Method for determining a compression joint defect of a tunnel cable joint based on parameter alternating iteration

Abstract

The application provides a tunnel cable joint crimping defect determination method based on parameter alternating iteration, adopts a transient thermal circuit model to determine a crimping coefficient, introduces a segmentation number and a segmentation weight coefficient to construct a non-uniform segmentation model in the determination process, constructs a coupling model between the segmentation weight coefficient and the crimping coefficient, and performs alternating iteration through the crimping contact coefficient and the segmentation weight coefficient, so as to determine the final crimping contact coefficient to judge the crimping defect of the cable joint, while ensuring the judgment calculation speed, considering the axial heat transfer attenuation characteristics between the joint and the cable body, and effectively ensuring the precision of the final judgment structure.

Description

Technical Field

[0001] This invention relates to a method for determining cable joint defects, and more particularly to a method for determining crimping defects in tunnel cable joints based on alternating parameter iteration. Background Technology

[0002] Cable joints are relatively weak points in power cable systems. Due to factors such as manufacturing processes, crimping pressure, installation quality, and long-term operating environment, insufficient mechanical clamping force may occur between the conductor and the crimping tube inside the joint, leading to contact resistance and crimping defects. These defects increase localized Joule heat loss, causing the joint area temperature to rise. In tunnel environments, where space is relatively limited and airflow and heat exchange conditions are complex, localized heat accumulation can more easily amplify the safety risks posed by joint defects.

[0003] In existing technologies, temperature-based methods for determining cable joint crimping defects typically require prior calculation or prediction of the joint's temperature response, such as using the finite element method. While the finite element model can accurately reflect the complex structure and multi-physics coupling relationships of the joint, the computational load is substantial, making it difficult to directly meet the speed requirements of online engineering assessments. On the other hand, existing technologies also employ transient thermal circuit models for calculations, which offer advantages such as high speed and clear physical meaning of parameters. However, these models divide the cable body into uniform or simplified sections, failing to adequately consider the axial heat transfer attenuation characteristics between the joint and the cable body. When the temperature gradient near the joint is large, uniform segmentation can result in insufficient node density near the joint, leading to increased temperature calculation errors. Furthermore, since crimping defect determination essentially relies on the difference between the calculated temperature curve and the reference temperature curve, these calculation errors are further transmitted to the contact coefficient identification results, reducing the final accuracy of crimping defect determination.

[0004] Therefore, in order to solve the above-mentioned technical problems, it is urgent to propose a new technical approach. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a method for determining crimping defects in tunnel cable joints based on alternating parameter iteration. This method uses a transient thermal circuit model to determine the crimping coefficient. During the determination process, a non-uniform segmented model is constructed by introducing the number of segments and segment weight coefficients, and a coupling model between the segment weight coefficients and the crimping coefficient is built. The final crimping contact coefficient is determined by alternating iterations of the crimping contact coefficient and the segment weight coefficients to judge the crimping defects of the cable joint. While ensuring the speed of the judgment calculation, the axial heat transfer attenuation characteristics between the joint and the cable body are taken into account, thereby effectively ensuring the accuracy of the final judgment structure.

[0006] This invention provides a method for determining crimping defects in tunnel cable joints based on alternating parameter iterations, comprising the following steps:

[0007] S1. Obtain the structural parameters and operating condition parameters of the tunnel cable joint to be estimated. The operating condition parameters include ambient temperature, load current, air velocity in the tunnel, and reference temperature curve of the joint surface.

[0008] The S2.MATLAB / Simulink platform establishes an initial two-dimensional transient thermal circuit model based on the principle of thermal-electrical analogy.

[0009] S3. Construct a cable axial segmentation model based on the segmentation weight coefficient γ and a model showing the coupling relationship between the cable joint crimping contact coefficient k and the segmentation weight coefficient;

[0010] S4. Set the initial value of the cable joint crimping contact coefficient k to k0, and substitute the initial value k0 into the coupling relationship model between the cable joint crimping contact coefficient k and the segment weight coefficient to obtain the initial segment weight coefficient γ0;

[0011] S5. Substitute the initial segmentation weight coefficient γ0 and the cable joint crimping contact coefficient k into the initial two-dimensional transient thermal circuit model and adjust the value of the cable joint crimping contact coefficient k. Calculate the temperature curve of the cable joint surface under different cable joint crimping contact coefficients. Subtract the calculated temperature curve of the cable joint surface from the reference temperature curve of the joint surface. Find the cable joint crimping contact coefficient corresponding to the calculated temperature curve of the cable joint surface with the smallest difference and record it as k1.

[0012] S6. Substitute the cable joint crimping contact coefficient, denoted as k1, into the coupling relationship model between the cable joint crimping contact coefficient k and the segment weight coefficient to calculate the segment weight coefficient and denot it as γ1;

[0013] S7. Determine whether |k1-k0|≤e is satisfied. k And |γ1-γ0|≤e γ If so, the current cable joint crimping contact coefficient k1 is used to determine the cable joint crimping defect; otherwise, the initial value of the cable joint crimping contact coefficient k is changed, and the process returns to step S4.

[0014] Furthermore, the coupling relationship model between the cable joint crimping contact coefficient k and the segment weight coefficient is specifically as follows:

[0015]

[0016] Where: γ represents the segmentation weight coefficient, v represents the air velocity inside the cable tunnel, and T env The value represents the ambient temperature inside the cable tunnel, and I represents the average load current of the cable.

[0017] Furthermore, the axial segmentation model of the cable is specifically as follows:

[0018] ;

[0019] Where: N represents the total number of segments, This indicates that a distance coefficient is set. This represents the distance of the i-th segment node from the center of the cable joint. This indicates the length of the cable joint, and the length of the i-th cable segment is:

[0020] .

[0021] Furthermore, the average load current is determined by the following method:

[0022] ;

[0023] in: Indicates the first The cable load current at any given time, t represents the exponential weighting coefficient; end This is the time when the sampling data ends.

[0024] Furthermore, the initial two-dimensional transient thermal circuit model established based on the thermal-electric analogy principle specifically includes:

[0025] Determine the ground capacitance C of each segment node. th Axial thermal resistance R a and radial thermal resistance;

[0026] The heat loss of the cable joint crimping section and the heat loss of other sections outside the crimping section are determined as follows:

[0027] ;

[0028] ;

[0029] in: This indicates the heat loss of the cable joint crimped pipe section. This indicates the heat loss in sections other than the pressurized pipe section. Indicates dielectric loss. Indicates the loss of the metal shielding layer;

[0030] Reduce heat loss in cable joint crimping pipe sections And heat loss in other sections besides the pressurized pipe section. Equivalent to a voltage source, and the equivalent voltage source is connected to the ground capacitance C of the corresponding node. th Axial thermal resistance R a And the radial thermal resistance forms the circuit.

[0031] Furthermore, the axial thermal resistance R a Determined using the following method:

[0032] ; This represents the length of the (i+1)th cable segment. This represents the outer diameter of the (j-1)th layer in the radial direction of the cable.

[0033] Furthermore, judging cable joint crimping defects based on the current cable joint crimping contact coefficient k1 specifically includes:

[0034] When the crimping contact coefficient k1 is equal to or close to 1, the cable joint is judged to have no crimping defect. When k1 > 1, the cable joint is judged to have crimping defect, and the larger the value, the more serious the defect.

[0035] The beneficial effects of this invention are as follows: This invention uses a transient thermal circuit model to determine the crimping coefficient. During the determination process, a non-uniform segmented model is constructed by introducing the number of segments and segment weight coefficients, and a coupling model between the segment weight coefficients and the crimping coefficient is built. The final crimping contact coefficient is determined by iteratively alternating the crimping contact coefficient and the segment weight coefficients to judge the crimping defects of the cable joint. While ensuring the speed of the judgment calculation, the axial heat transfer attenuation characteristics between the joint and the cable body are taken into account, so that the node distribution of the transient thermal circuit model matches the axial temperature attenuation law near the joint. Moreover, the coupling of the segment weight coefficients and the crimping contact coefficient avoids model parameter mismatch caused by fixed segments, thereby effectively ensuring the accuracy of the final judgment structure. Attached Figure Description

[0036] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0037] Figure 1 This is a schematic diagram of the process of the present invention.

[0038] Figure 2 A schematic diagram of the initial two-dimensional transient thermal circuit model structure for the thermal-electric analogy principle of this invention.

[0039] Figure 3 This is a schematic diagram of the non-uniform segmentation of the present invention.

[0040] Figure 4 The image shows the effect of segmented weighting coefficients on temperature prediction error and solution time.

[0041] Figure 5 The figure shows the fitting results of the optimal segmented weight coefficients under different contact coefficients.

[0042] Figure 6This is a schematic diagram illustrating the determination result of the method of the present invention under a constant load current of 400A in a specific example of the present invention.

[0043] Figure 7 This is a diagram showing the determination result of the method of the present invention under the dual-cycle load current condition in a specific example of the present invention. Detailed Implementation

[0044] The present invention will be further described in detail below:

[0045] This invention provides a method for determining crimping defects in tunnel cable joints based on alternating parameter iterations, comprising the following steps:

[0046] S1. Obtain the structural parameters and operating condition parameters of the tunnel cable joint to be estimated. The operating condition parameters include ambient temperature, load current, air velocity in the tunnel, and reference temperature curve of the joint surface (the reference temperature curve is determined by actual temperature detection of the crimped pipe section surface of the joint in the tunnel cable).

[0047] S2. The MATLAB / Simulink platform establishes an initial two-dimensional transient thermal circuit model based on the principle of thermal-electrical analogy; specifically:

[0048] The initial two-dimensional transient thermal circuit model established based on the thermal-electric analogy principle specifically includes:

[0049] Determine the ground capacitance C of each segment node. th Axial thermal resistance R a and radial thermal resistance;

[0050] The heat loss of the cable joint crimping section and the heat loss of other sections outside the crimping section are determined as follows:

[0051] ;

[0052] ;

[0053] in: This indicates the heat loss of the cable joint crimped pipe section. This indicates the heat loss in sections other than the pressurized pipe section. Indicates dielectric loss. This indicates the loss of the metal shielding layer; from these two formulas, it can be seen that a cable joint includes two parts: one part is the crimped section and the other part is the non-crimped section.

[0054] in: Indicates the heat loss of the cable conductor:

[0055] ;

[0056] Where: I represents the effective value of the cable's operating current, This indicates the core DC resistance at 293.15K. This indicates the temperature coefficient of resistance of the conductor core. This indicates the temperature of the cable conductor, which can be directly measured using existing technology. Represents the proximity effect coefficient. This represents the skin effect coefficient.

[0057] Reduce heat loss in cable joint crimping pipe sections And heat loss in other sections besides the pressurized pipe section. Equivalent to a voltage source, and the equivalent voltage source is connected to the ground capacitance C of the corresponding node. th Axial thermal resistance R a And the radial thermal resistance forms the circuit.

[0058] Therefore, in the radial thermal resistance, R is included in sequence. il R is the radial thermal resistance of the insulating layer. cb Radial thermal resistance of copper braided tape; R fl The radial thermal resistance of the filler layer; R os For the radial thermal resistance of the outer sheath; R cs R is the radial thermal resistance of the stress cone. sl The radial thermal resistance of the shield; R is For the radial thermal resistance of the inner sheath; R ar For the radial thermal resistance of the armor layer; R env For environmental heat exchange thermal resistance, such as Figure 2 As shown, these thermal resistances are determined using existing methods based on the structure and materials of the cable and cable connectors. The corresponding environmental heat transfer resistance R on the branch line (which is the section representing the pressure pipe) env The potential at the upper end point is the surface temperature of the cable joint crimping pipe section.

[0059] S3. Construct a cable axial segmentation model based on the segmentation weight coefficient γ and a coupling relationship model between the cable joint crimping contact coefficient k and the segmentation weight coefficient; although the above was established... The model is similar, but the thermal resistance R between adjacent segments along the axis is unknown. a Therefore, R needs to be determined based on the segmentation. a : ; This represents the length of the (i+1)th cable segment. This represents the outer diameter of the (j-1)th layer in the radial direction of the cable. Since the segmentation in this invention is not a traditional uniform segmentation, but a non-uniform segmentation, it is necessary to determine the length of each segment:

[0060] The specific axial segmentation model of the cable is as follows:

[0061] ;

[0062] Where: N represents the total number of segments, This indicates that a distance coefficient is set. This represents the distance of the i-th segment node from the center of the cable joint. This indicates the length of the cable joint, and the length of the i-th cable segment is:

[0063] In the piecewise model, it is necessary to determine the number of segments N and the segment weight coefficient γ; once these two parameters are determined, the thermal resistance R can be determined. a The value of is then used to determine the two-dimensional transient model;

[0064] The heat losses in this model, particularly the dielectric loss and the metal shielding layer loss, can be determined using existing methods and calculated through simulation based on structural and material characteristics. However, the heat loss of the conductor core requires the construction of a thermal balance equation in the MATLAB / Simulink platform.

[0065] ;

[0066] Cth(i, j) is the heat capacity of the node located at axial position i and radial layer j, in J / K; T(i, j) is the node temperature; Q(i, j) is the heat source power injected into the node; T(x) is the temperature of the adjacent nodes connected to the node, including the axially adjacent nodes i−1 and i+1 and the radially adjacent nodes j−1 and j+1; Rth(i, j→x) is the thermal resistance between node (i, j) and the adjacent node x; the heat source power Q(i, j) injected into the node is the heat power transferred through the axial and radial thermal resistances. By adjusting the value of T(i, j), Q can be adjusted. c The value of T(i, j) is determined, and then the values ​​of each heat loss power are adjusted. After adjustment, the heat transfer between each node can be determined, and it can be judged whether the above thermal circuit model has reached equilibrium. If so, the value of the current node T(i, j) can be determined, which can determine the temperature Tc of the conductor core, and then the heat loss of the conductor core can be determined. This calculation process can be completed in the MATLAB / Simulink platform. Once these parameters are determined, they can be substituted into... and The calculation process of the heat balance equation is an existing technique and will not be elaborated further here.

[0067] The coupling relationship model between the cable joint crimping contact coefficient k and the segment weight coefficient is as follows:

[0068]

[0069] Where: γ represents the segmentation weight coefficient, v represents the air velocity inside the cable tunnel, and T env The value represents the ambient temperature inside the cable tunnel, and I represents the average load current of the cable.

[0070] Wherein, the average load current is determined by the following method:

[0071] ;

[0072] in: Indicates the first The cable load current at any given time, t represents the exponential weighting coefficient. end This marks the end time of the sampling data. Using an exponentially weighted average current ensures high accuracy in determining the data even under transient temperature rise and load fluctuation conditions.

[0073] Specifically:

[0074] .

[0075] S4. Set the initial value of the cable joint crimping contact coefficient k to k0, and substitute the initial value k0 into the coupling relationship model between the cable joint crimping contact coefficient k and the segment weight coefficient to obtain the initial segment weight coefficient γ0;

[0076] S5. Substitute the initial segmentation weight coefficient γ0 and the cable joint crimping contact coefficient k into the initial two-dimensional transient thermal circuit model and adjust the value of the cable joint crimping contact coefficient k. Calculate the temperature curve of the cable joint surface under different cable joint crimping contact coefficients. Subtract the calculated temperature curve of the cable joint surface from the reference temperature curve of the joint surface. Find the cable joint crimping contact coefficient corresponding to the calculated temperature curve of the cable joint surface with the smallest difference and record it as k1. When subtracting the two curves, the absolute value of the difference needs to be taken.

[0077] S6. Substitute the cable joint crimping contact coefficient, denoted as k1, into the coupling relationship model between the cable joint crimping contact coefficient k and the segment weight coefficient to calculate the segment weight coefficient and denot it as γ1;

[0078] S7. Determine whether |k1-k0|≤e is satisfied. k And |γ1-γ0|≤e γIf so, the current cable joint crimping contact coefficient k1 is used to determine the cable joint crimping defect; otherwise, the initial value of the cable joint crimping contact coefficient k is changed, and the process returns to step S4. In steps S4 to S6, we first set an initial crimping contact coefficient k0. Then, this value is substituted into the coupling relationship model between the cable joint crimping contact coefficient k and the segmented weight coefficient to determine an initial weight coefficient γ0. This initial weight coefficient is then substituted into the axial thermal resistance to determine the thermal resistance R. a Conductive thermal resistance R a Parameters such as radial conduction thermal resistance and heat loss are input into the MATLAB / Simulink platform to calculate the surface temperature of the crimped section of the cable joint. It is important to note that in the MATLAB / Simulink platform, the calculation is not performed directly using the initial k value, but rather continuously within a set range [k...]. min ,k max Adjust the value of k in [the context]. From the formula, we can see that... (Affected by the k value), the surface temperature of the crimped pipe section is calculated with different k values. Then, the surface temperatures under different k values ​​are subtracted, and it is determined whether the absolute value of the difference is less than a set threshold. The k values ​​that are less than the set threshold are found and then substituted into the coupling relationship model between the crimping contact coefficient k and the segment weight coefficient to obtain the segment weight coefficients under each k value that currently meets the conditions, thus proceeding to step S7. If the current k values ​​and the segment weight coefficients under each k value cannot meet the conditions of step S7, the initial k value (i.e., the crimping contact coefficient) is reset, and the process returns to step S4. Steps S4 to S6 are repeated. If the conditions of step S7 are met, and if there are multiple conditions, the k value corresponding to the minimum value of |k1-k0| and |γ1-γ0| is selected as the optimal k value, and then defect judgment is performed.

[0079] Judging cable joint crimping defects based on the current cable joint crimping contact coefficient k1 (i.e., using the optimal k value) specifically includes:

[0080] When the crimping contact coefficient k1 is equal to or approaches 1 (where approaching 1 means the absolute value of k1-1 is less than a set threshold), the cable joint is judged to have no crimping defect. When k1 > 1 (where |k1-1| is greater than the set threshold and k1 > 1), the cable joint is judged to have a crimping defect, and the larger the value, the more severe the defect. Using the above method, a transient thermal circuit model is employed to determine the crimping coefficient. During the determination process, the number of segments and segment weight coefficients are introduced to construct a non-uniform segmented model, and a coupling model between the segment weight coefficients and the crimping coefficient is constructed. The crimping contact coefficient and segment weight coefficient are iteratively alternated to determine the final crimping contact coefficient, thus judging the crimping defect of the cable joint. While ensuring the calculation speed, the axial heat transfer attenuation characteristics between the joint and the cable body are considered, ensuring that the node distribution of the transient thermal circuit model matches the axial temperature attenuation law near the joint. Furthermore, the coupling of the segment weight coefficients and the crimping contact coefficient avoids model parameter mismatch caused by fixed segments, thereby effectively ensuring the accuracy of the final judgment.

[0081] like Figure 3 As shown, for the number of segments N, the axial temperature changes rapidly near the joint area and gradually decreases further away from the joint area. Based on this axial heat transfer attenuation law, this invention determines the non-uniform segmentation location of the cable body by jointly using the number of segments N and the segment weighting coefficient γ, as follows: Figure 4 As shown, the number of segments N and the segment weight coefficient γ jointly affect the temperature prediction error and solution time of the two-dimensional transient thermal circuit model. When N is small, the number of model nodes is small, and the calculation speed is fast, but the characterization of the axial heat transfer attenuation characteristics near the joint is insufficient, resulting in a large temperature prediction error. When N continues to increase, the model accuracy improves, but the increase in the number of nodes leads to an increase in solution time, and the improvement in accuracy gradually approaches saturation. Therefore, a balance needs to be struck between temperature prediction accuracy and computational efficiency to determine an appropriate number of segments N. In practice, N can be determined in advance by implementing cable joints and cables of different structures and materials. In actual working conditions, the value of N can be obtained directly by looking up a table based on the current cable structure, material, and operating current rating.

[0082] like Figure 5 As shown: Optimal stratification weighting coefficient γ under different contact coefficients k opt It is not a fixed constant, but varies with the contact coefficient k and operating parameters. The contact coefficient k is used to characterize the crimping state between the conductor and the crimping tube. When k is equal to or close to 1, it indicates a good crimping state; when k is greater than 1, it indicates a decrease in the contact performance of the crimping interface and the presence of crimping defects, and the larger the k, the more severe the crimping defect. Since crimping defects will change the heat intensity and axial heat transfer distribution in the joint area, causing changes in the temperature decay law of the cable body, therefore γ optThere is a coupling relationship between γ and k. If γ is fixed during the contact coefficient identification process... opt If this remains unchanged, the hierarchical structure of the thermal circuit model will not match the actual defect state, thus affecting the identification accuracy of k. This invention takes this coupling relationship into account, thereby ensuring the final accuracy.

[0083] Figure 6 In the case of a constant load current of 400 A, after adopting the parameter alternation iteration method of the present invention, the contact coefficient determination value can gradually stabilize with the iteration process, indicating that under constant load conditions, by using γ opt Alternating updates with k effectively eliminate the parameter coupling effect between the layering weight coefficient and the contact coefficient, enabling accurate identification of the contact coefficient. Compared with the thermal path model with fixed layering parameters, this invention can reduce temperature calculation errors, thereby improving the reliability of the crimping defect determination results.

[0084] Figure 7 In dual-cycle load current conditions, the load current changes periodically with time, and the joint temperature response is affected by both the current and historical currents, exhibiting significant thermal inertia. When using the method described in this invention, the thermal effect of fluctuating load is equivalently characterized by an exponentially weighted root-mean-square current I, and the contact coefficient optimization objective function is corrected by a weighted RMSE, enabling the parameter alternating iteration method of this invention to adapt to the dynamic temperature rise process under fluctuating loads. The judgment results show that this method can still output stable contact coefficient judgment values ​​under fluctuating load conditions, thereby achieving high-precision judgment of the degree of crimping defects in tunnel cable joints.

[0085] Figure 6 and 7 In the middle, T FEM The corresponding curve represents the existing technology, T TTCM The curve represents the result of this invention.

[0086] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for determining crimping defects in tunnel cable joints based on alternating parameter iteration, characterized in that: Includes the following steps: S1. Obtain the structural parameters and operating condition parameters of the tunnel cable joint to be estimated. The operating condition parameters include ambient temperature, load current, air velocity in the tunnel, and reference temperature curve of the joint surface. S2. An initial two-dimensional transient thermal circuit model was established on the MATLAB / Simulink platform based on the principle of thermal-electric analogy. S3. Construct a cable axial segmentation model based on the segmentation weight coefficient γ and a model showing the coupling relationship between the cable joint crimping contact coefficient k and the segmentation weight coefficient; S4. Set the initial value of the cable joint crimping contact coefficient k to k0, and substitute the initial value k0 into the coupling relationship model between the cable joint crimping contact coefficient k and the segment weight coefficient to obtain the initial segment weight coefficient γ0; S5. Substitute the initial segmentation weight coefficient γ0 and the cable joint crimping contact coefficient k into the initial two-dimensional transient thermal circuit model and adjust the value of the cable joint crimping contact coefficient k. Calculate the temperature curve of the cable joint surface under different cable joint crimping contact coefficients. Subtract the calculated temperature curve of the cable joint surface from the reference temperature curve of the joint surface. Find the cable joint crimping contact coefficient corresponding to the calculated temperature curve of the cable joint surface with the smallest difference and record it as k1. S6. Substitute the cable joint crimping contact coefficient, denoted as k1, into the coupling relationship model between the cable joint crimping contact coefficient k and the segment weight coefficient to calculate the segment weight coefficient and denot it as γ1; S7. Determine whether |k1-k0|≤e is satisfied. k And |γ1-γ0|≤e γ If so, the current cable joint crimping contact coefficient k1 is used to determine the cable joint crimping defect; otherwise, the initial value of the cable joint crimping contact coefficient k is changed, and the process returns to step S4.

2. The method for determining crimping defects in tunnel cable joints based on alternating parameter iteration as described in claim 1, characterized in that: The coupling relationship model between the cable joint crimping contact coefficient k and the segment weight coefficient is as follows: Where: γ represents the segmentation weight coefficient, v represents the air velocity inside the cable tunnel, and T env The value represents the ambient temperature inside the cable tunnel, and I represents the average load current of the cable.

3. The method for determining crimping defects in tunnel cable joints based on alternating parameter iteration as described in claim 1, characterized in that: The specific axial segmentation model of the cable is as follows: ； Where: N represents the total number of segments, This indicates that a distance coefficient is set. This represents the distance of the i-th segment node from the center of the cable joint. This indicates the length of the cable joint, and the length of the i-th cable segment is: 。 4. The method for determining crimping defects in tunnel cable joints based on alternating parameter iteration as described in claim 2, characterized in that: The average load current is determined by the following method: ； in: Indicates the first The cable load current at any given time, t represents the exponential weighting coefficient; end This is the time when the sampling data ends.

5. The method for determining crimping defects in tunnel cable joints based on alternating parameter iteration as described in claim 3, characterized in that: The initial two-dimensional transient thermal circuit model established based on the thermal-electric analogy principle specifically includes: Determine the ground capacitance C of each segment node. th Axial thermal resistance R a and radial thermal resistance; The heat loss of the cable joint crimping section and the heat loss of other sections outside the crimping section are determined as follows: ； ； in: This indicates the heat loss of the cable joint crimped pipe section. This indicates the heat loss in sections other than the pressurized pipe section. Indicates dielectric loss. Indicates the loss of the metal shielding layer; Reduce heat loss in cable joint crimping pipe sections And heat loss in other sections besides the pressurized pipe section. Equivalent to a voltage source, and the equivalent voltage source is connected to the ground capacitance C of the corresponding node. th Axial thermal resistance R a And the radial thermal resistance forms the circuit.

6. The method for determining crimping defects in tunnel cable joints based on alternating parameter iteration as described in claim 5, characterized in that: Axial thermal resistance R a Determined using the following method: ; This represents the length of the (i+1)th cable segment. This represents the outer diameter of the (j-1)th layer in the radial direction of the cable.

7. The method for determining crimping defects in tunnel cable joints based on alternating parameter iteration as described in claim 1, characterized in that: Judging cable joint crimping defects based on the current cable joint crimping contact coefficient k1 specifically includes: When the crimping contact coefficient k1 is equal to or close to 1, the cable joint is judged to have no crimping defect. When k1 > 1, the cable joint is judged to have crimping defect, and the larger the value, the more serious the defect.

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