Method for short circuit test of medium voltage non-crosslinked polyethylene power cable

Abstract

This invention discloses a short-circuit test method for medium-voltage non-crosslinked polyethylene power cables, relating to the cable industry. Before testing, the cable sample undergoes thermal stress relief to achieve a standardized initial state. Parameters are calculated and set according to current short-circuit test standards, and a short-circuit current is applied. During the test, the melting phenomenon on the insulation surface is monitored and recorded simultaneously. Combined with subsequent partial discharge, power frequency withstand voltage tests, and structural dimensional information data, a correlation evaluation system between changes in the physical state of the insulation and electrical performance is established. This application establishes the core of the short-circuit withstand capability evaluation for non-crosslinked polyethylene cables as the ability of its insulation layer to resist irreversible melting deformation under thermal stress. This allows the test results to comprehensively and realistically assess the loss of structural integrity, deterioration of electrical performance, and secondary safety risks caused by the low-melting-point insulation layer of non-crosslinked polyethylene cables under short-circuit current impact, thus making the evaluation conclusions more universal and of greater engineering reference value.

Description

Technical Field

[0001] This invention relates to the field of cables, and more particularly to a short-circuit test method for medium-voltage non-crosslinked polyethylene power cables. Background Technology

[0002] Currently, the methods, techniques, and objectives of short-circuit testing for medium-voltage power cables mainly follow the International Electrotechnical Commission (IEC) and its derivative national standards, and their core framework is relatively mature and standardized. Existing short-circuit tests (such as those specified in IEC 60502-4 and GB / T 12706.4 standards) are essentially a form of "conductor thermo-mechanical stability verification." They primarily simulate short-circuit faults occurring in the power grid, verifying whether the cable conductor, shield, and connecting hardware will melt, weld, or undergo permanent deformation that could jeopardize subsequent operation under the extreme instantaneous high temperatures caused by the short-circuit current, and verifying whether it can still withstand a certain power frequency voltage after a short circuit.

[0003] The test is usually conducted on brand-new cable samples. Based on the cable conductor type and cross-sectional area, the formula is used to calculate and apply one or more short-circuit currents of varying amplitude and duration, causing the conductor temperature to rise to the maximum permissible short-circuit temperature within a short time. The standard evaluation method after the test focuses on two points: visual inspection of the conductor for breaks, welds, or obvious external bulging, cracks, or other visual damage to the insulation or sheath; and after the short-circuit test, the cable sample undergoes power frequency withstand voltage and partial discharge tests, with the absence of breakdown as the pass criterion.

[0004] The aforementioned standard method is designed for thermosetting insulation materials such as XLPE, and its fundamental assumption is that the insulation layer only undergoes thermal decomposition under short-circuit high temperatures, while its physical morphology remains stable. However, when this method is directly applied to low-melting-point thermoplastic insulated cables such as non-crosslinked polyethylene, many significant drawbacks exist.

[0005] First, non-crosslinked polyethylene insulation softens and melts before reaching 250°C. Existing methods cannot assess the physical processes that directly lead to insulation failure, such as insulation deformation, thinning, adhesive overflow, or shrinkage, thus neglecting the risk of physical phase transitions in the insulation. Second, residual stress generated during cable manufacturing, cabling, and factory testing may exacerbate uncontrollable insulation deformation when superimposed with short-circuit thermal stress. The standard does not require pre-treatment to standardize the condition of the samples, resulting in large dispersion of results, reduced comparability, and neglect of the influence of the material's initial state. Furthermore, existing methods lack evaluation standards for key phenomena that may occur during short circuits, such as current waveform distortion, changes in cable eccentricity after a short circuit, and insulation shrinkage at joints, and fail to monitor these key characteristics. Finally, due to the single evaluation parameter, which only focuses on conductor continuity and withstand voltage, while ignoring the quantitative assessment of insulation integrity, even if the insulation has deformed due to melting but has not broken down immediately, it may be mistakenly judged as qualified. More multi-dimensional evaluation indicators are needed. The standard binary criterion of pass / fail cannot distinguish those cables that have passed electrical tests but have undergone significant thermoplastic deformation. Summary of the Invention

[0006] This invention provides a short-circuit test method for medium-voltage non-crosslinked polyethylene power cables. By adding standardized pretreatment, the core evaluation of the short-circuit withstand capability of non-crosslinked polyethylene cables is established as the ability of its insulation layer to resist irreversible melting deformation under thermal stress. This mechanism is transformed into a set of measurable physical deformation parameters, establishing a correlation evaluation system between changes in the physical state of the insulation and the final electrical performance. This allows the test results to more systematically evaluate and classify the thermoplastic deformation behavior and risk level of the insulation layer under extreme thermal stress, providing more refined data support for product design and selection. It can comprehensively and realistically assess the loss of structural integrity, degradation of electrical performance, and secondary safety risks caused by the low-melting-point insulation layer of non-crosslinked polyethylene cables under short-circuit current impact.

[0007] To address the aforementioned technical problems, one objective of this invention is to provide a short-circuit test method for medium-voltage non-crosslinked polyethylene power cables, comprising the following steps: (1) Measure the structural dimensions of the cable sample, then heat-treat the sample at 70-90 ℃ for 12-48 h, and then connect the two ends of the sample to a standard short-circuit current generator. (2) Set the maximum short-circuit temperature and short-circuit duration of the sample, calculate the effective value of the short-circuit cable and the short-circuit current of the sample, start the synchronous monitoring system, and apply the effective value of the short-circuit cable and a symmetrical short-circuit current with a duration equal to the set short-circuit duration to the cable sample, and record the melting phenomenon on the sample surface throughout the process. (3) After the test is completed, the sample is subjected to partial discharge level and power frequency withstand voltage breakdown test, and the structural dimensions of the sample are measured at the same time. (4) Establish an evaluation system for evaluating insulation and thermoplastic behavior, and classify them according to the evaluation system; In steps (1) and (3), the structural dimension information includes at least one of the following: permanent deformation of outer diameter, eccentricity, and shrinkage.

[0008] In some embodiments, in step (1), the sample is a non-crosslinked polyethylene cable.

[0009] In some implementations, in step (2), the short-circuit current and the effective value of the short-circuit cable are calculated according to the following formula: ; In the formula, I 2 t is the short-circuit current; I is the effective value of the short-time current, in kA; t is the short-circuit time, in seconds; S is the cross-sectional area of ​​the sample, in mm. 2 ;θ sc θ represents the short-circuit temperature of the sample during operation, in °C. i The temperature of the conductor before the test is expressed in °C.

[0010] In some embodiments, in step (2), the synchronous monitoring system includes a thermocouple for monitoring the surface temperature of the sample.

[0011] In some embodiments, the synchronous monitoring system includes a laser displacement meter for monitoring the displacement of the sample end.

[0012] In some embodiments, the synchronous monitoring system includes a high-temperature camera for monitoring melting phenomena on the sample surface.

[0013] In some implementations, in step (2), the synchronous monitoring system monitors and records data from 1 second before the short circuit to 30 seconds after the short circuit.

[0014] In some embodiments, in step (3), after the test is completed, the sample is left to stand at room temperature for 3-6 hours, and then partial discharge level and power frequency withstand voltage breakdown tests are performed, while the structural size information of the sample is measured.

[0015] In some implementations, the evaluation system includes grades A, B, C, and D; Among them, Class A refers to passing the electrical test of GB / T 12706.2-2020 standard. During the test, there are no visible signs of melting or dripping. After the test, the insulation shrinkage is ≤2 mm and the permanent deformation of the outer diameter is ≤3%, indicating that the insulation has excellent thermal stability. The Class B designation refers to passing the electrical test according to the GB / T 12706.2-2020 standard. During the test, there are slight signs of thermoplasticity but no dripping. After the test, the insulation shrinkage is 2-5 mm and the permanent deformation of the outer diameter is 3%-8%, indicating that the insulation has good thermal stability and the deformation is within a controllable range. The Class C refers to the electrical test that passes the GB / T 12706.2-2020 standard, but shows obvious melting during the process, possibly accompanied by slight dripping. After the test, the insulation shrinkage is >5 mm or the permanent deformation of the outer diameter is >8%, indicating that the insulation has suffered significant thermoplastic damage. Although there is no electrical breakdown, the insulation integrity has been seriously reduced. The term "Class D" refers to failure to pass the electrical test of GB / T 12706.2-2020 standard, or the occurrence of safety accidents such as insulation fire or severe dripping.

[0016] To address the aforementioned technical problems, a second objective of this invention is to provide a terminal device, including a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor, wherein the processor executes the computer program to implement the short-circuit test method for the medium-voltage non-crosslinked polyethylene power cable.

[0017] In some embodiments, the processor includes a programmable temperature control box, a standard short-circuit current generator, a high-temperature camera monitoring device, and a precision measuring device for measuring structural dimensional information.

[0018] To address the aforementioned technical problems, a third objective of this invention is to provide a computer-readable storage medium comprising a stored computer program, wherein the computer program, when running, controls the device containing the computer-readable storage medium to execute the short-circuit test method for medium-voltage non-crosslinked polyethylene power cables.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. Before testing, the present invention performs thermal stress relief pretreatment on the cable sample to bring it to a standardized initial state, obtains the structural dimension information of the pretreated sample, calculates and sets parameters according to the current short-circuit test standard and applies short-circuit current, and simultaneously monitors and records the melting phenomenon of the insulation surface during the test. Combined with subsequent partial discharge test, power frequency withstand voltage test and structural dimension information, a correlation evaluation system between the physical state change of the connection insulation and electrical performance is established. This system can more systematically evaluate and classify the thermoplastic deformation behavior and risk level of the insulation layer under extreme thermal stress, providing more refined data support for product optimization and engineering selection.

[0020] 2. This invention can meet the requirements for short-circuit resistance testing and evaluation of non-crosslinked polyethylene cable systems above their critical thermoplastic deformation temperature. It has important engineering significance for accurately verifying the short-circuit reliability of low-melting-point thermoplastic insulated cables and preventing operational failures caused by insulation softening and melting. Attached Figure Description

[0021] Figure 1 The short-circuit current waveforms of three different non-crosslinked polyethylene cable samples in Example 1 of this invention are shown. Figure 2 The sample is a non-crosslinked polyethylene cable specimen that exhibits insulation shrinkage after a short-circuit test; Figure 3 The sample is a non-crosslinked polyethylene cable specimen that shows glue overflow after a short-circuit test. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0024] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0025] As used in this article: In these embodiments, unless otherwise specified, the portions and percentages are all by weight.

[0026] "And / or" is used to indicate that one or both of the described situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0027] In the description of this invention, it should be understood that the terms "upper", "lower", "left", "right", "top", "bottom", etc., indicating orientation or positional relationship are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0028] This invention provides a short-circuit test method for medium-voltage non-crosslinked polyethylene power cables, comprising the following steps: (1) Measure the structural dimensions of the cable sample, then heat-treat the sample at 70-90 ℃ for 12-48 h, and then connect the two ends of the sample to a standard short-circuit current generator. (2) Set the maximum short-circuit temperature and short-circuit duration of the sample, calculate the effective value of the short-circuit cable and the short-circuit current of the sample, start the synchronous monitoring system, and apply the effective value of the short-circuit cable and a symmetrical short-circuit current with a duration equal to the set short-circuit duration to the cable sample, and record the melting phenomenon on the sample surface throughout the process. (3) After the test is completed, the sample is subjected to partial discharge level and power frequency withstand voltage breakdown test, and the structural dimensions of the sample are measured at the same time. (4) Establish an evaluation system for evaluating insulation and thermoplastic behavior, and classify them according to the evaluation system; In steps (1) and (3), the structural dimension information includes at least one of the following: permanent deformation of outer diameter, eccentricity, and shrinkage.

[0029] This invention, based on the current standard short-circuit test of non-crosslinked polyethylene cables, pre-treats the cable samples with thermal stress relief before testing to bring them to a standardized initial state, obtains the structural and dimensional information of the samples before and after the short-circuit test, and simultaneously monitors and records the melting phenomenon on the insulation surface during the test. This can be combined with subsequent partial discharge tests and power frequency withstand voltage tests to establish a correlation evaluation system between changes in the physical state of the connection insulation and its electrical performance, realizing a systematic evaluation of the insulation melting and deformation behavior, and providing more refined data support for product optimization and engineering selection.

[0030] In some embodiments, in step (1), the sample is a non-crosslinked polyethylene cable.

[0031] In some implementations, in step (1), the heat treatment temperature is any one of 70 ℃, 75 ℃, 80 ℃, 85 ℃, 90 ℃ or a range between any two.

[0032] In some implementations, in step (1), the heat treatment time is any one or a range between any two of 12 h, 18 h, 24 h, 30 h, 36 h, 42 h, and 48 h.

[0033] This application pre-heat-treats the cable samples, which can eliminate the residual thermomechanical stress accumulated in the early processing of the cable samples, so that all samples are in a more stable and comparable state before the test, reducing background noise. At the same time, this process can simulate and release some of the thermal stress in operation, relax the molecular chain segments of non-crosslinked polyethylene insulation, and avoid the unpredictable coupling of residual stress with thermal stress in the subsequent short-circuit impact, which would lead to abnormal dispersion of deformation data.

[0034] In some implementations, in step (1), the standard short-circuit current generator includes a test transformer and a three-phase short-circuit copper plate.

[0035] In some embodiments, in step (1), the length of the sample is 3-10 m.

[0036] In some embodiments, in step (1), the length of the sample is any one of 3 m, 4 m, 5 m, 6 m, 7 m, 8 m, 9 m, 10 m or a range between any two.

[0037] In some embodiments, in step (1), the cross-section of the sample is 100-500 mm. 2 .

[0038] In some embodiments, in step (1), the cross-section of the sample is 100 mm. 2 150 mm 2 200mm 2 250 mm 2 300 mm 2 350 mm 2 400 mm 2 450 mm 2 500 mm 2 The range of values ​​between any one of them or any two of them.

[0039] In some embodiments, in step (1), the insulating shielding layer is removed from both ends of the sample, and a copper pressure connector is inserted, with one end connected to the test transformer and the other end connected to the three-phase short-circuit copper plate.

[0040] In some embodiments, in step (1), thermocouples and laser measuring points are arranged on the surface of the sample, and a laser displacement meter is installed at the pressure joint.

[0041] In some implementations, in step (2), the short-circuit current and the effective value of the short-circuit current are calculated according to the following formula: ; In the formula, I 2 t is the short-circuit current; I is the effective value of the short-time current, in kA; t is the short-circuit time, in seconds; S is the cross-sectional area of ​​the sample, in mm. 2 ;θ sc θ represents the short-circuit temperature of the sample during operation, in °C. i The temperature of the conductor before the test is expressed in °C.

[0042] In some embodiments, in step (2), the maximum temperature of the sample is set to 160-250 °C and the short-circuit duration is 1-5 s.

[0043] In some embodiments, in step (2), the maximum temperature of the sample is set to any one or any two of 160 ℃, 170 ℃, 180 ℃, 190 ℃, 200 ℃, 210 ℃, 220 ℃, 230 ℃, 240 ℃, and 250 ℃.

[0044] In some implementations, in step (2), the short-circuit duration of the sample is set to any one of 1 s, 2 s, 3 s, 4 s, 5 s or a range between any two.

[0045] In some embodiments, in step (2), the synchronous monitoring system includes a thermocouple for monitoring the surface temperature of the sample.

[0046] In some embodiments, the synchronous monitoring system includes a laser displacement meter for monitoring the displacement of the sample end.

[0047] In some embodiments, the synchronous monitoring system includes a high-temperature camera for monitoring melting phenomena on the sample surface.

[0048] In some implementations, in step (2), the synchronous monitoring system monitors and records data from 1 second before the short circuit to 30 seconds after the short circuit.

[0049] In some embodiments, in step (3), after the test is completed, the sample is left to stand at room temperature for 3-6 hours, and then partial discharge level and power frequency withstand voltage breakdown tests are performed, while the structural size information of the sample is measured.

[0050] In some embodiments, in step (3), after the test is completed, the sample is left to stand at room temperature for a period of time that is any one of 3 h, 4 h, 5 h, 6 h or any range between two of these.

[0051] In some embodiments, in step (3), the sample is subjected to a partial discharge test in accordance with the standard GB / T 3048.12-2007.

[0052] In some embodiments, in step (3), the sample is subjected to a power frequency withstand voltage breakdown test in accordance with the GB / T 3048.8-2007 standard.

[0053] In some embodiments, in step (3), the permanent deformation of the outer diameter and the eccentricity of the specimen are tested in accordance with GB / T 2951.11-2008.

[0054] In some implementations, the evaluation system includes grades A, B, C, and D; Among them, Class A refers to passing the electrical test of GB / T 12706.2-2020 standard. During the test, there are no visible signs of melting or dripping. After the test, the insulation shrinkage is ≤2 mm and the permanent deformation of the outer diameter is ≤3%, indicating that the insulation has excellent thermal stability. The Class B designation refers to passing the electrical test according to the GB / T 12706.2-2020 standard. During the test, there are slight signs of thermoplasticity but no dripping. After the test, the insulation shrinkage is 2-5 mm and the permanent deformation of the outer diameter is 3%-8%, indicating that the insulation has good thermal stability and the deformation is within a controllable range. The Class C refers to the electrical test that passes the GB / T 12706.2-2020 standard, but shows obvious melting during the process, possibly accompanied by slight dripping. After the test, the insulation shrinkage is >5 mm or the permanent deformation of the outer diameter is >8%, indicating that the insulation has suffered significant thermoplastic damage. Although there is no electrical breakdown, the insulation integrity has been seriously reduced. The term "Class D" refers to failure to pass the electrical test of GB / T 12706.2-2020 standard, or the occurrence of safety accidents such as insulation fire or severe dripping.

[0055] In some implementations, for samples classified as Grade C, the report should explicitly state that "this type of cable passed the standard short-circuit test, but its insulation exhibited significant thermoplastic deformation during the test, and it is recommended to pay special attention to it in product design, selection, or application condition evaluation."

[0056] The present invention also provides a terminal device, including a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor, wherein the processor executes the computer program to implement the short-circuit test method for the medium-voltage non-crosslinked polyethylene power cable.

[0057] The present invention also provides a computer-readable storage medium comprising a stored computer program, wherein, when the computer program is executed, it controls the device containing the computer-readable storage medium to perform the short-circuit test method for the medium-voltage non-crosslinked polyethylene power cable.

[0058] In some embodiments, the processor includes a programmable temperature control box, a standard short-circuit current generator, a high-temperature camera monitoring device, and a precision measuring device for measuring structural dimensional information.

[0059] To further illustrate the present invention, the following detailed description is provided in conjunction with embodiments, but these should not be construed as limiting the scope of protection of the present invention. Unless otherwise specified, the raw materials used in the following embodiments and comparative examples are all commercially available, and the same raw materials were used in parallel experiments.

[0060] Example 1 A short-circuit test method for medium-voltage non-crosslinked polyethylene power cables, such as Figure 1 As shown, it includes the following steps: (1) Sample condition standardization pretreatment: Three non-crosslinked polyethylene cable products, each approximately 5 m long, were cut as samples A, B, and C, with a cross-section of 300 mm. 2 The structural dimensions of the cable samples were measured. The samples were placed in a temperature-controlled cycling test chamber and heated to 80 °C under no current load, and held at that temperature for 24 hours. They were then slowly cooled to room temperature. This process can eliminate residual thermomechanical stress accumulated in the early processing of the cable samples, so that all samples are in a more stable and comparable state before the test, reducing background noise. At the same time, this process can simulate and release some of the thermal stress in operation, relax the molecular chain segments of the non-crosslinked polyethylene insulation, and avoid unpredictable coupling between residual stress and thermal stress in the subsequent short-circuit impact, which would lead to abnormal dispersion of deformation data. Subsequently, the insulation shielding layer was stripped at both ends of the cable conductor, and copper crimp connectors were installed. A high-current circuit was connected to both ends of the sample conductor, one end to the test transformer and the other end to the three-phase short-circuit copper plate. Thermocouples and laser measuring points were arranged on the cable surface, and a laser displacement meter was installed at the crimp connector. (2) Perform standard short-circuit tests and process monitoring: Strictly calculate and apply the short-circuit current according to the standard to make the conductor reach the temperature required by the standard, measure the ambient initial temperature, set the maximum temperature of the conductor and the short-circuit duration. In this embodiment, the maximum temperature is 250 ℃ and the short-circuit duration is 1s. Refer to the formula of IEC60986 as shown in the following formula (1), calculate the effective value of the short-circuit cable of the copper conductor cable, and apply a symmetrical short-circuit current with a duration of 1s to the cable sample. Equation (1); In the formula, I is the effective value of the short-time current, in kA; t is the short-circuit time, in seconds; and S is the cross-sectional area of ​​the sample, in mm. 2 ;θ sc θ represents the short-circuit temperature of the sample during operation, in °C. i Temperature of the conductor before the test, in °C; The synchronous monitoring system was activated, and all monitoring devices recorded all data from 1 second before the short circuit to 30 seconds after the short circuit. This included the test control system that controlled and monitored the output short circuit current of the test transformer, multiple thermocouples that monitored the cable temperature, laser displacement gauges that monitored the insulation retraction at the crimp joint, and high-temperature camera equipment that monitored the melting phenomenon on the sample surface throughout the process. The focus was on recording whether there were any changes in gloss, melting, flow or dripping phenomena on the insulation surface, and recording the time of occurrence. (3) Post-test special inspection and measurement: After completing the standard electrical tests, allow the sample to cool naturally at room temperature for at least 4 hours. Apply a voltage of 15.1 kV (1.73 U0) to the sample and perform a partial discharge test, recording the discharge amount. The experimental method is in accordance with GB / T 3048.12-2007. Perform a power frequency withstand voltage breakdown test of 30.5 kV (3.5 U0, 5 min) on the sample, in accordance with GB / T 3048.8-2007. Then measure the structural dimensions of the sample, including shrinkage, outer diameter deformation, and eccentricity. The retraction test uses a laser displacement meter to accurately measure the axial retraction distance of the insulating end at the crimp joint relative to the marking line; the standard for testing the permanent deformation of the outer diameter and the eccentricity of the sample is GB / T 2951.11-2008. (4) Establish a four-level evaluation system that is independent of standard electrical criteria and focuses on evaluating the thermoplastic behavior of insulation: Grade A: Passes the electrical tests (including discharge quantity and power frequency withstand voltage test) of GB / T 12706.2-2020 standard. There are no visible signs of melting or dripping during the test. After the test, the insulation shrinkage is ≤2 mm and the permanent deformation of the outer diameter is ≤3%, indicating that the insulation has excellent thermal stability. Grade B: Passes the electrical tests (including discharge quantity and power frequency withstand voltage test) of GB / T 12706.2-2020 standard. There are slight thermoplastic signs during the test but no dripping. After the test, the insulation shrinkage is 2-5 mm and the permanent deformation of the outer diameter is 3%-8%, indicating that the insulation has good thermal stability and the deformation is within a controllable range. Grade C: Passes the electrical tests (including discharge quantity and power frequency withstand voltage test) of GB / T 12706.2-2020 standard, but obvious melting occurs during the process, possibly accompanied by slight dripping. After the test, the insulation shrinkage is >5 mm or the permanent deformation of the outer diameter is >8%, indicating that the insulation has suffered significant thermoplastic damage. Although there is no electrical breakdown, the insulation integrity has been seriously reduced. Class D: Failed to pass the electrical tests (including discharge quantity and power frequency withstand voltage test) of GB / T 12706.2-2020 standard, or caused safety accidents such as insulation fire or serious dripping; (5) Conclusion on the classification of thermoplastic behavior: Based on the above evaluation system, A, B, C, and D grades are given. For cables graded as C, the report should clearly state that "this type of cable passed the standard short-circuit test, but its insulation showed significant thermoplastic deformation during the test. It is recommended to pay special attention to this in product design, selection, or application condition evaluation." The test results are shown in Table 1 below.

[0061] Example 2 A short-circuit test method for medium-voltage cross-linked polyethylene power cables is provided. The steps, raw materials, equipment, and process parameters used in each step are the same as in Example 1. The difference lies in the sample state standardization pretreatment in step (1), where three cross-linked polyethylene cable products with a length of approximately 5 m are cut as samples A, B, and C, with a cross-sectional area of ​​300 mm. 2 The structural dimensions of the cable sample were measured. The cable sample was placed in a temperature-controlled cycling test chamber and heated to 80 ℃ under no current load and kept at that temperature for 24 h. Then it was slowly cooled to room temperature. Subsequently, the insulation shielding layer was stripped from both ends of the cable conductor, and copper crimp connectors were installed. A high current circuit was connected to both ends of the sample conductor, with one end connected to the test transformer and the other end connected to the three-phase short-circuit copper plate. Thermocouples and laser measuring points were arranged on the cable surface, and a laser displacement meter was installed at the crimp connector.

[0062] Table 1 - Short-circuit test results of non-crosslinked and crosslinked polyethylene cable samples in Examples 1-2 of this application Comparative Example 1 A short-circuit test method for medium-voltage non-crosslinked polyethylene power cables includes the following steps: (1) Sample condition standardization pretreatment: Three non-crosslinked polyethylene cable products, each approximately 5 m long, were cut as samples A, B, and C, with a cross-section of 300 mm. 2The structural dimensions of the cable sample were measured. Then, the insulation shielding layer was stripped from both ends of the cable conductor, and copper crimping connectors were installed. A high current circuit was connected to both ends of the sample conductor, with one end connected to the test transformer and the other end connected to the three-phase short-circuit copper plate. Thermocouples and laser measuring points were arranged on the cable surface, and a laser displacement meter was installed at the crimping connector. (2) Perform standard short-circuit tests and process monitoring: Strictly calculate and apply the short-circuit current according to the standard to make the conductor reach the temperature required by the standard, measure the ambient initial temperature, set the maximum temperature of the conductor and the short-circuit duration. In this comparative example, the maximum temperature is 250 ℃ and the short-circuit duration is 1s. Refer to the formula of IEC60986 as shown in the following formula (1), calculate the effective value of the short-circuit cable of the copper conductor cable, apply a symmetrical short-circuit current with a duration of 1s to the cable sample with an effective value of the short-circuit cable. Equation (1); In the formula, I is the effective value of the short-time current, in kA; t is the short-circuit time, in seconds; and S is the cross-sectional area of ​​the sample, in mm. 2 ;θ sc θ represents the short-circuit temperature of the sample during operation, in °C. i Temperature of the conductor before the test, in °C; The synchronous monitoring system was activated, and all monitoring devices recorded all data from 1 second before the short circuit to 30 seconds after the short circuit. This included the test control system that controlled and monitored the output short circuit current of the test transformer, multiple thermocouples that monitored the cable temperature, laser displacement gauges that monitored the insulation retraction at the crimp joint, and high-temperature camera equipment that monitored the melting phenomenon on the sample surface throughout the process. The focus was on recording whether there were any changes in gloss, melting, flow or dripping phenomena on the insulation surface, and recording the time of occurrence. (3) Post-test special inspection and measurement: After completing the standard electrical tests, allow the sample to cool naturally at room temperature for at least 4 hours. Apply a voltage of 15.1 kV (1.73 U0) to the sample and perform a partial discharge test, recording the discharge amount. The experimental method is in accordance with GB / T 3048.12-2007. Perform a power frequency withstand voltage breakdown test of 30.5 kV (3.5 U0, 5 min) on the sample, in accordance with GB / T 3048.8-2007. Then measure the structural dimensions of the sample, including shrinkage, outer diameter deformation, and eccentricity. The retraction test uses a laser displacement meter to accurately measure the axial retraction distance of the insulating end at the crimp joint relative to the marking line; the standard for testing the permanent deformation of the outer diameter and the eccentricity of the sample is GB / T 2951.11-2008. (4) Establish a four-level evaluation system that is independent of standard electrical criteria and focuses on evaluating the thermoplastic behavior of insulation: Grade A: Passes the electrical tests (including discharge quantity and power frequency withstand voltage test) of GB / T 12706.2-2020 standard. There are no visible signs of melting or dripping during the test. After the test, the insulation shrinkage is ≤2 mm and the permanent deformation of the outer diameter is ≤3%, indicating that the insulation has excellent thermal stability. Grade B: Passes the electrical tests (including discharge quantity and power frequency withstand voltage test) of GB / T 12706.2-2020 standard. There are slight thermoplastic signs during the test but no dripping. After the test, the insulation shrinkage is 2-5 mm and the permanent deformation of the outer diameter is 3%-8%, indicating that the insulation has good thermal stability and the deformation is within a controllable range. Grade C: Passes the electrical tests (including discharge quantity and power frequency withstand voltage test) of GB / T 12706.2-2020 standard, but obvious melting occurs during the process, possibly accompanied by slight dripping. After the test, the insulation shrinkage is >5 mm or the permanent deformation of the outer diameter is >8%, indicating that the insulation has suffered significant thermoplastic damage. Although there is no electrical breakdown, the insulation integrity has been seriously reduced. Class D: Failed to pass the electrical tests (including discharge quantity and power frequency withstand voltage test) of GB / T 12706.2-2020 standard, or caused safety accidents such as insulation fire or serious dripping; (5) Conclusion on the classification of thermoplastic behavior: Based on the above evaluation system, A, B, C, and D grades are given. For cables graded as C, the report should clearly state that "this type of cable passed the standard short-circuit test, but its insulation showed significant thermoplastic deformation during the test. It is recommended to pay special attention to this in product design, selection, or application condition evaluation." The test results are shown in Table 2 below.

[0063] Comparative Example 2 A short-circuit test method for medium-voltage cross-linked polyethylene power cables is provided. All steps, raw materials, equipment, and process parameters used in each step are the same as in Comparative Example 1. The difference lies in the sample condition standardization pretreatment in step (1), where three cross-linked polyethylene cable products with a length of approximately 5 m are cut as samples A, B, and C, with a cross-sectional area of ​​300 mm. 2 The structural dimensions of the cable sample were measured. Then, the insulation shielding layer was removed from both ends of the cable conductor, and copper crimp connectors were installed. A high-current circuit was connected to both ends of the sample conductor, with one end connected to the test transformer and the other end connected to the three-phase short-circuit copper plate. Thermocouples and laser measuring points were arranged on the cable surface, and a laser displacement meter was installed at the crimp connector.

[0064] Table 2 - Short-circuit test results of non-crosslinked and crosslinked polyethylene cable samples in Comparative Examples 1-2 of this application Comparative Example 3 A short-circuit test method for medium-voltage non-crosslinked polyethylene power cables includes the following steps: (1) Sample condition standardization pretreatment: Three non-crosslinked polyethylene cable products, each approximately 5 m long, were cut as samples A, B, and C, with a cross-section of 300 mm. 2 The structural dimensions of the cable sample were measured. The cable sample was placed in a temperature-controlled cycling test chamber and heated to 50 °C under no current load and kept at that temperature for 24 h. Then it was slowly cooled to room temperature. Subsequently, the insulation shielding layer was stripped from both ends of the cable conductor, and copper crimp connectors were installed. A high current circuit was connected to both ends of the sample conductor, with one end connected to the test transformer and the other end connected to the three-phase short-circuit copper plate. Thermocouples and laser measuring points were arranged on the cable surface, and a laser displacement meter was installed at the crimp connector. (2) Perform standard short-circuit tests and process monitoring: Strictly calculate and apply the short-circuit current according to the standard to make the conductor reach the temperature required by the standard, measure the ambient initial temperature, set the maximum temperature of the conductor and the short-circuit duration. In this comparative example, the maximum temperature is 250 ℃ and the short-circuit duration is 1s. Refer to the formula of IEC60986 as shown in the following formula (1), calculate the effective value of the short-circuit cable of the copper conductor cable, apply a symmetrical short-circuit current with a duration of 1s to the cable sample with an effective value of the short-circuit cable. Equation (1); In the formula, I is the effective value of the short-time current, in kA; t is the short-circuit time, in seconds; and S is the cross-sectional area of ​​the sample, in mm. 2 ;θ sc θ represents the short-circuit temperature of the sample during operation, in °C. i Temperature of the conductor before the test, in °C; The synchronous monitoring system was activated, and all monitoring devices recorded all data from 1 second before the short circuit to 30 seconds after the short circuit. This included the test control system that controlled and monitored the output short circuit current of the test transformer, multiple thermocouples that monitored the cable temperature, laser displacement gauges that monitored the insulation retraction at the crimp joint, and high-temperature camera equipment that monitored the melting phenomenon on the sample surface throughout the process. The focus was on recording whether there were any changes in gloss, melting, flow or dripping phenomena on the insulation surface, and recording the time of occurrence. (3) Post-test special inspection and measurement: After completing the standard electrical tests, allow the sample to cool naturally at room temperature for at least 4 hours. Apply a voltage of 15.1 kV (1.73 U0) to the sample and perform a partial discharge test, recording the discharge amount. The experimental method is in accordance with GB / T 3048.12-2007. Perform a power frequency withstand voltage breakdown test of 30.5 kV (3.5 U0, 5 min) on the sample, in accordance with GB / T 3048.8-2007. Then measure the structural dimensions of the sample, including shrinkage, outer diameter deformation, and eccentricity. The retraction test uses a laser displacement meter to accurately measure the axial retraction distance of the insulating end at the crimp joint relative to the marking line; the standard for testing the permanent deformation of the outer diameter and the eccentricity of the sample is GB / T 2951.11-2008. (4) Establish a four-level evaluation system that is independent of standard electrical criteria and focuses on evaluating the thermoplastic behavior of insulation: Grade A: Passes the electrical tests (including discharge quantity and power frequency withstand voltage test) of GB / T 12706.2-2020 standard. There are no visible signs of melting or dripping during the test. After the test, the insulation shrinkage is ≤2 mm and the permanent deformation of the outer diameter is ≤3%, indicating that the insulation has excellent thermal stability. Grade B: Passes the electrical tests (including discharge quantity and power frequency withstand voltage test) of GB / T 12706.2-2020 standard. There are slight thermoplastic signs during the test but no dripping. After the test, the insulation shrinkage is 2-5 mm and the permanent deformation of the outer diameter is 3%-8%, indicating that the insulation has good thermal stability and the deformation is within a controllable range. Grade C: Passes the electrical tests (including discharge quantity and power frequency withstand voltage test) of GB / T 12706.2-2020 standard, but obvious melting occurs during the process, possibly accompanied by slight dripping. After the test, the insulation shrinkage is >5 mm or the permanent deformation of the outer diameter is >8%, indicating that the insulation has suffered significant thermoplastic damage. Although there is no electrical breakdown, the insulation integrity has been seriously reduced. Class D: Failed to pass the electrical tests (including discharge quantity and power frequency withstand voltage test) of GB / T 12706.2-2020 standard, or caused safety accidents such as insulation fire or serious dripping; (5) Conclusion on the classification of thermoplastic behavior: Based on the above evaluation system, A, B, C, and D grades are given. For cables graded as C, the report should clearly state that "this type of cable passed the standard short-circuit test, but its insulation showed significant thermoplastic deformation during the test. It is recommended to pay special attention to this in product design, selection, or application condition evaluation." The test results are shown in Table 3 below.

[0065] Comparative Example 4 A short-circuit test method for medium-voltage cross-linked polyethylene power cables is described. All steps, raw materials, equipment, and process parameters used in each step are the same as in Comparative Example 3. The difference lies in the sample condition standardization pretreatment in step (1), where three cross-linked polyethylene cable products with a length of approximately 5 m are cut as samples A, B, and C, with a cross-sectional area of ​​300 mm. 2 The structural dimensions of the cable sample were measured. The cable sample was placed in a temperature-controlled cycling test chamber and heated to 50 °C under no current load and kept at that temperature for 24 h. Then it was slowly cooled to room temperature. Subsequently, the insulation shielding layer was stripped from both ends of the cable conductor, and copper crimp connectors were installed. A high current circuit was connected to both ends of the sample conductor, with one end connected to the test transformer and the other end connected to the three-phase short-circuit copper plate. Thermocouples and laser measuring points were arranged on the cable surface, and a laser displacement meter was installed at the crimp connector.

[0066] Table 3 - Short-circuit test results of non-crosslinked and crosslinked polyethylene cable samples in Comparative Examples 3-4 of this application By comparing the schemes of Example 1 and Comparative Examples 1-2, it can be seen that the application adds a thermal stress relief pretreatment step at a specific temperature to the cable sample, which effectively reduces the random interference of residual stress in the production or factory test on the test results of non-crosslinked polyethylene cables, making the short-circuit deformation data of different batches and different laboratories more comparable, and improving the comparability and accuracy of the data.

[0067] Meanwhile, existing standard methods fail to differentiate the thermal behavior of insulating materials, and their evaluation framework is based on the assumption that the insulation layer does not melt or flow. Furthermore, current methods primarily use the electrical withstand voltage after a short circuit as the final qualification criterion, resulting in a singular functional evaluation. This application, for the first time, explicitly identifies the core failure mechanism of thermoplastic insulations such as non-crosslinked polyethylene during short circuits as the thermal melting and deformation of the insulation layer. Based on this, a specialized experimental evaluation system is established, solving the problem of the lack of corresponding evaluation standards for the unique failure modes of this type of material. By introducing a set of directly observable and quantifiable physical deformation parameters as structural evaluation indicators, including the maximum deformation of the insulation outer diameter, insulation shrinkage at joints, changes in insulation eccentricity, adhesive overflow, and monitoring the thermal behavior of the sample surface, structural indicators are combined with functional electrical indicators to construct a multi-dimensional, systematic evaluation model that can more comprehensively and earlier reveal damage to the insulation system.

[0068] Furthermore, by introducing quantitative constraints on the physical morphology of insulation, the evaluation conclusions of this method can more realistically reflect the actual state of the cable after short-circuit thermal shock. It pioneers a four-level evaluation model (A / B / C / D) that departs from binary criteria and specifically targets thermoplastic behavior. This model clearly defines and classifies the state of insulation damage (Level C) achieved through standard testing, realizing a profound shift in evaluation from whether failure has occurred to how and to what extent it approaches failure. The classification results provide cable manufacturers with direct, quantitative feedback for improving material formulations and optimizing structural designs. The core of the indicator system defined by this method lies in monitoring and evaluating whether unacceptable melt-related deformation has occurred, thus making the evaluation conclusions more universal and of greater engineering reference value, closer to engineering practice, and highly reliable.

[0069] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention for those skilled in the art.

Claims

1. A short-circuit test method for medium-voltage non-crosslinked polyethylene power cables, characterized in that, Includes the following steps: (1) Measure the structural dimensions of the cable sample, then heat-treat the sample at 70-90 ℃ for 12-48 h, and then connect the two ends of the sample to a standard short-circuit current generator. (2) Set the maximum short-circuit temperature and short-circuit duration of the sample, calculate the effective value of the short-circuit cable and the short-circuit current of the sample, start the synchronous monitoring system, and apply the effective value of the short-circuit cable and a symmetrical short-circuit current with a duration equal to the set short-circuit duration to the cable sample, and record the melting phenomenon on the sample surface throughout the process. (3) After the test is completed, the sample is subjected to partial discharge level and power frequency withstand voltage breakdown test, and the structural dimensions of the sample are measured at the same time. (4) Establish an evaluation system for evaluating insulation and thermoplastic behavior, and classify them according to the evaluation system; In steps (1) and (3), the structural dimension information includes at least one of the following: permanent deformation of outer diameter, eccentricity, and shrinkage.

2. The short-circuit test method for medium-voltage non-crosslinked polyethylene power cables as described in claim 1, characterized in that, In some embodiments, in step (1), the sample is a non-crosslinked polyethylene cable.

3. The short-circuit test method for medium-voltage non-crosslinked polyethylene power cables as described in claim 1, characterized in that, In step (2), the short-circuit current and the effective value of the short-circuit cable are calculated according to the following formula: ； In the formula, I 2 t is the short-circuit current; I is the effective value of the short-time current, in kA; t is the short-circuit time in seconds; S is the cross-sectional area of ​​the sample in mm. 2 ;θ sc θ represents the short-circuit temperature of the sample during operation, in °C. i The temperature of the conductor before the test is expressed in °C.

4. The short-circuit test method for medium-voltage non-crosslinked polyethylene power cables as described in claim 1, characterized in that, In step (2), the synchronous monitoring system includes a thermocouple for monitoring the surface temperature of the sample.

5. The short-circuit test method for medium-voltage non-crosslinked polyethylene power cables as described in claim 1, characterized in that, In step (2), the synchronous monitoring system includes a laser displacement meter for monitoring the displacement of the sample end.

6. The short-circuit test method for medium-voltage non-crosslinked polyethylene power cables as described in claim 1, characterized in that, In step (2), the synchronous monitoring system monitors and records data from 1 second before the short circuit to 30 seconds after the short circuit.

7. The short-circuit test method for medium-voltage non-crosslinked polyethylene power cables as described in claim 1, characterized in that, The synchronous monitoring system includes a high-temperature camera device for monitoring melting phenomena on the sample surface.

8. The short-circuit test method for medium-voltage non-crosslinked polyethylene power cables as described in claim 1, characterized in that, The evaluation system includes grades A, B, C, and D; Among them, Class A refers to passing the electrical test of GB / T 12706.2-2020 standard. During the test, there are no visible signs of melting or dripping. After the test, the insulation shrinkage is ≤2 mm and the permanent deformation of the outer diameter is ≤3%, indicating that the insulation has excellent thermal stability. The Class B designation refers to passing the electrical test according to the GB / T 12706.2-2020 standard. During the test, there are slight signs of thermoplasticity but no dripping. After the test, the insulation shrinkage is 2-5 mm and the permanent deformation of the outer diameter is 3%-8%, indicating that the insulation has good thermal stability and the deformation is within a controllable range. The Class C refers to the electrical test that passes the GB / T 12706.2-2020 standard, but shows obvious melting during the process, possibly accompanied by slight dripping. After the test, the insulation shrinkage is >5 mm or the permanent deformation of the outer diameter is >8%, indicating that the insulation has suffered significant thermoplastic damage. Although there is no electrical breakdown, the insulation integrity has been seriously reduced. The term "Class D" refers to failure to pass the electrical test of GB / T 12706.2-2020 standard, or the occurrence of safety accidents such as insulation fire or severe dripping.

9. A terminal device, characterized in that, The device includes a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor, wherein the processor, when executing the computer program, implements the short-circuit test method for medium-voltage non-crosslinked polyethylene power cables as described in any one of claims 1 to 8.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium includes a stored computer program, wherein, when the computer program is executed, it controls the device containing the computer-readable storage medium to perform a short-circuit test method for a medium-voltage non-crosslinked polyethylene power cable as described in any one of claims 1 to 8.

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