CN115902546A - Cable insulation life index measuring method - Google Patents
Cable insulation life index measuring method Download PDFInfo
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- CN115902546A CN115902546A CN202211438085.2A CN202211438085A CN115902546A CN 115902546 A CN115902546 A CN 115902546A CN 202211438085 A CN202211438085 A CN 202211438085A CN 115902546 A CN115902546 A CN 115902546A
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Abstract
The invention discloses a method for measuring the insulation life index of a cable, which comprises the following steps: selecting the initial voltage and the interstage boosting rate of the step-by-step breakdown test, and selecting two groups of cable insulation samples to respectively perform the step-by-step breakdown test with different step-by-step boosting time according to a certain boosting proportion; step two: describing an interstage boosting electrical aging process by applying calculus and inverse power law, and respectively establishing a nonlinear life equation based on the inverse power law and electrical aging cumulative effect according to stage-by-stage breakdown test data of each sample; step three: considering that the step-by-step breakdown test has randomness, averaging the electrical aging accumulation quantities of a group of samples with the same pressurization time in each step, combining the life equations of the same group of samples into an equation, and respectively combining the life equations of the two groups of samples to obtain an equation set; step four: and solving a nonlinear life equation set in the third step by using matlab software to obtain the life index of the cable insulation sample.
Description
Technical Field
The invention relates to the technical field of solid insulation electrical life evaluation, in particular to a cable insulation life index measuring method.
Background
The construction of new energy and high-voltage AC/DC power transmission system in China is developed vigorously. As a key device of a high-voltage transmission system, the production and application of high-voltage and ultra-high-voltage power cables are rapidly increased, for example, high-voltage flexible direct-current cables have been widely used in long-distance offshore transmission, urban power grids, new energy access and the like. However, the insulation material of high voltage, especially ultra high voltage power cable is still monopolized by foreign suppliers, and in recent years, domestic manufacturers are increasing research and development investments. Because the electrical stress is a key aging factor of the insulation of the high-voltage power cable, the electrical life evaluation is very important for the research and development of cable insulation materials and the design of an insulation structure, and is also one of important means for evaluating the long-term operation reliability of the high-voltage power cable.
Cable insulation electrical lifetime assessment typically employs a lifetime model based on the inverse power law, where the lifetime index is a key parameter of the model. Knowing the lifetime index, the lifetime equation can be determined by rapidly measuring the time to failure by increasing the voltage. The direct method for measuring the life index is based on a constant voltage electrical aging test under different voltages, and is obtained by performing linear fitting on voltage and failure time data on a log-log coordinate. The method is accurate and reliable, but the test workload is large and time-consuming because at least 4-5 voltages are selected for electrical aging and the failure time under low-voltage aging is often too long.
In order to improve the testing efficiency and reduce the testing cost, a step-by-step boosting approximate simplification method is proposed for accelerating the determination of the life index. The method constructs a deformed inverse power life equation set based on two groups of step-by-step breakdown tests, and obtains a life index approximation formula by solving the equation set. However, since the interstage boosting usually adopts a step boosting mode, the steep rising edge of the voltage causes the space charge injection and migration characteristics to change, and the cable insulation is likely to fail prematurely; the electrical aging mechanism is also changed, so that the inverse power law is not applicable any more; in addition, the aging effect of the boosting process and the influence of the boosting rate are not reflected in the life equation. Finally, when calculating according to the life index formula, the last stage pressurization duration needs to be converted into the voltage application stage number, which is not strict, and the formula itself is also an approximate analytic solution of a nonlinear life equation, so that the final result is not accurate enough. Therefore, it is necessary to improve the step-by-step boosting approximate simplification method, and to provide a more scientific and accurate method for rapidly determining the lifetime index.
Disclosure of Invention
In order to overcome the defects in the prior art, the invention provides the method for measuring the cable insulation life index, which can enable the test process to be more scientific and reasonable, quickly and accurately obtain the measurement result and provide support for cable insulation development and reliability evaluation.
In order to achieve the above purpose, the technical solution for solving the technical problem is as follows:
a cable insulation life index measuring method comprises the following steps:
the method comprises the following steps: selecting the initial voltage and the interstage boosting rate of the step-by-step breakdown test, and selecting two groups of cable insulation samples to respectively carry out the step-by-step breakdown tests with different step-by-step boosting time according to a certain boosting proportion;
step two: describing an interstage boosting electrical aging process by applying calculus and inverse power law, and respectively establishing a nonlinear life equation based on the inverse power law and electrical aging cumulative effect according to stage-by-stage breakdown test data of each sample;
step three: considering that the step-by-step breakdown test has randomness, averaging the electrical aging accumulated quantity of a group of samples with the same pressurizing time of each step, combining the life equations of the samples in the same group into an equation, and respectively combining the life equations of the two groups of samples to obtain an equation group;
step four: and solving a nonlinear life equation set in the third step by using matlab software to obtain the life index of the cable insulation sample.
Preferably, the starting voltage of the step-by-step breakdown test in the step one is selected to be 40% of the short-time breakdown voltage of the sample.
Preferably, the interstage boost rate in step one is selected from 1, 2, 5, 10, 20, 50, 100V/s.
Further, if the sample breaks down during the inter-stage boosting, the nonlinear lifetime equation in step two is:
when the P =1, the signal is transmitted,
when the ratio of P =2,
when the P is more than or equal to 3,
in the formula u 0 The initial voltage is good, the interstage boosting rate is good, P is the voltage application stage number, T is the constant pressurization time of each stage, delta T is the boosting duration of the last stage, n is a service life index, c is a constant, and i is an integer;
if the sample breaks down in the constant pressurization process, the nonlinear life equation in the second step is as follows:
when the ratio of P =1,
when the P is more than or equal to 2,
where Δ T is the last stage constant pressurization duration.
Further, the method for combining the life equations of the set of samples with the same pressurization time of each stage in the third step is as follows:
the lifetime equation for a single sample in the group is recorded as: l is j (n)＝c，j∈[1，m]；
Then the life equation of the same group of samples is combined as:
wherein m is the number of samples, j is an integer, L j And (n) is the cumulative amount of electrical aging of the jth sample in the group.
Due to the adoption of the technical scheme, compared with the prior art, the invention has the following advantages and positive effects:
the invention adopts the slow interstage boosting rate to carry out the step-by-step breakdown test, avoids the defect of premature failure of cable insulation caused by changing space charge injection and migration characteristics along the steep rising edge of voltage, ensures that the mechanism of step-by-step boosting electrical aging and constant voltage electrical aging is consistent, and ensures the applicability of the inverse power law. By applying calculus and inverse power law to the interstage boosting process neglected by the traditional step-by-step boosting method, the constructed life equation can reflect the aging effect of the boosting process and the influence of the boosting rate. In addition, the service life index obtained by solving the nonlinear service life equation set by using matlab software is more accurate than the rough approximate value obtained by solving a step-by-step boosting approximate simplification method. The invention provides a more scientific and accurate method for rapidly measuring the insulation life index of a cable.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. It is obvious that the drawings in the following description are only some embodiments of the invention, and that for a person skilled in the art, other drawings can be derived from them without inventive effort. In the drawings:
fig. 1 is a flowchart illustrating steps of a method for measuring an insulation life index of a cable according to an embodiment of the present invention;
fig. 2 is a schematic diagram of slow step-by-step boosting according to an embodiment of the present invention.
Detailed Description
The technical solutions of the present invention will be described clearly and completely with reference to the accompanying drawings, and it should be understood that the described embodiments are some, but not all embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.
Referring to fig. 1, the present embodiment provides a method for determining an insulation life index of a cable, including the following steps:
the method comprises the following steps: selecting the initial voltage and the interstage boosting rate of the step-by-step breakdown test, and selecting two groups of cable insulation samples to respectively perform the step-by-step breakdown test with different step-by-step boosting time according to a certain boosting proportion;
in the embodiment, the cable insulation sample is an insulation test piece with the thickness of 0.5mm, and the short-time breakdown voltage of the sample is 37.5kV; the initial voltage of each stage of breakdown test is 40% of the short-time breakdown voltage of the sample, namely 15kV, the boosting proportion is 1.06, the constant pressurizing time of each stage is 1min and 20min respectively, the interstage boosting rate is selected from 1, 2, 5, 10, 20, 50 and 100V/s, and the interstage boosting rate is preferably 100V/s.
Step two: describing an interstage boosting electrical aging process by applying calculus and inverse power law, and respectively establishing a nonlinear life equation based on the inverse power law and electrical aging cumulative effect according to stage-by-stage breakdown test data of each sample;
specifically, if the sample breaks down during the inter-stage boosting, the nonlinear lifetime equation in step two is:
when the ratio of P =1,
when the ratio of the total weight of the powder P =2,
when P is more than or equal to 3,
in the formula u 0 Is the starting voltage, k is the interstage boosting rate, P is the voltage application progression, T is the constant pressurization time of each stage, Δ T is the last stage boosting duration, n is the life index, c is a constant, f is an integer;
if the sample breaks down in the constant pressurization process, the nonlinear life equation in the second step is as follows:
when P =1，
When P is more than or equal to 2,
where Δ T is the last stage constant pressurization duration.
In this embodiment, u 0 In the specification of =15kV, q =1.06, k =0.1kV/s, two groups of samples (10 samples in each group) are selected and respectively subjected to T 1 =60s and T 2 Step-by-step breakdown test of =1200s, test result will produce corresponding voltage application step number P 1 And P 2 And Δ t 1 (or. DELTA.T) 1 ) And Δ t 2 (or. DELTA.T) 2 ). The test results are shown in table 1:
TABLE 1 progressive breakdown test results
And substituting the step-by-step breakdown test result of each sample into the nonlinear life equation in the second step to establish 20 life equations.
Step three: considering that the step-by-step breakdown test has randomness, averaging the electrical aging accumulated quantity of a group of samples with the same pressurizing time of each step, combining the life equations of the samples in the same group into an equation, and respectively combining the life equations of the two groups of samples to obtain an equation group;
specifically, the method for combining the life equations of the set of samples with the same pressurization time of each stage in the third step is as follows:
the lifetime equation for a single sample in the group is recorded as: l is a radical of an alcohol j (n)＝c，j∈[1，m]；
Then the life equation of the same group of samples is combined as:
wherein m is the number of samples, j is an integer, L j And (n) is the cumulative amount of electrical aging of the jth sample in the group.
In the present embodiment, the number m of samples in each group is 10, and 20 lifetime equations are finally combined into a nonlinear equation set containing 2 lifetime equations.
Step four: and solving a nonlinear life equation set in the third step by using matlab software to obtain the life index of the cable insulation sample.
In the embodiment, the numerical solution of the nonlinear life equation set is solved by matlab software, the life index of the cable insulation sample is 10.8, and the measurement result of the life index is reasonable, so that the method is proved to be effective.
The invention adopts the slow interstage boosting rate to carry out the step-by-step breakdown test, avoids the defect that the cable insulation fails too early due to the change of space charge injection and migration characteristics on the steep rising edge of the voltage, ensures that the mechanisms of step-by-step boosting electrical aging and constant voltage electrical aging are consistent, and ensures the applicability of the inverse power law. By applying calculus and inverse power law to the interstage boosting process ignored by the traditional step-by-step boosting method, the constructed life equation can reflect the aging effect and the influence of the boosting rate in the boosting process. In addition, the service life index obtained by solving the nonlinear service life equation set by using matlab software is more accurate than the rough approximate value obtained by solving a step-by-step boosting approximate simplification method. The invention provides a more scientific and accurate method for rapidly measuring the insulation life index of a cable.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (5)
1. A cable insulation life index measuring method is characterized by comprising the following steps:
the method comprises the following steps: selecting the initial voltage and the interstage boosting rate of the step-by-step breakdown test, and selecting two groups of cable insulation samples to respectively perform the step-by-step breakdown test with different step-by-step boosting time according to a certain boosting proportion;
step two: describing an interstage boosting electrical aging process by applying calculus and inverse power law, and respectively establishing a nonlinear life equation based on the inverse power law and electrical aging cumulative effect according to stage-by-stage breakdown test data of each sample;
step three: considering that the step-by-step breakdown test has randomness, averaging the electrical aging accumulated quantity of a group of samples with the same pressurizing time of each step, combining the life equations of the samples in the same group into an equation, and respectively combining the life equations of the two groups of samples to obtain an equation group;
step four: and solving a nonlinear life equation set in the third step by using matlab software to obtain the life index of the cable insulation sample.
2. The method as claimed in claim 1, wherein the initial voltage of the step-by-step breakdown test in the first step is 40% of the short-time breakdown voltage of the sample.
3. The method as claimed in claim 1, wherein the step-up rate in step one is selected from 1, 2, 5, 10, 20, 50 and 100V/s.
4. The method for determining the insulation life index of the cable according to claim 1, wherein if the sample breaks down during the inter-stage boosting, the nonlinear life equation in the second step is as follows:
when the P =1, the signal is transmitted,
when the ratio of the total weight of the powder P =2,
when P is more than or equal to 3,
in the formula u 0 Is the initial voltage, k is the interstage boost rate, P is the voltage application stage number, T is the constant boost time of each stage, Δ T is the last stage boost duration, n is the life index, c is a constant, i is an integer;
if the sample breaks down in the constant pressurization process, the nonlinear life equation in the second step is as follows:
when the P =1, the signal is transmitted,
when the P is more than or equal to 2,
where Δ T is the last stage constant pressurization duration.
5. The method for determining the insulation life index of the cable according to claim 1, wherein the life equation of the group of samples with the same pressing time at each stage in the third step is combined as follows:
the lifetime equation for a single sample in the group is recorded as: l is j (n)＝c，j∈[1，m]；
Then the life equation of the same group of samples is combined as:
in which m is testNumber of samples, j is an integer, L j And (n) is the cumulative amount of electrical aging of the jth sample in the group.
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