An xlpe cable partial aging type diagnosis method based on pdc branch number characteristics

Abstract

The application relates to the technical field of cable insulation state detection, and is an XLPE cable local aging type diagnosis method and system based on PDC branch number characteristics, wherein the system comprises a polarization / depolarization test unit, which performs PDC test on a to-be-detected cable to obtain a polarization / depolarization current curve of the to-be-detected cable; a branch parameter fitting unit, which performs four-branch fitting on the polarization-depolarization current curve by adopting a Debye model to obtain time constants of each branch; wherein the time constants of each branch time include polarization branch time constants and depolarization branch time constants; and an aging type judgment unit, which judges whether the time constants of each branch are the same according to the time constants of each branch; the application determines by using the fourth branch characteristic parameter in the PDC response starting from the medium polarization mechanism, is simple and reliable, avoids the uncertainty of artificial experience, and significantly improves the diagnosis precision.

Description

Technical Field

[0001] This invention relates to the field of cable insulation condition detection technology, and in particular to a diagnostic method for local aging types of XLPE cables based on the characteristics of the number of PDC branches. Background Technology

[0002] Cross-linked polyethylene (XLPE) medium-voltage cables are critical equipment in power distribution networks, and their insulation performance is essential for the safe operation of the network and power system. During long-term operation, cables develop localized thermal aging defects under stress from voltage, current, and temperature. Furthermore, during manufacturing, laying, and operation, uneven insulation cooling, improper laying, or external damage can cause damage to the outer sheath and shielding layer, allowing moisture to seep into the insulation layer and leading to localized water treeing. These localized defects, such as thermal aging and water treeing, gradually weaken the cable's insulation performance, making the aged areas weak points and ultimately leading to breakdown accidents with serious consequences. Therefore, accurately diagnosing localized aging defects in cables and taking timely preventative measures is crucial for improving cable reliability and extending its service life.

[0003] Methods for diagnosing local defects in cables mainly include AC withstand voltage testing and partial discharge detection, dielectric loss tangent and capacitance measurement, and dielectric response methods (such as polarization-depolarization current PDC). Among these, AC withstand voltage testing and partial discharge detection are more sensitive to obvious defects, but their ability to identify early local aging defects such as water trees is limited. Figure 9 (A schematic diagram of an existing device for detecting water tree aging in short / long cables); the dielectric loss tangent (tanδ) and capacitance detection methods are simple and intuitive, but difficult to distinguish between local defects and overall aging; in contrast, the polarization-depolarization current (PDC) method (such as...) Figure 10 The diagram shows a schematic of a cable insulation testing system using the existing polarization-depolarization current (PDC) method. By applying a step voltage to obtain the time-domain current response of the insulating medium and fitting the parameters of the Debye equivalent circuit model, the aging characteristics of the insulation can be reflected. Furthermore, when using PDC testing to evaluate XLPE cable insulation, the testing procedure is simple, non-destructive, and highly sensitive to localized severe defects. Therefore, the high sensitivity of the PDC method to early and different types of defects is considered an important means of aging diagnosis for medium-voltage XLPE cables.

[0004] Currently, some research and patents have adopted polarization-depolarization current (PDC) testing combined with the extended Debye model to assess the insulation condition of cables and attempt to identify water tree aging characteristics. For example, Chinese patent CN201810968751.0 introduces a diode equivalent structure based on the traditional three-branch Debye model and identifies water tree aging through the nonlinear capacitance parameter (Cd3) in the third branch; Chinese patent CN202110400609.8 converts the PDC polarization current to the frequency domain, extracts the polarization loss tanδ, and judges water tree defects by combining the peak frequency change. Although these methods have a certain identification ability, they all rely on fixed branch structures, extract feature parameters only in local branches, and their fitting process depends on complex spectrum calculations or exponential model calculations. The parameter solution has a large degree of randomness, the diagnostic results lack uniformity, and there are shortcomings in distinguishing between thermal aging and water tree aging.

[0005] Furthermore, Chinese patent CN202010635703.7 proposes to calculate the conductivity current coefficient using multiple consecutive PDC tests and judge the aging state through its changing trend. Although this can reflect the asymmetric polarization behavior caused by water tree aging, the test cycle is long and the operation is complex, making it unsuitable for field applications. However, the diagnostic object is mainly the overall insulation state, and it cannot effectively distinguish different types of local defects (such as water treeing and thermal aging).

[0006] Although some studies have mentioned that increasing the number of branches by expanding the Debye model can improve model accuracy, these additional branches exist only as a mathematical fitting method and have not been used as a diagnostic basis, nor have they established a clear correspondence with the physical polarization behavior of water tree aging.

[0007] In summary, while existing technologies can diagnose cable aging to some extent, they still have significant shortcomings. On the one hand, the testing process generally relies on repeated tests or complex physical and chemical methods, making it cumbersome and impractical in the field. On the other hand, the diagnostic criteria of existing methods are not uniform, and the judgment system is incomplete, leading to results that are easily influenced by empirical factors. Furthermore, traditional testing methods have low differentiation between different aging modes, making it difficult to accurately determine the type of localized aging.

[0008] Therefore, it is essential to propose a novel cable insulation aging identification method with a clear diagnostic mechanism, clear criteria, simple operation, and the ability to accurately distinguish between multiple aging modes, in order to improve the accuracy of aging type judgment and the feasibility of field application. Summary of the Invention

[0009] This invention provides a method for diagnosing the local aging type of XLPE cables based on the characteristics of the number of PDC branches, which overcomes the shortcomings of the prior art and can effectively solve the problem that traditional cable local aging type diagnosis cannot accurately diagnose and distinguish different types of aging modules.

[0010] To solve the above problems, the technical solution of this invention is achieved through the following method: a method for diagnosing the local aging type of XLPE cables based on the characteristics of the number of PDC branches, comprising:

[0011] Perform PDC testing on the cable under test to obtain the polarization / depolarization current curve of the cable under test;

[0012] The polarization-depolarization current curve was fitted to four branches using the Debye model, and the time constants of each branch were obtained. The time constants of each branch include the time constants of the polarization branch and the time constants of the depolarization branch.

[0013] Determine whether the time constants of each branch are the same based on the time constants of each branch.

[0014] If the response is yes or no, the time constants of each branch are different, proving the existence of a fourth branch in the four-branch fitting, and determining that the cable insulation defect is a local water tree aging defect; let the time constant of the depolarized fourth branch in the fourth branch be denoted as . ;

[0015] After determining the existence of a fourth branch, i.e., the cable insulation defect is a localized water tree aging defect, the time constant for depolarizing the fourth branch is then determined. Is it greater than 50 and less than or equal to 150?

[0016] Therefore, the local aging degree of the cable under test is determined to be moderate water tree aging, and the risk of breakdown is moderate.

[0017] If the response is negative, then determine the time constant of the fourth branch depolarization. If the value is less than or equal to 50, the test cable is determined to have mild water treeing aging, with a low risk of breakdown; otherwise, the time constant of the fourth branch is depolarized. If the value is greater than 150, the local aging degree of the cable under test is determined to be severe water tree aging, with a high risk of breakdown.

[0018] A power-law term was fitted to the depolarization current curve of the cable under test during the long-term depolarization phase to obtain the power-law decay exponent. ;

[0019] Determining the power-law decay index Is it greater than 0.3 and less than or equal to 0.9?

[0020] The response indicates that the traps at the local aging site of the cable under test have become more dispersed and wider, and the number of deep traps has increased.

[0021] Whether or not it responds, determine the power-law decay exponent. If the response is greater than 0.9 and less than 1, then it is determined that the trap distribution at the local aging site of the cable under test is low and the density of deep traps is low; otherwise, the power-law decay index is... If the value is less than or equal to 0.3, it is determined that there are many traps and a high density of deep traps in the local aging area of ​​the cable under test.

[0022] The above-mentioned PDC test was performed on the cable under test, and the polarization-depolarization current curve of the cable under test was obtained, including:

[0023] The outer surface of the cable sample to be tested is cleaned and dried to reduce experimental errors;

[0024] Connect the first section of cable core 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.

[0025] A DC voltage U is applied to the cable to polarize it for a time of T, and the curve of polarization current changing with time is recorded.

[0026] After turning off the DC voltage, a depolarization test was performed. The depolarization time was also T, and the depolarization current decay curve over time was collected.

[0027] The above-mentioned Debye model was used to fit the polarization-depolarization current curve to four branches, and the time constants of each branch were obtained; including:

[0028] The four-branch fitting equation for the polarization current curve is as follows:

[0029] ,

[0030] In the formula, y p This refers to the instantaneous current during the polarization phase; This represents the time corresponding to the instantaneous current. I This is the steady-state conductivity component; A、 C, E, G These are the polarization intensity coefficients for different polarization branches; B, D, F, H These are the time constants of the polarization branches;

[0031] The four-branch fitting equation for the depolarization current curve is as follows:

[0032] ,

[0033] In the formula, This is the instantaneous current for depolarization; Let A, C, E, and G be the time variables corresponding to the depolarization current, respectively, and let B, D, F, and H be the polarization intensity coefficients of different depolarization branches.

[0034] The time constants for each branch are calculated as follows:

[0035] polarization current The expression is:

[0036] ,

[0037] Depolarization current The expression is:

[0038] ,

[0039] In the formula, To polarize the current amplitude of each RC branch, , To depolarize the current amplitude of each RC branch, ; A 0 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; t p Polarization time; t d The time for depolarization; For the first i The polarization time constant of the branch, ; For the first i Depolarization time constant of the branch, ; i Indicate each branch road; For polarization i The equivalent capacitance of the branch; For depolarization i The equivalent capacitance of the branch; U is the DC voltage applied to the high-voltage end of the PDC.

[0040] The above-mentioned determination of whether the time constants of each branch are the same based on the time constants of each branch includes:

[0041] Therefore, the time constants of each branch are partially repeated, meaning that some branches overlap. This proves that there is no fourth branch in the four-branch fitting, and the cable insulation defect is determined to be thermal aging or other general type of defect.

[0042] The above-mentioned power-law fitting was performed on the depolarization current curve of the long-term depolarization phase of the cable under test, and the power-law decay exponent was obtained. ;include:

[0043] The fitting equation for the power-law term is as follows:

[0044] ,

[0045] In the formula, This is the instantaneous current for depolarization; Let A, C, E, and G be the time variables corresponding to the depolarization current, respectively, and let B, D, F, and H be the polarization intensity coefficients of different depolarization branches. K The power-law coefficients, r It represents the power-law decay exponent.

[0046] The DC voltage U mentioned above is a DC voltage of 1-5kV; the polarization time T is 90s.

[0047] This invention achieves accurate identification of cable insulation aging by introducing the fourth branch feature of Debye. This method can accurately distinguish aging types even under conditions where multiple aging modes coexist. Compared with traditional methods that rely solely on changes in dielectric loss, insulation resistance, or conductivity, this invention starts from the dielectric polarization mechanism and uses the fourth branch feature parameters in the PDC response for judgment. This method is simple and reliable, avoids the uncertainty of human experience, and significantly improves diagnostic accuracy. According to the steps described above, the local aging state of the cable can be determined, thus preparing for subsequent treatment and maintenance, significantly improving the reliability of insulation aging diagnosis, and has broad engineering application prospects. Attached Figure Description

[0048] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0049] Figure 1 This is a flowchart of the judgment method of the present invention.

[0050] Figure 2 This is the Debye model of the four branches of the aging cable of the present invention.

[0051] Figure 3 This is the PDC polarization current curve in Embodiment 3 of the present invention.

[0052] Figure 4 This is the PDC depolarization current curve in Embodiment 3 of the present invention.

[0053] Figure 5 This is the PDC polarization current curve in Embodiment 4 of the present invention.

[0054] Figure 6This is the PDC depolarization current curve in Embodiment 4 of the present invention.

[0055] Figure 7 This is a schematic diagram of the thermally aged short cable in Embodiment 4 of the present invention.

[0056] Figure 8 This is a schematic diagram of the thermal aging long cable in Embodiment 3 of the present invention.

[0057] Figure 9 This is a schematic diagram of short / long cables with water tree aging in the background technology of this invention.

[0058] Figure 10 This is a schematic diagram of PDC testing in the background technology of this invention. Detailed Implementation

[0059] The present invention is not limited to the following embodiments, and specific implementation methods can be determined according to the technical solutions and actual conditions of the present invention.

[0060] Example 1: As Figure 1 As shown, this embodiment of the invention discloses a method for diagnosing the local aging type of XLPE cables based on the characteristics of the number of PDC branches, including:

[0061] S101, perform PDC test on the cable under test to obtain the polarization / depolarization current curve of the cable under test;

[0062] S102, the Debye model is used to fit the polarization-depolarization current curve to four branches, and the time constant of each branch is obtained; wherein, the time constant of each branch includes the time constant of the polarization branch and the time constant of the depolarization branch;

[0063] S103, determine whether the time constants of each branch are the same based on the time constants of each branch;

[0064] S104, if the response is yes or no, then the time constants of each branch are different, proving that a fourth branch exists in the four-branch fitting, and determining that the cable insulation defect is a local water tree aging defect; let the time constant of the depolarized fourth branch in the fourth branch be denoted as . ;

[0065] S105, after determining the existence of a fourth branch, i.e., the cable insulation defect is a local water tree aging defect, determine the depolarization time constant of the fourth branch. Is it greater than 50 and less than or equal to 150?

[0066] S106, in response, the local aging degree of the cable under test is determined to be moderate water tree aging, and the risk of breakdown is moderate;

[0067] S107, if the response is negative, then determine the time constant of the fourth branch depolarization. If the value is less than or equal to 50, the test cable is determined to have mild water treeing aging, with a low risk of breakdown; otherwise, the time constant of the fourth branch is depolarized. If the value is greater than 150, the local aging degree of the cable under test is determined to be severe water tree aging, with a high risk of breakdown.

[0068] S108, perform power-law fitting on the depolarization current curve of the long-term depolarization phase of the cable under test, and obtain the power-law decay exponent. ;

[0069] S109, Determine the power-law decay index Is it greater than 0.3 and less than or equal to 0.9?

[0070] S1010, in response, it is determined that the traps at the local aging point of the cable under test have widened and the number of deep traps has increased;

[0071] S1011, Response status, determine the power-law decay exponent. If the response is greater than 0.9 and less than 1, then it is determined that the trap distribution at the local aging site of the cable under test is low and the density of deep traps is low; otherwise, the power-law decay index is... If the value is less than or equal to 0.3, it is determined that there are many traps and a high density of deep traps in the local aging area of ​​the cable under test.

[0072] In step S101 above, a PDC test is performed on the cable under test to obtain the polarization-depolarization current curve of the cable under test, including:

[0073] The outer surface of the cable sample to be tested is cleaned and dried to reduce experimental errors;

[0074] Connect the first section of cable core 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; thereby shielding external interference.

[0075] A DC voltage U is applied to the cable to polarize it for a time of T, and the curve of polarization current changing with time is recorded.

[0076] After turning off the DC voltage, a depolarization test was performed. The depolarization time was also T, and the depolarization current decay curve over time was collected.

[0077] Wherein, the DC voltage U is a DC voltage of 1-5kV; the polarization time T is 90s. The polarization / depolarization current response obtained through step S101 can comprehensively reflect the dynamic changes of different polarization mechanisms in the insulating medium, providing basic data for subsequent aging type determination.

[0078] In polarization / depolarization current (PDC) testing, the polarization behavior of cable insulation can be characterized using a multi-branch Debye model. For cables that are not aged or have only undergone thermal aging and other general aging processes, the dielectric polarization process is relatively simple, typically exhibiting only 2-3 polarization branches. However, when water treeing occurs in the cable insulation, numerous water-containing microchannels and interfacial polarization layers form within the water tree region, causing significant interfacial polarization and space charge accumulation effects in the local dielectric. This process leads to slow dipole orientation and charge release behavior in the polarization and depolarization currents, resulting in a prolonged current hysteresis characteristic during the depolarization phase, i.e., a fourth branch.

[0079] In step S102 above, the Debye model is used to fit the polarization-depolarization current curve to four branches, and the time constant of each branch is obtained; including:

[0080] The four-branch fitting equation for the polarization current curve is as follows:

[0081] ,

[0082] In the formula, This refers to the instantaneous current during the polarization phase; This represents the time corresponding to the instantaneous current. I This is the steady-state conductivity component; A, C, E, G These are the polarization intensity coefficients for different polarization branches; B, D, F, H These are the time constants of the polarization branches, with the values ​​obtained from the fitting taken as the first, second, third, and fourth polarization branches, respectively, from smallest to largest.

[0083] The four-branch fitting equation for the depolarization current curve is as follows:

[0084] ,

[0085] In the formula, This is the instantaneous current for depolarization; Let A, C, E, and G be the polarization intensity coefficients of different depolarization branches, and B, D, F, and H be the time constants of the depolarization branches. The time constants of the first, second, third, and fourth depolarization branches are determined by the values ​​obtained from the fitting, from smallest to largest.

[0086] The time constants for each branch are calculated as follows:

[0087] polarization current The expression is:

[0088] ,

[0089] Depolarization current The expression is:

[0090] ,

[0091] In the formula, To polarize the current amplitude of each RC branch, , To depolarize the current amplitude of each RC branch, ,; A 0 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; t p Polarization time; t d The time for depolarization; For the first i The polarization time constant of the branch, ; For the first i Depolarization time constant of the branch, ; i Indicate each branch road; For polarization i The equivalent capacitance of the branch; For depolarization i The equivalent capacitance of the branch; U is the DC voltage applied to the high-voltage end of the PDC; and The least squares method was used to fit the data, and the four branches determined by the fitting were numbered as branches 1, 2, 3 and 4 in order of increasing time constant.

[0092] In step S103 above, determining whether the time constants of each branch are the same based on the time constants of each branch includes:

[0093] Therefore, the time constants of each branch are partially repeated, meaning that some branches overlap. This proves that there is no fourth branch in the four-branch fitting, and the cable insulation defect is determined to be thermal aging or other general type of defect.

[0094] Among them, the polarization / depolarization current (PDC) test data are fitted with a four-branch Debye model. If there are four different values ​​in the fitted time constant, it indicates that there is an additional slow polarization behavior in the insulating medium, that is, the equivalent model is considered to have a fourth branch, and the number of branches is determined to be 4. If there are identical or repeated values ​​in the fitted time constant, it is considered that some branches overlap. That is, if there are only three sets of different values ​​in the time constant, it indicates that the polarization process of the medium is relatively simple, and there is only general dipole orientation or interface polarization behavior. The actual effective number of branches is equal to the number of different values ​​in the time constant.

[0095] In step S108 above, the depolarization current curve of the long-term depolarization phase of the cable under test is fitted with a power-law term to obtain the power-law decay exponent. ;include:

[0096] The fitting equation for the power-law term is as follows:

[0097] ,

[0098] In the formula, This is the instantaneous current for depolarization; Let A, C, E, and G be the polarization intensity coefficients of different depolarization branches, and B, D, F, and H be the time constants of the depolarization branches. The time constants of the first, second, third, and fourth depolarization branches are determined by the values ​​obtained from the fitting, from smallest to largest. K The power-law coefficients, r It represents the power-law decay exponent.

[0099] The fourth branch reflects the polarization mechanism of water tree aging:

[0100] After water treeing occurs in cable insulation, a large number of microscale water-bearing channels and interface layers will form in the water tree region. The distortion of the local electric field causes the charge carriers to be repeatedly captured and released at the interface, forming a slow dipole orientation and space charge accumulation process. This slow polarization process is manifested as a long current lag in the depolarization stage, corresponding to the newly added fourth branch in the model.

[0101] To more accurately describe the long-tailed characteristics of the depolarization current after aging, a power-law decay term is introduced based on the four-branch model. When the cable insulation is in a healthy state, the polarization process is mainly governed by a finite number of time constants, and its response can be characterized by an exponential branch. However, in the water tree aging region, as the trap energy levels gradually become continuous, the charge release process spans multiple time scales, and the depolarization current exhibits obvious power-law tail characteristics.

[0102] Therefore, adding a power-law term to the four-branch model not only significantly improves the fitting accuracy in the long time region, but also improves the fitting accuracy through the power-law decay exponent. r The changes quantitatively reflect the degree of distribution of medium traps and the enhancement process of interface polarization, thus providing a more sensitive criterion for determining the aging state of cable insulation.

[0103] In summary, this invention discloses a method for diagnosing local aging types of XLPE cables based on the characteristics of the number of PDC branches. This method fits the polarization / depolarization current test data to a multi-branch Debye model, determines the existence of a fourth branch in the model, extracts its time constant characteristics, and establishes a quantitative relationship between water tree aging characteristic parameters and dielectric polarization behavior, thereby achieving accurate identification of different aging types. The method of this invention has a clear structure and well-defined mechanism, and can effectively distinguish aging types under the condition of multiple aging modes such as thermal aging and water tree aging coexisting, significantly improving the accuracy of cable insulation condition assessment and providing reliable technical support for online monitoring, operation and maintenance, and life prediction of power cables.

[0104] Example 2: This embodiment of the invention discloses a diagnostic system for local aging types of XLPE cables based on the characteristics of the number of PDC branches, including:

[0105] The polarization / depolarization test unit performs PDC testing on the cable under test to obtain the polarization / depolarization current curve of the cable under test;

[0106] The branch parameter fitting unit uses the Debye model to fit the polarization-depolarization current curve to four branches, and obtains the time constant of each branch. The time constant of each branch includes the time constant of the polarization branch and the time constant of the depolarization branch.

[0107] The aging type determination unit determines whether the time constants of each branch are the same based on the time constant of each branch.

[0108] Judging the response unit, if the response is positive or negative, then the time constants of each branch are different, proving the existence of a fourth branch in the four-branch fitting, and determining that the cable insulation defect is a local water tree aging defect; let the time constant of the depolarized fourth branch in the fourth branch be denoted as . ;

[0109] The aging degree judgment unit, after determining the existence of a fourth branch, i.e., the cable insulation defect is a local water tree aging defect, determines the depolarization time constant of the fourth branch. Is it greater than 50 and less than or equal to 150?

[0110] The first response unit responds, and then determines that the local aging degree of the cable under test is moderate water tree aging, and the risk of breakdown is moderate.

[0111] If the second response unit responds negatively, then the time constant of the fourth branch depolarization is determined. If the value is less than or equal to 50, the test cable is determined to have mild water treeing aging, with a low risk of breakdown; otherwise, the time constant of the fourth branch is depolarized. If the value is greater than 150, the local aging degree of the cable under test is determined to be severe water tree aging, with a high risk of breakdown.

[0112] The power-law fitting unit performs power-law fitting on the depolarization current curve of the cable under test during the long-term depolarization phase, and calculates the power-law decay exponent. ;

[0113] Trap distribution judgment unit, judging the power-law decay exponent Is it greater than 0.3 and less than or equal to 0.9?

[0114] The third response unit responds, and then determines that the traps at the local aging point of the cable under test have become more dispersed and the number of deep traps has increased.

[0115] The fourth response unit determines whether the power-law decay exponent is responsive. If the response is greater than 0.9 and less than 1, then it is determined that the trap distribution at the local aging site of the cable under test is low and the density of deep traps is low; otherwise, the power-law decay index is... If the value is less than or equal to 0.3, it is determined that there are many traps and a high density of deep traps in the local aging area of ​​the cable under test.

[0116] In summary, compared with the prior art, the present invention has the following advantages:

[0117] (1) The diagnostic mechanism is clear and the identification basis is reliable. By introducing a fourth branch into the Debye model that fits the polarization-depolarization current curve of the PDC test, the slow polarization and space charge accumulation characteristics caused by water tree aging can be reflected. The correspondence between water tree aging and model parameters is established from the physical level, which improves the accuracy of aging type identification.

[0118] (2) High differentiation of aging types. By identifying the presence of an independent fourth branch and its time constant characteristics in the PDC test, this invention can accurately distinguish between water tree aging and general defects such as thermal aging under the condition of multiple aging modes coexisting. Compared with traditional dielectric parameter detection methods, it significantly improves the sensitivity and accuracy of aging type diagnosis.

[0119] (3) Strong applicability and simple testing process. The method of the present invention can realize diagnostic analysis based on a small number of PDC test data, without the need for complex sample processing of cables, and is applicable to insulation status assessment of in-service cables, laboratory samples and different laying environments.

[0120] In summary, this invention achieves accurate identification of cable insulation aging by introducing the fourth branch feature of Debye. This method can accurately distinguish aging types even under conditions where multiple aging modes coexist. Compared to traditional methods that rely solely on changes in dielectric loss, insulation resistance, or conductivity, this invention starts from the dielectric polarization mechanism and uses the fourth branch feature parameters in the PDC response for judgment. This method is simple and reliable, avoids the uncertainty of human experience, and significantly improves diagnostic accuracy. Based on the steps described above, the local aging state of the cable can be determined, thus preparing for subsequent processing and maintenance, significantly improving the reliability of insulation aging diagnosis, and has broad engineering application prospects.

[0121] Example 3: To verify the effectiveness of the method of the present invention, three short cable samples, each 25cm in length and 3*95mm² in size, were selected for comparative testing.

[0122] The three sample sections underwent unaged, thermally aged, and water-tree aged treatments, respectively. The thermally aged samples were pressure-aged in a 140℃ oven for 10 days; the water-tree aged samples were aged at room temperature for the same 10 days, with all pressure conditions set at 6kV and 400Hz. After the aging test, the surfaces of each sample were wiped clean to minimize the impact of surface contamination on the polarization-depolarization current test results. The short cable thermal aging test apparatus is as follows: Figure 8 As shown.

[0123] After sample processing, PDC testing is performed. First, the cable core at the beginning of the cable under test is connected to the high-voltage end of the PDC tester. A 4kV DC voltage is applied to the cable through the PDC high-voltage end, and the polarization time is maintained at 90s. The curve of polarization current changing over time is recorded. Figure 3 As shown; after the polarization process ends, the DC voltage is disconnected and a depolarization current test is performed. The depolarization time is set to 90s, the same as the polarization time. Simultaneously, the curve of the depolarization current decaying over time is recorded, as shown. Figure 4 As shown.

[0124] In the process of identifying the local aging type of the cable, the number of fitted branches is used as the criterion. The statistical results of the branch fitting in this embodiment are shown in Table 1.

[0125] As shown in Table 1, no fourth branch appeared in the polarization / depolarization current fitting results of the unaged cable and the thermally aged cable; however, a fourth branch clearly appeared in the fitting results of the water-tree aged cable, and the depolarization time constant of the fourth branch was also high. The value is 106.77, indicating a moderate degree of aging and a moderate risk of breakdown. Furthermore, the power-law decay index for water tree aging is 0.08, suggesting that water tree-aged cables have the highest number of traps, with a relatively high density of deep traps.

[0126] Example 4: To verify the effectiveness of the method of the present invention, a defect-free three-phase cross-linked polyethylene insulated cable (model YJV22-10 kV 3×50, insulation thickness 4.5mm) with a voltage level of 10kV was selected, and a cable sample with a length of 12m was taken as the experimental object.

[0127] The 12m three-phase cable was stripped into three single-phase samples.

[0128] At a point 6.6m from the beginning of a single-phase cable, the outer sheath, steel armor, and inner sheath are stripped in sequence. The exposed portion is placed on a heating platform, and a 3.9kV power frequency voltage is applied to both ends of the cable while maintaining the heating platform temperature at a constant 80℃. This pressure is applied continuously for 5 days to induce accelerated thermal aging defects. Figure 7 This is a schematic diagram of an accelerated thermal aging testing device for long cables. The device places the XLPE cable to be tested on a heating platform and heats a localized area at a set temperature (e.g., 80°C). Simultaneously, an electric field is applied through a voltage source, and a water resistance is connected in series in the circuit to ensure that even if insulation breakdown occurs during the experiment, the current is limited to a safe range, preventing damage to equipment and personnel. Through aging under the combined effects of heat and electricity, the degradation characteristics of the cable insulation material can be obtained in a short time.

[0129] A water tree defect was artificially created 7.65m from the beginning of another phase cable. The hole depth was 3mm, and a heat shrink tube was wrapped around the defect location with a small opening for injecting saturated NaCl (1.8mol / L) solution. At the same time, a 3.9kV power frequency voltage was applied to the cable core, and a ground wire was drawn out from the NaCl solution to ground it. The pressure was applied for 5 days to accelerate the formation of water tree aging defects.

[0130] The third phase cable was kept intact as a control sample.

[0131] The above three single-phase samples were tested using the method described in this invention. The cable core at the beginning of the cable under test was connected to the high-voltage end of the PDC tester. A 1kV DC voltage U was applied to the cable through the high-voltage end of the PDC, and the polarization time was maintained at 90s. The curve of polarization current changing with time was recorded, as follows: Figure 5 As shown. After the polarization process ends, the DC voltage is disconnected and a depolarization current test is performed. The depolarization time is set to 90 seconds, the same as the polarization time. Simultaneously, the curve of the depolarization current decaying over time is recorded, as shown. Figure 6 As shown.

[0132] In the process of identifying cable aging types, the number of branches in the fitting results is used as a criterion to determine the local aging type of the cable; the statistical results of the branch fitting are shown in Table 2.

[0133] Table 2 shows that the Debye model fitting results for unaged cables and heat-aged cables are the same, exhibiting a three-branch characteristic. Only the fitting results for the branch time constants of water-tree aged cables are different, indicating the appearance of a fourth branch in water-tree aged cables. Furthermore, the depolarization time constant of the fourth branch in water-tree aged cables is... The value was 42.12, indicating mild aging. Furthermore, this aged cable had the lowest power-law attenuation index at 0.21, indicating a high number of traps and the highest density of deep traps. Among the aforementioned aged cables, only the water-tree aged cable exhibited a fourth branch, suggesting that this fourth branch is a characteristic response of the internal polarization behavior of the cable insulation material during water-tree aging, reflecting a novel slow polarization process introduced by water treeing. Identifying the presence of a fourth branch can effectively distinguish water-tree aging from general aging types such as thermal aging, thereby enabling accurate judgment of the local aging state of the cable.

[0134] Table 1. Statistical results of branch fitting in Example 3

[0135]

[0136] Table 2 Statistical results of branch fitting in Example 4

[0137]

Claims

1. A method for diagnosing local aging types of XLPE cables based on the characteristics of the number of PDC branches, characterized in that, include: Perform PDC testing on the cable under test to obtain the polarization / depolarization current curve of the cable under test; The polarization-depolarization current curve was fitted to four branches using the Debye model, and the time constants of each branch were obtained. The time constants of each branch include the time constants of the polarization branch and the time constants of the depolarization branch. Determine whether the time constants of each branch are the same based on the time constants of each branch. If the response is yes or no, the time constants of each branch are different, proving the existence of a fourth branch in the four-branch fitting, and determining that the cable insulation defect is a local water tree aging defect; let the time constant of the depolarized fourth branch in the fourth branch be denoted as . ; After determining the existence of a fourth branch, i.e., the cable insulation defect is a localized water tree aging defect, the time constant for depolarizing the fourth branch is then determined. Is it greater than 50 and less than or equal to 150? Therefore, the local aging degree of the cable under test is determined to be moderate water tree aging, and the risk of breakdown is moderate. If the response is negative, then determine the time constant of the fourth branch depolarization. If the value is less than or equal to 50, the test cable is determined to have mild water treeing aging, with a low risk of breakdown; otherwise, the time constant of the fourth branch is depolarized. If the value is greater than 150, the local aging degree of the cable under test is determined to be severe water tree aging, with a high risk of breakdown. A power-law term was fitted to the depolarization current curve of the cable under test during the long-term depolarization phase to obtain the power-law decay exponent. ; Determining the power-law decay index Is it greater than 0.3 and less than or equal to 0.9? The response indicates that the traps at the local aging site of the cable under test have become more dispersed and wider, and the number of deep traps has increased. Whether or not it responds, determine the power-law decay exponent. If the response is greater than 0.9 and less than 1, then it is determined that the trap distribution at the local aging site of the cable under test is low and the density of deep traps is low; otherwise, the power-law decay index is... If the value is less than or equal to 0.3, it is determined that there are many traps and a high density of deep traps in the local aging area of ​​the cable under test.

2. The method for diagnosing the local aging type of XLPE cable based on the characteristics of the number of PDC branches 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 first section of cable core 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 for a time of T, and the curve of polarization current changing with time is recorded. After turning off the DC voltage, a depolarization test was performed. The depolarization time was also T, and the depolarization current decay curve over time was collected.

3. The method for diagnosing the local aging type of XLPE cable based on the characteristics of the number of PDC branches according to claim 1, characterized in that, The method employs the Debye model to fit the polarization-depolarization current curve to four branches, obtaining the time constants for each branch; including: The four-branch fitting equation for the polarization current curve is as follows: ， In the formula, y p This refers to the instantaneous current during the polarization phase; This represents the time corresponding to the instantaneous current. I This is the steady-state conductivity component; A, C, E, G These are the polarization intensity coefficients for different polarization branches; B, D, F, H These are the time constants of the polarization branches; The four-branch fitting equation for the depolarization current curve is as follows: ， In the formula, y d This is the instantaneous current for depolarization; Let A, C, E, and G be the time variables corresponding to the depolarization current, respectively, and let B, D, F, and H be the polarization intensity coefficients of different depolarization branches. The time constants for each branch are calculated as follows: 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, ; A 0 The magnitude of the conduction current component; R pi For polarization i The equivalent resistance of a branch; R di For depolarization i The equivalent resistance of a branch; t p Polarization time; t d The time for depolarization; For the first i The polarization time constant of the branch, ; For the first i Depolarization time constant of the branch, ; i Indicate each branch road; C pi For polarization i The equivalent capacitance of the branch; C di For depolarization i The equivalent capacitance of the branch; U is the DC voltage applied to the high-voltage end of the PDC.

4. The method for diagnosing the local aging type of XLPE cable based on the characteristics of the number of PDC branches according to claim 1, characterized in that, The step of determining whether the time constants of each branch are the same based on the time constants of each branch includes: Therefore, the time constants of each branch are partially repeated, meaning that some branches overlap. This proves that there is no fourth branch in the four-branch fitting, and the cable insulation defect is determined to be thermal aging or other general type of defect.

5. The method for diagnosing the local aging type of XLPE cable based on the characteristics of the number of PDC branches according to claim 1, characterized in that, The power-law term fitting is performed on the depolarization current curve of the long-term depolarization phase of the cable under test to obtain the power-law decay index. ;include: The fitting equation for the power-law term is as follows: ， In the formula, y d This is the instantaneous current for depolarization; Let A, C, E, and G be the time variables corresponding to the depolarization current, respectively, and let B, D, F, and H be the polarization intensity coefficients of different depolarization branches. K The power-law coefficients, r It represents the power-law decay exponent.

6. The method for diagnosing the local aging type of XLPE cable based on the characteristics of the number of PDC branches according to claim 2, characterized in that, The DC voltage U is a DC voltage of 1-5kV; the polarization time T is 90s.

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