0046] V. Safe Recovery Procedures After all tests are completed, first discharge the cable sheath grounding circuit to the ground using a discharge rod. After the residual voltage drops to 0V, remove the test wiring, restore the original connection of the three-phase grounding wires at both ends of the cable, and restore the disconnected electrical connections of external secondary equipment one by one. Only after confirming that everything is correct can you leave the test site.

[0047] Example 1 This embodiment uses a 500-meter-long 110kV cable line as an example to illustrate the method for locating abnormal single-phase grounding loop resistance. The three-phase sheath grounding loops of the cable are tested one by one using a loop resistance tester. The test results are: Phase A 3000mΩ, Phase B 40mΩ, and Phase C 35mΩ. Therefore, the sheath grounding loop resistance of Phase A exceeds the standard, while the grounding loop resistances of Phases B and C are within acceptable limits.

[0048] like Figure 2 As shown, the three-phase grounding wires at the cable's tail end are short-circuited and connected to the grounding grid for reliable grounding, while all three-phase grounding wires at the cable's head end are left suspended and disconnected. A pulse traveling wave tester is connected to the head end, with the tester's signal terminal connected to the A-phase (abnormal phase) grounding wire and the reference terminal connected to the B-phase (qualified phase) grounding wire. A low-voltage pulse is injected into the grounding circuit, with a fixed pulse amplitude of 20V and pulse widths set to 20ns and 500ns respectively, and the first waveform is acquired.

[0049] Keeping the reference terminal wiring of the tester unchanged, switch the signal terminal to the C phase (qualified phase) ground wire, and acquire the second waveform under the same pulse parameters as the first group of tests.

[0050] like Figure 5 As shown, the first and second waveforms are aligned with the same time base, and the amplitude difference is calculated point by point to plot the difference curve. The abrupt change in amplitude in the difference curve is the impedance change point. In this embodiment, comparing the two sets of waveforms, it is found that the first waveform has a significant waveform bulge 10 meters from the beginning, while the second waveform has only a slight bulge at the same position. Therefore, the defect is located at the joint position 10 meters from the beginning.

[0051] Adjust the wiring topology, short-circuiting the three-phase grounding wire at the first end to ground, and leaving the three-phase grounding wire at the last end suspended. Repeat the above testing procedure at the last end in the same way. The retest results at the last end show that the defect point is 505 meters away from the last end. |10 meters + 505 meters - 500 meters| = 15 meters, 500 meters × 10% = 50 meters, 15 meters ≤ 50 meters, so the test results at both ends are located at the same joint, which can corroborate each other and confirm that the defect location is accurate. Only a small-scale exploratory pit repair is needed at this point, without excavating other joints.

[0052] Example 2 This embodiment takes the simultaneous occurrence of high resistance faults in the grounding circuits of two phase sheaths of an 800-meter 110kV cable line as an example to illustrate the location method under abnormal two-phase operating conditions.

[0053] The circuit resistance was tested in advance using a circuit resistance tester. The test results were 50mΩ for phase A, greater than 5000mΩ for phase B, and greater than 5000mΩ for phase C. It was confirmed that phase A was normal, while the sheath grounding circuit resistance of phases B and C was abnormal.

[0054] like Figure 3 As shown, the three-phase grounding wires at the tail end of the cable are short-circuited and grounded, while the three-phase grounding wires at the head end are left unconnected. A pulse traveling wave tester is connected to the head end, and the reference end is fixedly connected to the grounding wire of phase A (qualified phase). The signal end is connected to the grounding wires of phase B and phase C (abnormal phase) in sequence. The pulse amplitude is fixed at 20V, and the pulse width is set to 20ns and 1000ns respectively. The waveforms of phase B and phase C are collected respectively.

[0055] The waveforms acquired from phases B and C are aligned and superimposed using the same time reference, and compared with a typical waveform library. The defect location is directly determined based on the position of the abnormal bulge in the waveform. In this embodiment, a significant abnormal bulge appears in the waveform of phase B at a distance of 10 meters from the test end, which matches the waveform characteristics of a near-end high-resistivity defect. Therefore, the defect in phase B is located at the near-end joint. A significant abnormal bulge appears in the waveform of phase C at a distance of 780 meters from the test end, which matches the waveform characteristics of a far-end high-resistivity defect. Therefore, the defect in phase C is located at the far-end joint.

[0056] By employing a bidirectional detection and cross-verification method at both ends of the cable, the location of the defect is finally determined after comparing and verifying the positioning results at both ends. Precise positioning can be achieved without excavation. During maintenance, only the joint with the located defect needs to be excavated; other joints do not require excavation, significantly reducing costs.

[0057] Example 3 This embodiment takes the simultaneous occurrence of a high resistance fault in a 150-meter 220kV three-phase sheath grounding circuit as an example to illustrate the location method under abnormal three-phase operating conditions.

[0058] A loop resistance tester was used to test the resistance in advance. The test results showed that the resistance of phase A was greater than 5000mΩ, phase B was greater than 5000mΩ, and phase C was greater than 5000mΩ. It was confirmed that the grounding loop resistance of the three phases A, B, and C was abnormal.

[0059] like Figure 4 As shown, an independent auxiliary cable is laid along the cable's surface path as an external reference ground wire. The auxiliary cable is a copper conductor with an insulated sheath, with a cross-section of 6 square millimeters, and is laid on the ground along the path of the cable section to be tested. The three-phase grounding wire at the cable's tail end is short-circuited and grounded to the auxiliary cable. The three-phase grounding wire at the cable's head end and the auxiliary cable are both suspended. A pulse traveling wave tester is connected to the head end. The reference end is uniformly connected to the auxiliary cable, and the signal end is sequentially connected to the A, B, and C phase sheath grounding wires, respectively. The pulse amplitude is fixed at 20V, and the pulse width is set to 20ns and 200ns, respectively, and the waveforms of phases A, B, and C are collected.

[0060] The waveforms acquired from phases A, B, and C are aligned and superimposed using the same time reference. The three-phase waveforms are then compared with a typical waveform library, and the defect location is directly determined based on the location of any abnormal bulges in the waveforms. In this embodiment, phase A exhibits a significant abnormal bulge at a distance of 10 meters from the test end, consistent with the waveform characteristics of a near-end high-resistivity defect; therefore, the defect in phase A is located at the near-end joint. Similarly, phases B and C both exhibit significant abnormal bulges at a distance of 145 meters from the test end, consistent with the waveform characteristics of a far-end high-resistivity defect; therefore, the defects in phases B and C are located at the far-end joint.

[0061] By employing a bidirectional detection and cross-verification method at both ends of the cable, the location of the defect is finally determined after comparing and verifying the positioning results at both ends. Precise positioning can be achieved without excavation. During maintenance, only the joint with the located defect needs to be excavated; other joints do not require excavation, significantly reducing costs.

Claims

1. A non-destructive method for locating cable sheath grounding loop resistance defects based on low-voltage pulse traveling wave ranging, characterized in that, Includes the following steps: Step 1: In the event of a power outage on the high-voltage cable, adjust the cable grounding system to a single-end grounding method, and use a loop resistance tester to test the loop resistance of the three-phase sheath grounding loop of the cable to distinguish between phases with qualified resistance and phases with abnormal resistance. Step 2: First, perform testing at the cable end; if only one phase of the cable's sheath grounding circuit resistance is abnormal, execute the single-phase abnormality detection procedure; if two phases of the cable's sheath grounding circuit resistance is abnormal, execute the two-phase abnormality detection procedure; if all three phases of the cable's sheath grounding circuit resistance are abnormal, execute the three-phase abnormality detection procedure. The single-phase anomaly detection process is as follows: short-circuit and ground the three-phase grounding wires at the cable tail end, and leave the three-phase grounding wires at the cable head end suspended; connect a pulse traveling wave tester to the cable head end, connect the signal terminal of the pulse traveling wave tester to the grounding wire of the phase with abnormal resistance, and connect the reference terminal to the grounding wire of the first phase with qualified resistance, and acquire the first waveform; keep the reference terminal unchanged, switch the signal terminal to the grounding wire of the second phase with qualified resistance, and acquire the second waveform; compare the location and time delay of the distortion points of the first waveform and the second waveform, and calculate the distance L from the defect to the test point. x =v*Δt / 2, where v is the traveling wave velocity and Δt is the time delay at the waveform distortion point; The two-phase anomaly detection process is as follows: short-circuit and ground the three-phase grounding wires at the tail end of the cable, and leave the three-phase grounding wires at the head end suspended; connect a pulse traveling wave tester to the head end, connect the reference end of the pulse traveling wave tester to the grounding wire of the phase with qualified resistance, and connect the signal end to the grounding wires of the two phases with abnormal resistance in sequence, and collect the waveforms respectively. The measured waveform is compared with a pre-established typical waveform library to determine the location and time delay of waveform distortion points, and the distance L from the defect to the test point is calculated. x =v*Δt / 2, where v is the traveling wave velocity and Δt is the time delay at the waveform distortion point; The three-phase anomaly detection process is as follows: an independent auxiliary cable is laid along the cable laying path as an external reference ground wire; the three-phase ground wire at the end of the cable is short-circuited and grounded to the auxiliary cable, and the three-phase ground wire at the beginning of the cable and the auxiliary cable are both suspended; a pulse traveling wave tester is connected to the beginning of the cable, the reference end of the pulse traveling wave tester is connected to the auxiliary cable, and the signal end is connected to the three-phase ground wires A, B, and C in sequence to collect the three-phase waveforms respectively; The measured waveform is compared with a pre-established typical waveform library to determine the location and time delay of waveform distortion points, and the distance L from the defect to the test point is calculated. x =v*Δt / 2, where v is the traveling wave velocity and Δt is the time delay at the waveform distortion point; Step 3: After swapping the wiring status of the cable tail end and head end, repeat the test at the tail end according to the corresponding abnormal working condition test procedure, cross-check the two test results, and output the positioning result.

2. The non-destructive positioning method for cable sheath grounding loop resistance defects based on low-voltage pulse traveling wave ranging according to claim 1, characterized in that, In step two, the waveform acquisition adopts a multi-level pulse comprehensive detection strategy. The pulse amplitude is fixed at 20V, and the pulse width is adjustable from 20ns to 1000ns. The pulse width is increased from low to high to detect and record waveforms that can clearly reflect defect reflections. Narrow pulses are used to detect near-end defects, and wide pulses are used to detect far-end defects. If ultra-narrow pulse hardware is unavailable, the near-end connector can be moved out of the detection blind zone by adding an extension cable to the test end to effectively move the test endpoint outward.

3. The non-destructive positioning method for cable sheath grounding loop resistance defects based on low-voltage pulse traveling wave ranging according to claim 1, characterized in that, In step two, after acquiring the waveform, preprocessing and noise reduction are performed: under each pulse parameter, ≥10 sets of waveforms are repeatedly acquired at a single point for mean filtering, and combined with power frequency bandpass digital filtering to filter out 50Hz electromagnetic interference on site.

4. The non-destructive positioning method for cable sheath grounding loop resistance defects based on low-voltage pulse traveling wave ranging according to claim 1, characterized in that, Before step two, disconnect the electrical connection between the cable sheath grounding wire and the relay protection device and online monitoring device.

5. The non-destructive positioning method for cable sheath grounding loop resistance defects based on low-voltage pulse traveling wave ranging according to claim 1, characterized in that, The quantitative standard for judging waveform distortion in step two is as follows: at the same time delay position, if the abnormal loop waveform has a bulge and the difference between the reflection amplitude of the abnormal loop waveform and the reference loop waveform is greater than the preset threshold ΔU, it is determined that there is a loop resistance defect at this position, ΔU=10mV.

6. The non-destructive positioning method for cable sheath grounding loop resistance defects based on low-voltage pulse traveling wave ranging according to claim 1, characterized in that, The quantitative criteria for cross-checking the two test results in step three are as follows: set the defect distance L1 measured at the tail end, the defect distance L2 measured at the head end, the total length L of the cable under test, and the allowable error threshold ΔL; if |L1+L2-L|≤ΔL, it is determined that the two test results are the same defect; if the difference exceeds ΔL, the wiring is rechecked and the test is repeated; if the difference still exceeds the standard after the retest, it is determined that there are multiple impedance defects in the line. At this time, there is one defect at the waveform distortion point of the two tests at the head end and the tail end, respectively, and ΔL=L×10%.

7. The non-destructive positioning method for cable sheath grounding loop resistance defects based on low-voltage pulse traveling wave ranging according to claim 1, characterized in that, The conductor of the auxiliary cable is a copper or aluminum conductor with a cross-section of not less than 6 square millimeters. It is a bare wire or a wire with insulation and is laid on the ground along the path of the cable section to be tested.

8. The non-destructive positioning method for cable sheath grounding loop resistance defects based on low-voltage pulse traveling wave ranging according to claim 1, characterized in that, The typical waveform library detects and stores the waveform characteristics of typical defect locations by arranging typical defect locations on a simulated line. It includes at least four types of defects: near-end high resistance defects, near-end low resistance defects, far-end high resistance defects, and far-end low resistance defects. Near-end defects refer to defects within 20 meters of the current test end, and far-end defects refer to defects within 20 meters of the opposite end of the cable.

9. The non-destructive positioning method for cable sheath grounding loop resistance defects based on low-voltage pulse traveling wave ranging according to claim 1, characterized in that, In step two, the calculation of the distance between the defect and the test point is divided into two modes: Mode 1, which uses a fixed traveling wave velocity of 190m / μs to roughly calculate the distance between the defect and the test point, qualitatively determine whether the defect is located at the near end or the far end, and lock the joint or terminal to which the defect belongs; Mode 2, when there is an intact grounding loop and the cable length of the intact loop is known, the traveling wave propagation velocity is accurately calibrated on the intact loop, and the distance between the defect point and the test point is accurately calculated based on the time difference between the transmitted pulse and the reflected pulse and the calibrated wave velocity during the test.

10. The non-destructive positioning method for cable sheath grounding loop resistance defects based on low-voltage pulse traveling wave ranging according to claim 1, characterized in that, After the test is completed, discharge the residual charge in the circuit. Once the voltage drops to 0, restore all wiring and secondary equipment connections.

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