Method and device for diagnosing local aging state of cable based on PDC (Polycrystalline Diamond Compact) branch parameter method

Abstract

The invention relates to the technical field of cable insulation state detection, in particular to a PDC branch parameter method-based cable local aging state diagnosis method and device, and the device comprises a PDC test unit which carries out the PDC test of a to-be-tested cable, and obtains a polarization / depolarization current curve of the to-be-tested cable; the conductivity calculation unit is used for subtracting the steady-state value of the depolarization current curve from the steady-state value of the polarization current curve to obtain a conducting current in a steady state, and calculating a direct-current conductivity gamma according to the conducting current in the steady state; the asymmetry coefficient calculation unit is used for calculating an asymmetry coefficient; according to the method, the direct current conductivity change calculated by continuous multiple PDC tests is taken as a criterion, so that water tree aging, thermal aging and other general defects can be accurately identified; and meanwhile, the judgment on the local aging degree of the cable is realized by calculating the asymmetry coefficient, and the local aging type identification and the aging degree diagnosis are synchronously realized through the quantifiable diagnosis numerical value.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of cable insulation state detection, and in particular to a cable partial aging state diagnosis method and device based on a PDC branch parameter method. BACKGROUND

[0002] As an important power transmission and distribution equipment in the power system, the operation stability of the cable is directly related to the safety and reliability of the entire power system. However, due to non-standard laying, harsh operating environment, and external mechanical stress and other factors, the cable will often have partial defects such as outer sheath damage, water infiltration, copper shielding layer damage, and insulation aging. These defects will cause the cable partial insulation performance to decline and gradually develop into a breakdown failure under the action of a continuous electric field, thereby causing a power system accident.

[0003] With the increase of the cable operation time, the main insulation will generally have typical water tree aging and thermal aging defects, Figure 10 The existing cable water tree aging device is shown in the figure. The aging defects are very serious for the normal operation of the cable, and if the aging degree cannot be identified and evaluated in time and then countermeasures are taken, the risk of cable body breakdown will be greatly increased, thereby inducing a serious power accident. Therefore, the insulation state of the cable body needs to be accurately diagnosed and evaluated in engineering. The polarization / depolarization current (PDC) test method (such as Figure 11 The existing polarization / depolarization current (PDC) method is shown in the figure of the cable insulation test system schematic diagram) can obtain the time domain current response of the insulation medium, extract the equivalent model parameters, and then reflect the aging characteristics of the cable insulation.

[0004] Chinese invention patent CN201410691172.8 proposes to measure the polarization current by step voltage excitation, and to obtain the low-frequency dielectric loss spectrum by using the extended Debye model fitting and frequency domain conversion to evaluate the relative aging degree, but the determination of the index threshold under different field conditions still needs experience calibration.

[0005] Chinese invention patent CN201710611312.X proposes to calculate the "aging factor A" as a quantitative index through offline depolarization current test, which is simple and sensitive, but its applicability is limited to overall aging evaluation, but it cannot further reflect the severity of the local defects.

[0006] Chinese invention patent CN202010635703.7 proposes to calculate the change trend of the "conductivity current coefficient" through continuous PDC test to identify water tree aging, which has a targeted advantage for water tree detection, but its method is effective for a specific aging mode, and the applicability is limited for other aging types and the like.

[0007] The Chinese invention patent CN201911089429.1 proposes to use the change amount of low-frequency dielectric loss factor under different polarization voltages as the aging characteristic, which has high sensitivity and can reflect the overall insulation aging. However, it does not provide explicit threshold values or standardized discrimination processes to support engineering decisions.

[0008] As can be seen from the above disclosure, the existing methods mainly focus on the aging degree evaluation of the overall cable or the identification of specific aging types. Although they have certain effects on the test methods and index selection, they still have the following limitations: 1. Some methods lack unified and standardized discrimination criteria, and have insufficient field applicability. Most of the methods are only for water trees, and have insufficient judgment ability for other general aging types such as thermal aging; 2. The existing methods often focus on the trend judgment of the overall cable aging, and it is difficult to reflect the severity of local defects; 3. Some methods have not established a mature criterion system, and in certain cases, they still need to be tested and judged by experience, which has limited scope of application in actual cable operation and maintenance. SUMMARY

[0009] The present application provides a cable local aging state diagnosis method and device based on PDC branch parameter method, which overcomes the shortcomings of the prior art. It can effectively solve the problem that the existing cable local aging judgment method cannot judge the severity of cable local aging defects.

[0010] To solve the above problems, one of the technical solutions of the present application is realized by the following way: a cable local aging state diagnosis method based on PDC branch parameter method, comprising the following steps: performing PDC test on the cable to be tested to obtain the polarization / depolarization current curve of the cable to be tested; Subtracting the steady-state value of the depolarization current curve from the steady-state value of the polarization current curve to obtain the conductive current at steady state, and calculating the direct current conductivity from the conductive current at steady state ; Substituting the polarization / depolarization current curve into the Debye three-branch equivalent circuit model to fit the polarization third-branch time constant and the depolarization third-branch time constant , and calculating the asymmetry coefficient based on the polarization third-branch time constant and the depolarization third-branch time constant ; Calculating the direct current conductivity increment of the direct current conductivity of the two consecutive times, and judging whether the direct current conductivity increment is significantly changed or greater than zero; Response is, then, the DC conductivity increases with the increase of the number of tests, determine the presence of water tree aging inside the cable to be tested; Response is not, then, the DC conductivity increment The change is not significant, or equal to zero, indicating that the change of DC conductivity is not big, determine the presence of aging or the presence of thermal aging or the presence of other types of aging in the cable to be tested; After determining that the cable to be tested has water tree aging, the asymmetry coefficient is calculated , determine whether the asymmetry coefficient is greater than 1.3 and less than 1.6; Response is, then, determine the mild water tree aging, the risk of breakdown is small; Response is not, then, determine whether the asymmetry coefficient Is greater than 1.6 and less than 2, response is, then, determine the moderate water tree aging, the risk of breakdown is moderate; Response is not, then, the asymmetry coefficient Greater than 2, determine the severe water tree aging, the risk of breakdown is big.

[0011] The above PDC test is carried out on the cable to be tested, and the polarization / depolarization current curve of the cable to be tested is obtained, including: The outer surface of the cable sample to be tested is cleaned and dried to reduce experimental error; The core of the first section of the cable to be tested is connected to the high voltage end of the PDC tester, and the middle and both ends are wrapped with copper shielding tape to ground to shield the surface leakage current; A DC voltage U is applied to the cable for polarization, and the polarization time is At the same time, the curve of the polarization current changing with time is recorded; After the DC voltage is turned off, the depolarization test is carried out, and the depolarization time is At the same time, the decay curve of the depolarization current with time is collected.

[0012] The steady-state value of the polarization current curve minus the steady-state value of the depolarization current curve to obtain the conductive current at steady state, and the DC conductivity is calculated according to the conductive current at steady state , including: The formula for calculating the DC conductivity is: , Among them, The DC conductivity, The conductive current at steady state, The applied DC voltage.

[0013] The polarization / depolarization current curve is substituted into the Debye three-branch equivalent circuit model to fit the polarization third branch time constant And the depolarization third branch time constant , based on the polarization third branch time constant and depolarization third branch time constant the asymmetry coefficient is calculated , comprising: the polarization / depolarization current of the cable to be tested in the PDC test is represented by a direct current resistance branch and three parallel RC branches; polarization current the expression is: , depolarization current the expression is: , in the formula, is the current amplitude of the polarization RC branch, ; is the current amplitude of the depolarization RC branch, ; is the amplitude of the conductive current component; is the equivalent resistance of the polarization first branch; i is the equivalent resistance of the polarization second branch; is the equivalent resistance of the depolarization first branch; i is the equivalent resistance of the depolarization second branch; is the polarization time; is the depolarization time; is the polarization time constant of each branch, ; is the depolarization time constant of each branch, ; i denotes each branch; is the equivalent capacitance of the polarization first branch; i is the equivalent capacitance of the polarization second branch; is the equivalent capacitance of the depolarization first branch; i is the equivalent capacitance of the depolarization second branch; U is the direct current voltage applied at the high voltage end of the PDC; asymmetry coefficient the calculation formula of the asymmetry coefficient is: , in the formula, is the polarization third branch time constant, is the depolarization third branch time constant.

[0014] the above calculation continuously twice the direct current conductivity the direct current conductivity increment of the direct current conductivity , comprising, the calculation method is as follows: , in the formula, is the change of the direct current conductivity, , the first n +1 and the firstn direct current conductivity of the secondary test, n =1,2,…

[0015] After judging that the to-be-tested cable does not exist aging or exists thermal aging or exists other types of aging, the asymmetry coefficient is calculated , and it is judged whether the asymmetry coefficient is greater than 1.0 and less than 1.1; In response to yes, it is determined that the to-be-tested cable is in slight thermal aging, and the risk of breakdown is very small; In response to no, it is judged whether the asymmetry coefficient is greater than 1.1 and less than 1.2, in response to yes, it is determined that the to-be-tested cable is in moderate thermal aging, and the risk of breakdown is small; in response to no, the asymmetry coefficient is greater than 1.2, it is determined that the to-be-tested cable is in severe thermal aging, and the risk of breakdown rises.

[0016] The second technical scheme of the present application is realized by the following way: a cable local aging state diagnosis device based on a PDC branch parameter method, which uses a cable local aging state diagnosis method based on a PDC branch parameter method to realize, comprising: a PDC test unit, which performs PDC test on a to-be-tested cable to obtain a polarization / depolarization current curve of the to-be-tested cable; a conductivity calculation unit, which obtains a steady-state conductivity current by subtracting a steady-state value of the depolarization current curve from a steady-state value of the polarization current curve, and calculates a direct current conductivity based on the steady-state conductivity current; an asymmetry coefficient calculation unit, which substitutes the polarization / depolarization current curve into a Debye three-branch equivalent circuit model to fit to obtain a polarization third-branch time constant and a depolarization third-branch time constant , and calculates an asymmetry coefficient based on the polarization third-branch time constant and the depolarization third-branch time constant ; an aging type judgment unit, which calculates a direct current conductivity increment of the direct current conductivity of two consecutive times, and judges whether the direct current conductivity increment is significantly changed or greater than zero; a first response unit, which responds to yes, that is, the direct current conductivity increases with the increase of the test times, and determines that there exists water tree aging inside the to-be-tested cable; a second response unit, which responds to no, that is, the direct current conductivity increment is not significantly changed or equal to zero, which indicates that the direct current conductivity is not greatly changed, and determines that the to-be-tested cable does not exist aging or exists thermal aging or exists other types of aging; The aging type judging unit calculates the asymmetry coefficient after judging that the cable under test has water tree aging , and judges whether the asymmetry coefficient is greater than 1.3 and less than 1.6; The third response unit judges that the water tree aging is slight and the risk of breakdown is small in response to yes. The fourth response unit judges whether the asymmetry coefficient is greater than 1.6 and less than 2 in response to no, judges that the water tree aging is moderate and the risk of breakdown is medium in response to yes, and judges that the asymmetry coefficient is greater than 2 in response to no, judges that the water tree aging is severe and the risk of breakdown is great.

[0017] The PDC testing unit comprises a processing module, a connecting module, a polarization testing module and a depolarization testing module. The processing module cleans and dries the outer surface of the cable sample under test, so as to reduce experimental errors. The connecting module connects the core of the first section of the cable under test with the high-voltage end of the PDC tester, and winds copper shielding tape around the middle and both ends to ground, so as to shield the surface leakage current. The polarization testing module applies a direct current voltage U to the cable to polarize, and the polarization time is , and records the curve of the polarization current changing with time. The depolarization testing module carries out depolarization test after the direct current voltage is turned off, and the depolarization time is , and collects the decay curve of the depolarization current with time.

[0018] Compared with the prior art, the present application has the following advantages: 1. The present application uses the direct current conductivity change as the preliminary local aging type judging basis, realizes the preliminary accurate identification of water tree aging and other general defects such as thermal aging, and can further determine the local aging degree by calculating the asymmetry coefficient, so that the present application realizes the local aging type identification and aging degree diagnosis simultaneously through the quantifiable diagnosis value.

[0019] 2. The present application establishes a clear judgment threshold system, overcomes the shortcomings of the prior art which relies on experience judgment, and has strong reliability and engineering practicability. BRIEF DESCRIPTION OF DRAWINGS

[0020] The specific embodiments of the present application will be further described in detail below with reference to the accompanying drawings.

[0021] Figure 1 The method flowchart of the present application embodiment 1.

[0022] Figure 2 The aging cable three-branch Debye model in the present application embodiment 1. ​

[0023] Figure 3 This is a block diagram of the device structure in Embodiment 2 of the present invention.

[0024] Figure 4 This is a block diagram of the device structure of the PDC test unit in Embodiment 2 of the present invention.

[0025] Figure 5 This is the PDC polarization current curve in Embodiment 3 of the present invention.

[0026] Figure 6 This is the PDC depolarization current curve in Embodiment 3 of the present invention.

[0027] Figure 7 This is a schematic diagram of the thermally aged cable in Embodiment 3 of the present invention.

[0028] Figure 8 This is the PDC polarization current curve in Embodiment 4 of the present invention.

[0029] Figure 9 This is the PDC depolarization current curve in Embodiment 4 of the present invention.

[0030] Figure 10 This is a schematic diagram of a water tree aging cable in the background art of this invention.

[0031] Figure 11 This is a schematic diagram of PDC testing in the background technology of this invention. Detailed Implementation

[0032] The present invention is not limited to the following embodiments, and the specific implementation can be determined according to the technical solution of the present invention and the actual situation.

[0033] Example 1: As Figure 1 As shown in the figure, this invention discloses a method for diagnosing the local aging condition of cables based on the PDC branch parameter method, including the following steps: S101, perform PDC test on the cable under test to obtain the polarization / depolarization current curve of the cable under test; S102, subtract the steady-state value of the depolarization current curve from the steady-state value of the polarization current curve to obtain the steady-state conduction current, and calculate the DC conductivity based on the steady-state conduction current. ; S103, Substitute the polarization / depolarization current curves into the Debye three-branch equivalent circuit model to obtain the time constant of the third polarization branch. and the time constant of the third branch of depolarization Based on the time constant of the third polarization branch and the time constant of the third branch of depolarization The asymmetry coefficient was calculated. ; S104, calculating the direct current conductivity of the two consecutive times the direct current conductivity increment , determining whether the direct current conductivity increment is significantly changed or greater than zero; S105, in response to yes, that is, the direct current conductivity increases with the increase of the test times, determining that there is water treeing aging in the cable to be tested; S106, in response to no, that is, the direct current conductivity increment is not significantly changed or equal to zero, indicating that the direct current conductivity is not changed much, determining that there is no aging or there is thermal aging or there is other type of aging in the cable to be tested; S107, after determining that there is water treeing aging in the cable to be tested, calculating the asymmetry coefficient , determining whether the asymmetry coefficient is greater than 1.3 and less than 1.6; S108, in response to yes, determining that it is mild water treeing aging and the risk of breakdown is small; S109, determining whether the asymmetry coefficient is greater than 1.6 and less than 2, in response to yes, determining that it is moderate water treeing aging and the risk of breakdown is medium; in response to no, determining that the asymmetry coefficient is greater than 2, determining that it is severe water treeing aging and the risk of breakdown is large.

[0034] In the above step S101, the PDC test is performed on the cable to be tested to obtain the polarization / depolarization current curve of the cable to be tested, including: cleaning and drying the outer surface of the cable sample to be tested to reduce experimental errors; connecting the core of the first section of the cable to be tested to the high-voltage end of the PDC tester, winding copper shielding tape around the middle and both ends to ground to shield the surface leakage current; applying a direct current voltage U to the cable for polarization, and the polarization time is , and recording the curve of the polarization current changing with time; performing depolarization test after the direct current voltage is turned off, and the depolarization time is , and collecting the decay curve of the depolarization current with time.

[0035] Among them, the outer surface of the cable sample to be tested is cleaned to remove dust, dirt and oil stains; the cable surface is kept dry as much as possible to avoid the interference of the leakage current in the humid environment on the test results.

[0036] Among them, the direct current voltage U applied to the cable can be a direct current voltage of 1-5kV; when performing polarization-depolarization current (PDC) test, the cable to be tested needs to be tested for multiple times (3 times or more) after the cable stops running for a period of time; the polarization / depolarization time t is controlled to be more than 90s.

[0037] The PDC test was conducted after the cable had been out of service for a period of time (more than 20 minutes) because the relaxation time of the XLPE insulation material is approximately 1084 seconds (about 18 minutes). When the downtime exceeds the relaxation time, the internal polarization of the material tends to stabilize, and the water tree channels are in a closed state. When the PDC test is started at this time, during the polarization process, the water tree channels gradually reopen under the action of the applied electric field, and the water molecules in the channels reorient and participate in the conduction process. If multiple PDC tests are performed continuously and the corresponding DC conductivity is calculated, a gradual increasing trend of DC conductivity can be observed.

[0038] In step S102 above, the steady-state conduction current is obtained by subtracting the steady-state value of the depolarization current curve from the steady-state value of the polarization current curve, and the DC conductivity is calculated based on the steady-state conduction current. ,include: The formula for calculating DC conductivity is: , in, DC conductivity The conduction current in steady state The applied DC voltage.

[0039] In step S103 above, the polarization / depolarization current curves are substituted into the Debye three-branch equivalent circuit model to obtain the time constant of the third polarization branch. and the time constant of the third branch of depolarization Based on the time constant of the third polarization branch and the time constant of the third branch of depolarization The asymmetry coefficient was calculated. ,include: The polarization / depolarization current of the cable under test during PDC testing is represented by a DC resistance branch plus three parallel RC branches; polarization current The expression is: , Depolarization current The expression is: , In the formula, To polarize the current amplitude of each RC branch, ; To depolarize the current amplitude of each RC branch, ; The magnitude of the conduction current component; For polarization iThe equivalent resistance of the branch; The polarization time of the first branch; i The equivalent resistance of the branch; The polarization time of the second branch; The depolarization time of the first branch; The polarization time constant of each branch, ; The depolarization time constant of each branch, ; i Indicates each branch; The polarization time of the third branch; i The equivalent capacitance of the branch; The depolarization time of the third branch; i The DC voltage applied to the high voltage end of the PDC; The calculation formula of the asymmetry coefficient is: U , , In the formula, The polarization time constant of the third branch, The depolarization time constant of the third branch.

[0040] Where, , , And The three branches determined by fitting are numbered as the first branch, the second branch and the third branch in order of time constant from small to large, and the polarization time constant and the depolarization time constant of each branch are obtained by fitting; At the same time, the cable to be tested is tested continuously for multiple times, and the change of DC conductivity in the test is recorded; The fitting equation of the polarization current curve is: , In the formula, The instantaneous current (unit: A) in the polarization stage; The time corresponding to the instantaneous current (unit: s); The steady-state conductance component; A, C, D are the polarization intensity coefficients of different polarization branches respectively; B, E, F are the time constants of the branches respectively; The largest among B, E and F is the polarization time constant of the third branch ; The fitting equation of the depolarization current curve is: , In the formula, The instantaneous current (unit: A) in the depolarization stage; ​is the time corresponding to the instantaneous current (unit: s); A, C, D are polarization intensity coefficients of different depolarization branches; B, E, F are time constants of the branches, of which the largest is the time constant of the third depolarization branch .

[0041] In the step S104, the DC conductivity increment of the DC conductivity is calculated, including the calculation method as follows: , In the formula, is the change of the DC conductivity, , are the DC conductivities of the first n +1 and the first n test, n =1, 2, ….

[0042] In the step S106, after it is judged that the cable to be tested does not exist aging or exists thermal aging or exists other types of aging, the asymmetry coefficient is calculated, and it is judged whether the asymmetry coefficient is greater than 1.0 and less than 1.1; In response to yes, it is determined that the cable is in mild thermal aging, and the risk of breakdown is very small; In response to no, it is judged whether the asymmetry coefficient is greater than 1.1 and less than 1.2, in response to yes, it is determined that the cable is in moderate thermal aging, and the risk of breakdown is small; in response to no, the asymmetry coefficient is greater than 1.2, it is determined that the cable is in severe thermal aging, and the risk of breakdown rises.

[0043] In the step S106, whether the DC conductivity increment changes significantly includes that the DC conductivity increment changes within ±10% or remains zero; it is indicated that the change of the conductivity is not large, which means that the cable is not aged or exists thermal aging or other types of aging defects. Thermal aging or other general types of aging defects are usually caused by long-term exposure of the cable to high temperature or other environments, resulting in degradation of the insulation material, and the change of the conductivity is not obvious or small, because the aging process mainly affects the change of the molecular chain rupture of the material.

[0044] In step S106, water tree aging and other defects such as thermal aging are classified separately because water tree aging defects are caused by moisture gradually seeping into the insulating material along the micro-defects or impurities under the action of an electric field, forming tiny dendritic conductive channels containing a large amount of microscale moisture and ions. When the applied electric field is removed, these channels will "close" within a certain period of time, and when the pressure is applied and polarized again, the closed channels will gradually reopen, thus exhibiting significant conductivity fluctuations. In contrast, other general types of defects such as thermal aging mainly manifest as material chain segment breakage, oxidative cross-linking, and accumulation of crystalline defects. They do not form obvious through-conductive channels, and charge conduction mainly relies on the bulk conductivity mechanism. Therefore, their DC conductivity changes little in multiple tests, exhibiting high stability and not showing conductivity fluctuations similar to those caused by the opening and closing of water tree channels.

[0045] In this invention, the time constant of the third branch of depolarization is selected as the main criterion because, in the equivalent analysis of the Debye three-branch model, the first and second branches mainly correspond to short-time polarization and medium-time polarization processes, and are less affected by aging; while the local aging of the cable is a long-term process, so the third branch can characterize the aging state of the insulation; as the degree of aging increases, the resistance and capacitance parameters of the third branch change significantly; therefore, the characteristics of the time constant of the third branch can be used as a diagnostic indicator to characterize the aging of cable insulation.

[0046] In summary, this invention uses the change in DC conductivity calculated from multiple consecutive PDC tests as a criterion to accurately identify water treeing, thermal aging, and other general defects. At the same time, by calculating the asymmetry coefficient, it can determine the degree of local aging of the cable. This invention achieves simultaneous identification of local aging types and diagnosis of aging degree through quantifiable diagnostic values.

[0047] Furthermore, this invention establishes a clear judgment threshold system, overcoming the shortcomings of existing methods that rely on experience-based judgment, and has strong reliability and engineering applicability.

[0048] Example 2: As Figure 3 As shown, this invention discloses a cable local aging condition diagnosis device based on the PDC branch parameter method. The device enables the cable local aging condition diagnosis method based on the PDC branch parameter method, and includes: The PDC test unit performs PDC testing on the cable under test to obtain the polarization / depolarization current curve of the cable under test. The conductivity calculation unit obtains the steady-state conduction current by subtracting the steady-state value of the depolarization current curve from the steady-state value of the polarization current curve, and then calculates the DC conductivity based on the steady-state conduction current. ; The asymmetry coefficient calculation unit substitutes the polarization / depolarization current curve into the Debye three-branch equivalent circuit model to fit the polarization third-branch time constant and the depolarization third-branch time constant , calculates the asymmetry coefficient based on the polarization third-branch time constant and the depolarization third-branch time constant ; ; The aging type judgment unit calculates the direct current conductivity increment of the direct current conductivity of the cable sample in two consecutive times , judges whether the direct current conductivity increment is significantly changed or greater than zero ; The first response unit determines that there is water treeing in the cable to be tested in response to yes, that is, the direct current conductivity increases with the increase of the test times ; The aging degree judgment unit calculates the asymmetry coefficient after determining that there is water treeing in the cable to be tested, judges whether the asymmetry coefficient is greater than 1.3 and less than 1.6 ; The third response unit determines that there is mild water treeing and the risk of breakdown is small in response to yes ; The fourth response unit judges whether the asymmetry coefficient is greater than 1.6 and less than 2 in response to no, determines that there is moderate water treeing and the risk of breakdown is medium in response to yes, and determines that there is severe water treeing and the risk of breakdown is great in response to no

[0049] The PDC test unit includes a processing module, a connecting module, a polarization test module and a depolarization test module The processing module cleans and dries the outer surface of the cable sample to be tested to reduce experimental errors The connecting module connects the core of the first section of the cable to be tested to the high-voltage end of the PDC tester, and winds copper shielding tape around the middle and both ends to ground to shield the surface leakage current The polarization test module applies a direct current voltage U to the cable to polarize, and the polarization time is , and records the curve of the polarization current changing with time The depolarization test module performs depolarization test after the direct current voltage is turned off, and the depolarization time is , and collects the decay curve of the depolarization current with time

[0050] Example 3: To verify the effectiveness of the method of the present application, a 10 kV voltage grade three-phase cross-linked polyethylene insulated cable (model YJV22-10 kV 3x50, insulation thickness 4.5 mm) without defects was selected, and a cable sample with a length of 12 m was taken as the experimental object.

[0051] The 12 m three-phase cable was stripped into three single-phase samples.

[0052] The outer sheath, steel armor and inner sheath were sequentially removed at 6.6 m from the first end of the cable, and the exposed part was placed on the heating table. At the same time, a 3.9 kV power frequency voltage was applied to both ends of the cable, and the temperature of the heating table was kept constant at 80°C. The accelerated thermal aging defects were formed by continuous voltage application for 5 days. Figure 7 The structure diagram of the cable thermal aging acceleration test device is shown in Figure 1. The device is to place the XLPE cable to be tested on the heating table, heat the local position at a set temperature (such as 80°C), and apply an electric field through a voltage source, and a water resistance is connected in series in the circuit to ensure that the current is limited within a safe range even if insulation breakdown occurs during the experiment, avoiding harm to equipment and personnel. Through the aging under the combined action of heat and electricity, the degradation characteristics of the cable insulation material can be obtained in a short time.

[0053] An artificial water tree defect was made at 7.65 m from the first end of the other phase cable, with a hole depth of 3 mm, and a heat shrink tube was wrapped around the defect position, leaving a small opening for injection of saturated NaCl (1.8 mol / L) solution; At the same time, a 3.9 kV power frequency voltage was applied to the cable core, and the ground wire in the NaCI solution was grounded. The formation of water tree aging defects was accelerated by applying voltage for 5 days.

[0054] The third phase cable remained intact as a defect-free control sample.

[0055] The above three single-phase samples were tested by the above-mentioned method of the present application. The first end of the cable to be tested was connected to the high-voltage end of the PDC tester, and a 1 kV DC voltage U was applied to the cable through the high-voltage end of the PDC. The polarization time was 90 s, and the polarization current-time curve was recorded, as shown in Figure 2. Figure 5 After the polarization process was completed, the DC voltage was disconnected and the depolarization current was tested. The depolarization time was the same as the polarization time, which was set to 90 s, and the depolarization current-time decay curve was recorded, as shown in Figure 3. Figure 6

[0056] In the cable aging type identification process, the local aging type of the cable was identified by measuring the DC conductivity change multiple times. The DC conductivity statistical results are shown in Table 1, and the time constant and unbalance coefficient statistical results of the third branch are shown in Table 2.

[0057] ​​From Table 1, the DC conductivity of the unaged cable and the heat aged cable does not change significantly in multiple tests, while the DC conductivity of the water tree aged cable is continuously increasing in multiple tests. From Table 2, the asymmetry coefficient of the unaged cable is 1.01, the asymmetry coefficient of the heat aged cable is 1.12, and the asymmetry coefficient of the water tree aged cable is 1.15, which is within the interval of 1.05 to 1.15. , and the asymmetry coefficient of the water tree aged cable is 1.15, which is within the interval of 1.05 to 1.15. Through the analysis of the asymmetry coefficients of different aged cables, the severity of the aging defects can be quantitatively analyzed, which provides certain data support for the maintenance and repair of the line. Moreover, the method determines the threshold parameters of different aging types, which greatly improves the accuracy of the local aging type determination. , and the asymmetry coefficient of the water tree aged cable is 1.15, which is within the interval of 1.05 to 1.15. Through the analysis of the asymmetry coefficients of different aged cables, the severity of the aging defects can be quantitatively analyzed, which provides certain data support for the maintenance and repair of the line. Moreover, the method determines the threshold parameters of different aging types, which greatly improves the accuracy of the local aging type determination.

[0058] In order to verify the effectiveness of the method, a cable that was undergoing power outage maintenance in a substation was tested on site. The rated voltage level of the cable was 10 kV, the length was 1100 m, and the conductor cross-sectional size was 3 x 185 mm².

[0059] During the PDC test, the first end of the cable to be tested was connected to the high voltage end of the PDC tester. A 4 kV DC voltage was applied to the cable through the PDC high voltage end, and the polarization time was 90 s. The polarization current curve with time was recorded, as shown in Figure 1. Figure 8 After the polarization process was completed, the DC voltage was disconnected and the depolarization current test was performed. The depolarization time was the same as the polarization time, which was 90 s. The depolarization current decay curve with time was recorded, as shown in Figure 2. Figure 9

[0060] In the process of identifying the local aging type of the cable, the change of the DC conductivity in multiple measurements was used to identify the aging type of the cable. The DC conductivity statistical results of this embodiment are shown in Table 3, and the time constant and unbalance coefficient statistical results of the third branch are shown in Table 4.

[0061] From the DC conductivity statistical results obtained by multiple tests in Table 3, it can be seen that the DC conductivity of the three-phase cable after multiple tests does not change much, indicating that there is no water tree aging defect, and the aging defects of the three-phase cable are determined to be heat aging or other general defect types.

[0062] From Table 4, the asymmetry coefficient of the A-phase cable is 1.05, , the asymmetry coefficient of the B-phase cable is 1.06, , and the asymmetry coefficient of the C-phase cable is 1.05. ​The asymmetry coefficients of the three-phase cable are in the thermal aging interval, and the asymmetry coefficient of phase C is the largest. According to the aging degree judgment formula, the A-phase cable has mild thermal aging, and the B-phase and C-phase cables have moderate thermal aging. Therefore, in maintenance and repair, the operating state of the B-phase and C-phase cables should be focused on. In the three-phase cable, the local aging degree of the C-phase is also the most serious. Combined with the analysis of local staff and geographical environment, it is confirmed that the cable indeed only has general defects such as thermal aging, and does not have water treeing aging defects. It is proved that the method is effective. The method greatly improves the accuracy of judgment through the threshold parameters of different aging types and the aging degree judgment, and greatly improves the accuracy of judgment.

[0063] Table 1 DC conductivity of different local aging types of cables in Example 3

[0064] Table 2 Depolarization third branch time constant of different local aging types of cables in Example 3

[0065] Table 3 DC conductivity results of different phase cables in Example 4

[0066] Table 4 Polarization third branch time constant and asymmetry coefficient of different phase cables in Example 4

Claims

1. A method for diagnosing the local aging condition of cables based on the PDC branch parameter method, characterized in that, Includes the following steps: Perform PDC testing on the cable under test to obtain the polarization / depolarization current curve of the cable under test; The steady-state conductance current is obtained by subtracting the steady-state value of the depolarization current curve from the steady-state value of the polarization current curve. The DC conductivity is then calculated based on the steady-state conductance current. ; Substituting the polarization / depolarization current curves into the Debye three-branch equivalent circuit model, the time constant of the third polarization branch is obtained through fitting. and the time constant of the third branch of depolarization Based on the time constant of the third polarization branch and the time constant of the third branch of depolarization The asymmetry coefficient was calculated. ; Calculate DC conductivity in two consecutive tests DC conductivity increment Determine the increment of DC conductivity Whether the change is significant or greater than zero; Therefore, the DC conductivity increases with the number of tests, indicating that water treeing aging exists inside the cable under test. The increase in DC conductivity is determined by whether or not the response is positive. If the change is not significant or equal to zero, it indicates that the change in DC conductivity is not significant, and it is determined that the cable under test does not have aging, or has thermal aging or other types of aging. After determining that the cable under test has water tree aging, the asymmetry coefficient is calculated. Determine whether the asymmetry coefficient is greater than 1.3 and less than 1.6; The response indicates mild water tree aging, with a low risk of breakdown. If the response is negative, then determine the asymmetry coefficient. If the response is greater than 1.6 and less than 2, then it is determined to be moderate water tree aging, with a moderate risk of breakdown. The asymmetric coefficient depends on whether the response is positive or negative. A value greater than 2 indicates severe water tree aging and a high risk of breakdown.

2. The cable local aging condition diagnosis method based on PDC branch parameter method according to claim 1, characterized in that, The PDC test is performed on the cable under test to obtain the polarization / depolarization current curve of the cable under test, including: The outer surface of the cable sample to be tested is cleaned and dried to reduce experimental errors; Connect the core of the first section of the cable to be tested to the high-voltage end of the PDC tester, and wrap copper shielding tape around the middle and both ends to ground it in order to shield the surface leakage current. A DC voltage U is applied to the cable to polarize it, and the polarization time is... Meanwhile, the curve of polarization current changing with time was recorded; After shutting off the DC voltage, a depolarization test was performed. The depolarization time was [time missing]. Simultaneously, the depolarization current decay curve over time was collected.

3. The cable local aging condition diagnosis method based on PDC branch parameter method according to claim 2, characterized in that, The steady-state conduction current is obtained by subtracting the steady-state value of the depolarization current curve from the steady-state value of the polarization current curve. The DC conductivity is then calculated based on the steady-state conduction current. ,include: The formula for calculating DC conductivity is: ， in, DC conductivity The conduction current in steady state, The applied DC voltage.

4. The cable local aging condition diagnosis method based on PDC branch parameter method according to claim 2 or 3, characterized in that, The polarization / depolarization current curves are then substituted into the Debye three-branch equivalent circuit model to obtain the time constant of the third polarization branch. and the time constant of the third branch of depolarization Based on the time constant of the third polarization branch and the time constant of the third branch of depolarization The asymmetry coefficient was calculated. ,include: The polarization / depolarization current of the cable under test during PDC testing is represented by a DC resistance branch plus three parallel RC branches; polarization current The expression is: ， Depolarization current The expression is: ， In the formula, To polarize the current amplitude of each RC branch, ; To depolarize the current amplitude of each RC branch, ; The magnitude of the conduction current component; For polarization i The equivalent resistance of a branch; For depolarization i The equivalent resistance of a branch; Polarization time; The time for depolarization; Polarization time constants of each branch ; The depolarization time constant for each branch is... ; i Indicate each branch road; For polarization i The equivalent capacitance of the branch; For depolarization i The equivalent capacitance of the branch; U DC voltage applied to the high-voltage side of the PDC; asymmetry coefficient The calculation formula is: ， In the formula, The time constant of the third polarization branch. This is the time constant for the depolarization third branch.

5. The cable local aging condition diagnosis method based on PDC branch parameter method according to claim 3, characterized in that, The calculation of two consecutive DC conductivity values DC conductivity increment The calculation method is as follows: ， In the formula, For the change in DC conductivity, , They are the first n +1 and the n DC conductivity of this test n =1,2,…… 6. The cable local aging condition diagnosis method based on PDC branch parameter method according to claim 4, characterized in that, After determining that the cable under test does not have aging, or has thermal aging, or has other types of aging, the asymmetry coefficient is calculated. Determine whether the asymmetry coefficient is greater than 1.0 and less than 1.1; Therefore, the response indicates mild thermal aging, with a very low risk of breakdown. If the response is negative, then determine the asymmetry coefficient. If the response is greater than 1.1 and less than 1.2, then it is determined to be moderate thermal aging with a low risk of breakdown; if the response is not, then the asymmetry coefficient... A value greater than 1.2 indicates severe thermal aging, increasing the risk of breakdown.

7. A cable local aging condition diagnosis device based on the PDC branch parameter method, characterized in that, The device is implemented using the cable local aging condition diagnosis method based on the PDC branch parameter method as described in any one of claims 1 to 6, comprising: The PDC test unit performs PDC testing on the cable under test to obtain the polarization / depolarization current curve of the cable under test. The conductivity calculation unit obtains the steady-state conduction current by subtracting the steady-state value of the depolarization current curve from the steady-state value of the polarization current curve, and then calculates the DC conductivity based on the steady-state conduction current. ; The asymmetry coefficient calculation unit substitutes the polarization / depolarization current curves into the Debye three-branch equivalent circuit model and fits the time constant of the third polarization branch to obtain the result. and the time constant of the third branch of depolarization Based on the time constant of the third polarization branch and the time constant of the third branch of depolarization The asymmetry coefficient was calculated. ; The aging type determination unit calculates the DC conductivity of two consecutive tests. DC conductivity increment Determine the increment of DC conductivity Whether the change is significant or greater than zero; The first response unit responds by indicating that the DC conductivity increases with the number of tests, thus determining that water treeing aging exists inside the cable under test. The second response unit, in response to no, determines the increment of DC conductivity. If the change is not significant or equal to zero, it indicates that the change in DC conductivity is not significant, and it is determined that the cable under test does not have aging, or has thermal aging or other types of aging. The aging degree assessment unit calculates the asymmetry coefficient after determining that the cable under test exhibits water tree aging. Determine whether the asymmetry coefficient is greater than 1.3 and less than 1.6; The third response unit, upon responding, is then determined to be mild water tree aging with a low risk of breakdown. The fourth response unit, whether or not it responds, determines the asymmetry coefficient. If the response is greater than 1.6 and less than 2, then it is determined to be moderate water tree aging with a moderate risk of breakdown; if the response is not, then the asymmetry coefficient... A value greater than 2 indicates severe water tree aging and a high risk of breakdown.

8. The cable local aging condition diagnosis device based on the PDC branch parameter method according to claim 7, characterized in that, The PDC test unit includes a processing module, a connection module, a polarization test module, and a depolarization test module; The processing module cleans and dries the outer surface of the cable sample under test to reduce experimental errors; The connection module connects the cable core of the first section of the cable under test to the high-voltage end of the PDC tester, and wraps copper shielding tape around the middle and both ends to ground it in order to shield the surface leakage current. The polarization test module applies a DC voltage U to the cable to polarize it, and the polarization time is... Meanwhile, the curve of polarization current changing with time was recorded; The depolarization test module performs a depolarization test after the DC voltage is turned off. The depolarization time is... Simultaneously, the depolarization current decay curve over time was collected.

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