Device and method for live detection of defect subsection of cable grounding system by characteristic current method

Abstract

The invention provides a device and a method for live detection of a defect subsection of a cable grounding system by a characteristic current method. The device comprises a characteristic current signal generator, a characteristic current signal receiver and a non-contact current signal coupler, the characteristic current signal generator is connected with a transposition valve plate in the cross interconnection box and injects a coupling characteristic current signal, the non-contact current signal coupler measures current generated after the characteristic current signal generator applies the coupling characteristic current signal, the characteristic current signal receiver is connected with the non-contact current signal coupler, and the characteristic current signal receiver is connected with the non-contact current signal coupler. And receiving and processing a detection signal of the non-contact current signal coupler, and comparing amplitudes to determine a defective cable section of the grounding system. According to the method, the fault section can be quickly confirmed and the grounding defect of the high-voltage cable can be quickly positioned under the conditions of not hindering the electrified normal operation of equipment and being non-destructive.

Description

Technical Field

[0001] The present invention relates to the field of high-voltage cable grounding system defect detection, and in particular to a method and device for live detection of defective subsections in a cable grounding system using a characteristic current method. Background Art

[0002] When the cable line distance is long, a high induced voltage will be generated on the metal sheath of the single-core cable. In order to limit this induced voltage, a cross-connection method is generally adopted at the middle joint of the cable to interchange the metal layers of the three-phase cable. Under the effect of the phase difference between the three phases, the induced voltages cancel each other out, thereby limiting the induced voltage value on the metal sheath.

[0003] The cross-connection grounding system of high-voltage cables includes cable metal sheaths, accessories, lead seals, grounding cables and connectors between various parts. Among them, the cross-connection box is used in long cable lines. In order to reduce the induced voltage on the cable sheath, the metal sheath on one side of the insulating joint of one phase and the metal sheath on the other side of the insulating joint of the other phase are connected to each other in sequence, and then grounded in sections. It includes sheath overvoltage limiters, grounding bars, transposition bars, common grounding terminals, etc. The rationality of the design of the cross-connection grounding system and its good operating condition are prerequisites for ensuring the safe operation of high-voltage cables. The failure rate of high-voltage cables caused by defects in the grounding system is relatively high, which will cause cable failures, fires and other problems under long-term operating conditions. Therefore, in order to ensure the safe and reliable operation of high-voltage cable lines, it is of great significance to detect the cross-connection grounding system, locate and repair the defects in the grounding system, and reduce power outages and ensure the safe and reliable operation of the entire power grid. At present, there are mainly two methods for locating defects in the cross-connection grounding system of high-voltage cables: Coupling method: This method couples a voltage signal into the ground loop via a voltage transformer, measures the current in the ground loop via a current transformer, and calculates the loop resistance. Currently, most ground resistance testers on the market use the coupling method, which means there's no voltage in the measured loop. However, during operation, high-voltage cable ground loops can induce significant voltage. Therefore, measuring the ground loop resistance of high-voltage cables with cross-connected grounding requires a specialized high-voltage cable ground loop resistance tester. However, these instruments can only measure the loop resistance of a single branch consisting of three cable segments. While they can determine if there are any defects such as poor contact within the entire branch, they cannot determine the cable segment or its precise location.

[0004] DC direction method: This method is to add a DC source to the transposition valve plate in the cross-connection box, and use an active Hall sensor to measure the DC current in the cables on both sides of the connector. According to Ohm's law, the current is larger on the side with lower resistance. Through this method, it can be determined in which direction of the cable grounding loop on both sides of the DC power access point there is a poor contact defect. However, under normal operating conditions, the internal circulating current of the cable grounding wire has a strong interference with the DC signal acquisition. Therefore, this method can only detect the defect direction during a power outage and cannot be detected with power on. In actual engineering applications, after the cable is powered off, the Kelvin method is generally used to measure the grounding resistance of each cable section to determine whether there is a poor grounding defect. The DC direction method is often used to determine the defective cable section, and then destructively open the window of the cable outer insulation sheath to determine which end of the lead seal has a poor grounding defect.

[0005] In summary, under live conditions, these methods can only determine whether a branch of a high-voltage cable using a cross-connection grounding method has a poor grounding defect based on the resistance value of the grounding loop within the three cable segments of a branch. To determine the exact location of the defective cable segment, it is necessary to remove the transposition valve plate within the cross-connection box after a power outage and perform destructive window openings on multiple cable segments for further testing. Therefore, if a grounding system defect is detected in a cable branch under live conditions, a long power outage is required to locate and eliminate the defect. Therefore, the ability to quickly and accurately locate grounding system defects under live conditions and non-destructively is crucial for reducing power outages and improving power supply reliability. Summary of the Invention

[0006] The purpose of the present invention is to provide a device and method for live detection of defective sub-segments of a cable grounding system using the characteristic current method, which can accurately determine the cross-connected sub-segments with grounding system defects, thereby effectively reducing the power outage time required to eliminate the defects and improving power supply reliability.

[0007] In order to achieve the above-mentioned purpose, the present invention is implemented through the following technical solutions.

[0008] A device for live detection of defective subsections in a cable grounding system using a characteristic current method, comprising: a characteristic current signal generator, a characteristic current signal receiver, and a non-contact current signal coupler; The characteristic current signal generator is connected to the transposition valve plate in the cross-connection box and is used to inject the coupled characteristic current signal; The characteristic current signal receiver is connected to the contactless current signal coupler and is used to receive and process the detection signal of the contactless current signal coupler; The non-contact current signal coupler is an inductive current coupler used to measure the current generated after the characteristic current signal generator applies a coupled characteristic current signal.

[0009] Furthermore, the characteristic current signal generator includes a control unit, a driving circuit, a filtering circuit, a current regulating circuit, and a coupling transmission circuit; the power input end is connected to the mains or a rechargeable battery, and the signal output end emits current excitation signals of different frequencies.

[0010] Furthermore, the characteristic current signal receiver includes an input protection unit, an amplification processing unit, a microcomputer processing unit, a signal tracking and zeroing unit, and a display screen.

[0011] The present invention also discloses a method for detecting defective subsections of a cable grounding system using a characteristic current method, which is implemented based on the above-mentioned device and includes the following steps: S1. Select the cross-connected grounding box of the cable line as the test location and add the excitation characteristic current signal to the branch with the grounding system defect. S2. Use the induction method to measure the characteristic current amplitude on the cable surface on the small and large sides of the cross-connection box under test, and compare the amplitudes to determine the defective cable section in the grounding system.

[0012] Furthermore, the test location described in step S1 is the transposition valve plate in the cross-interconnected grounding box, and the characteristic current signal generator is directly connected to the transposition valve plate in the cross-interconnected grounding box with a wire clamp. During the measurement process, the relay in the three-phase measurement circuit connected in series with each phase load is controlled so that the corresponding grounding branch is connected to the measuring device to realize the measurement of the characteristic current of the grounding branch.

[0013] Furthermore, the characteristic current signal receiver obtains the characteristic current on the cables on both sides of the cross-connection box by induction through a non-contact current signal coupler, and compares and analyzes the amplitudes of the characteristic currents on both sides. The grounding system defect exists on the side where the characteristic current amplitude is significantly reduced.

[0014] Furthermore, the equivalent circuit of the same cross-connection branch is: Resistors R1, R2, and R3 are connected in series, with the other end of resistor R1 and resistor R3 grounded. During testing, excitation characteristic current signals are added to the connection point between resistors R1 and R2, and the connection point between resistors R2 and R3, respectively. Among them, the resistors R1, R2, and R3 represent the resistances of the three sections of the metal outer sheath of the high-voltage cable and its grounding connectors in the cross-connected grounding mode.

[0015] Furthermore, the method for determining defective cable segments by amplitude comparison is as follows: Add an excitation characteristic current signal at the connection point of resistor R1 and resistor R2. The current flowing through the branch of resistor R1 is I1, and the current flowing through the branches of resistors R2 and R3 is I2. If I1 << I2, it is determined that the defect of the branch cable grounding system is located in the cross-bonding sub-section where R1 is located. If I1 >> I2, it is determined that the defect of the branch cable grounding system is located in the cross-bonding sub-section where R2 and / or R3 are located; Add an excitation characteristic current signal at the connection point of resistor R2 and resistor R3. The current flowing through the branches of resistors R1 and R2 is I3, and the current flowing through the branch of resistor R3 is I4. If I3 >> I4, it is determined that the defect of the branch cable grounding system is located in the cross-bonding sub-section where R3 is located. If I3 << I4, it is determined that the defect of the branch cable grounding system is located in the cross-bonding sub-section where R1 and / or R2 are located; If I1 << I4, I2 << I4, and I3 << I4, it is determined that the defect of the branch cable grounding system is located in the cross-bonding sub-section where R1 and R2 are located; If I2 << I1, I3 << I1, and I4 << I1, it is determined that the defect of the branch cable grounding system is located in the cross-bonding sub-section where R2 and R3 are located.

[0016] The beneficial effects of the present invention are as follows: It can quickly confirm the fault section and locate the high-voltage cable grounding defect under the condition of not disturbing the normal operation of the equipment with electricity and non-destructive conditions. Determining the specific cross-bonding sub-section and confirming the defect location are both within 5 minutes. It can effectively improve the detection efficiency of the grounding system defect with strong concealment, reduce the power outage time and the impact on the power grid operation, and avoid irreparable damage to the insulation performance of the cable body caused by multiple window inspections. The present invention can effectively detect and accurately locate grounding system defects such as lead corrosion and warping, grounding bolt loosening, and inner surface corrosion of the grounding copper busbar of high-voltage cables. The equipment has a small volume, high measurement accuracy and reliability, and is suitable for outdoor and limited space operations in cable tunnels. Description of the Drawings

[0017] Figure 1 It is the equivalent circuit diagram of the same cross-bonding branch of the present invention; Figure 2 It is the schematic diagram of the device for detecting the defect sub-section of the cable grounding system by the characteristic current method in Embodiment 1 of the present invention; Figure 3 It is the schematic diagram of the device for detecting the defect sub-section of the cable grounding system by the characteristic current method in Embodiment 2 of the present invention; Figure 4 It is the connection schematic diagram of the characteristic current signal generator in Embodiment 1 of the present invention; Figure 5 It is the connection schematic diagram of the characteristic current signal receiver and the non-contact current signal coupler in Embodiment 1 of the present invention. Detailed Embodiment

[0018] The technical solutions in the embodiments of the present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0019] Example 1 This embodiment discloses a device for detecting defective sub-sections of a cable grounding system using a characteristic current method. Figure 2 , including: a characteristic current signal generator, a characteristic current signal receiver, and a contactless current signal coupler.

[0020] Please refer to Figure 4 One end of the characteristic current signal generator is connected to the transposition valve plate in the cross-connection box through a signal output line, and the other end is grounded. It is used to inject a coupled characteristic current signal, specifically including a control unit, a drive circuit, a filter circuit, a current regulation circuit, and a coupling transmission circuit; the power input end is connected to the AC power or a rechargeable battery, and the signal output end can emit current excitation signals of different frequencies, including but not limited to square waves, triangle waves, special-shaped waves, etc., which can be detected by the non-contact current signal coupler, and are connected to the transposition valve plate in the cross-connection box.

[0021] Please refer to Figure 5 The characteristic current signal receiver is connected to the contactless current signal coupler through a signal receiving line, and is used to receive and process the detection signal of the contactless current signal coupler, including an input protection unit, an amplification processing unit, a microcomputer processing unit, a signal tracking and zeroing unit and a display screen.

[0022] The contactless current signal coupler is an inductive current coupler used to measure and analyze the current generated after the characteristic current signal generator applies a coupled characteristic current signal.

[0023] Example 2 This embodiment discloses a method for detecting defective subsections in a cable grounding system using a characteristic current method, which is implemented based on the device of embodiment 1 and includes the following steps: S1. Select the transposition valve plate in the cross-interconnected grounding box of the cable line as the test location, directly connect it to the transposition valve plate in the cross-interconnected grounding box with a wire clamp, add an excitation characteristic current signal to the branch with a grounding system defect, and during the measurement process, control the relay in series with each phase load in the three-phase measurement circuit so that the corresponding grounding branch is connected to the measuring device to achieve the measurement of the characteristic current of the grounding branch.

[0024] S2. The characteristic current amplitude is measured on the surface of the cables on the small and large sides of the cross-connection box under test using the induction method. The characteristic current signal receiver obtains the characteristic current on the cables on both sides of the cross-connection box by induction using a non-contact current signal coupler. The characteristic current amplitudes on both sides are compared and analyzed. The grounding system defect exists on the side where the characteristic current amplitude is significantly reduced.

[0025] The principle of amplitude comparison is the characteristic current method. Please refer to Figure 1 , resistors R1, R2, and R3 are connected in series. The other end of resistor R1 is grounded, and the other end of resistor R3 is grounded. During the test, excitation characteristic current signals are applied to the connection points between resistor R1 and resistor R2 and between resistor R2 and resistor R3 respectively; Among them, resistors R1, R2, and R3 respectively represent the resistances of the metal outer sheaths of the three sections of the high-voltage cable cross-connected grounding method and their grounding connectors.

[0026] When an excitation characteristic current signal is applied to the connection point between resistor R1 and resistor R2, the current flowing through the branch of resistor R1 is I1, and the current flowing through the branches of resistors R2 and R3 is I2. If I1 << I2, it is determined that the defect of the branch cable grounding system is located in the cross-connected sub-section where R1 is located. If I1 >> I2, it is determined that the defect of the branch cable grounding system is located in the cross-connected sub-section where R2 and R3 are connected in series; When an excitation characteristic current signal is applied to the connection point between resistor R2 and resistor R3, the current flowing through the branches of resistors R1 and R2 is I3, and the current flowing through the branch of resistor R3 is I4. If I3 >> I4, it is determined that the defect of the branch cable grounding system is located in the cross-connected sub-section where R3 is located. If I3 << I4, it is determined that the defect of the branch cable grounding system is located in the cross-connected sub-section where R1 and R2 are connected in series.

[0027] In this embodiment, the cross-connected sub-section of cross-connected branch A1-B2-C3 is taken as an example. Please refer to Figure 2 , directly connect the output clip of the characteristic current signal generator to the A1-B2 cross-connected commutating varistor in the 2# cross-connected grounding box of the charged and normally operating cable line. After starting the generator, the control unit outputs a signal through the drive circuit and sends out a characteristic current signal through the signal output terminal, and the output current is constant.

[0028] Bring the non-contact current coupler close to the measured cable body, and measure the characteristic current amplitudes I1 and I2 on both sides of the cross-connected box. I1 is the current flowing in the direction of A1 (R1), and I2 is the current flowing through the B2-C3 section (R2 and R3 connected in series). At this time, if I1 >> I2, it is determined that the defect is located in the B2-C3 direction.

[0029] Connect the characteristic current signal generator to the B2-C3 commutating varistor of the 3# cross-connected grounding box with a clip, as Figure 3 shown. Bring the non-contact current coupler close to the measured cable body, and measure the characteristic current amplitudes I3 and I4 on both sides of the cross-connected box. I3 is the current flowing through the A1-B2 section (R1 and R2 connected in series), and I4 is the current flowing through the C3 section (R3). At this time, I3 << I4, and it is determined that the defect is located in the B2-A1 direction.

[0030] Based on the comprehensive measurement results, it is determined that the defect of the branch cable grounding system is located in the B2 cross-bonding sub-section.

[0031] Embodiment 3 This embodiment provides a method for determining the section with a grounding system defect in the cross-bonding sub-section A1 of the cross-bonding branch A1-B2-C3 of the present invention during live detection, and the specific steps are as follows: The same as the steps in Example 2, select the conversion valve plate inside the cross-bonding grounding box of the cable line as the test site, directly connect it to the conversion valve plate of the cross-bonding grounding box with a wire clamp, add an excitation characteristic current signal to the branch with a grounding system defect, and during the measurement process, control the relays connected in series with each phase load in the three-phase measurement circuit to make the corresponding grounding branch access the measurement device, so as to measure the characteristic current of this grounding branch.

[0032] Measure the amplitude of the characteristic current on the cable surfaces on the small-size side and large-size side of the measured cross-bonding box respectively by the induction method. The characteristic current signal receiver obtains the characteristic current on the cables on both sides of the cross-bonding box by induction through a non-contact current signal coupler.

[0033] Compare the amplitude of the characteristic current of the same branch of the measured cable line operating normally with electricity. If I1 << I2 at this time, it can be determined that the defect of the branch cable grounding system is located in the A1 cross-bonding sub-section.

[0034] Embodiment 4 This embodiment provides a method for determining the section with a grounding system defect in the cross-bonding sub-section C3 of the cross-bonding branch A1-B2-C3 of the present invention during live detection, and the specific steps are as follows: The same as the steps in Example 2, select the conversion valve plate inside the cross-bonding grounding box of the cable line as the test site, directly connect it to the conversion valve plate of the cross-bonding grounding box with a wire clamp, add an excitation characteristic current signal to the branch with a grounding system defect, and during the measurement process, control the relays connected in series with each phase load in the three-phase measurement circuit to make the corresponding grounding branch access the measurement device, so as to measure the characteristic current of this grounding branch.

[0035] Measure the amplitude of the characteristic current on the cable surfaces on the small-size side and large-size side of the measured cross-bonding box respectively by the induction method. The characteristic current signal receiver obtains the characteristic current on the cables on both sides of the cross-bonding box by induction through a non-contact current signal coupler.

[0036] Compare the amplitude of the characteristic current of the same branch of the measured cable line operating normally with electricity. If I3 >> I4 at this time, it can be determined that the defect of the branch cable grounding system is located in the C3 cross-bonding sub-section.

[0037] Example 5 This embodiment provides a method for determining the section where there are grounding system defects in both the cross - connection sub - sections A1 and B2 of the cross - connection branch A1 - B2 - C3 for on - line detection of the present invention. The specific steps are as follows: The same as the steps in Example 2. Select the commutating valve piece in the cross - connection grounding box of the cable line as the test part. Connect it directly to the commutating valve piece in the cross - connection grounding box with a wire clamp. Add an excitation characteristic current signal to the branch with a grounding system defect. During the measurement process, control the relays connected in series with each phase load in the three - phase measurement circuit so that the corresponding grounded branch is connected to the measuring device to measure the characteristic current of the grounded branch.

[0038] Measure the amplitude of the characteristic current on the cable surface on the small - side and large - side of the measured cross - connection box by induction method respectively. The characteristic current signal receiver obtains the characteristic current on the cables on both sides of the cross - connection box by induction through a non - contact current signal coupler.

[0039] Add an excitation characteristic current signal at the connection point of resistor R1 and resistor R2. The current flowing through the branch of resistor R1 is I1, and the current flowing through the branches of resistor R2 and R3 is I2. If I1 << I2, it is determined that the grounding system defect of this branch cable is in the cross - connection sub - section where R1 is located; if I1 >> I2, it is determined that the grounding system defect of this branch cable is in the cross - connection sub - section where R2 and R3 are in series. Add an excitation characteristic current signal at the connection point of resistor R2 and resistor R3. The current flowing through the branches of resistor R1 and R2 is I3, and the current flowing through the branch of resistor R3 is I4. If I3 >> I4, it is determined that the grounding system defect of this branch cable is in the cross - connection sub - section where R3 is located; if I3 << I4, it is determined that the grounding system defect of this branch cable is in the cross - connection sub - section where R1 and R2 are in series.

[0040] If there are defects in two cross - connection sub - sections in the same branch, it may be the case that both A1 and B2 are defective, both B2 and C3 are defective, or both A1 and C3 are defective.

[0041] Compare the amplitudes of the characteristic currents of the same branch of the measured cable line operating normally with electricity. If I1 << I4, I2 << I4, and I3 << I4, it can be determined that only the resistance of the C3 branch has not increased, indicating that the grounding system defect of this branch cable is in the cross - connection sub - sections of A1 and B2.

[0042] If I2 << I1, I3 << I1, and I4 << I1, it can be determined that only the resistance of the A1 branch has not increased, indicating that the grounding system defect of this branch cable is in the cross - connection sub - sections of B2 and C3.

[0043] If the currents I1, I2, I3, and I4 are all much smaller than the injected excitation characteristic current signal, it can be determined that the defect of the branch cable grounding system is located in the A1 and C3 cross-bonding sub-section, or defects occur in all of the A1, B2, and C3 cross-bonding sub-sections. At this time, the B2 cross-bonding sub-section can be directly grounded, and then the magnitudes of I1, I2, I3, and I4 can be compared. If I1 << I2 and I4 << I3, it can be determined that the defect of the grounding system is located in the A1 and C3 cross-bonding sub-section; if the values of I1, I2, I3, and I4 are still close, it can be determined that defects occur in all of the A1, B2, and C3 cross-bonding sub-sections.

[0044] Finally, it should be noted that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A device for detecting defective subsections in a cable grounding system using a characteristic current method, characterized in that: Comprising: A characteristic current signal generator, a characteristic current signal receiver, and a non-contact current signal coupler; The characteristic current signal generator is connected to the commutation valve plate in the cross-bonding box and is used to inject a coupled characteristic current signal; The characteristic current signal receiver is connected to the non-contact current signal coupler and is used to receive and process the detection signal of the non-contact current signal coupler; The non-contact current signal coupler is an inductive current coupler and is used to measure the current generated after the characteristic current signal generator applies a coupled characteristic current signal.

2. The device for detecting defective subsections of a cable grounding system using a characteristic current method according to claim 1 is characterized in that: The characteristic current signal generator includes a control unit, a drive circuit, a filter circuit, a current regulation circuit, and a coupled transmission circuit; the power input terminal is connected to the mains or a rechargeable battery, and the signal output terminal emits current excitation signals of different frequencies.

3. The device for detecting defective subsections in a cable grounding system using a characteristic current method according to claim 2 is characterized in that: The characteristic current signal receiver includes an input protection unit, an amplification and processing unit, a microcomputer processing unit, a signal tracking and zeroing unit, and a display screen.

4. A method for live detection of defective subsections in a cable grounding system using a characteristic current method, implemented based on the device for live detection of defective subsections in a cable grounding system using a characteristic current method according to any one of claims 1 to 3, characterized in that: Including the steps: S1. Select the cross-bonding grounding box of the cable line as the test site, and add an excitation characteristic current signal to the branch with a grounding system defect; S2. Measure the amplitudes of the characteristic currents on the cable surfaces on the small-size side and the large-size side of the tested cross-bonding box by the induction method respectively, and compare the amplitudes to determine the cable section with a grounding system defect.

5. The method for live detection of defective subsections in a cable grounding system using the characteristic current method according to claim 4 is characterized in that: In step S1, the test site is the commutation valve plate in the cross-bonding grounding box. The characteristic current signal generator is directly connected to the commutation valve plate of the cross-bonding grounding box with a wire clamp. During the measurement process, the relays connected in series with each phase load in the three-phase measurement circuit are controlled so that the corresponding grounding branch is connected to the measurement device to realize the measurement of the characteristic current of the grounding branch.

6. The method for live detection of defective subsections in a cable grounding system using the characteristic current method according to claim 4 is characterized in that: The characteristic current signal receiver obtains the characteristic currents on the cables on both sides of the cross-bonding box in an inductive manner through the non-contact current signal coupler, and compares and analyzes the amplitudes of the characteristic currents on both sides. The grounding system defect exists on the side where the amplitude of the characteristic current is significantly reduced.

7. The method for live detection of defective subsections in a cable grounding system using the characteristic current method according to claim 6 is characterized in that: The equivalent circuit of the same cross-bonding branch is: Resistors R1, R2, and R3 are connected in series. The other end of resistor R1 is grounded, and the other end of resistor R3 is grounded. During the test, excitation characteristic current signals are added to the connection points of resistor R1 and resistor R2 and the connection points of resistor R2 and resistor R3 respectively; Among them, resistors R1, R2, and R3 respectively represent the resistances of the metal outer sheaths of the three sections of the high-voltage cable cross-bonding grounding method and their grounding connectors.

8. The method for live detection of defective subsections in a cable grounding system using the characteristic current method according to claim 7 is characterized in that: The method for judging the defective cable section by amplitude comparison is: An excitation characteristic current signal is added to the connection point of resistor R1 and resistor R2. The current flowing through the branch of resistor R1 is I1, and the current flowing through the branches of resistors R2 and R3 is I2. If I1 << I2, it is determined that the grounding system defect of this branch cable is located in the cross-bonding sub-section where R1 is located. If I1 >> I2, it is determined that the grounding system defect of this branch cable is located in the cross-bonding sub-section where R2 and R3 are connected in series; Add an excitation characteristic current signal at the connection point of resistor R2 and resistor R3. The current flowing through the branch of resistors R1 and R2 is I3, and the current flowing through the branch of resistor R3 is I4. If I3 >> I4, it is determined that the defect of the branch cable grounding system is located in the cross-bonding subsection where R3 is located. If I3 << I4, it is determined that the defect of the branch cable grounding system is located in the cross-bonding subsection where R1 and R2 are in series; If I1 << I4, I2 << I4 and I3 << I4, it is determined that the defect of the branch cable grounding system is located in the cross-bonding subsection where R1 and R2 are located; If I2 << I1, I3 << I1 and I4 << I1, it is determined that the defect of the branch cable grounding system is located in the cross-bonding subsection where R2 and R3 are located.

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