A method for determining the life index of cable insulation

Abstract

The application discloses a kind of cable insulation life index determination methods, comprising the following steps: step one: selecting the starting voltage and interstage voltage increasing rate of step-by-step breakdown test, two groups of cable insulation samples are selected to carry out step-by-step breakdown test with different voltage increasing time in each stage according to certain voltage increasing ratio;Step two: applying calculus and inverse power law to describe interstage voltage increasing electrical aging process, based on inverse power law and electrical aging cumulative effect, a nonlinear life equation is established according to the step-by-step breakdown test data of each sample;Step three: considering the randomness of step-by-step breakdown test, the electrical aging cumulative amount of a group of samples with the same voltage increasing time in each stage is averaged, the life equations of samples in the same group are combined into one equation, and after the life equations of two groups of samples are combined respectively, an equation set is obtained;Step four: solve the nonlinear life equation set in step three by using matlab software, and obtain the life index of cable insulation sample.

Description

Technical Field

[0001] This invention relates to the field of solid insulation electrical life assessment technology, and in particular to a method for determining the insulation life index of cables. Background Technology

[0002] Guided by dual carbon targets, my country's new energy and high-voltage AC / DC transmission system construction is booming. As key equipment in high-voltage transmission systems, the production and application of high-voltage and ultra-high-voltage power cables are growing rapidly. For example, high-voltage flexible DC cables have been widely used in long-distance offshore power transmission, urban power grids, and new energy integration. Since electrical stress is a key aging factor for high-voltage power cable insulation, electrical life assessment is crucial for the research and development of cable insulation materials and the design of insulation structures. It is also one of the important means of evaluating the long-term operational reliability of high-voltage power cables.

[0003] Cable insulation electrical life assessment typically employs a life model based on the inverse power law, where the life index is a key parameter. Knowing the life index allows for rapid measurement of failure time by increasing the voltage, thus determining the life equation. A direct method for determining the life index is based on constant-voltage electrical aging tests at different voltages, with the index obtained by linearly fitting voltage and failure time data onto a double logarithmic coordinate system. This method is accurate and reliable, but it requires at least 4-5 voltages for electrical aging, and the failure time under low-voltage aging is often too long, resulting in a large workload and significant time consumption.

[0004] To improve testing efficiency and reduce testing costs, a simplified step-up voltage approach has been proposed to accelerate the determination of the lifetime index. This method constructs a modified inverse power law lifetime equation system based on two sets of step-down tests, and obtains an approximate lifetime index by solving the equation system. However, since inter-stage voltage increases typically employ a step-up approach, the steep voltage rise time can cause changes in space charge injection and migration characteristics, potentially leading to premature cable insulation failure. The electrical aging mechanism also changes, rendering the inverse power law inapplicable. Furthermore, the lifetime equation does not reflect the aging effect of the voltage increase process or the influence of the voltage increase rate. Finally, when calculating the lifetime index using the formula, the duration of the final voltage application stage needs to be converted into the number of voltage application stages, which is imprecise. Additionally, the formula itself is an approximate analytical solution of a nonlinear lifetime equation, resulting in inaccurate final results. Therefore, it is necessary to improve the simplified step-up voltage approach and propose a more scientific and accurate method for rapidly determining the lifetime index. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention provides a method for determining the cable insulation life index, which makes the testing process more scientific and reasonable, and obtains test results quickly and accurately, providing support for cable insulation development and reliability evaluation.

[0006] To achieve the aforementioned objectives of the invention, the technical solution adopted to solve its technical problems is as follows:

[0007] A method for determining the insulation life index of cables includes the following steps:

[0008] Step 1: Select the starting voltage and inter-stage voltage ramp rate for the step-by-step breakdown test, and select two sets of cable insulation samples to conduct step-by-step breakdown tests with different voltage ramp times for each stage according to a certain voltage ramp ratio;

[0009] Step 2: Apply calculus and inverse power law to describe the interstage voltage boosting electrical aging process. Based on the step-by-step breakdown test data of each sample, establish a nonlinear lifetime equation based on the inverse power law and the cumulative effect of electrical aging.

[0010] Step 3: Considering the randomness of the step-by-step breakdown test, the cumulative electrical aging of a group of samples with the same pressurization time at each stage is averaged, and the lifetime equations of the same group of samples are combined into one equation. The lifetime equations of two groups of samples are then combined to obtain a set of equations.

[0011] Step 4: Use MATLAB software to solve the nonlinear life equations from Step 3 to obtain the life index of the cable insulation sample.

[0012] Preferably, the starting voltage of the step-by-step breakdown test in step one is selected as 40% of the short-time breakdown voltage of the sample.

[0013] Preferably, the interstage boost rate in step one is selected from 1, 2, 5, 10, 20, 50, and 100 V / s.

[0014] Furthermore, if the sample breaks down during the interstage pressurization process, the nonlinear lifetime equation in step two becomes:

[0015] when hour,

[0016] when hour,

[0017] when hour,

[0018]

[0019] In the formula, The starting voltage, For interstage boost rate, Apply a series of voltage levels, For each constant pressurization time, For the duration of the final stage boost, Life expectancy index It is a constant. It is an integer;

[0020] If the sample breaks down during constant pressure application, the nonlinear lifetime equation in step two is:

[0021] when hour,

[0022] when hour,

[0023]

[0024] In the formula, The duration of constant pressure application in the final stage.

[0025] Furthermore, the method for merging the lifetime equations of a group of samples with the same pressurization time in step three is as follows:

[0026] The lifetime equation for a single sample within a group is denoted as: , ;

[0027] The combined lifetime equations for the same group of samples are then: ;

[0028] In the formula, Number of samples It is an integer. For the first in the group The cumulative amount of electrical aging of each sample.

[0029] By employing the above technical solutions, this invention has the following advantages and positive effects compared with the prior art:

[0030] This invention employs a slow inter-stage voltage ramp rate for step-by-step breakdown testing, avoiding the premature cable insulation failure caused by steep voltage rise times altering space charge injection and migration characteristics. This ensures that the mechanism of step-by-step voltage ramp electrical aging is consistent with that of constant-voltage electrical aging, guaranteeing the applicability of the inverse power law. By applying calculus and the inverse power law to the inter-stage voltage ramp process neglected by traditional step-by-step ramp methods, the constructed lifetime equation reflects the aging effect of the voltage ramp process and the influence of the ramp rate. Furthermore, the lifetime index obtained by solving the nonlinear lifetime equations using MATLAB software is more accurate than the coarse approximation obtained by the simplified step-by-step voltage ramp method. This invention provides a more scientific and accurate method for rapid determination of cable insulation lifetime index. Attached Figure Description

[0031] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. In the drawings:

[0032] Figure 1 A flowchart illustrating the steps of a method for determining the insulation life index of a cable according to an embodiment of the present invention;

[0033] Figure 2 This is a schematic diagram of a slow, step-by-step voltage increase according to a specific embodiment of the present invention. Detailed Implementation

[0034] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. 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.

[0035] See Figure 1 As shown in the figure, this embodiment provides a method for determining the insulation life index of cables, including the following steps:

[0036] Step 1: Select the starting voltage and inter-stage voltage ramp rate for the step-by-step breakdown test, and select two sets of cable insulation samples to conduct step-by-step breakdown tests with different voltage ramp times for each stage according to a certain voltage ramp ratio;

[0037] In this embodiment, the cable insulation sample is an insulation test piece with a thickness of 0.5 mm, and the short-time breakdown voltage of the sample is 37.5 kV. The starting voltage of the two progressive breakdown tests is selected as 40% of the short-time breakdown voltage of the sample, i.e., 15 kV, and the step-up ratio is 1.06. The constant voltage application time for each step is 1 min and 20 min, respectively. The inter-stage step-up rate is selected from 1, 2, 5, 10, 20, 50, and 100 V / s, and the preferred inter-stage step-up rate is 100 V / s.

[0038] Step 2: Apply calculus and inverse power law to describe the interstage voltage boosting electrical aging process. Based on the step-by-step breakdown test data of each sample, establish a nonlinear lifetime equation based on the inverse power law and the cumulative effect of electrical aging.

[0039] Specifically, if the sample breaks down during the interstage pressurization process, the nonlinear lifetime equation in step two is:

[0040] when hour,

[0041] when hour,

[0042] when hour,

[0043]

[0044] In the formula, The starting voltage, For interstage boost rate, Apply a series of voltage levels, For each constant pressurization time, For the duration of the final stage boost, Life expectancy index It is a constant. It is an integer;

[0045] If the sample breaks down during constant pressure application, the nonlinear lifetime equation in step two is:

[0046] when hour,

[0047] when hour,

[0048]

[0049] In the formula, The duration of constant pressure application in the final stage.

[0050] In this embodiment, , , Two groups of samples (10 samples in each group) were selected and tested separately. and The step-by-step breakdown test will produce a corresponding number of voltage application stages as the test results are obtained. and ,as well as (or )and (or The experimental results are shown in Table 1:

[0051]

[0052] Table 1 Results of step-by-step breakdown test

[0053] By substituting the results of the step-by-step breakdown test for each sample into the nonlinear lifetime equation described in step two, 20 lifetime equations can be established.

[0054] Step 3: Considering the randomness of the step-by-step breakdown test, the cumulative electrical aging of a group of samples with the same pressurization time at each stage is averaged, and the lifetime equations of the same group of samples are combined into one equation. The lifetime equations of two groups of samples are then combined to obtain a set of equations.

[0055] Specifically, the method for merging the lifetime equations of a group of samples with the same pressurization time in step three is as follows:

[0056] The lifetime equation for a single sample within a group is denoted as: , ;

[0057] The combined lifetime equations for the same group of samples are then: ;

[0058] In the formula, Number of samples It is an integer. For the first in the group The cumulative amount of electrical aging of each sample.

[0059] In this embodiment, the number of samples m in each group is 10, and the 20 lifetime equations are eventually merged into a nonlinear equation set containing 2 lifetime equations.

[0060] Step 4: Use MATLAB software to solve the nonlinear life equations from Step 3 to obtain the life index of the cable insulation sample.

[0061] In this embodiment, the numerical solution of the nonlinear life equation system was obtained using MATLAB software, and the life index of the cable insulation sample was found to be 10.8. This life index measurement result is reasonable, proving that the method of the present invention is effective.

[0062] This invention employs a slow inter-stage voltage ramp rate for step-by-step breakdown testing, avoiding the premature cable insulation failure caused by steep voltage rise times altering space charge injection and migration characteristics. This ensures that the mechanism of step-by-step voltage ramp electrical aging is consistent with that of constant-voltage electrical aging, guaranteeing the applicability of the inverse power law. By applying calculus and the inverse power law to the inter-stage voltage ramp process neglected by traditional step-by-step ramp methods, the constructed lifetime equation reflects the aging effect of the voltage ramp process and the influence of the ramp rate. Furthermore, the lifetime index obtained by solving the nonlinear lifetime equations using MATLAB software is more accurate than the coarse approximation obtained by the simplified step-by-step voltage ramp method. This invention provides a more scientific and accurate method for rapid determination of cable insulation lifetime index.

[0063] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for determining the insulation life index of cables, characterized in that, Includes the following steps: Step 1: Select the starting voltage and inter-stage voltage ramp rate for the step-by-step breakdown test, and select two sets of cable insulation samples to conduct step-by-step breakdown tests with different voltage ramp times for each stage according to a certain voltage ramp ratio; Step 2: Apply calculus and inverse power law to describe the interstage voltage boosting electrical aging process. Based on the step-by-step breakdown test data of each sample, establish a nonlinear lifetime equation based on the inverse power law and the cumulative effect of electrical aging. If the sample breaks down during the interstage pressurization process, the nonlinear lifetime equation in step two is: when hour, when hour, when hour, In the formula, The starting voltage, For interstage boost rate, Apply a series of voltage levels, For each constant pressurization time, For the duration of the final stage boost, Life expectancy index It is a constant. It is an integer; If the sample breaks down during constant pressure application, the nonlinear lifetime equation in step two is: when hour, when hour, In the formula, The duration of constant pressure application in the final stage; Step 3: Considering the randomness of the step-by-step breakdown test, the cumulative electrical aging of a group of samples with the same pressurization time at each stage is averaged, and the lifetime equations of the same group of samples are combined into one equation. The lifetime equations of two groups of samples are then combined to obtain a set of equations. Step 4: Use MATLAB software to solve the nonlinear life equations from Step 3 to obtain the life index of the cable insulation sample.

2. The method for determining the insulation life index of a cable according to claim 1, characterized in that, The starting voltage of the step-by-step breakdown test in step one is selected as 40% of the short-time breakdown voltage of the sample.

3. The method for determining the insulation life index of a cable according to claim 1, characterized in that, The interstage boost rate in step one is selected from 1, 2, 5, 10, 20, 50, and 100 V / s.

4. The method for determining the insulation life index of a cable according to claim 1, characterized in that, The method for merging the lifetime equations of a group of samples with the same pressurization time in step three is as follows: The lifetime equation for a single sample within a group is denoted as: , ; The combined lifetime equations for the same group of samples are then: ; In the formula, Number of samples It is an integer. For the first in the group The cumulative amount of electrical aging of each sample.

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