Cable terminal tail pipe breakdown fault early warning method based on total current component
Through the method based on full current component, combined with simulation model and high-frequency signal detection, an effective warning of breakdown fault of the cable terminal tail tube is achieved, and the problem of low sensitivity to determining abnormal signal of grounding current in the prior art is solved, and the operation reliability and maintenance cost of cable lines are reduced.
Patent Information
- Application Number
- CN202510400571.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-01
- Publication Date
- 2025-07-01
AI Technical Summary
The prior art lacks a cable terminal tail tube breakdown fault warning method based on ground current abnormal signal, and relies solely on high-frequency discharge pulse signals to determine that the insulation healthy state under the tail tube is not sensitive and is susceptible to external noise interference.
The cable terminal tailpipe breakdown fault warning method is adopted based on the full current component, and the grounding current amplitude and high-frequency characteristic components under different grounding states are determined through simulation models, and the connection between abnormal electrical signals and tailpipe grounding system faults is established to achieve an effective early warning of the cable terminal tailpipe breakdown faults.
The online assessment of the health status of insulation under the tail tube of the cable terminal is realized, and equipment maintenance can be carried out in a timely manner before a fault occurs, reducing maintenance costs and improving the operating reliability of cable lines.
Smart Images
Figure CN120233195A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of fault diagnosis and online monitoring of high-voltage cable accessories, and particularly to a method for warning of breakdown faults of the tail pipe of a cable terminal based on the total current component. Background Art
[0002] With the advancement of urban construction, high-voltage overhead lines have gradually been replaced by cable lines, and high-voltage cable lines have been widely used in the power system. The reliable operation of cable equipment is the premise for the stable operation of the line. As an important component of the power equipment in the transmission network, once a cable equipment fails, it will cause great economic losses and safety problems. A cable includes a cable body and cable accessories, and a cable terminal belongs to the cable accessories and is installed at the end of the line to connect the cable to other electrical equipment.
[0003] Due to the influence of factors such as the design, installation process, and actual operating environment of the cable terminal itself, it often becomes a part where insulation faults occur frequently. As one of the weak links in the cable system, the operating reliability of the cable terminal will directly affect the overall safe operation of the cable line. Therefore, studying the breakdown fault principle of the tail pipe of an outdoor cable terminal and then proposing a method for warning of breakdown faults of the tail pipe of an outdoor cable terminal is of great significance for improving the operating reliability of the cable line.
[0004] Existing research has analyzed the formation mechanism of the breakdown fault of the tail pipe by disassembling and inspecting the faulty cable terminal samples, and pointed out that the failure of the cable aluminum sleeve grounding caused by the cracking of the lead seal is the key inducement of the fault. The cracking process of the lead seal can be divided into three stages: intact lead seal, partial cracking of the lead seal, and through cracking of the lead seal. When the lead seal is partially cracked, it is equivalent to an increase in the grounding resistance of the cable aluminum sleeve. At this time, the loop current path into the ground of the aluminum sleeve has not changed, but the loop current amplitude has decreased. When the lead seal is through-cracked, the direct electrical connection between the cable aluminum sleeve and the tail pipe is disconnected, and the loop current path of the aluminum sleeve changes. The aluminum sleeve loop current is released through the cable insulation outer shield - terminal bottom plate support cylinder - tail pipe - grounding wire. At this time, the cable aluminum sleeve is equivalent to being grounded in series with a large resistor, and the aluminum sleeve loop current amplitude decreases sharply. And existing research also points out that when the lead seal cracks, partial discharge phenomena will appear on the lower insulation surface of the tail pipe, and the high-frequency current generated by the discharge will also be released through the insulation outer shield - terminal bottom plate support cylinder - tail pipe - grounding wire. Therefore, it is possible to judge the health status of the insulation under the tail pipe of the cable terminal by detecting the amplitude change and component composition of the current on the grounding wire of the cable terminal tail pipe, realize effective warning of the breakdown fault of the cable terminal tail pipe, and achieve the effect of timely equipment maintenance before the fault occurs to reduce the maintenance cost.
[0005] Existing research has provided a variety of detection methods for internal discharge defects in cable systems. Among them, the high-frequency partial discharge detection method detects discharge signals to achieve the detection of fault defects, which is a commonly used detection means for cable system faults. Currently, the detection frequency generally ranges from 1 to 300 MHz. The partial discharge source in the cable system can be regarded as a point pulse signal source. When partial discharge occurs inside the cable system, the high-frequency current pulse generated by the discharge propagates in different directions along the cable core and the metal shielding layer, thereby generating unbalanced current and changing magnetic fields. By using the high-frequency current transformer CT sensor installed on the cable body or the grounding wire, when the magnetic field at the detection position changes, the sensor senses the partial discharge pulse signal, and then realizes the detection of the fault.
[0006] Although the existing technology has noticed the correlation between the abnormal grounding current signal and the cable system fault, there is currently no early warning method for the breakdown fault of the cable terminal tail pipe based on the abnormal grounding current signal. The existing technology system lacks research on the correlation between the abnormal grounding current signal and different development stages of the tail pipe breakdown fault, and only relying on the high-frequency discharge pulse signal in the grounding current to determine the health state of the lower insulation of the tail pipe has low sensitivity and is easily affected by external noise. Summary of the Invention
[0007] The purpose of the present invention is to overcome the shortcomings and deficiencies of the existing technology, and provide an early warning method for the breakdown fault of the cable terminal tail pipe based on the total current component. Considering comprehensively the grounding state, discharge state, and changes in the grounding path in different typical fault stages during the development process of the cable terminal tail pipe breakdown fault, based on the simulation model, the grounding current amplitude and high-frequency characteristic components under different grounding states are determined, and then the connection between the abnormal electrical signal and the tail pipe grounding system fault is established, realizing the effective early warning of the cable terminal tail pipe breakdown fault, achieving the effect of timely equipment maintenance before the fault occurs to reduce the maintenance cost, and finally realizing the online assessment of the health state of the lower insulation of the cable terminal tail pipe.
[0008] To achieve the above purpose, the technical solution provided by the present invention is: an early warning method for the breakdown fault of the cable terminal tail pipe based on the total current component, including the following steps:
[0009] S1. Prepare the cable terminal tail pipe lead seals in multiple typical grounding states as samples to be measured, and build a test platform for measuring the grounding resistance of the lead seals in these multiple typical grounding states. The platform is equipped with a voltage source, a sampling resistor, and two voltmeters. The voltage source, the sampling resistor, and the sample to be measured are connected in series in sequence to form a series circuit. One voltmeter is used to measure the voltage across the sampling resistor, and the other voltmeter is used to measure the voltage across the sample to be measured. Based on the measurement of the voltages across the sampling resistor and the sample to be measured by the two voltmeters, the grounding resistance values of the cable terminal tail pipe lead seals in the above multiple typical grounding states are obtained, that is, the resistance values of the samples to be measured are obtained;
[0010] S2. Based on the key parameters of the cable terminal, combined with the analysis of the change of the current path into the ground of the aluminum sleeve ring of the cable line at different development stages of the breakdown fault of the terminal tail pipe, establish a simulation calculation model of the aluminum sleeve ring current of the cable line applicable to different fault development stages, input the lead seal grounding resistance values of the cable terminal tail pipe under different typical grounding states, and realize the simulation analysis of the grounding current characteristics of the cable terminal at different fault development stages;
[0011] S3. Considering that there may be discharge phenomena on the lower insulation surface of the tail pipe after the lead seal is completely cracked, which damages the insulation and ultimately leads to the occurrence of insulation breakdown accidents, combined with the analysis of the current path into the ground of the high-frequency current generated by the discharge under the tail pipe, propose a detection method for the high-frequency current signal generated by the discharge phenomenon under the tail pipe. Specifically, for the current signal collected by the coupling coil installed on the terminal grounding wire, use a high-pass filter and a signal amplifier to extract the high-frequency components in the current signal as the characteristic state quantity for judging whether there is a discharge phenomenon under the tail pipe and the severity of the discharge phenomenon;
[0012] S4. Judge the health state of the insulation under the tail pipe by detecting the amplitude change and high-frequency component composition of the terminal grounding current.
[0013] Further, the step S1 includes the following steps:
[0014] S11. Select the lead seals of the cable terminal tail pipe in three typical grounding states of intact lead seal, partially cracked lead seal, and through-cracked lead seal as the samples to be tested. The samples to be tested are connected to the sampling resistor R1, voltage source U, voltmeters V1 and V2 through copper connecting wires to finally form a lead seal grounding resistance test platform. Among them, the voltmeter V1 is used to measure the voltage across the sampling resistor R1, and the voltmeter V2 is used to measure the voltage across the sample to be tested;
[0015] S12. Based on the voltage results measured by the voltmeters V1 and V2, the resistance value of the sample to be tested can be obtained. The calculation formula is as follows:
[0016]
[0017] In the formula: U1 is the measured value of the voltmeter V1, U2 is the measured value of the voltmeter V2, R1 represents the resistance value of the sampling resistor, and R2 represents the resistance value of the sample to be tested.
[0018] Further, the step S2 includes the following steps:
[0019] S21. Based on the key parameters of the cable line, including the cable voltage grade, cable structure parameters, cable line length, cable laying spacing, and cable grounding method, a simulation calculation model of the aluminum sheath circulating current of the cable line is built in PSCAD. This model uses a three-phase AC power supply. A, B, and C are the corresponding three-phase AC voltage sources in the software's built-in model library. C1, C2, and C3 are the cable groups in the software's built-in model library. Set R3 as the terminal grounding resistance and R4 as the cable load resistance. There are three grounding resistors respectively connected to C1, C2, and C3 in R3. Let Ias4, Ibs4, and Ics4 represent the cable terminal grounding currents flowing through the three grounding resistors respectively. There are three load resistors respectively connected to C1, C2, and C3 in R4. Let Ias1, Ias2, and Ias3 represent the cable conductor load currents flowing through the three load resistors respectively. In PSCAD, there are multiple transmission line models. Here, the Bergeron lumped resistance model is used for calculation and simulation, which can decouple the three-phase network into three independent single-phase networks by using the modal decomposition method, directly obtain the voltage at any position, and the Bergeron lumped resistance model is applicable to short-distance transmission lines;
[0020] S22. In the simulation calculation model of the aluminum sheath circulating current of the cable line, in order to simulate the actual load current change situation, the cable conductor load currents Ias1, Ias2, and Ias3 are adjusted by changing the value of the R4 resistor.
[0021] S23. In order to analyze the cable terminal grounding current state variables under different typical grounding states, the cable terminal grounding current simulation under different typical grounding states is realized by changing the value of the R3 resistor.
[0022] S24. According to the user's requirement for the calculation duration, set the simulation duration and step size of the simulation calculation model of the aluminum sheath circulating current of the cable line in the PSCAD software to obtain the dynamic change data of the cable terminal grounding current under different typical grounding states. By comparing the amplitude differences of the grounding currents under the three typical grounding states, the grounding current thresholds for judging the three typical grounding states are obtained.
[0023] Further, the step S4 includes the following steps:
[0024] S41. Install a cable terminal tailpipe breakdown fault warning system on the grounding wire of the terminal tailpipe. The system includes a current coupling coil, control switch K1, control switch K2, a grounding current amplitude determination module, a high-frequency signal monitoring module, and an alarm module. Among them: The current coupling coil is used to collect the total current component of the grounding current. One end is connected to the grounding current amplitude determination module through a wire, and the other end is respectively connected to control switch K1 and the high-frequency signal monitoring module through wires; The grounding current amplitude determination module is used to determine the amplitude of the collected grounding current, control the opening and closing of K1 and K2, and make the alarm module send a warning signal for complete cracking of the lead seal. The grounding current amplitude determination module is respectively connected to control switches K1 and K2 through wires, and signal lines are connected between control switches K1 and K2 and the alarm module and the grounding current amplitude determination module for reflecting control; The high-frequency signal monitoring module controls the alarm module to send an alarm signal by detecting the amplitude of the high-frequency component, jointly realizing the online monitoring of the insulation health status under the in-service cable terminal tailpipe. The high-frequency signal monitoring module is connected to control switch K2 through a wire and a signal line is connected between it and the alarm module;
[0025] S42. Take the grounding current thresholds in two typical grounding states of intact lead seals and partially cracked lead seals as the normal operating values for the grounding current amplitude determination module to act. When the grounding current amplitude determination module detects that the grounding current is less than or equal to the normal operating value, it controls K1 to close and K2 to open. At this time, the normal operating value is the theoretically maximum current; Take the grounding current threshold in the typical grounding state of a completely cracked lead seal as the warning value for the grounding current amplitude determination module to act. When the grounding current amplitude determination module detects that the grounding current is greater than or equal to the warning value, the grounding current amplitude determination module does not act. When the grounding current amplitude determination module detects that the grounding current is less than the warning value, it controls K1 to open, K2 to close, and at the same time controls the alarm module to send a warning signal for complete cracking of the lead seal;
[0026] S43. When the high-frequency signal monitoring module detects the continuous occurrence of high-frequency signal components or detects a high-amplitude high-frequency component signal, it is determined that there is a serious insulation damage state inside the tailpipe. At this time, the high-frequency signal monitoring module controls the alarm module to send a signal of abnormal insulation health status under the tailpipe to the cloud, finally realizing the timely warning of the breakdown fault of the in-service cable terminal tailpipe.
[0027] Compared with the prior art, the present invention has the following advantages and beneficial effects:
[0028] The present invention fully considers the changes in the magnitude of the grounding current and the high-frequency component under different development stages of the breakdown fault of the cable terminal tail pipe, and proposes an assessment method for the insulation health state under the cable terminal tail pipe by combining the magnitude of the grounding current and the high-frequency component signal, which can more directly and accurately reflect the true operating state of the cable terminal, realize the effective early warning of the breakdown fault of the cable terminal tail pipe, achieve the effect of timely equipment maintenance before the fault occurs to reduce the maintenance cost, fill the gap in the existing research, realize the support for the health monitoring of the cable terminal, thereby reducing the maintenance cost, and ultimately achieve the improvement of the operation reliability of the cable line and promote the construction of the smart grid. BRIEF DESCRIPTION OF THE DRAWINGS
[0029] Figure 1 FIG. is a schematic diagram of a lead seal grounding resistance test platform under different typical grounding states.
[0030] Figure 2 FIG. is a schematic diagram of an aluminum sheath circulating current simulation calculation model for a cable line.
[0031] Figure 3 FIG. is a schematic diagram of a breakdown fault warning system for a cable terminal tail pipe. DETAILED DESCRIPTION OF THE EMBODIMENTS
[0032] The present invention will be further described in detail below in conjunction with the embodiments and the drawings, but the embodiments of the present invention are not limited thereto.
[0033] This embodiment discloses a method for warning of breakdown faults of a cable terminal tail pipe based on full current components, and the specific situation is as follows:
[0034] S1. Prepare lead seals of cable terminal tail pipes in various typical grounding states as samples to be tested, and build a lead seal grounding resistance test platform in these various typical grounding states. The platform is equipped with a voltage source, a sampling resistor, and two voltmeters. The voltage source, the sampling resistor, and the sample to be tested are connected in series in sequence to form a series circuit. One voltmeter is used to measure the voltage across the sampling resistor, and the other voltmeter is used to measure the voltage across the sample to be tested. Based on the measurement of the voltages across the sampling resistor and the sample to be tested by the two voltmeters, the grounding resistance values of the lead seals of the cable terminal tail pipes in the above various typical grounding states are obtained, that is, the resistance values of the samples to be tested are obtained. The method includes the following steps:
[0035] S11. Select lead seals of cable terminal tail pipes in three typical grounding states, namely intact lead seals, partially cracked lead seals, and through-cracked lead seals, as samples to be tested. As Figure 1 shown, the sample to be tested is connected to the sampling resistor R1, the voltage source U, the voltmeters V1 and V2 through copper connecting wires to finally form a lead seal grounding resistance test platform. Among them, the voltmeter V1 is used to measure the voltage across the sampling resistor R1, and the voltmeter V2 is used to measure the voltage across the sample to be tested.
[0036] S12. Based on the voltage results measured by voltmeters V1 and V2, the resistance value of the sample to be measured can be obtained. The calculation formula is as follows:
[0037]
[0038] In the formula: U1 is the measured value of voltmeter V1, U2 is the measured value of voltmeter V2, R1 represents the resistance value of the sampling resistor, and R2 represents the resistance value of the sample to be measured.
[0039] S2. Based on the key parameters of the cable terminal, combined with the analysis of the change of the current path flowing into the ground of the cable line aluminum sheath ring at different development stages of the terminal tail pipe breakdown fault, a cable line aluminum sheath current simulation calculation model applicable to different fault development stages is established. By inputting the lead seal grounding resistance values of the cable terminal tail pipe under different typical grounding states, the simulation analysis of the cable terminal grounding current characteristics at different fault development stages is realized; it includes the following steps:
[0040] S21. Based on the key parameters of the cable line, including the cable voltage level, cable structure parameters, cable line length, cable laying spacing, and cable grounding method, a cable line aluminum sheath current simulation calculation model is built in PSCAD. As Figure 2 shown, this model uses a three-phase AC power supply. A, B, and C are the corresponding three-phase AC voltage sources in the software's built-in model library. C1, C2, and C3 are the cable groups in the software's built-in model library. Set R3 as the terminal grounding resistance and R4 as the cable load resistance. There are three grounding resistors respectively connected to C1, C2, and C3 in R3. Let Ias4, Ibs4, and Ics4 represent the cable terminal grounding currents flowing through the three grounding resistors respectively. There are three load resistors respectively connected to C1, C2, and C3 in R4. Let Ias1, Ias2, and Ias3 represent the cable conductor load currents flowing through the three load resistors respectively. In PSCAD, there are multiple transmission line models. Here, the Bergeron concentrated resistance model is used for calculation and simulation, which can decouple the three-phase network into three independent single-phase networks by using the modal decomposition method, directly obtain the voltage at any position, and the Bergeron concentrated resistance model is applicable to short-distance transmission lines;
[0041] S22. In the cable line aluminum sheath current simulation calculation model, in order to simulate the actual load current change situation, the cable conductor load currents Ias1, Ias2, and Ias3 are adjusted by changing the value of the R4 resistor;
[0042] S23. In order to realize the analysis of the cable terminal grounding current state quantities under different typical grounding states, the cable terminal grounding current simulation under different typical grounding states is realized by changing the value of the R3 resistor;
[0043] S24. According to the user's requirement for the calculation duration, set the simulation duration and step size of the aluminum sheath circulating current simulation calculation model of the cable line in the PSCAD software to obtain the dynamic change data of the grounding current at the cable terminal under different typical grounding states. By comparing the amplitude differences of the grounding currents in the three typical grounding states, the grounding current thresholds for judging the three typical grounding states are obtained.
[0044] S3. Considering that there may be discharge phenomena on the lower insulation surface of the tail pipe after the lead seal is completely cracked, which may damage the insulation and ultimately lead to insulation breakdown accidents. Combining the analysis of the high-frequency current into the ground path generated by the discharge under the tail pipe, a detection method for the high-frequency current signal generated by the discharge under the tail pipe is proposed. Specifically, for the current signal collected by the coupling coil installed on the terminal grounding wire, the high-pass filter and signal amplifier are used to extract the high-frequency components in the current signal as the characteristic state quantity for judging whether there is a discharge phenomenon under the tail pipe and the severity of the discharge phenomenon.
[0045] S4. Judge the insulation health state under the tail pipe by detecting the amplitude change and high-frequency component composition of the terminal grounding current, including the following steps:
[0046] S41. Install a cable terminal tail pipe breakdown fault warning system on the terminal tail pipe grounding wire, as Figure 3 shown. The system includes a current coupling coil, control switches K1 and K2, a grounding current amplitude determination module, a high-frequency signal monitoring module, and an alarm module. Among them: The current coupling coil is used to collect the total current component of the grounding current. One end is connected to the grounding current amplitude determination module through a wire, and the other end is respectively connected to the control switch K1 and the high-frequency signal monitoring module through wires; The grounding current amplitude determination module is used to determine the amplitude of the collected grounding current, control the opening and closing of K1 and K2, and the alarm module issues a lead seal complete cracking warning signal. The grounding current amplitude determination module is respectively connected to the control switches K1 and K2 through wires, and signal lines are connected between the control switches K1 and K2 and the alarm module and the grounding current amplitude determination module for reflecting control; The high-frequency signal monitoring module controls the alarm module to issue an alarm signal by detecting the amplitude of the high-frequency component, and jointly realizes the online monitoring of the insulation health state under the tail pipe of the in-service cable terminal. The high-frequency signal monitoring module is connected to the control switch K2 through a wire and is connected to the alarm module through a signal line;
[0047] S42. Take the grounding current thresholds in two typical grounding states, namely intact lead seals and partially cracked lead seals, as the normal operating values for the grounding current amplitude determination module to operate. When the grounding current amplitude determination module detects that the grounding current is less than or equal to the normal operating value, control K1 to close and K2 to open. At this time, the normal operating value is the theoretically maximum current. Take the grounding current threshold in the typical grounding state of a through-cracked lead seal as the warning value for the grounding current amplitude determination module to operate. When the grounding current amplitude determination module detects that the grounding current is greater than or equal to the warning value, the grounding current amplitude determination module does not operate. When the grounding current amplitude determination module detects that the grounding current is less than the warning value, control K1 to open, K2 to close, and at the same time control the alarm module to send a lead seal through-crack warning signal.
[0048] S43. When the high-frequency signal monitoring module detects the continuous occurrence of high-frequency signal components or detects a large-amplitude high-frequency component signal, it is determined that a relatively serious insulation damage state has occurred in the tail pipe. At this time, the high-frequency signal monitoring module controls the alarm module to send a signal of abnormal insulation health status under the tail pipe to the cloud, ultimately realizing the timely warning of the breakdown fault of the in-service cable terminal tail pipe.
[0049] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.
Claims
1. A cable terminal tail tube breakdown fault early warning method based on full current component, characterized in that: The following steps are involved: S1. Prepare cable terminal tail pipe lead seals under various typical grounding conditions as samples to be tested, and build a lead seal grounding resistance test platform under the various typical grounding conditions, the platform is configured with a voltage source, a sampling resistor and two voltmeters, the voltage source, the sampling resistor and the sample to be tested are sequentially connected to form a series circuit, one voltmeter is used to measure the voltage across the sampling resistor, and the other voltmeter is used to measure the voltage across the sample to be tested, based on the measurement of the voltage across the sampling resistor and the sample to be tested by the two voltmeters, the cable terminal tail pipe lead seal grounding resistance values under the above-mentioned various typical grounding conditions are obtained, that is, the resistance value of the sample to be tested is obtained; S2. Based on the key parameters of the cable terminal and the analysis of the changes in the path of the aluminum sleeve ring of the cable line flowing into the ground at different development stages of the terminal tail tube breakdown fault, a simulation calculation model for the aluminum sleeve circulating current of the cable line at different fault development stages is established. The lead seal grounding resistance values of the cable terminal tail tube under different typical grounding conditions are input to realize the simulation analysis of the grounding current characteristics of the cable terminal at different fault development stages. S3. Considering that after the lead seal is completely cracked, there may be discharge on the insulation surface under the tail pipe, which may damage the insulation and eventually lead to insulation breakdown. Combined with the analysis of the ground path of the high-frequency current generated by the discharge under the tail pipe, a detection method for the high-frequency current signal generated by the discharge under the tail pipe is proposed. Specifically, the high-frequency component of the current signal collected by the coupling coil installed on the terminal grounding wire is extracted by using a high-pass filter and a signal amplifier as a characteristic state quantity to judge whether there is a discharge phenomenon under the tail pipe and the severity of the discharge phenomenon. S4. The healthy state of the insulation under the tail pipe is determined by detecting the amplitude change and high-frequency component of the terminal grounding current.
2. A cable terminal tail pipe breakdown fault early warning method based on full current component according to claim 1, characterized in that: The step S1 comprises the following steps: S11, selecting cable terminal tail pipe lead seals in three typical grounding states, namely intact lead seal, partially cracked lead seal and through-cracked lead seal, as samples to be tested, and connecting the samples to be tested with sampling resistor R1, voltage source U, voltmeters V1 and V2 through copper connecting wires to finally form a lead seal grounding resistance test platform, wherein the voltmeter V1 is used to measure the voltage across the sampling resistor R1, and the voltmeter V2 is used to measure the voltage across the sample to be tested; S12. Based on the voltage results measured by voltmeters V1 and V2, the resistance value of the sample to be tested can be obtained. The calculation formula is as follows: Wherein: U1 is the actual measured value of voltmeter V1, U2 is the actual measured value of voltmeter V2, R1 represents the resistance value of the sampling resistor, and R2 represents the resistance value of the sample to be tested.
3. A cable terminal tail pipe breakdown fault early warning method based on full current component according to claim 2, characterized in that: The step S2 comprises the following steps: S21. Based on the key parameters of the cable line, including cable voltage level, cable structure parameters, cable line length, cable laying spacing and cable grounding method, a cable line aluminum sleeve circulating current simulation calculation model is built in PSCAD. The model adopts a three-phase AC power supply. A, B, and C are the corresponding three-phase AC voltage sources in the software's own model library. C1, C2, and C3 are cable groups in the software's own model library. R3 is set as the terminal grounding resistor, and R4 is the cable load resistor. Three grounding resistors connected to C1, C2, and C3 are set in R3, with Ias4, Ibs4, and Ic s4 represents the cable terminal grounding current flowing through the three grounding resistors. Three load resistors connected to C1, C2 and C3 are set in R4. Ias1, Ias2 and Ias3 represent the cable conductor load current flowing through the three load resistors. In PSCAD, there are multiple transmission line models. The Bergeron concentrated resistance model is used for calculation and simulation. The modal decomposition method can be used to decouple the three-phase network into three independent single-phase networks, and the voltage at any position can be directly obtained. The Bergeron concentrated resistance model is suitable for short-distance transmission lines. S22. In the simulation calculation model of the aluminum sheath circulating current of the cable line, in order to simulate the actual load current change, the load current Ias1, Ias2, and Ias3 of the cable conductor are adjusted by changing the value of the R4 resistor; S23. In order to analyze the grounding current state of the cable terminal under different typical grounding conditions, the grounding current simulation of the cable terminal under different typical grounding conditions is realized by changing the value of the R3 resistor; S24. According to the user's demand for calculation time, the simulation time and step length of the cable line aluminum sheath circulating current simulation calculation model are set in the PSCAD software to obtain the dynamic change data of the cable terminal grounding current under different typical grounding conditions. By comparing the differences in grounding current amplitudes under three typical grounding conditions, the grounding current thresholds for judging the three typical grounding conditions are obtained.
4. A cable terminal tail pipe breakdown fault early warning method based on full current component according to claim 3, characterized in that: The step S4 comprises the following steps: S41. Install a cable terminal tail tube breakdown fault warning system on the terminal tail tube grounding wire. The system includes a current coupling coil, a control switch K1, a control switch K2, a grounding current amplitude determination module, a high-frequency signal monitoring module and an alarm module, wherein: the current coupling coil is used to collect the full current component of the grounding current, one end of which is connected to the grounding current amplitude determination module through a wire, and the other end is connected to the control switch K1 and the high-frequency signal monitoring module through a wire; the grounding current amplitude determination module is used to determine the collected grounding current amplitude and control K1, K2 The disconnection and alarm module sends out a warning signal of complete cracking of the lead seal. The grounding current amplitude determination module is connected to the control switches K1 and K2 respectively through wires, and a signal line is connected between the control switches K1 and K2 and the alarm module and the grounding current amplitude determination module to reflect the control; the high-frequency signal monitoring module controls the alarm module to send out an alarm signal by detecting the amplitude of the high-frequency component, and jointly realizes the online monitoring of the insulation health status under the tail tube of the in-service cable terminal. The high-frequency signal monitoring module is connected to the control switch K2 through wires, and a signal line is connected to the alarm module; S42, using the grounding current thresholds under the two typical grounding conditions of intact lead seal and partially cracked lead seal as the normal working values of the grounding current amplitude determination module. When the grounding current amplitude determination module detects that the grounding current is less than or equal to the normal working value, K1 is controlled to be closed and K2 is disconnected. At this time, the normal working value is the theoretical maximum current value. Using the grounding current threshold under the typical grounding condition of through-cracked lead seal as the warning value of the grounding current amplitude determination module. When the grounding current amplitude determination module detects that the grounding current is greater than or equal to the warning value, the grounding current amplitude determination module does not act. When the grounding current amplitude determination module detects that the grounding current is less than the warning value, K1 is controlled to be disconnected and K2 is closed. At the same time, the alarm module is controlled to send out a warning signal of through-cracked lead seal. S43. When the high-frequency signal monitoring module detects that the high-frequency signal component continues to appear or detects a high-frequency component signal with a large value, it determines that serious insulation damage occurs in the tail pipe. At this time, the high-frequency signal monitoring module controls the alarm module to send a signal to the cloud indicating that the insulation health status under the tail pipe is abnormal, ultimately achieving timely warning of the breakdown failure of the tail pipe of the operating cable terminal.