CN-121432091-B - XLPE cable local aging type diagnosis method based on PDC branch number characteristics

CN121432091BCN 121432091 BCN121432091 BCN 121432091BCN-121432091-B

Abstract

The invention relates to the technical field of cable insulation state detection, in particular to a XLPE cable local aging type diagnosis method and system based on PDC branch number characteristics, wherein the system comprises a polarization/depolarization test unit, and the PDC test is carried out on a cable to be tested to obtain a polarization/depolarization current curve of the cable to be tested; the invention discloses a method for diagnosing a polarization-depolarization current curve, which comprises a branch parameter fitting unit, a aging type judging unit and a PDC response unit, wherein the branch parameter fitting unit is used for performing four-branch fitting on the polarization-depolarization current curve by adopting a Debye model to respectively obtain time constants of each branch, the time constants of each branch comprise a polarization branch time constant and a depolarization branch time constant, the aging type judging unit is used for judging whether the time constants of each branch are identical or not according to the time constants of each branch, and the method is simple and reliable by utilizing a fourth branch characteristic parameter in the PDC response from a medium polarization mechanism, meanwhile, the uncertainty of manual experience is avoided, and the diagnosis precision is remarkably improved.

Inventors

ZHU LIANG
JIN XIN
ZHANG GUANGRU
REN HAODONG
CHEN JIE
LIU CHUN
LIU BINYU
GOU JUN
ZHANG YANLI
YANG JUNTING
ZHANG YU
MA ZHENQI
FAN DILONG
WANG XIAOLONG
CHEN MINGLONG
Li Xueken
CHEN XIAOTONG
SUN BO
BAI AIDONG

Assignees

国网甘肃省电力公司电力科学研究院
国网甘肃省电力公司兰州供电公司

Dates

Publication Date: 20260508
Application Date: 20251201

Claims (6)

1. The XLPE cable local aging type diagnosis method based on PDC branch number characteristics is characterized by comprising the following steps of: PDC test is carried out on the cable to be tested, and a polarization/depolarization current curve of the cable to be tested is obtained; Performing four-branch fitting on a polarization-depolarization current curve by adopting a Debye model to respectively obtain time constants of all branches, wherein the time constants of all branch times comprise a polarization branch time constant and a depolarization branch time constant; judging whether the time constants of the branches are the same according to the time constants of the branches; In response to the failure, the time constants of the branches are different, the fourth branch is proved to exist in the four-branch fitting, the cable insulation defect is judged to be a local water tree aging defect, and the depolarization fourth branch time constant in the fourth branch is recorded as ; After judging that the fourth branch exists, namely the cable insulation defect is a local water tree aging defect, judging the depolarization fourth branch time constant Whether greater than 50 and less than or equal to 150; In response, judging that the local aging degree of the cable to be tested is moderate water tree aging and breakdown risk is moderate; in response to no, determine a depolarization fourth leg time constant Whether 50 or not, and, in response to, judging that the local aging degree of the cable to be tested is mild water tree aging, and the breakdown risk is low; in response to no, depolarize the fourth leg time constant The cable to be tested is judged to be aged in a serious water tree, and the breakdown risk is high; Performing power law item fitting on a depolarization current curve of a long-time section of a cable depolarization stage to be tested, and obtaining a power law attenuation index ; Judging power law attenuation index Whether greater than 0.3 and less than or equal to 0.9; In response, judging that traps at the local ageing position of the cable to be tested are dispersed and widened, and increasing deep traps; In response to no, determine a power law decay exponent Whether the trap density is more than 0.9 and less than 1, if so, judging that the trap distribution at the local ageing position of the cable to be tested is less, and if not, judging that the deep trap density is lower, and if not, the power law attenuation index is the same And the trap distribution at the local ageing position of the cable to be tested is judged to be more than or equal to 0.3, and the density of deep traps is high.
2. The XLPE cable partial aging type diagnosis method based on PDC branch number characteristics according to claim 1, wherein the PDC test is performed on the cable to be tested to obtain a polarization-depolarization current curve of the cable to be tested, comprising: Cleaning and drying the outer surface of the cable sample to be tested, so that experimental errors are reduced; connecting the first-section cable core with the high-voltage end of the PDC tester, and winding copper shielding belts at the middle part and the two ends to be grounded so as to shield surface leakage current; Applying a direct-current voltage U to the cable for polarization, wherein the polarization time is T, and simultaneously recording the curve of the change of the polarization current along with time; and after the direct-current voltage is turned off, performing a depolarization test, wherein the depolarization time is also T, and simultaneously acquiring a depolarization current decay curve along with time.
3. The method for diagnosing the local aging type of the XLPE cable based on the PDC branch number characteristics according to claim 1, wherein the four-branch fitting is performed on the polarization-depolarization current curve by adopting a Debye model to obtain the time constants of the branches respectively, and the method comprises the following steps: the four branch fitting equation for the polarized current curve is as follows: , Wherein y p is the instantaneous current in the polarization stage; the current is the time corresponding to the instantaneous current, I is a steady-state conductivity component, A, C, E, G is the polarization intensity coefficients of different polarization branches respectively, and B, D, F, H is the time constant of the polarization branches respectively; the four branch fitting equation for the depolarization current curve is as follows: , wherein y d is the depolarized instantaneous current; For the time variable corresponding to the depolarization current, A, C, E, G is the polarization intensity coefficient of different depolarization branches, and B, D, F, H is the time constant of the depolarization branches; the calculation mode of the time constant of each branch is as follows: Polarized current The expression is: , Depolarization current The expression of (2) is: , in the formula, To polarize the current amplitude of each RC leg, , To depolarize the current magnitudes of the various RC branches, A 0 is the magnitude of the conduction current component, R pi is the equivalent resistance of the polarized ith branch, R di is the equivalent resistance of the depolarized ith branch, t p is the polarization time, and t d is the depolarization time; Is the polarization time constant of the i-th branch, ; Is the depolarization time constant of the i-th leg, I represents each branch, C pi is the equivalent capacitance of the polarized ith branch, C di is the equivalent capacitance of the depolarized ith branch, and U is the direct current voltage applied by the high voltage end of the PDC.
4. The method for diagnosing the local aging type of the XLPE cable based on the PDC branch number characteristics according to claim 1, wherein the method for judging whether the time constants of the branches are the same according to the time constants of the branches comprises the following steps: In response, the time constant of each branch is partially repeated, namely, partial branches are overlapped, the fourth branch is proved to be absent in the four-branch fitting, and the cable insulation defect is judged to be thermal aging or other common defects.
5. The method for diagnosing the local aging type of the XLPE cable based on the PDC branch number characteristics according to claim 1, wherein the depolarization current curve of the long-time section of the depolarization stage of the cable to be tested is subjected to power law item fitting, and a power law attenuation index is obtained The method comprises the following steps: the power law term fitting equation is as follows: , wherein y d is the depolarized instantaneous current; The time variable corresponding to the depolarization current is A, C, E, G, the polarization intensity coefficients of different depolarization branches are B, D, F, H, the time constant of the depolarization branches is B, D, F, H, K is a power law coefficient, and r is a power law attenuation index.
6. The XLPE cable local aging type diagnosis method based on PDC branch number characteristics is characterized in that the direct current voltage U is 1-5kV direct current voltage, and the polarization time T is 90s.

Description

XLPE cable local aging type diagnosis method based on PDC branch number characteristics Technical Field The invention relates to the technical field of cable insulation state detection, in particular to an XLPE cable local aging type diagnosis method based on PDC branch number characteristics. Background Crosslinked polyethylene (XLPE) medium voltage cables are key devices for distribution networks, and their insulating properties are critical to the safe operation of the distribution network and the power system. In the process of manufacturing, laying and running, the outer sheath and the shielding layer are damaged due to uneven insulation cooling, irregular laying or damage caused by external force, so that moisture can infiltrate into the insulating layer to cause the aging of local water tree. The insulation performance of the cable is gradually weakened by local defects such as heat aging, water tree and the like, so that the aged part becomes a local weak point, and finally breakdown accidents are caused, thereby causing serious consequences. Therefore, the method can accurately diagnose the local aging defect of the cable and timely make preventive measures, and has important significance for improving the reliability of the cable and prolonging the service life. The cable local defect diagnosis method mainly comprises alternating current withstand voltage test, partial discharge detection, dielectric loss tangent and capacitance measurement, a dielectric response method (such as polarization-depolarization current PDC) and the like. The alternating-current withstand voltage and partial discharge detection is sensitive to obvious defects, but has limited identification capability to early partial aging of water trees and the like (fig. 9 is a schematic diagram of an existing short/long cable water tree aging device), the dielectric loss tangent (tan delta) and the capacitance detection method are simple and visual, but are difficult to distinguish the partial defects from the overall aging, and compared with the polarization-depolarization current (PDC) method (a schematic diagram of a cable insulation test system of the existing polarization-depolarization current (PDC) method, which is shown in fig. 10), the time-domain current response of an insulation medium is obtained by applying step voltage, and the Debye equivalent circuit model parameters are fitted, so that the aging characteristics of insulation can be reflected. And when the PDC test is used for evaluating the insulation of the XLPE cable, the test flow is simple and convenient, is nondestructive, and is sensitive to the reflection of local serious defects. Thus, the high sensitivity of the PDC method to early and different types of defects is considered to be an important means for the diagnosis of medium voltage XLPE cable ageing. Currently, some studies and patents have employed polarization-depolarization current (PDC) tests in combination with extended Debye models to evaluate cable insulation 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, performs water tree aging identification by using a nonlinear capacitance parameter (Cd 3) in the third branch, and chinese patent CN202110400609.8 converts PDC polarized current into a frequency domain, extracts polarization loss tan δ, and determines a water tree defect in combination with peak frequency variation. The method has certain identification capability, but relies on a fixed branch structure, only extracts characteristic parameters in a local branch, and the fitting process relies on complex spectrum operation or exponential model calculation, so that the parameter solving has high randomness, the diagnosis result lacks uniformity, and the method has defects in distinguishing thermal aging from water tree aging. In addition, chinese patent CN202010635703.7 proposes that continuous multiple PDC tests are adopted to calculate the conductance current coefficient, and the aging state is determined according to the variation trend, which can reflect the asymmetric polarization behavior caused by the aging of the water tree, but has long test period and complex operation, and is not suitable for field application. However, the diagnosis object is mainly in an overall insulation state, and cannot effectively distinguish different types of local defects (such as water tree and thermal aging). Although some studies mention increasing the number of branches by extending the Debye model to optimize model accuracy, these additional branches exist only as mathematical fit means, are not used as diagnostic basis, and do not establish a clear correspondence with the physical polarization behavior of the water tree aging. In summary, although the diagnosis of the aging state of the cable can be realized to a certain extent in the prior art, the whole cab