Method and system for testing sheath impedance in high-voltage cable cross-bonding system

Abstract

A method for testing sheath impedance in a high-voltage cable cross-bonding system. Three-phase current data and three-phase grounding circulating current data from a first cable body segment and a last cable body segment in cross-bonding units, and three-phase current data, three-phase grounding circulating current data, and three-phase induced voltage data from a cable body at a cross-bonding box are detected and collected, thereby directly calculating three-phase sheath impedance of the three cable segments and determining whether the three-phase sheath impedance is abnormal (step 5), thus determining whether the tested cross-bonding units have a defect. The present invention involves a simple operation, achieves an accurate testing result, and can achieve energized testing without an external excitation source. By performing real-time testing on the cross-bonding units and calculating the sheath impedance of the three cable segments in the cross-bonding units in real time, the states of the cross-bonding units can be determined in real time, which makes it possible to detect a cable fault immediately, thereby preventing a loss caused due to the cable fault.

Description

Method and system for testing the sheath impedance of a high-voltage cable cross-bonding system

[0001] This application claims priority to the Chinese patent application No. 202410411499.9, filed on April 8, 2024, with the Chinese Patent Office, the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of power transmission and transformation equipment, in particular to a method and system for testing the sheath impedance of a high-voltage cable cross-bonding system. BACKGROUND

[0003] In the 1990s, urban power transmission cable lines entered a stage of rapid development. As of now, some power transmission cables have been in operation for more than 30 years, and are in a period of frequent faults. Long-distance power transmission cables often use cross-bonding transposition to reduce the sheath induced voltage and ground loop current. Defects such as poor connection of metal parts of cross-bonding boxes, joint dampness leading to lead corrosion, and the like frequently occur after long-term operation, and in severe cases, the sheath loop can be open, which can further cause cable breakdown and cause great economic losses and adverse social impacts.

[0004] In the related art, there are usually two troubleshooting methods for the above-mentioned faults. One is to stop power supply and troubleshoot in sections. However, this method is time-consuming and laborious, and the fault troubleshooting efficiency is low. The other is live detection troubleshooting. For example, the invention patent with the application number CN202210793900.0 discloses a "cable cross-interconnected grounding system connection state live testing device and method". It obtains the loop impedance of the first branch plus the second branch and the third branch parallel impedance, the loop impedance of the second branch plus the first branch and the third branch parallel impedance, and the loop impedance of the third branch plus the first branch and the second branch parallel impedance under two different frequencies through testing. It uses Ohm's law and the characteristics that resistance and inductance are independent of frequency to form a system of equations and calculate the impedance and resistance of each branch. According to the impedance and resistance, the state of the grounding system is determined, so as to detect the electrical connection state of the cable cross-interconnected grounding system under the live state of the cable. For example, the invention patent with the application number CN202211197408.3 discloses a "cable cross-interconnected grounding system loop resistance high-precision testing method and device". It installs a loop resistance high-precision testing device in the cross-interconnected grounding system, tests the response current when the coupler is installed in the three-phase branch of the cross-interconnected grounding system, and the excitation frequency is ω1, ω2 and ω3. It uses electromagnetic induction law and Ohm's law to form a system of equations to calculate the resistance and inductance of each phase branch, so as to realize the detection of loop resistance under the live state of the system and determine the current line electrical connection state. However, for the above-mentioned two live detection methods, an external excitation source is needed to realize detection, and the external excitation source has uncontrollable factors such as excitation source failure, which may cause problems such as excessive cable body, grounding box and grounding line heating due to excessive sheath current, which has a certain impact on the normal operation state of the cable.

[0005] Therefore, how to realize live detection and troubleshooting of the high-voltage cable cross-interconnected system without external excitation source is a problem to be solved at present. SUMMARY

[0006] The purpose of the present application is to overcome the shortcomings of the related art and provide a high-voltage cable cross-interconnected system sheath impedance testing method and system. When applied, the three-phase current data, three-phase grounding loop current data of the first and last two sections of the cross-interconnected unit, and the three-phase current data, three-phase grounding loop current data and three-phase induced voltage data of the cable body at the cross-interconnected box are detected and collected. The sheath impedance of the three sections of the cable is directly calculated, and it is judged whether the sheath impedance is abnormal, so as to judge whether the detected cross-interconnected unit has defects. No external excitation source is needed, the operation is simple, and the detection result is accurate.

[0007] The purpose of the present application is mainly realized by the following technical solutions:

[0008] In a first aspect, the application provides a method for testing the sheath impedance of a high-voltage cable cross-linking system, comprising the following steps:

[0009] Step 1: Select a group of cross-linking units in the power cable line as test points, wherein the group of cross-linking units includes a first cable, a second cable, a last cable, a first cross-linking box, and a second cross-linking box;

[0010] Step 2: Obtain three-phase current data, three-phase ground loop current data, and three-phase induced voltage data of the cross-linking units, including: three-phase currents IA1, IB1, IC1 of the first cable body; three-phase currents IA2, IB2, IC2 of the last cable body; three-phase currents IA3, IB3, IC3 of the cable body at the first cross-linking box; three-phase currents IA4, IB4, IC4 of the cable body at the second cross-linking box; three-phase ground loop currents Ia1, Ib1, Ic1 of the direct grounding point of the first cable; three-phase ground loop currents Ia2, Ib2, Ic2 of the direct grounding point of the last cable; three-phase ground loop currents Ia3, Ib3, Ic3 at the first cross-linking box; three-phase ground loop currents Ia4, Ib4, Ic4 at the second cross-linking box; three-phase induced voltages Ua3, Ub3, Uc3 at the first cross-linking box; three-phase induced voltages Ua4, Ub4, Uc4 at the second cross-linking box; wherein IA1, IB1, IC1, IA2, IB2, IC2, IA3, IB3, IC3, IA4, IB4, IC4, Ia1, Ib1, Ic1, Ia2, Ib2, Ic2, Ia3, Ib3, Ic3, Ia4, Ib4, Ic4 are vectors with units of A, and Ua3, Ub3, Uc3, Ua4, Ub4, Uc4 are vectors with units of V;

[0011] Step 3: Determine whether there is a sheath grounding fault in the cross-linking units, if there is a sheath grounding fault, terminate the determination, and determine that the cross-linking units have a sheath grounding fault problem; if there is no sheath grounding fault, proceed to the next step;

[0012] Step 4: Phase synchronize the three-phase current data, three-phase ground loop current data, and three-phase induced voltage data of the cross-linking units;

[0013] Step 5: Based on the phase-synchronized three-phase current data, three-phase ground loop current data, and three-phase induced voltage data, calculate the sheath impedance Za1, Zb1, Zc1, Za2, Zb2, Zc2, Za3, Zb3, Zc3 of the three cables with units of Ω, and determine whether the sheath impedance of the three cables is abnormal:

[0014] Normal state: the impedance values of the three-phase sheath of the same cable in the cross-connection unit meet the condition that the phase-to-phase imbalance is less than or equal to 3 and the maximum impedance is less than or equal to 2Ω.

[0015] Attention state: the impedance of the same cable sheath meets the condition that the phase-to-phase imbalance is greater than 3 and less than or equal to 5 and the maximum impedance is less than or equal to 5Ω, or the phase-to-phase imbalance is less than or equal to 5 and the maximum impedance is greater than 2 and less than or equal to 5Ω.

[0016] Abnormal state: the impedance of the same cable sheath meets the condition that the phase-to-phase imbalance is greater than 5 or the maximum impedance is greater than 5Ω.

[0017] In the implementation of the present application, first, a group of cross-connection units in the power transmission cable line to be tested is selected as the test point, the cross-connection unit including a first cable, a second cable, a last cable, a first cross-connection box and a second cross-connection box. Specifically, the cross-connection unit further includes direct grounding boxes at the first end and the last end. Specifically, the three cables of the cross-connection unit each include an A phase, a B phase and a C phase, and one end of the first cable is connected to the direct grounding box at the first end, the other end is connected to the second cable in the first cross-connection box, one end of the last cable is connected to the direct grounding box at the last end, and the other end is connected to the second cable in the second cross-connection box. In order to calculate the three-phase sheath impedance of the three cables, the first cable, the last cable and the two cross-connection boxes of the cross-connection unit are selected as the test point, and the three-phase currents IA1, IB1, IC1 of the first cable body, the three-phase currents IA2, IB2, IC2 of the last cable body, the three-phase currents IA3, IB3, IC3 of the cable body at the first cross-connection box, the three-phase currents IA4, IB4, IC4 of the cable body at the second cross-connection box, the three-phase grounding loop currents Ia1, Ib1, Ic1 of the direct grounding point of the first cable, the three-phase grounding loop currents Ia2, Ib2, Ic2 of the direct grounding point of the last cable, the three-phase grounding loop currents Ia3, Ib3, Ic3 at the connecting piece of the first cross-connection box, the three-phase grounding loop currents Ia4, Ib4, Ic4 at the connecting piece of the second cross-connection box, the three-phase induced voltages Ua3, Ub3, Uc3 at the connecting piece of the first cross-connection box, and the three-phase induced voltages Ua4, Ub4, Uc4 at the connecting piece of the second cross-connection box are obtained. After obtaining the above current and voltage data, it is judged whether the cable of the cross-connection unit has a sheath grounding fault. If there is a sheath grounding fault, the judgment is terminated, and it is determined that the cross-connection unit has a sheath grounding fault problem. If there is no sheath grounding fault, the phase synchronization of the above current and voltage data is performed, and the three-phase sheath impedance Za1, Zb1, Zc1, Za2, Zb2, Zc2, Za3, Zb3, Zc3 of the three cables is calculated, and the three-phase sheath impedance of the three cables is judged to determine whether the three-phase sheath impedance of the three cables is abnormal.

[0018] Normal state: the impedance values of the cross-connection unit of the same section of cable three-phase sheath meet the condition that the phase-to-phase imbalance is less than or equal to 3, and the maximum impedance is less than or equal to 2Ω.

[0019] Attention state: the phase-to-phase imbalance of the impedance of the same section of cable sheath is greater than 3 and less than or equal to 5, and the maximum impedance is less than or equal to 5Ω, or the phase-to-phase imbalance is less than or equal to 5, and the maximum impedance is greater than 2 and less than or equal to 5Ω.

[0020] Abnormal state: the phase-to-phase imbalance of the impedance of the same section of cable sheath is greater than 5, or the maximum impedance is greater than 5Ω.

[0021] Differently from the related art, the detection method proposed in the present application only needs to collect the three-phase current data of the first and last two sections of the cable body of the cross interconnection unit, the three-phase ground loop current data of the direct grounding points of the first and last two sections of the cable, and the three-phase current data of the cable body at the two cross interconnection boxes, the three-phase ground loop current data and the three-phase induced voltage data at the connecting piece of the two cross interconnection boxes, to calculate the three-phase sheath impedance of the three sections of the cable, so as to realize live detection and troubleshooting, without the need for any external excitation source. In the related art, in order to realize live detection and troubleshooting of the cross interconnection unit, an external excitation source is usually needed to realize detection of the current line, such as the invention patent "Cable Cross Interconnection Grounding System Connection State Live Testing Device and Method" with application number CN202210793900.0 and the invention patent "Cable Cross Interconnection Grounding System Loop Resistance High Precision Testing Method and Device" with application number CN202211197408.3. The above two detection methods are both detected by means of external excitation source, so as to judge the cable state. However, the external excitation source has uncontrollable factors such as excitation source failure, which may cause problems such as overheating of the cable body, grounding box and grounding wire due to excessive sheath current, and has certain influence on the normal operation state of the cable. Therefore, in the present application, the inventors detect and collect the three-phase current data of the first and last two sections of the cable body of the cross interconnection unit, the three-phase ground loop current data of the direct grounding points of the first and last two sections of the cable, and the three-phase current data of the cable body at the two cross interconnection boxes, the three-phase ground loop current data and the three-phase induced voltage data at the connecting piece of the two cross interconnection boxes, to directly calculate the sheath impedance of the three sections of the cable, and judge whether the sheath impedance is abnormal, so as to judge whether the detected cross interconnection unit has defects. The impedance detection method proposed in the present application does not need any external excitation source, so there is no uncontrollable factor such as excitation source failure, and it will not affect the normal operation of the cable. Furthermore, the impedance detection method proposed in the present application can detect the three-phase current data of the first and last two sections of the cable body of the cross interconnection unit, the three-phase ground loop current data of the direct grounding points of the first and last two sections of the cable, and the three-phase current data of the cable body at the two cross interconnection boxes, the three-phase ground loop current data and the three-phase induced voltage data at the connecting piece of the two cross interconnection boxes in real time, so as to realize real-time calculation of the three-phase sheath impedance of the three sections of the cable of the detected cross interconnection unit, to judge whether the detected cross interconnection unit is abnormal, has good real-time performance, can timely find cable faults, and prevent losses caused by cable faults.

[0022] Further, the method for judging whether the cable of the cross interconnection unit has a sheath ground fault in step 3 is: if the difference between the three-phase currents of the first cable body and the three-phase currents of the last cable body is less than or equal to 10 A, that is, if |IA1+IB1+IC1-IA2-IB2-IC2|≤10 A, it is judged that the cable of the cross interconnection unit does not have a sheath ground fault.

[0023] Further, step 4 is specifically:

[0024] Step 4.1: Select the phase with the maximum amplitude of the three-phase current of the first cable body as the reference quantity Im for phase synchronization: Im=MAX(IA1, IB1, IC1)

[0025] Step 4.2: Calculate the phase difference:

[0026] If the maximum phase of the three-phase current of the first cable body is phase A, that is, Im=IA1, then

[0027] If the maximum phase of the three-phase current of the first cable body is phase B, that is, Im=IB1, then

[0028] If the maximum phase of the three-phase current of the first cable body is phase C, that is, Im=IC1, then

[0029] Step 4.3: Perform phase compensation: the phases of the three-phase currents of the cable body at the first cross interconnection box, the three-phase ground loop currents at the first cross interconnection box, and the three-phase induced voltages at the first cross interconnection box are all subtracted by the phases of the three-phase currents of the cable body at the second cross interconnection box, the three-phase ground loop currents at the second cross interconnection box, and the three-phase induced voltages at the second cross interconnection box are all subtracted by the phases of the three-phase currents of the last cable body and the three-phase ground loop currents of the direct ground point of the last cable are all subtracted by Complete phase synchronization.

[0030] Further, step 5 is specifically:

[0031] Step 5.1: Calculate the three-phase leakage currents of the three sections of cables:

[0032] Let the three-phase leakage currents of the first cable be Iac1, Ibc1, and Icc1, with units of A:

[0033] The A-phase leakage current Iac1 of the first cable is: Iac1=Ia1-Ib3

[0034] The B-phase leakage current Ibc1 of the first section cable: Ibc1 = Ib1 - Ic3

[0035] The C-phase leakage current Icc1 of the first section cable: Icc1 = Ic1 - Ia3

[0036] Let the three-phase leakage currents of the second section cable be Iac2, Ibc2, and Icc2, respectively, in A:

[0037] The A-phase leakage current Iac2 of the second section cable: Iac2 = Ia3 - Ib4

[0038] The B-phase leakage current Ibc2 of the second section cable: Ibc2 = Ib3 - Ic4

[0039] The C-phase leakage current Icc2 of the second section cable: Icc2 = Ic3 - Ia4

[0040] Let the three-phase leakage currents of the final section cable be Iac3, Ibc3, and Icc3, respectively, in A:

[0041] The A-phase leakage current Iac3 of the final section cable: Iac3 = Ia4 - Ia2

[0042] The B-phase leakage current Ibc3 of the final section cable: Ibc3 = Ib4 - Ib2

[0043] The C-phase leakage current Icc3 of the final section cable: Icc3 = Ic4 - Ic2

[0044] Step 5.2: Determine the composition of the three-phase ground loop currents Ia1, Ib1, Ic1 at the direct grounding point of the first section cable of the cross-connection unit, the three-phase ground loop currents Ia2, Ib2, Ic2 at the direct grounding point of the final section cable, the three-phase ground loop currents Ia3, Ib3, Ic3 at the joint of the first cross-connection box, and the three-phase ground loop currents Ia4, Ib4, Ic4 at the joint of the second cross-connection box:

[0045] The composition formulas of the three-phase ground loop currents Ia1, Ib1, Ic1 at the direct grounding point of the first section cable are as follows:

[0046] The A-phase ground loop current Ia1 at the direct grounding point of the first section cable:

[0047] The B-phase ground loop current Ib1 at the direct grounding point of the first section cable:

[0048] The C-phase ground loop current Ic1 at the direct grounding point of the first section cable:

[0049] The constitutions of three-phase ground loop currents Ia2, Ib2, Ic2 of the end cable directly connected to the ground are as follows:

[0050] The A-phase ground loop current Ia2 of the end cable directly connected to the ground is:

[0051] The B-phase ground loop current Ib2 of the end cable directly connected to the ground is:

[0052] The C-phase ground loop current Ic2 of the end cable directly connected to the ground is:

[0053] The constitutions of three-phase ground loop currents Ia3, Ib3, Ic3 at the joint of the first cross interconnection box are as follows:

[0054] The A-phase ground loop current Ia3 at the joint of the first cross interconnection box is:

[0055] The B-phase ground loop current Ib3 at the joint of the first cross interconnection box is:

[0056] The C-phase ground loop current Ic3 at the joint of the first cross interconnection box is:

[0057] The constitutions of three-phase ground loop currents Ia4, Ib4, Ic4 at the joint of the second cross interconnection box are as follows:

[0058] The A-phase ground loop current Ia4 at the joint of the second cross interconnection box is:

[0059] The B-phase ground loop current Ib4 at the joint of the second cross interconnection box is:

[0060] The C-phase ground loop current Ic4 at the joint of the second cross interconnection box is:

[0061] Wherein, Ig1, Ig2, Ig3 are the induced current components of the three sheath circuits of A-B-C, B-C-A and C-A-B respectively, and the unit is A;

[0062] Step 5.3: Determine the constitutions of three-phase induced voltages at the joint of the first cross interconnection box and the joint of the second cross interconnection box:

[0063] The constitutions of the three-phase induction voltages at the joint of the first cross-connection box are as follows:

[0064] The A-phase induction voltage Ua3 at the joint of the first cross-connection box is:

[0065] The B-phase induction voltage Ub3 at the joint of the first cross-connection box is:

[0066] The C-phase induction voltage Uc3 at the joint of the first cross-connection box is:

[0067] Wherein, Za1, Zb1 and Zc1 represent the three-phase sheath impedances of the first cable, and the unit is Ω;

[0068] The constitutions of the three-phase induction voltages at the joint of the second cross-connection box are as follows:

[0069] The B-phase induction voltage Ub4 at the joint of the second cross-connection box is:

[0070] The C-phase induction voltage Uc4 at the joint of the second cross-connection box is:

[0071] The A-phase induction voltage Ua4 at the joint of the second cross-connection box is:

[0072] Wherein, Za2, Zb2 and Zc2 represent the three-phase sheath impedances of the second cable, and the unit is Ω;

[0073] Step 5.4: The formulas obtained in steps 5.1, 5.2 and 5.3 are solved to obtain the three-phase sheath impedances Za1, Zb1 and Zc1 of the first cable and the three-phase sheath impedances Za2, Zb2 and Zc2 of the second cable:

[0074] The three-phase sheath impedances of the first cable are respectively:

[0075] The A-phase sheath impedance Za1 of the first cable is:

[0076] The B-phase sheath impedance Zb1 of the first cable is:

[0077] The C-phase sheath impedance Zc1 of the first cable is:

[0078] The three-phase sheath impedances of the second cable are respectively:

[0079] The A-phase sheath impedance Za2 of the second cable is:

[0080] The B-phase sheath impedance Zb2 of the second section cable:

[0081] The C-phase sheath impedance Zc2 of the second section cable:

[0082] Step 5.5: Calculate the three-phase sheath impedances Za3, Zb3, Zc3 of the last section cable, in units of Ω:

[0083] The constitutive formula of the three-phase induced voltage at the joint of the second cross-connection box can also be expressed as:

[0084] The A-phase induced voltage Ua4 at the joint of the second cross-connection box:

[0085] The B-phase induced voltage Ub4 at the joint of the second cross-connection box:

[0086] The C-phase induced voltage Uc4 at the joint of the second cross-connection box:

[0087] By combining the formulas obtained in steps 5.1, 5.2, and 5.3, we get:

[0088] The A-phase sheath impedance Za3 of the last section cable:

[0089] The B-phase sheath impedance Zb3 of the last section cable:

[0090] The C-phase sheath impedance Zc3 of the last section cable:

[0091] Step 5.6: According to the calculation results of steps 5.4 and 5.5, determine whether the sheath impedance of the three-section cable is abnormal:

[0092] Normal state: The impedance values of the three-phase sheaths of the same section cable of the cross-connection unit satisfy that the phase-to-phase imbalance is less than or equal to 3, and the maximum impedance is less than or equal to 2 Ω, i.e.:

[0093] When and MAX(Za1, Zb1, Zc1) ≤ 2, it is determined that the sheath impedance of the first section cable is normal.

[0094] When and MAX(Za2, Zb2, Zc2) ≤ 2, it is determined that the sheath impedance of the second section cable is normal.

[0095] When and MAX(Za3, Zb3, Zc3)≤2, the sheath impedance of the last section cable is judged to be normal;

[0096] Attention state: the interphase imbalance of the sheath impedance of the same section cable is greater than 3 and less than or equal to 5, and the maximum impedance is less than or equal to 5Ω, or the interphase imbalance is less than or equal to 5, and the maximum impedance is greater than 2 and less than or equal to 5Ω, that is:

[0097] When and MAX(Za1, Zb1, Zc1)≤5, or when and 2<MAX(Za1, Zb1, Zc1)≤5, the sheath impedance of the first section cable is judged to be in the attention state, and the phase with the sheath impedance greater than 2Ω in the first section cable is the abnormal phase;

[0098] When and MAX(Za2, Zb2, Zc2)≤5, or when and 2<MAX(Za2, Zb2, Zc2)≤5, the sheath impedance of the second section cable is judged to be in the attention state, and the phase with the sheath impedance greater than 2Ω in the second section cable is the abnormal phase;

[0099] When and MAX(Za3, Zb3, Zc3)≤5, or when and 2<MAX(Za3, Zb3, Zc3)≤5, the sheath impedance of the last section cable is judged to be in the attention state, and the phase with the sheath impedance greater than 2Ω in the last section cable is the abnormal phase;

[0100] Abnormal state: the interphase imbalance of the sheath impedance of the same section cable is greater than 5, or the maximum impedance is greater than 5Ω, that is:

[0101] When or MAX(Za1, Zb1, Zc1)>5, the sheath impedance of the first section cable is judged to be abnormal, and the phase impedance of the sheath impedance greater than 5Ω in the first section cable is abnormal;

[0102] When or MAX(Za2, Zb2, Zc2)>5, the sheath impedance of the second section cable is judged to be abnormal, and the phase impedance of the sheath impedance greater than 5Ω in the second section cable is abnormal;

[0103] When or MAX(Za3, Zb3, Zc3)>5, the sheath impedance of the last section cable is judged to be abnormal, and the phase impedance of the sheath impedance greater than 5Ω in the last section cable is abnormal.

[0104] Further, when the first end and / or the last end of the cross-connection unit is a straight-through head, the three-phase ground loop current of the first section cable and / or the last section cable directly grounding to the ground is the sum current of the previous cross-connection unit and / or the next cross-connection unit, at this time, the leakage current of the first section cable and / or the last section cable is replaced by the leakage current of the second section cable, the lengths of the three section cables of the cross-connection unit are defined as L1, L2, L3 respectively, in meters, the leakage current is proportional to the length, and the three-phase sheath impedance of the first section cable and / or the three-phase sheath impedance of the last section cable is:

[0105] When the first end of the cross-connection unit is a straight-through head, the three-phase sheath impedance of the first section cable is:

[0106] The A-phase sheath impedance Za1 of the first section cable is:

[0107] The B-phase sheath impedance Zb1 of the first section cable is:

[0108] The C-phase sheath impedance Zc1 of the first section cable is:

[0109] When the last end of the cross-connection unit is a straight-through head, the three-phase sheath impedance of the last section cable is:

[0110] The A-phase sheath impedance Za3 of the last section cable is:

[0111] The B-phase sheath impedance Zb3 of the last section cable is:

[0112] The C-phase sheath impedance Zc3 of the last section cable is:

[0113] Specifically, when the first end of the cross-connection unit is a straight-through head, the ground loop current of the first section cable of the cross-connection unit is the sum current of the previous cross-connection unit, at this time, the leakage current of the first section cable is replaced by the leakage current of the second section cable; when the last end of the cross-connection unit is a straight-through head, the ground loop current of the last section cable of the cross-connection unit is the sum current of the next cross-connection unit, at this time, the leakage current of the last section cable is replaced by the leakage current of the second section cable; when the first end and the last end of the cross-connection unit are straight-through heads, the ground loop current of the first section cable of the cross-connection unit is the sum current of the previous cross-connection unit, and the ground loop current of the last section cable is the sum current of the next cross-connection unit, at this time, the leakage current of the first section cable and the leakage current of the last section cable are replaced by the leakage current of the second section cable.

[0114] It should be noted that the cross-connection unit is divided into A-B-C and A-C-B transposition modes, and in this application, the overall scheme is described in A-B-C transposition mode, and the test method is also applicable to A-C-B transposition mode. For A-C-B transposition mode, in the calculation, the phase B in the phase A calculation formula of each cable is replaced by phase C, and the phase C is replaced by phase A, and the remaining parameters remain unchanged; the phase C in the phase B calculation formula is replaced by phase A, and the phase A is replaced by phase B, and the remaining parameters remain unchanged; the phase A in the phase C calculation formula is replaced by phase B, and the phase B is replaced by phase C, and the remaining parameters remain unchanged.

[0115] In a second aspect, the application provides a test system for the sheath impedance of a high-voltage cable cross-connection system, which applies the test method for the sheath impedance of a high-voltage cable cross-connection system as described above, comprising:

[0116] A first cable body detection unit for detecting the three-phase current of the first cable body and the three-phase ground loop current of the direct grounding point of the first cable;

[0117] A last cable body detection unit for detecting the three-phase current of the last cable body and the three-phase ground loop current of the direct grounding point of the last cable;

[0118] A first cross-connection box detection unit for detecting the three-phase current of the cable body at the first cross-connection box, the three-phase ground loop current at the first cross-connection box, and the three-phase induced voltage at the first cross-connection box;

[0119] A second cross-connection box detection unit for detecting the three-phase current of the cable body at the second cross-connection box, the three-phase ground loop current at the second cross-connection box, and the three-phase induced voltage at the second cross-connection box.

[0120] It should be noted that if the first end and / or the last end of the cross-connection unit is a straight-through head, i.e., the first end and / or the last end of the cross-connection unit is a common grounding end, then the three-phase sheath impedance of the three-section cable of the cross-connection unit can be calculated without detecting the three-phase ground loop current of the direct grounding point of the first cable and / or the three-phase ground loop current of the direct grounding point of the last cable.

[0121] In a possible implementation, the first cable body detection unit includes six current sensors, respectively for detecting three-phase currents of the first cable body and three-phase ground loop currents of the first cable direct grounding point; the last cable body detection unit includes six current sensors, respectively for detecting three-phase currents of the last cable body and three-phase ground loop currents of the last cable direct grounding point; the first cross-connection box detection unit includes six current sensors and three voltage sensors, the six current sensors are respectively for detecting three-phase currents of the cable body at the first cross-connection box and three-phase ground loop currents at the first cross-connection box connecting piece, and the three voltage sensors are respectively for detecting three-phase induced voltages at the first cross-connection box connecting piece; the second cross-connection box detection unit includes six current sensors and three voltage sensors, the six current sensors are respectively for detecting three-phase currents of the cable body at the second cross-connection box and three-phase ground loop currents at the second cross-connection box connecting piece, and the three voltage sensors are respectively for detecting three-phase induced voltages at the second cross-connection box connecting piece. It should be noted that, if the first end of the cross-connection unit is a straight-through head, that is, the first end of the cross-connection unit is a common grounding end, the first cable body detection unit does not need to detect the three-phase ground loop currents of the first cable direct grounding point, and at this time, the first cable body detection unit only needs three current sensors to complete the detection requirement; if the last end of the cross-connection unit is a straight-through head, that is, the last end of the cross-connection unit is a common grounding end, the last cable body detection unit does not need to detect the three-phase ground loop currents of the last cable direct grounding point, and at this time, the last cable body detection unit only needs three current sensors to complete the detection requirement; if the first end and the last end of the cross-connection unit are straight-through heads, that is, the first end and the last end of the cross-connection unit are common grounding ends, the first cable body detection unit does not need to detect the three-phase ground loop currents of the first cable direct grounding point, and the last cable body detection unit does not need to detect the three-phase ground loop currents of the last cable direct grounding point, and at this time, the first cable body detection unit and the last cable body detection unit only need three current sensors to complete the detection requirement.

[0122] Further, the test system for the sheath impedance of the high-voltage cable cross-connection system further comprises a data receiving unit connected with the first cable body detection unit, the last cable body detection unit, the first cross-connection box detection unit and the second cross-connection box detection unit respectively, for receiving the three-phase current data of the first cable body, the three-phase ground loop current data of the direct grounding point of the first cable, the three-phase current data of the last cable body, the three-phase ground loop current data of the direct grounding point of the last cable, the three-phase current data of the cable body at the first cross-connection box, the three-phase ground loop current data at the first cross-connection box, the three-phase induced voltage data at the first cross-connection box, the three-phase current data of the cable body at the second cross-connection box, the three-phase ground loop current data at the second cross-connection box, the three-phase induced voltage data at the second cross-connection box, and performing operation and processing on the received current and voltage data.

[0123] In a possible implementation, the data receiving unit is a computer.

[0124] In conclusion, compared with the related art, the present application has the following advantages: the present application detects and collects the three-phase current data of the first and last cable bodies of the cross-connection unit, the three-phase ground loop current data of the direct grounding points of the first and last cables, and the three-phase current data of the cable bodies at the two cross-connection boxes, the three-phase ground loop current data and the three-phase induced voltage data at the two cross-connection boxes, directly calculates the three-phase sheath impedance of the three cables, and judges whether the three-phase sheath impedance is abnormal, so as to judge whether the detected cross-connection unit has defects; the present application is simple to operate, the detection result is accurate, and the live detection can be realized without external excitation source; the present application performs real-time detection on the cross-connection unit, calculates the sheath impedance of the three cables of the cross-connection unit in real time, judges the state of the cross-connection unit in real time, and can find the cable fault in time, thereby preventing the loss caused by the cable fault. BRIEF DESCRIPTION OF DRAWINGS

[0125] The accompanying drawings, which are included to provide a further understanding of the embodiments of the present application and are incorporated in and constitute a part of this application, illustrate embodiments of the present application and do not limit the present application. In the drawings:

[0126] FIG. 1 is a flowchart of the embodiment of the present application;

[0127] FIG. 2 is an impedance calculation schematic diagram of the A-B-C connection mode of the embodiment of the present application;

[0128] FIG. 3 is an impedance calculation schematic diagram of the A-C-B connection mode of the embodiment of the present application;

[0129] FIG. 4 is a cable sheath impedance equivalent schematic diagram of the A-B-C loop in the A-B-C connection mode of the embodiment of the present application. DETAILED DESCRIPTION

[0130] In order to make the purpose, technical solutions and advantages of the present application clearer, further detailed description will be made to the present application in combination with embodiments and drawings, the illustrative embodiments and the description thereof are only used to explain the present application, and do not limit the present application.

[0131] Embodiment

[0132] As shown in FIG. 1, the present embodiment provides a test method for the sheath impedance of a high-voltage cable cross-connection system, comprising the following steps:

[0133] Step 1: Select a group of cross-connection units in a power transmission cable line as test points, wherein the group of cross-connection units comprises a first section of cable, a second section of cable, a last section of cable, a first cross-connection box and a second cross-connection box;

[0134] Step 2: Obtain three-phase current data, three-phase ground loop current data and three-phase induced voltage data of the cross-connection units, including three-phase currents IA1, IB1, IC1 of the first section of cable body, three-phase currents IA2, IB2, IC2 of the last section of cable body, three-phase currents IA3, IB3, IC3 of the cable body at the first cross-connection box, three-phase currents IA4, IB4, IC4 of the cable body at the second cross-connection box, three-phase ground loop currents Ia1, Ib1, Ic1 of the direct grounding point of the first section of cable, three-phase ground loop currents Ia2, Ib2, Ic2 of the direct grounding point of the last section of cable, three-phase ground loop currents Ia3, Ib3, Ic3 at the first cross-connection box, three-phase ground loop currents Ia4, Ib4, Ic4 at the second cross-connection box, three-phase induced voltages Ua3, Ub3, Uc3 at the first cross-connection box, and three-phase induced voltages Ua4, Ub4, Uc4 at the second cross-connection box; wherein IA1, IB1, IC1, IA2, IB2, IC2, IA3, IB3, IC3, IA4, IB4, IC4, Ia1, Ib1, Ic1, Ia2, Ib2, Ic2, Ia3, Ib3, Ic3, Ia4, Ib4, Ic4 are all vectors, with the unit of A, and Ua3, Ub3, Uc3, Ua4, Ub4, Uc4 are all vectors, with the unit of V;

[0135] Step 3: Determine whether there is a sheath grounding fault in the cable of the cross-connection unit, if there is a sheath grounding fault, terminate the determination, and determine that the cross-connection unit has a sheath grounding fault problem; if there is no sheath grounding fault, proceed to the next step;

[0136] Step 4: Perform phase synchronization on the three-phase current data, three-phase ground loop current data and three-phase induced voltage data of the cross-connection unit.

[0137] Step 5: Based on the phase-synchronized three-phase current data, three-phase ground loop current data and three-phase induced voltage data, the sheath impedance Za1, Zb1, Zc1, Za2, Zb2, Zc2, Za3, Zb3, Zc3 of the three sections of the cable is calculated, with the unit of Ω, to determine whether the sheath impedance of the three sections of the cable is abnormal:

[0138] Normal state: the impedance values of the three-phase sheaths of the same section of the cable of the cross-connection unit meet the condition that the phase-to-phase imbalance is less than or equal to 3, and the maximum impedance is less than or equal to 2Ω.

[0139] Attention state: the phase-to-phase imbalance of the sheath impedance of the same section of the cable is greater than 3 and less than or equal to 5, and the maximum impedance is less than or equal to 5Ω, or the phase-to-phase imbalance is less than or equal to 5, and the maximum impedance is greater than 2 and less than or equal to 5Ω.

[0140] Abnormal state: the phase-to-phase imbalance of the sheath impedance of the same section of the cable is greater than 5, or the maximum impedance is greater than 5Ω.

[0141] In the implementation of the embodiment, first, a group of cross-connection units in a power transmission cable line to be tested is selected as test points, the cross-connection unit including a first cable, a second cable, a last cable, a first cross-connection box and a second cross-connection box. Specifically, the cross-connection unit further includes direct grounding boxes at the first and last ends. Specifically, the three cables of the cross-connection unit each include A-phase, B-phase and C-phase, one end of the first cable is connected to the direct grounding box at the first end, the other end is connected to the second cable in the first cross-connection box, one end of the last cable is connected to the direct grounding box at the last end, and the other end is connected to the second cable in the second cross-connection box. In order to calculate the three-phase sheath impedance of the three cables, the first cable, the last cable and the two cross-connection boxes of the cross-connection unit are selected as test points, and the three-phase currents IA1, IB1 and IC1 of the first cable body, the three-phase currents IA2, IB2 and IC2 of the last cable body, the three-phase currents IA3, IB3 and IC3 of the cable body at the first cross-connection box, the three-phase currents IA4, IB4 and IC4 of the cable body at the second cross-connection box, the three-phase grounding loop currents Ia1, Ib1 and Ic1 of the direct grounding point of the first cable, the three-phase grounding loop currents Ia2, Ib2 and Ic2 of the direct grounding point of the last cable, the three-phase grounding loop currents Ia3, Ib3 and Ic3 at the connecting piece of the first cross-connection box, the three-phase grounding loop currents Ia4, Ib4 and Ic4 at the connecting piece of the second cross-connection box, the three-phase induced voltages Ua3, Ub3 and Uc3 at the connecting piece of the first cross-connection box, and the three-phase induced voltages Ua4, Ub4 and Uc4 at the connecting piece of the second cross-connection box are obtained. After obtaining the above current and voltage data, it is judged whether the cable of the cross-connection unit has a sheath grounding fault. If there is a sheath grounding fault, the judgment is terminated, and it is determined that the cross-connection unit has a sheath grounding fault problem. If there is no sheath grounding fault, the phase synchronization of the above current and voltage data is performed, and the three-phase sheath impedances Za1, Zb1, Zc1, Za2, Zb2, Zc2, Za3, Zb3 and Zc3 of the three cables are calculated, and the three-phase sheath impedances of the three cables are judged to determine whether the three-phase sheath impedances of the three cables are abnormal. The specific method is as follows:

[0142] Normal state: the impedance values of the three-phase sheaths of the same cable of the cross-connection unit satisfy that the phase-to-phase imbalance degree is less than or equal to 3, and the maximum impedance is less than or equal to 2Ω.

[0143] Attention state: the sheath impedance of the same cable satisfies that the phase-to-phase imbalance degree is greater than 3 and less than or equal to 5, and the maximum impedance is less than or equal to 5Ω, or the phase-to-phase imbalance degree is less than or equal to 5, and the maximum impedance is greater than 2 and less than or equal to 5Ω.

[0144] Abnormal state: the sheath impedance of the same cable satisfies that the phase-to-phase imbalance degree is greater than 5, or the maximum impedance is greater than 5Ω.

[0145] Differently from the related art, the detection method proposed in the embodiment only needs to collect the three-phase current data of the cable bodies at the first and last sections of the cross-connection unit, the three-phase ground loop current data of the direct grounding points at the first and last sections of the cable, the three-phase current data of the cable bodies at the two cross-connection boxes, the three-phase ground loop current data and the three-phase induced voltage data at the connecting sections of the two cross-connection boxes, and then calculate the three-phase sheath impedance of the three sections of the cable, so as to realize live detection and troubleshooting without any external excitation source. In the related art, if the live detection and troubleshooting of the cross-connection unit are to be realized, an external excitation source is usually needed to realize the detection of the current line, such as the invention patent "Cable Cross-Connection Grounding System Connection State Live Testing Device and Method" with the application number CN202210793900.0 and the invention patent "Cable Cross-Connection Grounding System Loop Resistance High-Precision Testing Method and Device" with the application number CN202211197408.3. The above two detection methods are both for detection through the way of external excitation source, so as to judge the cable state. However, the external excitation source has uncontrollable factors such as excitation source failure, which may cause problems such as overheating of the cable body, grounding box and grounding wire due to excessive sheath current, and has certain influence on the normal operation state of the cable. Therefore, in the embodiment, the inventor detects and collects the three-phase current data of the cable bodies at the first and last sections of the cross-connection unit, the three-phase ground loop current data of the direct grounding points at the first and last sections of the cable, the three-phase current data of the cable bodies at the two cross-connection boxes, the three-phase ground loop current data and the three-phase induced voltage data at the connecting sections of the two cross-connection boxes, and then directly calculates the three-phase sheath impedance of the three sections of the cable, and judges whether the three-phase sheath impedance is abnormal, so as to judge whether the detected cross-connection unit has defects. The impedance detection method proposed in the embodiment does not need any external excitation source, so there are no uncontrollable factors such as excitation source failure, and the normal operation of the cable will not be affected. Furthermore, the impedance detection method proposed in the embodiment can detect the three-phase current data of the cable bodies at the first and last sections of the cross-connection unit, the three-phase ground loop current data of the direct grounding points at the first and last sections of the cable, the three-phase current data of the cable bodies at the two cross-connection boxes, the three-phase ground loop current data and the three-phase induced voltage data at the connecting sections of the two cross-connection boxes in real time when it is applied, so as to realize real-time calculation of the sheath impedance of the three sections of the cable of the detected cross-connection unit, and judge whether the detected cross-connection unit is abnormal, which is good in real-time performance and can timely find the cable fault, so as to prevent the loss caused by the cable fault.

[0146] Further, the method for judging whether the cable of the cross-connection unit has a sheath ground fault in step 3 is: if the difference between the three-phase currents of the first cable body and the three-phase currents of the last cable body is less than or equal to 10 A, that is, if |IA1+IB1+IC1-IA2-IB2-IC2|≤10 A, it is judged that the cable of the cross-connection unit does not have a sheath ground fault.

[0147] Further, the step 4 is specifically:

[0148] Step 4.1: Select the maximum phase of the three-phase current of the first cable body as the reference quantity Im for phase synchronization: Im=MAX(IA1, IB1, IC1)

[0149] Step 4.2: Calculate the phase difference:

[0150] If the maximum phase of the three-phase current of the first cable body is A phase, that is, Im=IA1, then

[0151] If the maximum phase of the three-phase current of the first cable body is B phase, that is, Im=IB1, then

[0152] If the maximum phase of the three-phase current of the first cable body is C phase, that is, Im=IC1, then

[0153] Step 4.3: Perform phase compensation: the phases of the three-phase currents of the cable body at the first cross-connection box, the three-phase ground loop currents at the first cross-connection box, and the three-phase induced voltages at the first cross-connection box are all subtracted by the phases of the three-phase currents of the cable body at the second cross-connection box, the three-phase ground loop currents at the second cross-connection box, and the three-phase induced voltages at the second cross-connection box are all subtracted by the phases of the three-phase currents of the last cable body and the three-phase ground loop currents of the direct ground point of the last cable are all subtracted by Complete phase synchronization.

[0154] As shown in FIG. 2, the impedance calculation schematic diagram of the A-B-C connection mode in the present embodiment, the step 5 is specifically:

[0155] Step 5.1: Calculate the three-phase leakage currents of the three cables:

[0156] Let the three-phase leakage currents of the first cable be Iac1, Ibc1, and Icc1, units A:

[0157] The A-phase leakage current Iac1 of the first cable: Iac1=Ia1-Ib3

[0158] The B-phase leakage current Ibc1 of the first section cable: Ibc1 = Ib1 - Ic3

[0159] The C-phase leakage current Icc1 of the first section cable: Icc1 = Ic1 - Ia3

[0160] Let the three-phase leakage currents of the second section cable be Iac2, Ibc2, and Icc2, respectively, with units of A:

[0161] The A-phase leakage current Iac2 of the second section cable: Iac2 = Ia3 - Ib4

[0162] The B-phase leakage current Ibc2 of the second section cable: Ibc2 = Ib3 - Ic4

[0163] The C-phase leakage current Icc2 of the second section cable: Icc2 = Ic3 - Ia4

[0164] Let the three-phase leakage currents of the final section cable be Iac3, Ibc3, and Icc3, respectively, with units of A:

[0165] The A-phase leakage current Iac3 of the final section cable: Iac3 = Ia4 - Ia2

[0166] The B-phase leakage current Ibc3 of the final section cable: Ibc3 = Ib4 - Ib2

[0167] The C-phase leakage current Icc3 of the final section cable: Icc3 = Ic4 - Ic2

[0168] Step 5.2: Determine the composition of the three-phase ground loop currents Ia1, Ib1, Ic1 at the direct grounding point of the first section cable of the cross-connection unit, the three-phase ground loop currents Ia2, Ib2, Ic2 at the direct grounding point of the final section cable, the three-phase ground loop currents Ia3, Ib3, Ic3 at the joint of the first cross-connection box, and the three-phase ground loop currents Ia4, Ib4, Ic4 at the joint of the second cross-connection box:

[0169] The composition of the three-phase ground loop currents Ia1, Ib1, Ic1 at the direct grounding point of the first section cable is as follows:

[0170] The A-phase ground loop current Ia1 at the direct grounding point of the first section cable:

[0171] The B-phase ground loop current Ib1 at the direct grounding point of the first section cable:

[0172] The C-phase ground loop current Ic1 at the direct grounding point of the first section cable:

[0173] The three-phase ground loop currents Ia2, Ib2, Ic2 of the end cable directly grounded are constituted as follows:

[0174] The A-phase ground loop current Ia2 of the end cable directly grounded is constituted as follows:

[0175] The B-phase ground loop current Ib2 of the end cable directly grounded is constituted as follows:

[0176] The C-phase ground loop current Ic2 of the end cable directly grounded is constituted as follows:

[0177] The three-phase ground loop currents Ia3, Ib3, Ic3 of the first cross-connection box joint are constituted as follows:

[0178] The A-phase ground loop current Ia3 of the first cross-connection box joint is constituted as follows:

[0179] The B-phase ground loop current Ib3 of the first cross-connection box joint is constituted as follows:

[0180] The C-phase ground loop current Ic3 of the first cross-connection box joint is constituted as follows:

[0181] The three-phase ground loop currents Ia4, Ib4, Ic4 of the second cross-connection box joint are constituted as follows:

[0182] The A-phase ground loop current Ia4 of the second cross-connection box joint is constituted as follows:

[0183] The B-phase ground loop current Ib4 of the second cross-connection box joint is constituted as follows:

[0184] The C-phase ground loop current Ic4 of the second cross-connection box joint is constituted as follows:

[0185] Wherein, Ig1, Ig2, Ig3 are the induced current components of the three A-B-C, B-C-A and C-A-B shielded circuits, and the unit is A;

[0186] It should be noted that the size of each leakage current is allocated according to the resistance size in step 5.2, that is, the size of the leakage current is allocated according to the impedance of the cable sheath when the cross-connection unit is normal. As shown in FIG. 4, it is an equivalent schematic diagram of the cable sheath impedance of the A-B-C loop in the A-B-C connection mode of the cross-connection unit. In FIG. 4, the leakage current of each cable is equivalent to a direct grounding point flowing from the middle position of the sheath to both sides. Since the sheath impedance of each cable is uniform when there is no defect, when the A-phase leakage current Iac1 of the first cable flows from the middle position of the sheath to both sides of the direct grounding point, the sheath impedance on the left side of the middle position of the sheath is Z, and the sheath impedance on the right side is 5Z, so according to Ohm's law, the current flowing through the left side of the middle position of the sheath is and the current flowing through the right side of the middle position of the sheath is Similarly, the B-phase leakage current Ibc2 of the second cable flows through the left side of the middle position of the sheath and the current flowing through the right side of the middle position of the sheath is The C-phase leakage current Icc3 of the last cable flows through the left side of the middle position of the sheath and the current flowing through the right side of the middle position of the sheath is Therefore, the composition formula of the A-phase grounding loop current of the first cable direct grounding point is Similarly, the composition formula of the remaining grounding loop currents Ib1, Ic1, Ia2, Ib2, Ic2, Ia3, Ib3, Ic3, Ia4, Ib4, Ic4 can be obtained.

[0187] Step 5.3: Determine the composition of the three-phase induced voltage at the first cross-connection box joint and the second cross-connection box joint.

[0188] The composition formula of the three-phase induced voltage at the first cross-connection box joint is as follows:

[0189] The A-phase induced voltage Ua3 at the first cross-connection box joint is:

[0190] The B-phase induced voltage Ub3 at the first cross-connection box joint is:

[0191] The C-phase induced voltage Uc3 at the first cross-connection box joint is:

[0192] Wherein, Za1, Zb1, Zc1 represent the three-phase sheath impedance of the first cable, with the unit of Ω;

[0193] The composition formula of the three-phase induced voltage at the second cross-connection box joint is as follows:

[0194] The B-phase induced voltage Ub4 at the second cross-connection box joint is:

[0195] C-phase induced voltage Uc4 at the joint of the second cross-connection box:

[0196] A-phase induced voltage Ua4 at the joint of the second cross-connection box:

[0197] wherein Za2, Zb2, Zc2 represent the three-phase sheath impedances of the second section of cable, respectively, in units of Ω;

[0198] Step 5.4: the formula obtained by combining steps 5.1, 5.2 and 5.3 is used to obtain the three-phase sheath impedances Za1, Zb1, Zc1 of the first section of cable and the three-phase sheath impedances Za2, Zb2, Zc2 of the second section of cable:

[0199] The three-phase sheath impedances of the first section of cable are respectively:

[0200] A-phase sheath impedance Za1 of the first section of cable:

[0201] B-phase sheath impedance Zb1 of the first section of cable:

[0202] C-phase sheath impedance Zc1 of the first section of cable:

[0203] The three-phase sheath impedances of the second section of cable are respectively:

[0204] A-phase sheath impedance Za2 of the second section of cable:

[0205] B-phase sheath impedance Zb2 of the second section of cable:

[0206] C-phase sheath impedance Zc2 of the second section of cable:

[0207] Step 5.5: calculate the three-phase sheath impedances Za3, Zb3, Zc3 of the last section of cable, in units of Ω:

[0208] The constitutive formula of the three-phase induced voltage at the joint of the second cross-connection box can also be expressed as:

[0209] A-phase induced voltage Ua4 at the joint of the second cross-connection box:

[0210] B-phase induced voltage Ub4 at the joint of the second cross-connection box:

[0211] C-phase induced voltage Uc4 at the joint of the second cross-connection box:

[0212] Solving the equations obtained from steps 5.1, 5.2 and 5.3 simultaneously, we get:

[0213] A-phase sheath impedance Za3 of the last section cable:

[0214] B-phase sheath impedance Zb3 of the last section cable:

[0215] C-phase sheath impedance Zc3 of the last section cable:

[0216] Step 5.6: According to the calculation results of steps 5.4 and 5.5, determine whether the sheath impedance of the three-section cable is abnormal:

[0217] Normal state: the impedance values of the three-phase sheaths of the same section cable of the cross-connection unit meet the condition that the phase-to-phase imbalance is less than or equal to 3, and the maximum impedance is less than or equal to 2Ω, i.e.:

[0218] When and MAX(Za1, Zb1, Zc1)≤2, it is determined that the sheath impedance of the first section cable is normal.

[0219] When and MAX(Za2, Zb2, Zc2)≤2, it is determined that the sheath impedance of the second section cable is normal.

[0220] When and MAX(Za3, Zb3, Zc3)≤2, it is determined that the sheath impedance of the last section cable is normal.

[0221] Attention state: the phase-to-phase imbalance of the sheath impedance of the same section cable is greater than 3 and less than or equal to 5, and the maximum impedance is less than or equal to 5Ω, or the phase-to-phase imbalance is less than or equal to 5 and the maximum impedance is greater than 2 and less than or equal to 5Ω, i.e.:

[0222] When and MAX(Za1, Zb1, Zc1)≤5, or when and 2<MAX(Za1, Zb1, Zc1)≤5, it is determined that the sheath impedance of the first section cable is in the attention state, and the phase with a sheath impedance greater than 2Ω in the first section cable is the abnormal phase.

[0223] When and MAX(Za2, Zb2, Zc2)≤5, or when and 2 < MAX(Za2, Zb2, Zc2) ≤ 5, the sheath impedance of the second section cable is determined to be in the attention state, and the phase of the second section cable with the sheath impedance greater than 2 Ω is the abnormal phase;

[0224] When and MAX(Za3, Zb3, Zc3) ≤ 5, or when and 2 < MAX(Za3, Zb3, Zc3) ≤ 5, the sheath impedance of the last section cable is determined to be in the attention state, and the phase of the last section cable with the sheath impedance greater than 2 Ω is the abnormal phase;

[0225] Abnormal state: the inter-phase imbalance of the sheath impedance of the same section cable is greater than 5, or the maximum impedance is greater than 5 Ω, that is:

[0226] When or MAX(Za1, Zb1, Zc1) > 5, the sheath impedance of the first section cable is determined to be abnormal, and the phase of the first section cable with the sheath impedance greater than 5 Ω is the abnormal phase;

[0227] When or MAX(Za2, Zb2, Zc2) > 5, the sheath impedance of the second section cable is determined to be abnormal, and the phase of the second section cable with the sheath impedance greater than 5 Ω is the abnormal phase;

[0228] When or MAX(Za3, Zb3, Zc3) > 5, the sheath impedance of the last section cable is determined to be abnormal, and the phase of the last section cable with the sheath impedance greater than 5 Ω is the abnormal phase.

[0229] Further, when the first end and / or the last end of the cross interconnection unit is a straight-through head, the three-phase ground loop of the first section cable and / or the last section cable directly grounded to the ground is the sum of the current of the previous cross interconnection unit and / or the next cross interconnection unit, at this time, the leakage current of the first section cable and / or the last section cable is equivalent to the leakage current of the second section cable, the lengths of the three section cables of the cross interconnection unit are defined as L1, L2, and L3 respectively, in meters, the size of the leakage current is in a proportional relationship with the length, and the three-phase sheath impedance of the first section cable and / or the three-phase sheath impedance of the last section cable is obtained:

[0230] When the first end of the cross interconnection unit is a straight-through head, the three-phase sheath impedance of the first section cable is respectively:

[0231] The A-phase sheath impedance Za1 of the first section cable is:

[0232] The B-phase sheath impedance Zb1 of the first section cable is:

[0233] The C-phase sheath impedance Zc1 of the first section cable is:

[0234] When the end of the cross-connection unit is a straight-through head, the three-phase sheath impedance of the end segment cable is respectively:

[0235] The A-phase sheath impedance Za3 of the end segment cable is:

[0236] The B-phase sheath impedance Zb3 of the end segment cable is:

[0237] The C-phase sheath impedance Zc3 of the end segment cable is:

[0238] Specifically, when the head of the cross-connection unit is a straight-through head, the ground loop current of the first segment cable of the cross-connection unit is the sum current of the previous cross-connection unit, at this time the leakage current of the first segment cable is equivalent to the leakage current of the second segment cable; when the end of the cross-connection unit is a straight-through head, the ground loop current of the end segment cable of the cross-connection unit is the sum current of the next cross-connection unit, at this time the leakage current of the end segment cable is equivalent to the leakage current of the second segment cable; when the head and the end of the cross-connection unit are both straight-through heads, the ground loop current of the first segment cable of the cross-connection unit is the sum current of the previous cross-connection unit, and the ground loop current of the end segment cable is the sum current of the next cross-connection unit, at this time the leakage current of the first segment cable and the end segment cable are both equivalent to the leakage current of the second segment cable.

[0239] It should be noted that the cross-connection unit is divided into A-B-C and A-C-B transposition modes, in this embodiment, the overall scheme is specifically described in the A-B-C transposition mode, and the test method is also applicable to the A-C-B transposition mode, and for the A-C-B transposition mode, the phase B in the calculation formula of the A phase of each segment cable is replaced by the phase C, and the phase C is replaced by the phase A, and the remaining parameters remain unchanged; the phase C in the calculation formula of the B phase is replaced by the phase A, and the phase A is replaced by the phase B, and the remaining parameters remain unchanged; the phase A in the calculation formula of the C phase is replaced by the phase B, and the phase B is replaced by the phase C, and the remaining parameters remain unchanged. The impedance calculation diagram of the A-C-B connection mode is shown in FIG. 3.

[0240] Based on the above-mentioned high-voltage cable cross-connection system sheath impedance test method, the embodiment further provides a high-voltage cable cross-connection system sheath impedance test system, comprising:

[0241] The first segment cable body detection unit is used for detecting the three-phase current of the first segment cable body and the three-phase ground loop current of the direct grounding point of the first segment cable;

[0242] The end cable body detection unit is configured to detect three-phase currents of the end cable body and three-phase ground loop currents of the end cable direct grounding points.

[0243] The first cross-connection box detection unit is configured to detect three-phase currents of the cable body at the first cross-connection box, three-phase ground loop currents at the first cross-connection box connecting piece, and three-phase induced voltages at the first cross-connection box connecting piece.

[0244] The second cross-connection box detection unit is configured to detect three-phase currents of the cable body at the second cross-connection box, three-phase ground loop currents at the second cross-connection box connecting piece, and three-phase induced voltages at the second cross-connection box connecting piece.

[0245] It is worth mentioning that if the first end and / or the last end of the cross-connection unit is a straight-through head, that is, the first end and / or the last end of the cross-connection unit is a common grounding end, then the three-phase ground loop currents of the first end cable direct grounding points and / or the three-phase ground loop currents of the last end cable direct grounding points do not need to be detected, and the three-phase sheath impedance of the three-section cable of the cross-connection unit can be calculated.

[0246] In a possible implementation, the first cable body detection unit includes six current sensors, respectively for detecting three-phase currents of the first cable body and three-phase ground loop currents of the first cable direct grounding point; the last cable body detection unit includes six current sensors, respectively for detecting three-phase currents of the last cable body and three-phase ground loop currents of the last cable direct grounding point; the first cross-connection box detection unit includes six current sensors and three voltage sensors, the six current sensors are respectively for detecting three-phase currents of the cable body at the first cross-connection box and three-phase ground loop currents at the first cross-connection box connecting piece, and the three voltage sensors are respectively for detecting three-phase induced voltages at the first cross-connection box connecting piece; the second cross-connection box detection unit includes six current sensors and three voltage sensors, the six current sensors are respectively for detecting three-phase currents of the cable body at the second cross-connection box and three-phase ground loop currents at the second cross-connection box connecting piece, and the three voltage sensors are respectively for detecting three-phase induced voltages at the second cross-connection box connecting piece. It should be noted that, if the first end of the cross-connection unit is a straight-through head, that is, the first end of the cross-connection unit is a common grounding end, the first cable body detection unit does not need to detect the three-phase ground loop currents of the first cable direct grounding point, and at this time, the first cable body detection unit only needs three current sensors to complete the detection requirement; if the last end of the cross-connection unit is a straight-through head, that is, the last end of the cross-connection unit is a common grounding end, the last cable body detection unit does not need to detect the three-phase ground loop currents of the last cable direct grounding point, and at this time, the last cable body detection unit only needs three current sensors to complete the detection requirement; if the first end and the last end of the cross-connection unit are straight-through heads, that is, the first end and the last end of the cross-connection unit are common grounding ends, the first cable body detection unit does not need to detect the three-phase ground loop currents of the first cable direct grounding point, and the last cable body detection unit does not need to detect the three-phase ground loop currents of the last cable direct grounding point, and at this time, the first cable body detection unit and the last cable body detection unit only need three current sensors to complete the detection requirement.

[0247] Further, the test system for the sheath impedance of the high-voltage cable cross-connection system further comprises a data receiving unit connected with the first cable body detection unit, the last cable body detection unit, the first cross-connection box detection unit and the second cross-connection box detection unit respectively, for receiving the three-phase current data of the first cable body, the three-phase ground loop current data of the direct grounding point of the first cable, the three-phase current data of the last cable body, the three-phase ground loop current data of the direct grounding point of the last cable, the three-phase current data of the cable body at the first cross-connection box, the three-phase ground loop current data at the first cross-connection box, the three-phase induced voltage data at the first cross-connection box, the three-phase current data of the cable body at the second cross-connection box, the three-phase ground loop current data at the second cross-connection box, the three-phase induced voltage data at the second cross-connection box, and performing operation and processing on the received current and voltage data.

[0248] In a possible implementation manner, the data receiving unit is a computer.

[0249] In order to better understand and describe the test method and system for the sheath impedance of the high-voltage cable cross-connection system, the present application will be described below in combination with a specific embodiment.

[0250] The test method for the sheath impedance of the high-voltage cable cross-connection system comprises the following steps.

[0251] Step 1: selecting a group of cross-connection units in the power transmission cable line as test points, wherein the group of cross-connection units comprises a first cable, a second cable, a last cable, a first cross-connection box and a second cross-connection box, the first and last ends of the selected cross-connection units are both common grounding ends, and the connection mode is A-C-B.

[0252] Step 2: Install the first cable body detection unit, the last cable body detection unit, the first cross-connection box detection unit and the second cross-connection box detection unit at the first end and the last end of the cross-connection unit and the two cross-connection boxes one by one, that is, install three current sensors at the first cable body of the cross-connection unit to monitor the three-phase currents of the first cable body respectively; install three current sensors at the last cable body of the cross-connection unit to monitor the three-phase currents of the last cable body respectively; install six current sensors and three voltage sensors at the first cross-connection box of the cross-connection unit, wherein the six current sensors are used to detect the three-phase currents and three-phase ground loop currents of the cable body at the first cross-connection box respectively; install six current sensors and three voltage sensors at the second cross-connection box of the cross-connection unit, wherein the six current sensors are used to detect the three-phase currents and three-phase ground loop currents of the cable body at the second cross-connection box respectively. It is worth noting that, since the selected cross-connection unit in this embodiment has a common ground terminal at both ends, that is, the ground loop currents of the first and last cables of the cross-connection unit are the sum of the currents of the two cross-connection units, and at this time the leakage currents of the first and last cables are equivalent to the leakage current of the second cable, therefore, it is not necessary to monitor the ground loop currents of the first and last cables, and at this time the first cable body detection unit and the last cable body detection unit only need three current sensors to meet the monitoring requirements.

[0253] The parameters obtained by collection are as follows:

[0254] IA1 = 128.1 ∠ 305°, IB1 = 130.8 ∠ 186°, IC1 = 122.6 ∠ 66°

[0255] IA2 = 120.5 ∠ 32°, IB2 = 119.4 ∠ 277°, IC2 = 124.3 ∠ 158°

[0256] IA3 = 125.8 ∠ 200°, IB3 = 131.6 ∠ 80°, IC3 = 124.7 ∠ 321°

[0257] Ia3 = 5.2 ∠ 115°, Ib3 = 6.3 ∠ 65°, Ic3 = 0

[0258] Ua3 = 6.5 ∠ 182°, Ub3 = 6.4 ∠ 48°, Uc3 = 6.4 ∠ 295°

[0259] IA4 = 120.7 ∠ 29°, IB4 = 121.6 ∠ 273°, IC4 = 117.6 ∠ 155°

[0260] Ia4 = 6.5 ∠ 235°, Ib4 = 2.8 ∠ 68°, Ic4 = 7.5 ∠ 305°

[0261] Ua4 = 5.4 ∠ 304°, Ub4 = 4.2 ∠ 178°, Uc4 = 7.4 ∠ 64°;

[0262] Step 3: judging whether the cable of the cross-connection unit has a sheath ground fault or not: |IA1 + IB1 + IC1 - IA2 - IB2 - IC2| = 8.37A < 10A

[0263] That is, the cable of the cross-connection unit does not have a sheath ground fault;

[0264] Step 4.1: selecting the phase with the maximum three-phase current amplitude of the first cable body as the reference quantity Im for phase synchronization: Im = MAX(IA1, IB1, IC1) = IB1

[0265] Therefore, the B phase is selected as the reference quantity Im;

[0266] Step 4.2: calculating the phase difference:

[0267] Since the phase with the maximum three-phase current of the first cable body is the B phase, that is, Im = IB1, then

[0268] Since the connection mode of the cable in this embodiment is A-C-B, the above formula is rewritten as:

[0269] Substituting the parameters, we get:

[0270] Step 4.3: performing phase compensation: the phases of the three-phase currents of the cable body at the first cross-connection box, the three-phase ground loop currents at the first cross-connection box, and the three-phase induced voltages at the first cross-connection box are all subtracted by the phases of the three-phase currents of the cable body at the second cross-connection box, the three-phase ground loop currents at the second cross-connection box, and the three-phase induced voltages at the second cross-connection box the phases of the three-phase currents of the cable body at the end of the cable body The parameters after phase compensation are obtained as:

[0271] IA1 = 128.1 ∠ 305°, IB1 = 130.8 ∠ 186°, IC1 = 122.6 ∠ 66°

[0272] IA2 = 120.5 ∠ 301.3°, IB2 = 119.4 ∠ 186.3°, IC2 = 124.3 ∠ 67.3°

[0273] IA3 = 125.8 ∠ 304°, IB3 = 131.6 ∠ 184°, IC3 = 124.7 ∠ 65°

[0274] Ia3 = 5.2 ∠ 219°, Ib3 = 6.3 ∠ 169°, Ic3 = 0

[0275] Ua3 = 6.5 ∠ 286°, Ub3 = 6.4 ∠ 152°, Uc3 = 6.4 ∠ 39°

[0276] IA4 = 120.7 ∠ 302°, IB4 = 121.6 ∠ 186°, IC4 = 117.6 ∠ 68°

[0277] Ia4 = 6.5 ∠ 148.3°, Ib4 = 2.8 ∠ 341.3°, Ic4 = 7.5 ∠ 218.3°

[0278] Ua4 = 5.4 ∠ 217.3°, Ub4 = 4.2 ∠ 91.3°, Uc4 = 7.4 ∠ 337.3°.

[0279] Step 5: Calculate the sheath impedance of each cable segment according to the formula:

[0280] Since the connection mode of the cross interconnection unit in this embodiment is A-C-B, the sheath impedance calculation formula of each cable segment is adjusted:

[0281] The lengths of the cable segments are L1 = 548 m, L2 = 576 m, and L3 = 550 m.

[0282] Za1 = 5.26 Ω, Zb1 = 1.06 Ω, Zc1 = 0.84 Ω; Za2 = 0.96 Ω, Zb2 = 1.02 Ω, Zc2 = 3.63 Ω; Za3 = 0.77 Ω, Zb3 = 1.42 Ω, Zc3 = 1.08 Ω.

[0283] For the first cable segment:

[0284] And: MAX(Za1, Zb1, Zc1) = Za1 = 5.26 Ω > 5 Ω

[0285] At this time, it is determined that the sheath impedance of the first cable segment is abnormal, and the A phase of the first cable segment is the abnormal phase.

[0286] For the second cable segment:

[0287] Since: 3 < 3.78 < 5

[0288] And: MAX(Za2, Zb2, Zc2) = Zc2 = 3.63 > 2

[0289] At this time, it is determined that the sheath impedance of the second cable segment is in the attention state, and the C phase impedance of the second cable segment is in the attention state.

[0290] For the end cable:

[0291] And: MAX(Za3, Zb3, Zc3) = Zb3 = 1.42 Ω < 2 Ω

[0292] At this time, it is judged that the sheath impedance of the end cable is normal.

[0293] The above specific embodiments further illustrate the purpose, technical solutions and beneficial effects of the present application. It should be understood that the above description is only a specific embodiment of the present application and is not intended to limit the protection scope of the present application. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for testing the sheath impedance of a high-voltage cable cross-connection system, comprising the following steps: Step 1: Select a group of cross-connection units in a transmission cable line as test points, wherein the group of cross-connection units includes a first cable segment, a second cable segment, a last cable segment, a first cross-connection box, and a second cross-connection box; Step 2: Acquire the three-phase current data, three-phase grounding loop current data and three-phase induced voltage data of the cross-connection unit, including: the three-phase currents IA1, IB1 and IC1 of the first cable body; the three-phase currents IA2, IB2 and IC2 of the last cable body; the three-phase currents IA3, IB3 and IC3 of the cable body at the first cross-connection box; the three-phase currents IA4, IB4 and IC4 of the cable body at the second cross-connection box; the three-phase grounding loop currents Ia1, Ib1 and Ic1 of the direct grounding point of the first cable section; the three-phase grounding loop currents Ia2, Ib2 and Ic2 of the direct grounding point of the last cable section; the three-phase grounding loop currents Ia3, Ib3 and Ic4 at the connecting piece of the first cross-connection box. c3; the three-phase ground loop currents Ia4, Ib4, Ic4 at the second cross-connection box connection; the three-phase induced voltages Ua3, Ub3, Uc3 at the first cross-connection box connection; the three-phase induced voltages Ua4, Ub4, Uc4 at the second cross-connection box connection; among them, IA1, IB1, IC1, IA2, IB2, IC2, IA3, IB3, IC3, IA4, IB4, IC4, Ia1, Ib1, Ic1, Ia2, Ib2, Ic2, Ia3, Ib3, Ic3, Ia4, Ib4, Ic4 are all vectors, with the unit of A, and Ua3, Ub3, Uc3, Ua4, Ub4, Uc4 are all vectors, with the unit of V; Step 3: Determine whether a sheath grounding fault exists in the cable of the cross-connection unit. If so, terminate the determination and determine that the cross-connection unit has a sheath grounding fault problem. If not, proceed to the next step. Step 4: performing phase synchronization on the three-phase current data, three-phase ground loop current data, and three-phase induced voltage data of the cross-connection unit; Step 5: Based on the phase-synchronized three-phase current data, three-phase ground loop current data, and three-phase induced voltage data, calculate the sheath impedances Za1, Zb1, Zc1, Za2, Zb2, Zc2, Za3, Zb3, and Zc3 of the three cable segments (in Ω). Determine whether the sheath impedances of the three cable segments are abnormal: Normal state: The impedance values ​​of the three-phase sheaths of the same cable section in the cross-connection unit all meet the requirements of phase imbalance less than or equal to 3, and the maximum impedance less than or equal to 2Ω. Note: The phase imbalance of the cable sheath impedance of the same section is greater than 3 and less than or equal to 5, and the maximum impedance is less than or equal to 5Ω, or the phase imbalance is less than or equal to 5 and the maximum impedance is greater than 2 and less than or equal to 5Ω. Abnormal state: The phase imbalance of the sheath impedance of the same section of cable is greater than 5, or the maximum impedance is greater than 5Ω.

2. A method for testing the sheath impedance of a high-voltage cable cross-connection system according to claim 1, wherein: The method for determining whether a sheath grounding fault exists in the cable of the cross-connection unit in step 3 is as follows: if the difference between the three-phase current of the first section of the cable body and the three-phase current of the last section of the cable body is less than or equal to 10A, that is, if |IA1+IB1+IC1-IA2-IB2-IC2|≤10A, then it is determined that no sheath grounding fault exists in the cable of the cross-connection unit.

3. A method for testing the sheath impedance of a high-voltage cable cross-connection system according to claim 1, wherein: The step 4 is specifically as follows: Step 4.1: Select the phase with the maximum amplitude of the three-phase current of the first section of the cable as the reference quantity Im for phase synchronization: Im=MAX(IA1,IB1,IC1) Step 4.2: Calculate the phase difference: If the maximum three-phase current of the first section of the cable is phase A, that is, Im = IA1, then If the maximum three-phase current of the first section of the cable is phase B, that is, Im = IB1, then If the maximum three-phase current of the first section of the cable is phase C, that is, Im = IC1, then Step 4.3: Perform phase compensation: Subtract the phases of the three-phase current of the cable body at the first cross-connection box, the three-phase ground loop current at the first cross-connection box connection, and the three-phase induced voltage at the first cross-connection box connection. The three-phase current of the cable body at the second cross-connection box, the three-phase ground loop current at the second cross-connection box connection piece, and the phase of the three-phase induced voltage at the second cross-connection box connection piece are all subtracted The three-phase current of the end cable body and the three-phase grounding loop current phase of the end cable direct grounding point are subtracted Complete phase synchronization.

4. A method for testing the sheath impedance of a high-voltage cable cross-connection system according to claim 1, wherein: The step 5 is specifically as follows: Step 5.1: Calculate the three-phase leakage current of the three cable segments: Assume that the three-phase leakage currents of the first cable section are Iac1, Ibc1, and Icc1, respectively, in A: Phase A leakage current Iac1 of the first cable section: Iac1=Ia1-Ib3 Phase B leakage current Ibc1 of the first cable section: Ibc1=Ib1-Ic3 Phase C leakage current Icc1 of the first cable section: Icc1=Ic1-Ia3 Assume that the three-phase leakage currents of the second cable section are Iac2, Ibc2, and Icc2, respectively, in A: Phase A leakage current of the second cable section Iac2: Iac2=Ia3-Ib4 Phase B leakage current of the second cable section Ibc2: Ibc2=Ib3-Ic4 Phase C leakage current of the second cable section Icc2: Icc2=Ic3-Ia4 Assume that the three-phase leakage currents of the last cable section are Iac3, Ibc3, and Icc3, respectively, in A: Phase A leakage current Iac3 of the last cable segment: Iac3=Ia4-Ia2 Phase B leakage current Ibc3 of the last cable segment: Ibc3=Ib4-Ib2 Phase C leakage current Icc3 of the last cable segment: Icc3=Ic4-Ic2 Step 5.2: Determine the composition of the three-phase grounding circulation currents Ia1, Ib1, Ic1 at the direct grounding point of the first cable section of the cross-connection unit, the three-phase grounding circulation currents Ia2, Ib2, Ic2 at the direct grounding point of the last cable section, the three-phase grounding circulation currents Ia3, Ib3, Ic3 at the first cross-connection box connection piece, and the three-phase grounding circulation currents Ia4, Ib4, Ic4 at the second cross-connection box connection piece: The three-phase grounding circulating currents Ia1, Ib1, and Ic1 at the direct grounding point of the first cable section are constructed as follows: Phase A grounding circulating current Ia1 at the direct grounding point of the first cable section: Phase B grounding circulating current Ib1 at the direct grounding point of the first cable section: Phase C grounding circulating current Ic1 at the direct grounding point of the first cable section: The three-phase grounding circulating currents Ia2, Ib2, and Ic2 at the direct grounding point of the last cable segment are constructed as follows: Phase A grounding circulating current Ia2 at the direct grounding point of the last cable segment: Phase B grounding circulating current Ib2 at the direct grounding point of the last cable segment: Phase C grounding circulating current Ic2 at the direct grounding point of the last cable segment: The three-phase grounding circulating currents Ia3, Ib3, and Ic3 at the first cross-connection box connection are constructed as follows: Phase A grounding circulating current Ia3 at the first cross-connection box connection: Phase B grounding circulating current Ib3 at the first cross-connection box connection: Phase C grounding circulating current Ic3 at the first cross-connection box connection: The three-phase grounding circulating currents Ia4, Ib4, and Ic4 at the second cross-connection box connection are constructed as follows: Phase A grounding circulating current Ia4 at the second cross-connection box connection: Phase B grounding circulating current Ib4 at the second cross-connection box connection: Phase C grounding circulating current Ic4 at the second cross-connection box connection: Among them, Ig1, Ig2, and Ig3 are the induced current components of the three sheath loops ABC, BCA, and CAB, respectively, and the unit is A; Step 5.3: Determine the composition of the three-phase induced voltage at the connection point of the first cross-connection box and the connection point of the second cross-connection box: The three-phase induced voltage at the connection of the first cross-connection box is constructed as follows: Phase A induced voltage Ua3 at the connection of the first cross-connection box: The B-phase induced voltage Ub3 at the connection of the first cross-connection box: The C-phase induced voltage Uc3 at the connection of the first cross-connection box: Among them, Za1, Zb1, and Zc1 respectively represent the three-phase sheath impedances of the first-section cable, with the unit of Ω; The constitutive formulas of the three-phase induced voltages at the second cross-bonding box connection are as follows: The B-phase induced voltage Ub4 at the second cross-connection box connection: The C-phase induced voltage Uc4 at the second cross-connection box connection: Phase A induced voltage Ua4 at the second cross-connection box connection: Among them, Za2, Zb2, and Zc2 respectively represent the three-phase sheath impedances of the second-section cable, with the unit of Ω; Step 5.4:联立步骤5.1、步骤5.2和步骤5.3所得的公式,求得首段电缆的三相护层阻抗Za1、Zb1、Zc1和第二段电缆的三相护层阻抗Za2、Zb2、Zc2: Solve the formulas obtained in Steps 5.1, 5.2, and 5.3 simultaneously to obtain the three-phase sheath impedances Za1, Zb1, Zc1 of the first-section cable and the three-phase sheath impedances Za2, Zb2, Zc2 of the second-section cable: Phase A sheath impedance Za1 of the first cable segment: Phase B sheath impedance Zb1 of the first cable segment: Impedance Zc1 of the C-phase sheath of the first cable segment: The three-phase sheath impedances of the first-section cable are respectively: The sheath impedance Za2 of phase A of the second cable segment is: The B-phase sheath impedance Zb2 of the second cable segment is: The C-phase sheath impedance Zc2 of the second cable segment is: The three-phase sheath impedances of the second-section cable are respectively: Step 5.5: Calculate the three-phase sheath impedances Za3, Zb3, Zc3 of the last-section cable, with the unit of Ω: Phase A induced voltage Ua4 at the second cross-connection box connection: The B-phase induced voltage Ub4 at the second cross-connection box connection: The C-phase induced voltage Uc4 at the second cross-connection box connection: Phase A sheath impedance Za3 of the last cable segment: Phase B sheath impedance Zb3 of the last cable segment: Impedance Zc3 of the C-phase sheath of the last cable segment: The constitutive formulas of the three-phase induced voltages at the second cross-bonding box connection can also be expressed as: 联立步骤5.1、步骤5.2和步骤5.3所得的公式,求得: when Solve the formulas obtained in Steps 5.1, 5.2, and 5.3 simultaneously to obtain: when Step 5.6: According to the calculation results of Steps 5.4 and 5.5, determine whether there are any abnormalities in the sheath impedances of the three sections of cables: when Normal state: The impedance values of the three-phase sheaths of the same section of cable in the cross-bonding unit all satisfy that the inter-phase unbalance degree is less than or equal to 3, and the maximum impedance is less than or equal to 2Ω, that is: When MAX(Za1, Zb1, Zc1) ≤ 2, it is judged that the sheath impedance of the first-section cable is normal; when And MAX(Za1,Zb1,Zc1)≤5, or when When MAX(Za2, Zb2, Zc2) ≤ 2, it is judged that the sheath impedance of the second-section cable is normal; when And MAX(Za2,Zb2,Zc2)≤5, or when When MAX(Za3, Zb3, Zc3) ≤ 2, it is judged that the sheath impedance of the last-section cable is normal; when And MAX(Za3,Zb3,Zc3)≤5, or when Attention state: The inter-phase unbalance degree of the sheath impedance of the same section of cable is greater than 3 and less than or equal to 5, and the maximum impedance is less than or equal to 5Ω, or the inter-phase unbalance degree is less than or equal to 5, and the maximum impedance is greater than 2 and less than or equal to 5Ω, that is: When 2 < MAX(Za1, Zb1, Zc1) ≤ 5, it is judged that the sheath impedance of the first-section cable is in the attention state, and the phase with a sheath impedance greater than 2Ω in the first-section cable is the abnormal phase; when When 2 < MAX(Za2, Zb2, Zc2) ≤ 5, it is judged that the sheath impedance of the second-section cable is in the attention state, and the phase with a sheath impedance greater than 2Ω in the second-section cable is the abnormal phase; when When 2 < MAX(Za3, Zb3, Zc3) ≤ 5, it is judged that the sheath impedance of the last-section cable is in the attention state, and the phase with a sheath impedance greater than 2Ω in the last-section cable is the abnormal phase; when Abnormal state: The inter-phase unbalance degree of the sheath impedance of the same section of cable is greater than 5, or the maximum impedance is greater than 5Ω, that is: When MAX(Za1, Zb1, Zc1) > 5, it is judged that the sheath impedance of the first-section cable is abnormal, and the phase impedance of the phase with a sheath impedance greater than 5Ω in the first-section cable is abnormal; When MAX(Za2, Zb2, Zc2) > 5, it is judged that the sheath impedance of the second-section cable is abnormal, and the phase impedance of the phase with a sheath impedance greater than 5Ω in the second-section cable is abnormal; When MAX(Za3, Zb3, Zc3) > 5, it is judged that the sheath impedance of the last-section cable is abnormal, and the phase impedance of the phase with a sheath impedance greater than 5Ω in the last-section cable is abnormal.

5. A method for testing the sheath impedance of a high-voltage cable cross-connection system according to claim 4, wherein: When the head end and / or the tail end of the cross-connection unit are straight-through connectors, the three-phase grounding loop current at the direct grounding point of the first cable segment and / or the last cable segment of the cross-connection unit is the sum of the currents of the previous cross-connection unit and / or the next cross-connection unit. In this case, the leakage current of the first cable segment and / or the last cable segment is equivalently replaced by the leakage current of the second cable segment. The lengths of the three cable segments of the cross-connection unit are defined as L1, L2, and L3, respectively, in meters. The magnitude of the leakage current is proportional to the segment length. The three-phase sheath impedance of the first cable segment and / or the three-phase sheath impedance of the last cable segment are calculated as follows: When the first end of the cross-connection unit is a straight-through connector, the three-phase sheath impedances of the first section of cable are: Phase A sheath impedance Za1 of the first cable segment: Phase B sheath impedance Zb1 of the first cable segment: Impedance Zc1 of the C-phase sheath of the first cable segment: When the end of the cross-connection unit is a straight-through connector, the three-phase sheath impedances of the final cable segment are: Phase A sheath impedance Za3 of the last cable segment: Phase B sheath impedance Zb3 of the last cable segment: Impedance Zc3 of the C-phase sheath of the last cable segment:

6. A system for testing the sheath impedance of a high-voltage cable cross-connection system, applying the method for testing the sheath impedance of a high-voltage cable cross-connection system according to any one of claims 1 to 5, comprising: The first section cable body detection unit is used to detect the three-phase current of the first section cable body and the three-phase grounding loop current of the direct grounding point of the first section cable; The end cable body detection unit is used to detect the three-phase current of the end cable body and the three-phase grounding loop current of the end cable direct grounding point; A first cross-connection box detection unit is used to detect the three-phase current of the cable body at the first cross-connection box, the three-phase ground loop current at the first cross-connection box connection piece, and the three-phase induced voltage at the first cross-connection box connection piece; The second cross-connection box detection unit is used to detect the three-phase current of the cable body at the second cross-connection box, the three-phase ground loop current at the second cross-connection box connection piece, and the three-phase induced voltage at the second cross-connection box connection piece.

7. A system for testing the sheath impedance of a high-voltage cable cross-connection system according to claim 6, wherein: The first-section cable body detection unit includes six current sensors, which are respectively used to detect the three-phase current of the first-section cable body and the three-phase grounding loop current at the direct grounding point of the first-section cable; the last-section cable body detection unit includes six current sensors, which are respectively used to detect the three-phase current of the last-section cable body and the three-phase grounding loop current at the direct grounding point of the last-section cable; the first cross-connection box detection unit includes six current sensors and three voltage sensors, and the six current sensors are respectively used to detect the three-phase current of the cable body at the first cross-connection box and the three-phase grounding loop current at the first cross-connection box connecting piece, and the three voltage sensors are respectively used to detect the three-phase induced voltage at the first cross-connection box connecting piece; the second cross-connection box detection unit includes six current sensors and three voltage sensors, and the six current sensors are respectively used to detect the three-phase current of the cable body at the second cross-connection box and the three-phase grounding loop current at the second cross-connection box connecting piece, and the three voltage sensors are respectively used to detect the three-phase induced voltage at the second cross-connection box connecting piece.

8. A testing system for the sheath impedance of a high-voltage cable cross-connection system according to claim 6, further comprising a data receiving unit, wherein the data receiving unit is connected to the first-section cable body detection unit, the last-section cable body detection unit, the first cross-connection box detection unit and the second cross-connection box detection unit respectively, and is used to receive the three-phase current data of the first-section cable body, the three-phase grounding loop current data of the first-section cable direct grounding point, the three-phase current data of the last-section cable body, the three-phase grounding loop current data of the last-section cable direct grounding point, the three-phase current data of the cable body at the first cross-connection box, the three-phase grounding loop current data at the first cross-connection box connection piece, the three-phase induced voltage data at the first cross-connection box connection piece, the three-phase current data of the cable body at the second cross-connection box, the three-phase grounding loop current data at the second cross-connection box connection piece, and the three-phase induced voltage data at the second cross-connection box connection piece, and to calculate and process the received current and voltage data.

9. A system for testing the sheath impedance of a high-voltage cable cross-connection system according to claim 8, wherein: The data receiving unit is a computer.

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