Non-destructive measurement method of by-product residual content in high voltage cable insulation layer

Abstract

A non-destructive method for measuring the residual content of by-products in the insulation layer of high-voltage cables is disclosed. In this method, the high-voltage cable is placed in a degassing chamber. Under constant temperature, the high-voltage cable is cut, cleaned, and connected to test the current. The copper conductor of the high-voltage cable is connected to the high-voltage electrode. The outer shielding layer of the high-voltage cable is circumferentially stripped to form a spaced outer shielding layer, which is then connected to the shielding electrode and grounded. A copper strip on the outer shielding layer outside the spaced outer shielding layer is connected to the measuring electrode. The current in the high-voltage cable insulation layer is measured to obtain test results, including multiple sets of test currents, test times, test voltages, and test frequencies. Based on the test results, a three-exponential fitting is performed. Using the fitted continuous function and the corresponding test time and frequency, the polarization parameter values ​​in a fixed frequency band are obtained. These values ​​change as degassing progresses, and the polarization parameters at different stages are recorded. p and q are degassing factors. When p and q are less than 0.3, the by-product content is within the acceptable range.

Description

Technical Field

[0001] This invention belongs to the field of power equipment testing, and in particular, it is a non-destructive method for measuring the residual content of by-products in the insulation layer of high-voltage cables. Background Technology

[0002] With the deepening development of new energy sources and the increasing complexity and diversity of electricity demand across regions, higher requirements are being placed on cables, which serve as crucial carriers of power dispatch. Ensuring cable insulation performance and improving the safety and reliability of the insulation layer are fundamental to cable manufacturing and operation. Currently, XLPE, a commonly used material, undergoes cross-linking through cross-linking agents, leaving behind residual cross-linking byproducts. Although these byproducts are present in low concentrations, they can still affect the safety and reliability of cable insulation. Currently, degassing treatment of cross-linking byproducts in extruded cables can improve insulation performance, but the degassing process is difficult to assess non-destructively, significantly increasing the industrial cost and uncertainty of cable manufacturing. Infrared spectroscopy, DC conductivity, space charge analysis, and gas chromatography are commonly used methods to determine the content of byproducts. Besides common chemical testing methods, broadband dielectric spectroscopy is also an important means of investigating cable insulation effectiveness. Liu Ying et al. proposed an assessment method for cable aging based on dielectric spectroscopy. However, since detection methods in the 0.1Hz to high-frequency range are insufficient to detect the presence of byproducts, there is currently no research on how to determine the content of byproducts and their impact on electrical performance. While the above methods can evaluate the degassing effect of cables, they all cause damage to the cables. The aim is to establish a quantitative evaluation method to characterize the content of by-products, which can assess the degassing of cables without damage and demonstrate the impact of by-products on the cable insulation layer using the ultra-low frequency domain.

[0003] The information disclosed in the background section is only intended to enhance the understanding of the background of the present invention, and therefore may contain information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention proposes a non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables. This method effectively assesses the residual content of by-products in the insulation layer of high-voltage cables online without damaging the overall cable, ensuring cable integrity while precisely controlling the degassing time, improving the safety and stability of the cable before it is put into production, and reducing the cost of by-product removal.

[0005] The objective of this invention is achieved through the following technical solution: a non-destructive method for measuring the residual content of by-products in the insulation layer of high-voltage cables includes:

[0006] Step 1: The high-voltage cable is placed in the degassing chamber. Under constant temperature, the high-voltage cable is cut, cleaned and wired to test the current. The copper conductor of the high-voltage cable is connected to the high-voltage electrode. The outer shielding layer of the high-voltage cable is stripped circumferentially to form a spaced outer shielding layer to connect to the shielding electrode and ground. The copper strip of the outer shielding layer outside the spaced outer shielding layer is connected to the measuring electrode.

[0007] Step 2: Test the insulation current of the high-voltage cable to obtain test results, which include multiple sets of test currents, test times, test voltages, and test frequencies;

[0008] Step 3: Perform a three-exponential fit based on the test results, where the fitting function is:

[0009] ,

[0010] in:

[0011] I is the test current, A; t is the test time, s; I0 is the steady-state current at steady state, A, which is the average value of the current test results in the last minute after 1 hour of testing, representing the steady-state current after polarization ends; A i Let A and τ be polarization parameters that include the degree of polarization. i The polarization parameter includes the polarization time, representing different relaxation types, s;

[0012] Step 4: Using the fitted continuous function and the corresponding test time and test frequency, obtain the value of the polarization parameter in a fixed frequency band, the expression of which is shown below:

[0013] ,

[0014] in:

[0015] ε' represents the real part of the relative permittivity at different frequencies in the ultra-low frequency band;

[0016] ε'' represents the imaginary part of the relative permittivity at different frequencies in the ultra-low frequency band, eliminating the influence of conductivity loss;

[0017] ε ∞ This represents the value of the real part of the relative permittivity at the corresponding frequency in steady state.

[0018] C0 is the capacitance value of the test cable, in F;

[0019] U c The test voltage is in V;

[0020] ω is the angular frequency, in rad / s;

[0021] Step 5: As degassing progresses, the polarization parameters change at different stages. p and q are degassing factors, and their expressions are:

[0022] ,

[0023] in:

[0024] ε'(0) is the maximum value of the real part of the relative permittivity after 0 days of degassing in the ultra-low frequency band;

[0025] ε''(0) is the maximum value of the imaginary part of the relative permittivity after 0 days of degassing in the ultra-low frequency band;

[0026] ε'(n) is the maximum value of the real part of the relative permittivity after n days of degassing in the ultra-low frequency band;

[0027] ε''(n) is the maximum value of the imaginary part of the relative permittivity after n days of degassing in the ultra-low frequency band;

[0028] When p and q are less than 0.3, the content of by-products is within the acceptable range.

[0029] In the non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables, in step 1, the temperature in the degassing chamber is set to 70-90℃.

[0030] In the non-destructive measurement method for the residual content of by-products in the insulation layer of a high-voltage cable, in step 1, before wiring, the copper conductor is treated with water-blocking adhesive with acetone and the oxide layer is polished before wiring.

[0031] In the non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables, a non-metallic frame is used to raise the cable under test by more than 10cm off the ground to avoid direct contact with the degassing chamber floor and metal equipment.

[0032] In the non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables, in step 2, the test current is recorded every second, each test lasts for 1-3 hours, the temperature fluctuation is less than ±1℃ during the test, and the outdoor temperature, humidity and degassing time are recorded simultaneously during the current test.

[0033] In the aforementioned non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables, step 4 involves a fixed frequency band covering 10... -5 -1Hz frequency band.

[0034] In the non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables, in step 4, the ultra-low frequency band is 10. -2 -10 -4 Hz band.

[0035] In the non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables, the ratio c between the steady-state current I0 and the result after 1 hour of current testing represents the degree of closeness between the fitted result and the actual result.

[0036] ,

[0037] in:

[0038] I 60 To measure the average test current (A) in the last minute of a 1-hour test,

[0039] If c is between 0.7 and 1.2, the fit is considered to be within a reasonable range.

[0040] Compared with existing technologies, this invention has the following advantages: This invention utilizes the cable charging current for triple-exponential fitting, then plots the ultra-low frequency polarization response spectrum of by-products in XLPE through the Fourier transform of the fitting parameters, establishing the quantity-effect relationship of by-products. By identifying the causes of polarization at different frequencies, ε' and ε'' characterize the content of by-products involved. The degassing compliance is determined by the ratio of parameter changes in the early and later stages of the test. This invention proposes a non-destructive testing method for evaluating the degassing status of full-size cables, helping to improve the stability and safety of cables after production, and effectively controlling time to achieve cost savings. It provides real-time monitoring of the residual content of by-products inside the insulation, clarifies the evaluation method for degassing effectiveness, and not only saves degassing time and reduces processing costs, but also improves the safety and reliability of heat treatment. Attached Figure Description

[0041] Various other advantages and benefits of the present invention will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. Furthermore, the same reference numerals denote the same parts throughout the drawings.

[0042] In the attached diagram:

[0043] Figure 1 This is a schematic diagram of the current test circuit for a non-destructive measurement method of the residual content of by-products in the insulation layer of a high-voltage cable provided in one embodiment of the present disclosure;

[0044] Figure 2 This is a schematic diagram showing the test results of cable insulation current at different degassing times;

[0045] Figure 3 This is a schematic diagram showing the relationship between different frequencies and polarization parameters in the ultra-low frequency band after the three-exponential Fourier transform;

[0046] Figure 4 This is a schematic diagram illustrating the Fourier transform infrared spectroscopy results for characterizing the residual content of acetophenone at different degassing times;

[0047] Figure 5 This is a schematic diagram showing the results of Fourier transform infrared spectroscopy characterization of cumyl alcohol residue content at different degassing times.

[0048] The present invention will be further explained below with reference to the accompanying drawings and embodiments. Detailed Implementation

[0049] The following will refer to the appendix. Figures 1 to 5 Specific embodiments of the invention will be described in more detail below. While specific embodiments of the invention are shown in the accompanying drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0050] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions are preferred embodiments for carrying out the invention; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of the invention. The scope of protection of this invention is determined by the appended claims.

[0051] To facilitate understanding of the embodiments of the present invention, further explanations and descriptions will be provided below with reference to the accompanying drawings and specific embodiments. The accompanying drawings do not constitute a limitation on the embodiments of the present invention.

[0052] like Figure 1 As shown, the non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables includes:

[0053] Step 1: The high-voltage cable is placed in the degassing chamber. Under constant temperature, the high-voltage cable is cut, cleaned and wired to test the current. The copper conductor of the high-voltage cable is connected to the high-voltage electrode. The outer shielding layer of the high-voltage cable is stripped circumferentially to form a spaced outer shielding layer to connect to the shielding electrode and ground. The copper strip of the outer shielding layer outside the spaced outer shielding layer is connected to the measuring electrode.

[0054] Step 2: Test the insulation current of the high-voltage cable to obtain test results, which include multiple sets of test currents, test times, test voltages, and test frequencies;

[0055] Step 3: Perform a three-exponential fit based on the test results, where the fitting function is:

[0056] ,

[0057] in:

[0058] I is the test current, A; t is the test time, s; I0 is the steady-state current at steady state, A, which is the average value of the current test results in the last minute after 1 hour of testing, representing the steady-state current after polarization ends; A i Let A and τ be polarization parameters that include the degree of polarization. i The polarization parameter includes the polarization time, representing different relaxation types, s;

[0059] Step 4: Using the fitted continuous function and the corresponding test time and test frequency, obtain the value of the polarization parameter in a fixed frequency band, the expression of which is shown below:

[0060] ,

[0061] in:

[0062] ε' represents the real part of the relative permittivity at different frequencies in the ultra-low frequency band;

[0063] ε'' represents the imaginary part of the relative permittivity at different frequencies in the ultra-low frequency band, eliminating the influence of conductivity loss;

[0064] ε ∞ ε is the value of the real part of the relative permittivity at the frequency corresponding to the steady state. ∞ The values ​​obtained are obtained through testing using the THz-TDS system, or can be adopted based on the type of base material and commonly used values ​​in engineering.

[0065] C0 is the capacitance value of the test cable, in F;

[0066] U c The test voltage is in V;

[0067] ω is the angular frequency, in rad / s;

[0068] Step 5: As degassing progresses, the polarization parameters change at different stages. p and q are degassing factors, and their expressions are:

[0069] ,

[0070] in:

[0071] ε'(0) is the maximum value of the real part of the relative permittivity after 0 days of degassing in the ultra-low frequency band;

[0072] ε''(0) is the maximum value of the imaginary part of the relative permittivity after 0 days of degassing in the ultra-low frequency band;

[0073] ε'(n) is the maximum value of the real part of the relative permittivity after n days of degassing in the ultra-low frequency band;

[0074] ε''(n) is the maximum value of the imaginary part of the relative permittivity after n days of degassing in the ultra-low frequency band;

[0075] When p and q are less than 0.3, the content of by-products is within the acceptable range.

[0076] In a preferred embodiment of the non-destructive measurement method for the residual content of by-products in the insulation layer of a high-voltage cable, in step 1, the temperature in the degassing chamber is set to 70-90℃.

[0077] In a preferred embodiment of the non-destructive measurement method for the residual content of by-products in the insulation layer of a high-voltage cable, in step 1, before wiring, the copper conductor is treated with acetone-based water-blocking adhesive and the oxide layer is polished before wiring.

[0078] In a preferred embodiment of the non-destructive measurement method for the residual content of by-products in the insulation layer of a high-voltage cable, a non-metallic frame is used to raise the cable under test by more than 10cm off the ground to avoid direct contact with the degassing chamber floor and metal equipment.

[0079] In a preferred embodiment of the non-destructive measurement method for the residual content of by-products in the insulation layer of a high-voltage cable, in step 2, the test current is recorded every second, each test lasts for 1-3 hours, the temperature fluctuation during the test is less than ±1℃, and the outdoor temperature, humidity and degassing time are recorded simultaneously during the current test.

[0080] In a preferred embodiment of the non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables, in step 4, the fixed frequency band covers 10... -5 -1Hz frequency band.

[0081] In a preferred embodiment of the non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables, in step 4, the ultra-low frequency band is 10. -2 -10 -4 Hz band.

[0082] In a preferred embodiment of the non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables, the ratio c between the steady-state current I0 and the result after 1 hour of current testing in the fitting result represents the degree of closeness between the fitting and the actual result.

[0083] ,

[0084] in:

[0085] I 60 To measure the average test current (A) in the last minute of a 1-hour test,

[0086] The value of c is 0.7-1.2, indicating that the fit is within a reasonable range.

[0087] In one embodiment, the exponential-Fourier transform non-destructive measurement method for by-product residual content consists of three parts: current testing, triple-exponential fitting-Fourier transform, and by-product residual content assessment. This scheme is designed for high-voltage cables, and its testing and calculation steps are as follows.

[0088] (1) According to Figure 1 Cut, clean, and connect the cables;

[0089] (2) Test the insulation current for 1-3 hours;

[0090] (3) Perform a three-exponential fit based on the test results;

[0091] (4) Perform Fourier transform on the fitting results;

[0092] (5) Use the Fourier transform results to evaluate the degassing status.

[0093] Step (1) should ensure wiring stability and the cable should not come into contact with the degassing chamber.

[0094] In step (2), the measurement time should be determined according to the type of cable and the accuracy requirements. During the test, the temperature fluctuation should be kept within ±1℃.

[0095] Step (3) can be performed using general fitting software such as MATLAB or Ogin.

[0096] Step (4) should cover 10 -5 -1Hz frequency band, to obtain the data required in step (5).

[0097] The byproduct residue in step (5) can cause XLPE to polarize at very low frequencies, at 10 -3 -10 -4 Hz band and 10 -2 -10 -3 The Hz peak exhibited a polarization loss peak and Debye phenomenon closely related to the byproduct content. Using the previously described test and calculation results, ε' and ε'' can be obtained to characterize the byproduct participation content and can be used to assess whether the degassing meets the standards.

[0098] The test needs to meet the following environmental requirements:

[0099] The testing site should have sufficient power supply equipment or power interfaces to power the testing equipment and sensors, and the power supply should be as stable as possible. During each set of tests, the cable temperature change should not exceed ±1℃. The test object should be fully isolated from other cables, especially other energized test circuits, to reduce magnetic field interference and measurement errors. When the testing environment is selected as a high-temperature degassing chamber, the testing equipment should be able to perform stable testing at 70℃.

[0100] First, let me explain the current testing scheme:

[0101] During the degassing process of high-voltage cables, the cable is placed in the degassing chamber, and testing can be carried out after the temperature stabilizes. All geometric parameters except the cable sample length are measured. The main parameters to be measured are the dimensions of the critical layer, the position of the stranded layers, and the position of all testing equipment. Pre-treatment of the cable is required before testing to ensure wiring stability and testing accuracy. The cable ends are stripped in layers before the experiment to facilitate wiring and testing.

[0102] The conductor layer is treated with acetone-treated water-blocking adhesive, and the oxide layer is polished before wiring to minimize wiring resistance, reduce the possibility of local overheating, and reduce errors. The cut surfaces of the insulation layer are cleaned to prevent flashover caused by stains from cutting.

[0103] according to Figure 1 Connect the test circuitry, ensuring stable wiring. To reduce wiring resistance, use copper conductor wires with a sufficiently large cross-section. If a single wire is insufficient, multiple wires can be connected in parallel, with the wire lengths kept as short as possible. Use a temperature sensor near the test circuit to monitor ambient temperature. Ensure stable grounding within the test circuit.

[0104] The test current is recorded every second, and each test lasts for 1 hour. The temperature fluctuation during the test is less than ±1℃. The test temperature and sample degassing time should be recorded simultaneously during the current test.

[0105] Triple-exponential fitting - Fourier transform part:

[0106] The test results are discrete functions of time and current. First, outliers and negative values ​​are removed using MATLAB. The standard deviation method can be used to remove outliers that are greater than three times the standard deviation within the window period.

[0107] The discrete test circuit was then fitted using the three-exponential method, transforming it into a continuous curve of a fixed function. The fitting formula is as follows:

[0108] (1)

[0109] in:

[0110] I is the test current, in A;

[0111] t is the test time, in seconds;

[0112] I0 is the steady-state current at steady state, in A;

[0113] A i A is a polarization parameter that includes the degree of polarization.

[0114] τ i Let s be the polarization parameter that includes the polarization time;

[0115] In the fitting results, I0 can be compared with the results after 1 hour of current testing, and the ratio c can represent the degree of closeness between the fitting and the actual results:

[0116] (2)

[0117] in:

[0118] I 60 The average value of the test current in the last minute of the 1-hour test is given in A.

[0119] The closer c is to 1, the higher the degree of fit. Since the cable charging time is relatively long, I0 is usually small. It is considered that when c is 0.7-1.2, the measurement results are meaningful, the fit is within a reasonable range, and subsequent calculations can be performed.

[0120] Fitting parameter τ i Representing different relaxation types, the larger the value, the longer the relaxation time; A i This represents the degree of relaxation; the higher the value, the more pronounced the relaxation.

[0121] By using the continuous function and coefficients obtained through fitting, corresponding to the test time and test frequency, we can obtain the values ​​of the relevant polarization parameters in a fixed frequency band, as shown in the following expression:

[0122] (3)

[0123] in:

[0124] ε' represents the real part of the relative permittivity at different frequencies in the ultra-low frequency band;

[0125] ε'' represents the imaginary part of the relative permittivity at different frequencies in the ultra-low frequency band, eliminating the influence of conductivity loss;

[0126] ε ∞ This represents the value of the real part of the relative permittivity at the corresponding frequency in steady state.

[0127] C0 is the capacitance value of the test cable, in F;

[0128] U c The test voltage is in V;

[0129] ω is the angular frequency, in rad / s.

[0130] The above polarization parameters have corresponding values ​​at different frequencies, which can be used for subsequent comparison and judgment.

[0131] The polarization parameters obtained are different at different frequencies, where t i The polarization parameter at a given frequency represents the degree of loss corresponding to that polarization, and its expression at different frequencies is as follows:

[0132] (4)

[0133] in:

[0134] ε i ' represents the real part of the relative permittivity for the i-th polarization at different frequencies in the ultra-low frequency band;

[0135] ε i '' represents the imaginary part of the relative permittivity for the i-th polarization at different frequencies in the ultra-low frequency band, after removing the influence of conductivity loss.

[0136] Finally, an explanation is given regarding the assessment of byproduct residue content.

[0137] Using the above polarization parameters, which change as degassing progresses, the parameters at different stages are recorded. p and q are defined as degassing factors, and their expressions are:

[0138] (5)

[0139] in:

[0140] ε'(0) is the maximum value of the real part of the relative permittivity after 0 days of degassing in the ultra-low frequency band;

[0141] ε''(0) is the maximum value of the imaginary part of the relative permittivity after 0 days of degassing in the ultra-low frequency band;

[0142] ε'(n) is the maximum value of the real part of the relative permittivity after n days of degassing in the ultra-low frequency band;

[0143] ε''(n) is the maximum value of the imaginary part of the relative permittivity after n days of degassing in the ultra-low frequency band.

[0144] We believe that when p and q are less than 0.3, the byproduct content is within the acceptable range.

[0145] In actual measurements, we need to test various parameters of the cable to obtain relevant dimensions and electrical parameters, ensuring the safety and accuracy of current testing. At the same time, we should ensure the stability of the wiring to avoid generating electric sparks during the experiment that could ignite byproduct gases.

[0146] Here is a test case, taking a 500kV DC cable as an example, to determine the degassing status through test current data.

[0147] Example 1

[0148] Preparations and current testing procedures for a 500kV DC cable before testing, and current results under different degassing times are as follows: Figure 2 As shown.

[0149] After filtering using MATLAB and removing outliers, the results are fitted using formula (1) and then substituted into formula (3). The relationship between polarization parameters and frequency in the ultra-low frequency band is as follows: Figure 3 As shown.

[0150] according to Figure 3 The p and q values ​​were calculated as follows: p = 0.43 and q = 0.42 after 15 days of degassing; p = 0.20 and q = 0.17 after 45 days of degassing; and p = 0.19 and q = 0.16 after 75 days of degassing. According to the judgment criteria, the p and q values ​​did not meet the standard after 15 days of degassing; after 45 days of degassing, the standard was met and degassing could be terminated. Degassing from 45 to 75 days was unnecessary and could be omitted.

[0151] To demonstrate the reliability of the results, we used Fourier transform infrared spectroscopy to monitor the content of by-products in the cable insulation layer at different degassing times, and the results are as follows: Figure 4 Figure 5 As shown, the content of by-products decreased rapidly in the first 45 days before degassing, and then decreased slowly after 45 days, indicating that degassing had little effect on this.

[0152] Although embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments and application fields described above. The specific embodiments described above are merely illustrative and instructive, and not restrictive. Those skilled in the art can make many other forms based on the guidance of this specification and without departing from the scope of protection of the claims of the present invention, and all of these are within the scope of protection of the present invention.

Claims

1. A non-destructive method for measuring the residual content of by-products in the insulation layer of high-voltage cables, characterized in that, It includes the following steps, Step 1: The high-voltage cable is placed in the degassing chamber. Under constant temperature, the high-voltage cable is cut, cleaned and wired to test the current. The copper conductor of the high-voltage cable is connected to the high-voltage electrode. The outer shielding layer of the high-voltage cable is stripped circumferentially to form a spaced outer shielding layer to connect to the shielding electrode and ground. The copper strip of the outer shielding layer outside the spaced outer shielding layer is connected to the measuring electrode. Step 2: Test the insulation current of the high-voltage cable to obtain test results, which include multiple sets of test currents, test times, test voltages, and test frequencies; Step 3: Perform a three-exponential fit based on the test results, where the fitting function is: ， in: I is the test current, A; t is the test time, s; I0 is the steady-state current at steady state, A, which is the average value of the current test results in the last minute after 1 hour of testing, representing the steady-state current after polarization ends; A i Let A and τ be polarization parameters that include the degree of polarization. i The polarization parameter includes the polarization time, representing different relaxation types, s; Step 4: Using the fitted continuous function and the corresponding test time and test frequency, obtain the value of the polarization parameter in a fixed frequency band, the expression of which is shown below: ， in: ε' represents the real part of the relative permittivity at different frequencies in the ultra-low frequency band; ε'' represents the imaginary part of the relative permittivity at different frequencies in the ultra-low frequency band, eliminating the influence of conductivity loss; ε ∞ This represents the value of the real part of the relative permittivity at the corresponding frequency in steady state. C0 is the capacitance value of the test cable, in F; U c The test voltage is in V; ω is the angular frequency, in rad / s; Step 5: As degassing progresses, the polarization parameters at different stages are recorded. p and q are degassing factors. When p and q are less than 0.3, the byproduct content reaches the acceptable range. The expression is: ， in: ε'(0) is the maximum value of the real part of the relative permittivity after 0 days of degassing in the ultra-low frequency band; ε''(0) is the maximum value of the imaginary part of the relative permittivity after 0 days of degassing in the ultra-low frequency band; ε'(n) is the maximum value of the real part of the relative permittivity after n days of degassing in the ultra-low frequency band; ε''(n) is the maximum value of the imaginary part of the relative permittivity after n days of degassing in the ultra-low frequency band.

2. The non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables according to claim 1, characterized in that, In step 1, the temperature in the degassing chamber is set to 70-90℃.

3. The non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables according to claim 1, characterized in that, In step 1, before wiring, the copper conductor is treated with acetone to remove the water-blocking adhesive and the oxide layer is polished before wiring.

4. The non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables according to claim 1, characterized in that, Use a non-metallic frame to raise the cable under test to a height of more than 10cm off the ground to avoid direct contact with the degassing chamber floor and metal equipment.

5. The non-destructive measurement method for the residual content of by-products in the insulation layer of high-voltage cables according to claim 1, characterized in that, In step 2, the test current is recorded every second, and each test lasts for 1-3 hours. The temperature fluctuation during the test is less than ±1℃. The outdoor temperature, humidity and degassing time are recorded simultaneously during the current test.

6. The method according to claim 1, characterized in that, In step 4, the fixed frequency band covers 10 -5 -1Hz frequency band.

7. The method according to claim 1, characterized in that, In step 4, the ultra-low frequency band is 10. -2 -10 -4 Hz frequency band.

8. The method according to claim 1, characterized in that, The ratio c between the steady-state current I0 and the current test result after 1 hour in the fitting results represents the degree of closeness between the fitting and the actual results. If c is 0.7-1.2, the fitting is considered to be within a reasonable range.

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