Underground cable fault detection method

Abstract

The invention relates to an underground cable fault detection method, and the method comprises the steps: S1, installing voltage and current detection modules at all entrances and exits of a cable, measuring the voltage and current of a cable guide core layer, a metal shielding layer and an armor layer in real time, transmitting the measurement data back to a remote monitoring unit, and turning to S2; s2, performing fault identification on the uploaded measurement data according to cable structure information stored in the remote monitoring unit, and if a fault is identified, performing fault recording and turning to S3; s3, judging the fault type according to the voltage and current of the cable guide core layer, the metal shielding layer and the armor layer, and turning to S4; and S4, constructing a fault point circuit model based on the cable 3pi structure circuit model and the fault type, calculating according to the criterion current to obtain an actual fault distance, and determining a fault position. Compared with the prior art, the method has the advantages of high accuracy, good real-time performance and the like.

Description

Technical Field

[0001] The invention relates to the field of cable detection, and in particular to a method for detecting underground cable faults. Background Art

[0002] Underground cables are indispensable tools for transporting power resources in cities. They are widely used in reality, but the cables themselves will have some defects during the manufacturing process, such as gaps in the cables, uneven insulation layers, and losses during transportation. At the same time, underground cables are prone to being in a humid state for a long time, which makes it easier for the cables to age and fail, affecting the power supply of the urban power grid. Therefore, when a cable fails, it is very important to measure the distance and repair the fault. However, power cables are deeply buried underground, and it is difficult to locate the fault after it occurs. Power cable fault identification and fault location technology has always been a hot research topic at home and abroad.

[0003] Currently, common distance measurement algorithms can be divided into two categories: impedance method and traveling wave injection method. The traveling wave method uses the return phenomenon of the voltage wave and current wave transmitted on the cable after the fault occurs, combined with the time difference of reaching one or both ends of the cable and the speed of the traveling wave transmission to achieve distance measurement. Under ideal circumstances, the traveling wave method will not be affected by the fault type and fault resistance, and there is no need to consider too many specific parameters of the line. Especially for small current grounding IT systems, it can effectively avoid the problem of small fault current. It is currently a popular application technology in the field of power cable distance measurement. However, the traveling wave method has a high sampling frequency, is easily interfered by high-frequency signals, and the wave speed is difficult to measure. It is also overly dependent on the accuracy of the detection equipment. Therefore, the use of the traveling wave method for cable fault distance measurement has the defect of low reliability. The impedance method uses the relationship that the impedance size on the transmission line is proportional to its transmission distance to measure the voltage and current values ​​at one or both ends of the transmission line after a fault occurs. By analyzing the transmission line equivalent circuit model, KVL and KCL equations are written, and then the distance from the fault location to the relay protection device detection location is calculated by solving the circuit equation group or assuming the initial value of the fault distance for multiple iterations. In terms of data reliability, the impedance method has high accuracy, and its relative error value is generally less than 3%, but the traditional impedance method has cumbersome steps, and generally requires the power cable to be offline, and related measurement equipment and circuits are added to complete the distance measurement work, and the time response to the fault is slow.

[0004] There are many types of power cable faults and fault causes. The current distance measurement methods mainly include impedance method and traveling wave method. The present invention adopts impedance method. Most of the existing impedance methods require the cable to be offline and relevant measurement equipment to be installed. The process is complicated and time-consuming, but the positioning result is relatively reliable.

[0005] In the process of establishing the model of power cable, its special conductive structure should be taken into account, that is, when a fault occurs, not only the fault current will flow through the cable core, but also a considerable current will flow through the metal sheath layer connected in parallel with it. Similar to the present invention, scholars such as Tang Jin used concentrated parameters to model the cable, established a double π structure model of capacitive coupling between the core and the sheath, and between the sheath and the earth, and used the continuity of the fault current before and after the fault point of the non-fault phase, and the discontinuity of the fault current at the fault phase as the positioning criterion, eliminating the influence of the fault resistance and achieving fault location.

[0006] However, due to the complex structure of underground cables, hidden line directions, and the possible existence of other unknown factors, the accuracy of the detection process is poor, which increases the time for cable maintenance and reduces labor efficiency. Summary of the invention

[0007] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a method for detecting underground cable faults with high accuracy and high efficiency.

[0008] The purpose of the present invention can be achieved by the following technical solutions:

[0009] A method for detecting underground cable faults, the method comprising:

[0010] S1. Install voltage and current detection modules at each entrance and exit of the cable to measure the voltage and current of the cable core layer, metal shielding layer and armor layer in real time, and transmit the measurement data back to the remote monitoring unit, and then go to S2;

[0011] S2. According to the cable structure information stored in the remote monitoring unit, the uploaded measurement data is subjected to fault identification. If a fault is identified, the fault is recorded and the process goes to S3.

[0012] S3, judging the fault type according to the voltage and current of the cable core layer, metal shielding layer and armor layer, and then going to S4;

[0013] S4. Construct a fault point circuit model based on the cable 3π structure circuit model and the fault type, calculate the fault distance according to the criterion current, and determine the fault location.

[0014] Preferably, the cable fault is identified based on the real-time collection of the voltage and current of the cable core layer, metal shielding layer and armor layer, specifically: when the voltage and current of the cable core layer, metal shielding layer and armor layer suddenly change and exceed the set threshold, it is determined that the cable has a fault.

[0015] Preferably, the cable structure information includes basic structure, grounding method, total cable length, laying method, series resistance, inductance and parallel capacitance.

[0016] Preferably, judging the fault type according to the voltage and current of the cable core layer, the metal shielding layer and the armor layer includes:

[0017] When a phase-to-phase short circuit occurs, the voltage between the two phases changes suddenly and the current is discontinuous;

[0018] When a ground short circuit occurs, the voltage of the fault layer involved changes suddenly and the current at the fault point becomes discontinuous.

[0019] Preferably, the fault types include single-phase conductor core layer-metal shielding layer short circuit, single-phase conductor core layer-metal shielding layer-armor layer short circuit, single-phase conductor core layer-metal shielding layer-armor layer grounding short circuit, and two-phase short circuit.

[0020] Preferably, the process of solving the fault distance includes:

[0021] The step size and the initial value of the fault point distance are set. According to the fault point circuit model constructed based on the cable 3π structure circuit model and the fault type, the forward and backward currents of the fault point are calculated. After traversing all distances, all the positioning criterion currents are obtained, and the distance corresponding to the lowest point is selected, which is the fault distance.

[0022] Preferably, the fault point circuit model constructed based on the cable 3π structure circuit model and the fault type is specifically:

[0023] When a fault occurs, the voltage at the fault point is expressed as:

[0024]

[0025] In the formula, are the fault point voltages of the conductor layer, metal shielding layer and armor layer respectively. are the first-end voltages of the conductor layer, metal shielding layer and armor layer respectively. are the first-end currents of the conductor core layer, metal shielding layer and armor layer respectively, D is the fault distance with solution, Z is the impedance matrix, Y is the admittance matrix, and D is the fault distance;

[0026] When a fault occurs, the expression of the forward current at the fault point is:

[0027]

[0028] In the formula, They are the forward currents of the fault points of the conductor layer, metal shielding layer, and armor layer respectively;

[0029] When a fault occurs, the expression of the voltage at the end of each layer is:

[0030]

[0031] In the formula, They are the terminal voltages of the conductor layer, metal shielding layer, and armor layer respectively. They are the backward currents of the fault point of the conductor layer, metal shielding layer and armor layer respectively;

[0032] When a fault occurs, the expression of the current at the end of each layer is:

[0033]

[0034] In the formula, They are the end currents of the conductor layer, metal shielding layer and armor layer respectively.

[0035] Preferably, the impedance matrix Z and the admittance matrix Y are expressed as:

[0036]

[0037] In the formula, z cc is the impedance matrix after taking into account the mutual inductance between the conductors, z cs is the impedance matrix after taking into account the mutual inductance between the conductor core and the metal shielding layer, z ca is the impedance matrix after taking into account the mutual inductance between the conductor core and the armor layer, z ss is the impedance matrix after taking into account the mutual inductance between the metal shielding layers, z sa is the impedance matrix after taking into account the mutual inductance between the metal shielding layer and the armor layer, z aa It is the impedance matrix after taking into account the mutual inductance between the armor layers;

[0038] Each element represents a matrix:

[0039]

[0040] In the formula, z is the impedance value, m is the mutual inductance value, and y is the admittance value; in the subscript, c represents the conductor layer, s represents the metal shielding layer, and a represents the armor layer; the superscript ABC represents the three phases; z cc The first row element z in the matrix c , They represent the total impedance of the conductor core layer after considering the self-inductance of phase A, the mutual inductance of phases AB, and the mutual inductance of phases AC; cs The first row of elements in the matrix They respectively represent the mutual inductance between the A-phase core layer and the A-phase metal shielding layer, the mutual inductance between the A-phase core layer and the B-phase metal shielding layer, and the mutual inductance between the A-phase core layer and the C-phase metal shielding layer.

[0041] Preferably, when a single-phase or two-phase fault occurs, the fault current before and after the fault point of the fault phase is unequal, while the fault current before and after the fault point of the healthy phase is continuous as the criterion for fault location, and the location criterion current calculation expression is:

[0042]

[0043] In the formula, It is the criterion current when a single-phase fault occurs. The superscript K represents the fault phase, and the superscripts K1 and K2 represent the healthy phases. It is the criterion current when a two-phase fault occurs. The superscripts T1 and T2 represent the faulty phases, and the superscript T represents the healthy phase.

[0044] Preferably, after the fault distance is obtained in S4, the method further includes calculating the fault resistance according to the fault point data, establishing a complete fault circuit according to the fault distance and the fault resistance, and verifying the fault circuit using Kirchhoff's law according to the voltage and current of the first section of the three phases before and after the fault;

[0045] Among them, when a single-phase fault occurs, the calculation expression of the fault resistance is:

[0046]

[0047] In the formula, the superscript K represents the fault phase, R fsa , R fcs , R fag They are the metal shielding layer-armoring layer fault resistance, the conductor layer-metal shielding layer fault resistance, and the armor layer grounding fault resistance. If the calculated value of one of the resistances is infinite, it means that there is no fault in the corresponding layer.

[0048] When calculating the fault resistance, if a two-phase fault occurs, the fault resistance R f The calculation expression is:

[0049]

[0050] In the formula, the superscripts T1 and T2 represent the fault phase.

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] (1) Based on the mutual influence of the currents in the cable core layer, metal shielding layer and armor layer, the present invention constructs a 3π structure circuit model of the cable and a fault point circuit model according to the fault type. The fault distance is obtained by iterative calculation of the judgment current, which can more accurately identify the location of the fault and overcome the problem of incomplete detection data leading to errors in fault location.

[0053] (2) The present invention can realize online real-time cable fault detection by installing voltage and current detection modules at each entrance and exit of the cable to measure the voltage and current of the cable core layer, metal shielding layer and armor layer in real time, and transmit the measurement data back to the remote monitoring unit for fault identification, fault type judgment and fault distance judgment. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 is a flow chart of the method of the present invention;

[0055] Figure 2 This is a schematic diagram of the fault location process;

[0056] Figure 3 Schematic diagram of the 3π equivalent model of the cable;

[0057] Figure 4 This is a schematic diagram of the fault distance prediction results;

[0058] Figure 5 The structural model of the power distribution system used for the validation process;

[0059] Figure 6 This is a schematic diagram of the relative error rate results. DETAILED DESCRIPTION

[0060] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0061] Example

[0062] like Figure 1 As shown, this embodiment provides an underground cable fault detection method, the method comprising:

[0063] S1. Install voltage and current detection modules at each entrance and exit of the cable to measure the voltage and current of the cable core layer, metal shielding layer and armor layer in real time, and transmit the measurement data back to the remote monitoring unit.

[0064] S2. According to the cable structure information stored in the remote monitoring unit, the cable structure information includes the basic structure, grounding method, total cable length, laying method, series resistance, inductance and parallel capacitance, etc., the uploaded measurement data is fault identified. When it is identified that the voltage and current of the cable core layer, metal shielding layer and armor layer suddenly change and exceed the set threshold, it is determined that there is a fault in the cable and the fault recording is performed.

[0065] S3. Determine the fault type based on the voltage and current of the cable core layer, metal shielding layer and armor layer.

[0066] In this embodiment, the fault type is determined by using the characteristic that the insulation resistance value of the fault layer at the fault point changes, and the voltage and current changes of each layer before and after the fault are used as the criterion for preliminary identification of the fault type. When a phase-to-phase short circuit occurs, a fault resistance will be formed between the two phases, and the voltages of the two phases will have a large mutation, and the current at the fault point will be discontinuous; when a ground short circuit occurs, a fault resistance will be formed between the fault layers, and the voltages of the fault layers involved will have a large mutation, and the current at the fault point will be discontinuous.

[0067] The fault types in this embodiment include single-phase conductor layer-metal shielding layer short circuit, single-phase conductor layer-metal shielding layer-armor layer short circuit, single-phase conductor layer-metal shielding layer-armor layer grounding short circuit, and two-phase short circuit.

[0068] S4. Construct a fault point circuit model based on the cable 3π structure circuit model and the fault type, calculate the fault distance according to the criterion current, and determine the fault location.

[0069] The process of solving the fault distance includes:

[0070] The step size and the initial value of the fault point distance are set. According to the fault point circuit model constructed based on the cable 3π structure circuit model and the fault type, the forward and backward currents of the fault point are calculated. After traversing all distances, all the positioning criterion currents are obtained, and the distance corresponding to the lowest point is selected, which is the fault distance.

[0071] The fault point circuit model constructed based on the cable 3π structure circuit model and fault type is as follows:

[0072] When a fault occurs, the voltage at the fault point is expressed as:

[0073]

[0074] In the formula, are the fault point voltages of the conductor layer, metal shielding layer and armor layer respectively. are the first-end voltages of the conductor layer, metal shielding layer and armor layer respectively. are the first-end currents of the conductor core layer, metal shielding layer and armor layer respectively, D is the fault distance with solution, Z is the impedance matrix, Y is the admittance matrix, and D is the fault distance;

[0075] The expressions of impedance matrix Z and admittance matrix Y are:

[0076]

[0077] In the formula, z cc is the impedance matrix after taking into account the mutual inductance between the conductors, z cs is the impedance matrix after taking into account the mutual inductance between the conductor core and the metal shielding layer, zca is the impedance matrix after taking into account the mutual inductance between the conductor core and the armor layer, z ss is the impedance matrix after taking into account the mutual inductance between the metal shielding layers, z sa is the impedance matrix after taking into account the mutual inductance between the metal shielding layer and the armor layer, z aa It is the impedance matrix after taking into account the mutual inductance between the armor layers;

[0078] Each element represents a matrix:

[0079]

[0080]

[0081] In the formula, z is the impedance value, m is the mutual inductance value, and y is the admittance value; in the subscript, c represents the conductor layer, s represents the metal shielding layer, and a represents the armor layer; the superscript ABC represents the three phases; z cc The first row element z in the matrix c , They represent the total impedance of the conductor core layer after considering the self-inductance of phase A, the mutual inductance of phases AB, and the mutual inductance of phases AC; cs The first row of elements in the matrix They respectively represent the mutual inductance between the A-phase core layer and the A-phase metal shielding layer, the mutual inductance between the A-phase core layer and the B-phase metal shielding layer, and the mutual inductance between the A-phase core layer and the C-phase metal shielding layer.

[0082] When a fault occurs, the expression of the forward current at the fault point is:

[0083]

[0084] In the formula, They are the forward currents of the fault points of the conductor layer, metal shielding layer, and armor layer respectively;

[0085] When a fault occurs, the expression of the voltage at the end of each layer is:

[0086]

[0087] In the formula, They are the terminal voltages of the conductor layer, metal shielding layer, and armor layer respectively. They are the backward currents of the fault point of the conductor layer, metal shielding layer and armor layer respectively;

[0088] When a fault occurs, the expression of the current at the end of each layer is:

[0089]

[0090] In the formula, They are the end currents of the conductor layer, metal shielding layer and armor layer respectively.

[0091] When a single-phase or two-phase fault occurs, the fault current before and after the fault point of the fault phase is not equal, while the fault current before and after the fault point of the healthy phase is continuous as the criterion for fault location. The calculation expression of the location criterion current is:

[0092]

[0093] In the formula, It is the criterion current when a single-phase fault occurs. The superscript K represents the fault phase, and the superscripts K1 and K2 represent the healthy phases. It is the criterion current when a two-phase fault occurs. The superscripts T1 and T2 represent the faulty phases, and the superscript T represents the healthy phase.

[0094] After solving the fault distance, the fault resistance is calculated according to the fault point data. The complete fault circuit is established according to the fault distance and fault resistance. The fault circuit is verified by Kirchhoff's law according to the voltage and current of the first section of the three phases before and after the fault.

[0095] Among them, when a single-phase fault occurs, the calculation expression of the fault resistance is:

[0096]

[0097] In the formula, the superscript K represents the fault phase, R fsa , R fcs , R fag They are the metal shielding layer-armoring layer fault resistance, the conductor layer-metal shielding layer fault resistance, and the armor layer grounding fault resistance. If the calculated value of one of the resistances is infinite, it means that there is no fault in the corresponding layer.

[0098] When calculating the fault resistance, if a two-phase fault occurs, the fault resistance R f The calculation expression is:

[0099]

[0100] In the formula, the superscripts T1 and T2 represent the fault phases.

[0101] This embodiment is verified by simulation experiments. The fault distance set in the simulation is 500m, the total line length is 1km, the fault type is a single-phase conductor layer-metal shielding layer-armor layer grounding fault, and the fault resistance is 10 ohms. First, the voltage and current acquisition module at the head end transmits the collected information to the remote computer, and preliminarily determines whether a fault has occurred by checking whether the voltage and current of each layer are normal. When a fault occurs, the computer then obtains factory information and laying information based on the location of the faulty cable segment, and establishes a remote computer. Figure 33π model. Then, we first judge the abnormal conditions of voltage and current in each layer, preliminarily determine the fault type, and use appropriate formulas to iterate the fault location according to the cable information, and calculate the fault resistance. The final result is obtained through inspection. Figure 4 shown.

[0102] Finally, the fault distance was determined to be 497m, with a relative error of 0.6%. The fault type was a single-phase conductor layer-metal shield layer-armor layer grounding fault, which was consistent with the fault type set in the simulation. Figure 5 The distribution system structure model used in the verification process of the present invention is used to set the fault location to multiple, set the CSAG fault and the CS fault, and analyze the relative error rate, such as Figure 6 As shown, it can be found that the error values ​​of the results are all within 1%, which shows that the model has good performance and proves the reliability of the present invention.

[0103] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed by the present invention, and these modifications or replacements should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A method for detecting underground cable faults, characterized in that: The method includes: S1. Install voltage and current detection modules at each entrance and exit of the cable to measure the voltage and current of the cable core layer, metal shielding layer and armor layer in real time, and transmit the measurement data back to the remote monitoring unit, and then go to S2; S2. According to the cable structure information stored in the remote monitoring unit, the uploaded measurement data is subjected to fault identification. If a fault is identified, the fault is recorded and the process goes to S3. S3, judging the fault type according to the voltage and current of the cable core layer, metal shielding layer and armor layer, and then going to S4; S4. Construct a fault point circuit model based on the cable 3π structure circuit model and the fault type, calculate the fault distance according to the criterion current, and determine the fault location.

2. The underground cable fault detection method according to claim 1, characterized in that: The cable fault is identified based on the real-time collection of the voltage and current of the cable core layer, metal shielding layer and armor layer. Specifically, when the voltage and current of the cable core layer, metal shielding layer and armor layer suddenly exceed the set threshold, it is determined that the cable has a fault.

3. The underground cable fault detection method according to claim 1, characterized in that: The cable structure information includes basic structure, grounding method, total cable length, laying method, series resistance, inductance and parallel capacitance.

4. The underground cable fault detection method according to claim 1, characterized in that: The fault type is determined based on the voltage and current of the cable core layer, metal shielding layer and armor layer, including: When a phase-to-phase short circuit occurs, the voltage between the two phases changes suddenly and the current is discontinuous; When a ground short circuit occurs, the voltage of the fault layer involved changes suddenly and the current at the fault point becomes discontinuous.

5. The underground cable fault detection method according to claim 4, characterized in that: The fault types include single-phase conductor core layer-metal shielding layer short circuit, single-phase conductor core layer-metal shielding layer-armor layer short circuit, single-phase conductor core layer-metal shielding layer-armor layer grounding short circuit, and two-phase short circuit.

6. The underground cable fault detection method according to claim 1, characterized in that: The process of solving the fault distance includes: The step size and the initial value of the fault point distance are set. According to the fault point circuit model constructed based on the cable 3π structure circuit model and the fault type, the forward and backward currents of the fault point are calculated. After traversing all distances, all the positioning criterion currents are obtained, and the distance corresponding to the lowest point is selected, which is the fault distance.

7. The underground cable fault detection method according to claim 6, characterized in that: The fault point circuit model constructed based on the cable 3π structure circuit model and the fault type is specifically: When a fault occurs, the voltage at the fault point is expressed as: In the formula, are the fault point voltages of the conductor layer, metal shielding layer and armor layer respectively. are the first-end voltages of the conductor layer, metal shielding layer and armor layer respectively. are the first-end currents of the conductor core layer, metal shielding layer and armor layer respectively, D is the fault distance with solution, Z is the impedance matrix, Y is the admittance matrix, and D is the fault distance; When a fault occurs, the expression of the forward current at the fault point is: In the formula, They are the forward currents of the fault points of the conductor layer, metal shielding layer, and armor layer respectively; When a fault occurs, the expression of the voltage at the end of each layer is: In the formula, They are the terminal voltages of the conductor layer, metal shielding layer, and armor layer respectively. They are the backward currents of the fault point of the conductor layer, metal shielding layer and armor layer respectively; When a fault occurs, the expression of the current at the end of each layer is: In the formula, They are the end currents of the conductor layer, metal shielding layer and armor layer respectively.

8. The underground cable fault detection method according to claim 7, characterized in that: The impedance matrix Z and admittance matrix Y are expressed as: In the formula, z cc is the impedance matrix after taking into account the mutual inductance between the conductors, z cs is the impedance matrix after taking into account the mutual inductance between the conductor core and the metal shielding layer, z ca is the impedance matrix after taking into account the mutual inductance between the conductor core and the armor layer, z ss is the impedance matrix after taking into account the mutual inductance between the metal shielding layers, z sa is the impedance matrix after taking into account the mutual inductance between the metal shielding layer and the armor layer, z aa It is the impedance matrix after taking into account the mutual inductance between the armor layers; Each element represents a matrix: In the formula, z is the impedance value, m is the mutual inductance value, and y is the admittance value; in the subscript, c represents the conductor layer, s represents the metal shielding layer, and a represents the armor layer; the superscript ABC represents the three phases; z cc The first row element z in the matrix c , They represent the total impedance of the conductor core layer after considering the self-inductance of phase A, the mutual inductance of phases AB, and the mutual inductance of phases AC; cs The first row of elements in the matrix They respectively represent the mutual inductance between the A-phase core layer and the A-phase metal shielding layer, the mutual inductance between the A-phase core layer and the B-phase metal shielding layer, and the mutual inductance between the A-phase core layer and the C-phase metal shielding layer.

9. The underground cable fault detection method according to claim 7, characterized in that: When a single-phase or two-phase fault occurs, the fault current before and after the fault point of the fault phase is not equal, while the fault current before and after the fault point of the healthy phase is continuous as the criterion for fault location. The calculation expression of the location criterion current is: In the formula, It is the criterion current when a single-phase fault occurs. The superscript K represents the fault phase, and the superscripts K1 and K2 represent the healthy phases. It is the criterion current when a two-phase fault occurs. The superscripts T1 and T2 represent the faulty phases, and the superscript T represents the healthy phase.

10. The underground cable fault detection method according to claim 7, characterized in that: After the fault distance is obtained in S4, the fault resistance is calculated according to the fault point data, a complete fault circuit is established according to the fault distance and the fault resistance, and the fault circuit is verified by Kirchhoff's law according to the voltage and current of the first section of the three phases before and after the fault; Among them, when a single-phase fault occurs, the calculation expression of the fault resistance is: In the formula, the superscript K represents the fault phase, R fsa , R fcs , R fag They are the metal shielding layer-armoring layer fault resistance, the conductor layer-metal shielding layer fault resistance, and the armor layer grounding fault resistance. If the calculated value of one of the resistances is infinite, it means that there is no fault in the corresponding layer. When calculating the fault resistance, if a two-phase fault occurs, the fault resistance R f The calculation expression is: In the formula, the superscripts T1 and T2 represent the fault phases.

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