Method for calculating cable contact resistance based on a probe coil

Abstract

The present application relates to a contact resistance calculation method based on a detection coil, a first detection coil is arranged at the contact surface between the first cable core and the second cable core on the inner surface of the crimped copper tube, a second detection coil is arranged on the outer surface of the crimped copper tube, an alternating voltage is applied between the A end and the B end, the voltage effective value U1 output by the first detection coil and the voltage effective value U2 output by the second detection coil are recorded, and then the contact resistance R is calculated j1 , j2 , j3 . The method can measure and calculate the contact resistance of each contact surface of the cable intermediate joint, and facilitate the discovery and analysis of problems existing in the cable intermediate joint.

Description

Technical Field

[0001] This invention belongs to the field of power cables, specifically relating to a method for calculating cable contact resistance based on a detection coil. Background Technology

[0002] The service life of a cable primarily depends on the aging degree of its insulation material, and the temperature of the insulation material is a crucial factor affecting its aging rate. Among these, the cable joint is the area most prone to heat generation in the entire cable line, mainly affected by its contact resistance. Therefore, accurately measuring the contact resistance of cable joints is of great significance for effectively assessing cable service life. However, current methods for calculating or measuring the contact resistance of cable joints can only measure the overall contact resistance of the joint, not the contact resistance of each individual contact surface. Chinese patent CN104635056B, entitled "A Method for Calculating the Contact Resistance of the Crimping Connector at a Three-Core Cable Joint," discloses a method for calculating the contact resistance of the crimping connector at a three-core cable joint. The method includes: 1) taking two equal-length sections of a 10kV three-core cable, one section containing the cable joint and the other the cable body; 2) drilling holes in the same core wire within both sections of the cable using an electric drill, with the holes located at both ends of each section; 3) applying a large current to the 10kV three-core cable using a current booster, and then measuring the voltage at both ends of the core wires in the two cable sections using an ohmmeter, recording the current and voltage data; 4) calculating the contact resistance between the crimping connector and the core wire at the cable joint based on the measured voltage at both ends of the core wires and the current flowing through the cable. Clearly, the contact resistance obtained by this method is the overall contact resistance of the joint. Chinese patent CN107203688A, entitled "A Method for Calculating Contact Resistance at the Crimped Connector of a Cable Intermediate Joint," discloses a method for calculating the contact resistance at the crimped connector of a cable intermediate joint. The method includes: S1, calculating the outer diameter D1 of the cable core based on the standard stranding process; S2, calculating the outer diameter D2 of the conductor's circumcircle after the cable core is crimped; S3, assuming the deformation of each conductor layer is the same after crimping, calculating the difference dc between the inscribed and circumscribed radii; S4, calculating the effective contact length le of the outer layer of the cross-section of the crimped conductor; S5, assuming full contact between the conductor and the sleeve, calculating the actual contact area As of the crimped connection of the intermediate joint conductor; and S6, calculating the contact resistance Rj between the cable core and the crimped connector. However, this method can only measure the overall contact resistance of the cable intermediate joint and cannot measure the contact resistance of each contact surface. Summary of the Invention

[0003] The technical problem of this invention is that existing methods for measuring and calculating cable intermediate joints can only measure the overall contact resistance of the cable intermediate joint, and cannot obtain the contact resistance of each contact surface of the cable intermediate joint.

[0004] The purpose of this invention is to solve the above-mentioned problems and provide a method for calculating cable contact resistance based on a detection coil. This method can measure the contact resistance of each contact surface of a cable joint, thereby discovering and analyzing problems existing in the cable joint and improving the safety of cable operation.

[0005] The technical solution of this invention is a cable contact resistance calculation method based on a detection coil. A first detection coil is installed at the contact surface between the first and second cable cores on the inner surface of the crimped copper tube, and a second detection coil is installed on the outer surface of the crimped copper tube. The contact resistance calculation method includes the following steps:

[0006] Step 1: Select a 1-meter-long cable core of the same specification as the cable in section AB without any intermediate joints, and measure the resistance R0 of the cable core per unit length.

[0007] Step 2: Measure the geometric parameters of the AB section cable, including the length L1 of the first cable core, the length L2 of the second cable core, the length L3 of the overlapping part of the crimped copper tube and the first cable core, and the length L4 of the overlapping part of the crimped copper tube and the second cable core.

[0008] Step 3: Construct the equivalent circuit of cable segment AB, and establish the mathematical relationship between the contact resistance of each contact surface of the intermediate joint and the total resistance Rs of cable segment AB.

[0009] Step 4: Apply an AC voltage between ends A and B of the AB section cable to form an AC current in the AB section cable, and record the effective voltage value U1 output by the first detection coil and the effective voltage value U2 output by the second detection coil.

[0010] Step 5: Calculate the contact resistance of each contact surface of the cable intermediate joint based on U1 and U2 obtained in Step 4.

[0011] Furthermore, in step 3, the contact resistance of the intermediate joint contact surface includes R. j1 R j2 R j3 , where R j1 R is the contact resistance of the mating contact surface between the first cable core and the second cable core. j2 R is the contact resistance of the crimped contact surface between the first cable core and the crimped copper tube. j3 This is the contact resistance of the crimped contact surface between the second cable core and the crimped copper tube.

[0012] In step 3, R s =R1+R j1 *(R j2 +R j3 ) / (R j1 +R j2 +R j3 )+R2,Rj2 / R j3 =L3 / L4

[0013] The conductor resistance of the first cable core is R1 = R0 * L1, and the conductor resistance of the second cable core is R2 = R0 * L2.

[0014] In step 4,

[0015] In step 5, contact resistance

[0016] Contact resistance

[0017] Contact resistance

[0018] Compared to existing technologies, the advantages of this invention are that it can measure and calculate the contact resistance of each contact surface of a cable joint, facilitating the discovery and analysis of problems existing in the cable joint. Furthermore, the method for calculating the contact resistance of the detection coil is easy to operate, requires minimal calculation, and is convenient to implement. Attached Figure Description

[0019] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0020] Figure 1 This is a schematic diagram of the AB section cable with an intermediate joint.

[0021] Figure 2 This is the equivalent circuit diagram for a cable segment AB with a mid-joint.

[0022] Figure 3 This is a schematic diagram of the AB section cable for setting up the detection coil in Example 1.

[0023] Figure 4 This is a flowchart of the cable contact resistance calculation method based on the detection coil in Example 1.

[0024] Figure 5 This is a flowchart of another method for calculating cable contact resistance according to the present invention.

[0025] Figure 6 This is a two-dimensional finite element model of the AB segment cable in Embodiment 2 of the present invention.

[0026] The markings in the figure are as follows: 1. First cable core; 2. Second cable core; 3. Crimped copper tube; 4. Butt joint contact surface between the first cable core and the second cable core; 5. Crimped contact surface between the first cable core and the crimped copper tube; 6. Crimped contact surface between the second cable core and the crimped copper tube; 7. First detection coil; 8. Second detection coil; 9. Air region of the two-dimensional finite element model. Detailed Implementation

[0027] Example 1

[0028] Figure 1 The image shows a cable segment AB with an intermediate joint, where the first cable core 1 and the second cable core 2 have the same specifications.

[0029] Figure 3 The diagram shows a cable segment AB with a detection coil. The first cable core 1 and the second cable core 2 have the same specifications. A first detection coil 7 is provided at the mating contact surface 4 between the first cable core and the second cable core on the inner surface of the crimped copper tube 3, and a second detection coil 8 is provided on the outer surface of the crimped copper tube 3.

[0030] like Figure 4 As shown, the method for calculating cable contact resistance based on a detection coil includes the following steps:

[0031] Step 1: Select a 1-meter-long cable core of the same specification as the cable in section AB without any intermediate joints, and measure the resistance R0 of the cable core per unit length.

[0032] Step 2: Measure the geometric parameters of the AB section cable, including the length L1 of the first cable core 1, the length L2 of the second cable core 2, the length L3 of the overlapping part of the crimped copper tube 3 and the first cable core 1, and the length L4 of the overlapping part of the crimped copper tube 3 and the second cable core 2.

[0033] Step 3: Construct the equivalent circuit of cable segment AB, such as... Figure 2 As shown, construct the intermediate joint contact resistance R. j1 R j2 R j3 The total resistance R of cable section AB s The mathematical relationship, where R j1 R is the contact resistance of the mating contact surface 4 between the first cable core and the second cable core. j2 R is the contact resistance of the crimping contact surface 5 between the first cable core and the crimped copper tube. j3 The contact resistance is the contact resistance between the crimping contact surface 6 of the second cable core and the crimped copper tube.

[0034] Step 4: Apply an AC voltage between ends A and B of the AB section cable to form an AC current in the AB section cable, and record the effective voltage value U1 output by the first detection coil 7 and the effective voltage value U2 output by the second detection coil 8.

[0035] Step 5: Calculate the contact resistance R of each contact surface of the cable joint. j1 ,R j2 ,R j3 .

[0036] In step 3, Rs =R1+R j1 *(R j2 +R j3 ) / (R j1 +R j2 +R j3 )+R2,R j2 / R j3 =L3 / L4; The conductor resistance of the first cable core 1 is R1 = R0 * L1, and the conductor resistance of the second cable core 2 is R2 = R0 * L2.

[0037] In step 4,

[0038] In step 5, contact resistance

[0039] Contact resistance

[0040] Contact resistance

[0041] Example 2

[0042] like Figure 5 As shown, a method for calculating cable contact resistance based on a detection coil is used to measure and calculate the contact resistance of an AB segment cable with an intermediate joint. The method selects point C on the outer surface of the crimped copper tube 3, corresponding to the mating contact surface 4 of the first and second cable cores, as the magnetic induction intensity measurement point. The method includes the following steps:

[0043] Step 1: Select a 1-meter-long cable core of the same specification as the cable in section AB without any intermediate joints, and measure the resistance R0 of the cable core per unit length.

[0044] Step 2: Measure the total resistance R of cable segment AB. s ;

[0045] Step 3: Measure the geometric parameters of the AB section cable, including the diameter d of the cable core, the length L1 of the first cable core 1, the length L2 of the second cable core 2, the diameter D of the crimped copper tube 3, the length L3 of the overlapping part of the crimped copper tube 3 and the first cable core 1, and the length L4 of the overlapping part of the crimped copper tube 3 and the second cable core 2.

[0046] Step 4: Based on the rated current effective value I of cable section AB. m A current of I is inserted into cable segment AB. m The DC current was measured using a gaussmeter to determine the magnetic induction intensity B at point C on the outer surface of the crimped copper tube. m ;

[0047] Step 5: Construct the equivalent circuit of cable segment AB, such as... Figure 2As shown, construct the intermediate joint contact resistance R. j1 R j2 R j3 The total resistance R of cable section AB s The mathematical relationship, where R j1 R is the contact resistance of the mating contact surface 4 between the first cable core and the second cable core. j2 R is the contact resistance of the crimping contact surface 5 between the first cable core and the crimped copper tube. j3 The contact resistance of the crimping contact surface 6 between the second cable core and the crimped copper tube; the total resistance R of the AB section cable. s =R1+R j1 *(R j2 +R j3 ) / (R j1 +R j2 +R j3 )+R2,R j2 / R j3 =L3 / L4, the conductor resistance of the first cable core 1 is R1 = R0 * L1, and the conductor resistance of the second cable core 2 is R2 = R0 * L2;

[0048] Step 6: Use finite element software to establish a two-dimensional finite element model of cable segment AB, and calculate the magnetic induction intensity B at the magnetic induction intensity measurement point. j And by changing the current Ix of the cable core and the current I of the crimped copper tube 3 m -I x This makes B j =B m ;

[0049] Step 7: Calculate the contact resistance R j1 R j2 R j3 .

[0050] Step 6, which involves establishing a two-dimensional finite element model of cable segment AB using finite element software, includes the following steps:

[0051] Step 6.1: Based on the axisymmetry of cable segment AB, establish a two-dimensional finite element model of cable segment AB, as follows: Figure 6 As shown, it includes a first cable core 1, a second cable core 2, a crimped copper tube 3, and an air area 9;

[0052] Step 6.2: Set the material properties of the first cable core 1, the second cable core 2, and the crimped copper tube 3 respectively. Set the relative permeability of the first cable core 1, the second cable core 2, the crimped copper tube 3, and the air region 9 to 1. Set the conductivity of the first cable core 1 and the second cable core 2 to the conductivity of the cable conductor. Set the conductivity of the crimped copper tube 3 to the conductivity of copper. Set the conductivity of the air region 9 to 0.

[0053] Step 6.3: Set the boundary conditions for the two-dimensional finite element model;

[0054] Step 6.3.1: Set axisymmetric boundary conditions at the cable axis of the two-dimensional finite element model;

[0055] Step 6.3.2: Set boundary conditions with zero magnetic induction at the boundary of the two-dimensional finite element model where the magnetic induction intensity is zero;

[0056] Step 6.4: Set the current source of the two-dimensional finite element model to I. m That is, the current flowing through ends A and B of the AB section cable is I. m The current in the area where the first cable core 1 overlaps with the crimped copper tube 3 is set to I. x The current in the area where the second cable core 2 overlaps with the crimped copper tube 3 is set to I. x The current for crimping copper tube 3 is set to I. m -I x ;

[0057] Step 6.5: Let I x =0, start the finite element software to calculate the magnetic field strength distribution of the two-dimensional finite element model, and record the magnetic field strength B at point C of the two-dimensional finite element model. j0 ;

[0058] Step 6.6: Let I x =I m Start the finite element software to calculate the magnetic field strength distribution of the two-dimensional finite element model, and record the magnetic field strength B at point C of the two-dimensional finite element model. jm ;

[0059] Step 6.7: Change I x Size, such that B j =B m .

[0060] B j =B m When, current I x =I m *(B m -B j0 ) / (B jm -B j0 );

[0061] In step 7,

[0062] Contact resistance

[0063] Contact resistance

[0064] Contact resistance

Claims

1. A method for calculating cable contact resistance based on a detection coil, characterized in that, A first detection coil is installed at the contact surface between the first and second cable cores on the inner surface of the crimped copper tube, and a second detection coil is installed on the outer surface of the crimped copper tube. The method for calculating the contact resistance of the contact surface includes the following steps. Step 1: Select a cable core of the same specification as the cable in section AB without any intermediate joints, and measure the resistance R0 of the cable core per unit length; Step 2: Measure the geometric parameters of the AB section cable, including the length L1 of the first cable core, the length L2 of the second cable core, the length L3 of the overlapping part of the crimped copper tube and the first cable core, and the length L4 of the overlapping part of the crimped copper tube and the second cable core. Step 3: Construct the equivalent circuit of cable segment AB, and establish the mathematical relationship between the contact resistance of each contact surface of the intermediate joint and the total resistance Rs of cable segment AB. Step 4: Apply an AC voltage between ends A and B of the AB section cable to form an AC current in the AB section cable, and record the effective voltage value U1 output by the first detection coil and the effective voltage value U2 output by the second detection coil. Step 5: Calculate the contact resistance of each contact surface of the cable intermediate joint based on U1 and U2 obtained in Step 4; In step 3, the contact resistance of the intermediate joint contact surface includes R. j1 R j2 R j3 , where R j1 R is the contact resistance of the mating contact surface between the first cable core and the second cable core. j2 R is the contact resistance of the crimped contact surface between the first cable core and the crimped copper tube. j3 The contact resistance is the contact resistance between the crimped contact surface of the second cable core and the crimped copper tube. R s =R1+R j1 (R j2 +R j3 ) / (R j1 +R j2 +R j3 )+R2,R j2 / R j3 =L3 / L4; The conductor resistance of the first cable core is R1 = R0. L1, the conductor resistance of the second cable core is R2 = R0 L2; In step 4, ; In step 5, contact resistance , Contact resistance , Contact resistance .

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