Cable partial aging state diagnosis method and device based on PDC branch parameter method

Abstract

The application relates to the technical field of cable insulation state detection, and is a cable partial aging state diagnosis method and device based on a PDC branch parameter method.The device comprises a PDC test unit, which performs PDC test on a cable to be tested to obtain a polarization / depolarization current curve of the cable to be tested; a conductance calculation unit, which obtains a steady-state conductance current by subtracting a steady-state value of a depolarization current curve from a steady-state value of a polarization current curve, and calculates a direct current conductance gamma according to the steady-state conductance current; and an asymmetry coefficient calculation unit, which calculates an asymmetry coefficient; the application takes the change of the direct current conductance calculated through continuous PDC tests as a criterion to accurately identify water tree aging and other general defects such as thermal aging; meanwhile, the asymmetry coefficient is calculated to judge the partial aging degree of the cable; and the application synchronously realizes partial aging type identification and aging degree diagnosis through quantifiable diagnosis values.

Description

Technical Field

[0001] This invention relates to the field of cable insulation condition detection technology, and in particular to a method and apparatus for diagnosing local aging conditions of cables based on the PDC branch parameter method. Background Technology

[0002] As crucial transmission and distribution equipment in power systems, the operational stability of cables directly impacts the safety and reliability of the entire power system. However, during long-term operation, due to factors such as improper laying, harsh operating environments, and external mechanical stresses, cables frequently exhibit localized defects such as outer sheath damage, moisture infiltration, copper shielding layer damage, and insulation aging. These defects lead to a decline in the cable's local insulation performance, which, under the influence of a continuous electric field, gradually develops into a breakdown fault, thereby triggering power system accidents.

[0003] As the service life of cables increases, the main insulation commonly exhibits typical defects such as water treeing and thermal aging. Figure 10 This is a schematic diagram of an existing cable water treeing aging device. Aging defects pose a serious threat to the normal operation of cables. If the degree of aging is not identified and assessed in a timely manner and countermeasures are not taken, the risk of cable breakdown will greatly increase, potentially leading to serious power accidents. Therefore, accurate diagnostic assessment of the cable insulation condition is necessary in engineering. Polarization / depolarization current (PDC) testing methods (such as...) Figure 11 As shown in the diagram, this is a schematic diagram of a cable insulation testing system based on the existing polarization / depolarization current (PDC) method. It can extract equivalent model parameters by acquiring the time-domain current response of the insulation medium, thereby reflecting the aging characteristics of the cable insulation.

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

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

[0006] Chinese invention patent CN202010635703.7 proposes to identify water tree aging by calculating the changing trend of the "conductivity current coefficient" through multiple consecutive PDC tests. It has a targeted advantage in water tree detection, but its method is effective for specific aging modes and has limited applicability to other aging types and other problems.

[0007] Chinese invention patent CN201911089429.1 proposes to use the change in low-frequency dielectric loss factor under different polarization voltages as an aging characteristic. It has high sensitivity and can reflect the overall insulation aging. However, it does not provide a clear threshold or standardized discrimination process to support engineering decisions.

[0008] As can be seen from the publicly available information, existing methods mainly focus on assessing the overall aging degree of cables or identifying specific aging types. While they play a certain role in the selection of testing methods and indicators, they still have the following limitations: 1. Some methods lack unified and standardized judgment criteria and have insufficient field applicability. Most methods are only applicable to water treeing and are insufficient in judging other general aging types such as thermal aging; 2. Existing methods often focus on judging the overall aging trend of cables and are difficult to reflect the severity of local defects; 3. Some methods have not yet established a mature judgment system and, under certain circumstances, still require testing experience to make judgments, thus limiting their applicability in actual cable operation and maintenance. Summary of the Invention

[0009] This invention provides a method and apparatus for diagnosing the local aging state of cables based on the PDC branch parameter method, which overcomes the shortcomings of the prior art and can effectively solve the problem that existing methods for judging the local aging of cables cannot judge the severity of local aging defects.

[0010] To address the above problems, one of the technical solutions of this invention is implemented through the following method: a method for diagnosing the local aging state of cables based on the PDC branch parameter method, comprising the following steps:

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

[0012] 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, and the DC conductance γ is calculated based on the steady-state conductance current.

[0013] 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. ;

[0014] Calculate the DC conductance increment of DC conductance γ in two consecutive calculations. Determine the increase in DC conductance Whether the change is significant or greater than zero;

[0015] Therefore, if the DC conductivity increases with the number of tests, it is determined that water treeing aging exists inside the cable under test.

[0016] The DC conductance increment is either positive or negative. If the change is not significant or equal to zero, it indicates that the change in DC conductance 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.

[0017] 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;

[0018] The response indicates mild water tree aging, with a low risk of breakdown.

[0019] 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; 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.

[0020] The above-mentioned PDC test was performed on the cable under test to obtain the polarization / depolarization current curve of the cable under test, including:

[0021] The outer surface of the cable sample to be tested is cleaned and dried to reduce experimental errors;

[0022] 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.

[0023] A DC voltage U is applied to the cable to polarize it, and the polarization time is... t p Meanwhile, the curve of polarization current changing with time was recorded;

[0024] After shutting off the DC voltage, a depolarization test was performed. The depolarization time was [time missing]. t d Simultaneously, the depolarization current decay curve over time was collected.

[0025] 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 conductance γ is then calculated based on this steady-state conductance current, including:

[0026] The formula for calculating DC conductance is:

[0027] ,

[0028] Where γ is the DC conductance, The conduction current in steady state The applied DC voltage.

[0029] The above method substitutes the polarization / depolarization current curves into the Debye three-branch equivalent circuit model and fits 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:

[0030] The polarization / depolarization current of the cable under test during PDC testing is represented by a DC resistance branch plus three parallel RC branches;

[0031] polarization current The expression is:

[0032] ,

[0033] Depolarization current The expression is:

[0034] ,

[0035] 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; 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 The DC voltage applied to the high-voltage side of the PDC;

[0036] asymmetric coefficient The calculation formula is:

[0037] ,

[0038] In the formula, The time constant of the third polarization branch. This is the time constant for the depolarization third branch.

[0039] The above calculations determine the DC conductance increment of DC conductance γ over two consecutive calculations. The calculation method is as follows:

[0040] ,

[0041] In the formula, For changes in DC conductance, They are the first n +1 and the n The DC conductance of this test n =1,2,……

[0042] After determining that the cable under test does not have aging, or has thermal aging, or other types of aging, the asymmetry coefficient is calculated. Determine whether the asymmetry coefficient is greater than 1.0 and less than 1.1;

[0043] Therefore, the response indicates mild thermal aging, with a very low risk of breakdown.

[0044] 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.

[0045] The second technical solution of the present invention is achieved through the following method: a cable local aging condition diagnosis device based on the PDC branch parameter method, wherein the device is implemented using a cable local aging condition diagnosis method based on the PDC branch parameter method, comprising:

[0046] The PDC test unit performs PDC testing on the cable under test to obtain the polarization / depolarization current curve of the cable under test.

[0047] The conductivity calculation unit takes the steady-state value of the polarization current curve and subtracts the steady-state value of the depolarization current curve to obtain the steady-state conductance current, and calculates the DC conductance γ based on the steady-state conductance current.

[0048] 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. ;

[0049] The aging type determination unit calculates the DC conductance increment of DC conductance γ in two consecutive measurements. Determine the increase in DC conductance Whether the change is significant or greater than zero;

[0050] The first response unit responds by indicating that the DC conductivity increases with the number of tests, thus indicating that water treeing aging exists inside the cable under test.

[0051] The second response unit, in response to no, then the DC conductance increment. If the change is not significant or equal to zero, it indicates that the change in DC conductance 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.

[0052] 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;

[0053] The third response unit, upon responding, is then determined to be mild water tree aging with a low risk of breakdown.

[0054] 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.

[0055] The aforementioned PDC test unit includes a processing module, a connection module, a polarization test module, and a depolarization test module;

[0056] The processing module cleans and dries the outer surface of the cable sample under test to reduce experimental errors;

[0057] 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.

[0058] The polarization test module applies a DC voltage U to the cable to polarize it, and the polarization time is... t p Meanwhile, the curve of polarization current changing with time was recorded;

[0059] The depolarization test module performs a depolarization test after the DC voltage is turned off. The depolarization time is...t d Simultaneously, the depolarization current decay curve over time was collected.

[0060] Compared with the prior art, the present invention has the following advantages:

[0061] 1. This invention uses changes in DC conductivity as a preliminary basis for judging the type of local aging, thereby achieving a preliminary and accurate identification of water tree aging and other general defects such as thermal aging. Furthermore, by calculating the asymmetry coefficient, the degree of local aging can be further determined. Therefore, this invention simultaneously achieves the identification of local aging type and the diagnosis of aging degree through quantifiable diagnostic values.

[0062] 2. This invention establishes a clear judgment threshold system, which overcomes the shortcomings of existing methods that rely on experience-based judgment, and has strong reliability and engineering applicability. Attached Figure Description

[0063] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0064] Figure 1 This is a flowchart of the method in Embodiment 1 of the present invention.

[0065] Figure 2 This is the Debye model of the three branches of the aging cable in Embodiment 1 of the present invention.

[0066] Figure 3 This is a block diagram of the device structure in Embodiment 2 of the present invention.

[0067] Figure 4 This is a block diagram of the device structure of the PDC test unit in Embodiment 2 of the present invention.

[0068] Figure 5 This is the PDC polarization current curve in Embodiment 3 of the present invention.

[0069] Figure 6 This is the PDC depolarization current curve in Embodiment 3 of the present invention.

[0070] Figure 7 This is a schematic diagram of the thermally aged cable in Embodiment 3 of the present invention.

[0071] Figure 8 This is the PDC polarization current curve in Embodiment 4 of the present invention.

[0072] Figure 9 This is the PDC depolarization current curve in Embodiment 4 of the present invention.

[0073] Figure 10 This is a schematic diagram of a water tree aging cable in the background art of this invention.

[0074] Figure 11This is a schematic diagram of PDC testing in the background technology of this invention. Detailed Implementation

[0075] 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.

[0076] 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:

[0077] S101, perform PDC test on the cable under test to obtain the polarization / depolarization current curve of the cable under test;

[0078] S102, take the steady-state value of the polarization current curve and subtract the steady-state value of the depolarization current curve to obtain the steady-state conductance current, and calculate the DC conductance γ based on the steady-state conductance current.

[0079] 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. ;

[0080] S104, Calculate the DC conductance increment of DC conductance γ in two consecutive cycles. Determine the increase in DC conductance Whether the change is significant or greater than zero;

[0081] S105, the response is that the DC conductivity increases with the number of tests, indicating that water treeing aging exists inside the cable under test;

[0082] S106, in response to no, the DC conductance increment If the change is not significant or equal to zero, it indicates that the change in DC conductance 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.

[0083] S107, after determining that the cable under test has water tree aging, calculate the asymmetry coefficient. Determine whether the asymmetry coefficient is greater than 1.3 and less than 1.6;

[0084] S108, if the response is as follows, it is determined to be mild water tree aging, with a low risk of breakdown;

[0085] S109, 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; 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.

[0086] 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:

[0087] The outer surface of the cable sample to be tested is cleaned and dried to reduce experimental errors;

[0088] 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.

[0089] A DC voltage U is applied to the cable to polarize it, and the polarization time is... t p Meanwhile, the curve of polarization current changing with time was recorded;

[0090] After shutting off the DC voltage, a depolarization test was performed. The depolarization time was [time missing]. t d Simultaneously, the depolarization current decay curve over time was collected.

[0091] The outer surface of the cable sample to be tested should be cleaned to remove dust, dirt and oil; the cable surface should be kept as dry as possible to avoid interference from leakage current in a humid environment.

[0092] The DC voltage U applied to the cable can be 1-5kV; when performing polarization-depolarization current (PDC) testing, the cable under test needs to be tested multiple times (3 times or more) after the cable has stopped running for a period of time; the polarization / depolarization time t is controlled to be above 90s.

[0093] 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 conductance is calculated, the DC conductance can be observed to gradually increase.

[0094] In step S102 above, 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 conductance γ is then calculated based on the steady-state conductance current, including:

[0095] The formula for calculating DC conductance is:

[0096] ,

[0097] Where γ is the DC conductance, The conduction current in steady state The applied DC voltage.

[0098] 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:

[0099] The polarization / depolarization current of the cable under test during PDC testing is represented by a DC resistance branch plus three parallel RC branches;

[0100] polarization current The expression is:

[0101] ,

[0102] Depolarization current The expression is:

[0103] ,

[0104] 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; 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 The DC voltage applied to the high-voltage side of the PDC;

[0105] asymmetric coefficient The calculation formula is:

[0106] ,

[0107] In the formula, The time constant of the third polarization branch. This is the time constant for the depolarization third branch.

[0108] in, , , and All were determined using the least squares fitting method. The three branches determined by the fitting were numbered as the first branch, the second branch, and the third branch in ascending order of time constant. The polarization time constant and depolarization time constant of the third branch were obtained by fitting. At the same time, the cable under test was tested multiple times and the change of DC conductance during the test was recorded.

[0109] The fitting equation for the polarization current curve is:

[0110] ,

[0111] In the formula, The instantaneous current during the polarization phase (unit: A); The instantaneous current corresponds to the time (in seconds). Here, A represents the steady-state conductance component; A, C, and D represent the polarization intensity coefficients of different polarization branches; B, E, and F represent the time constants of the branches; the largest of B, E, and F is the time constant of the third polarization branch. ;

[0112] The fitting equation for the depolarization current curve is:

[0113] ,

[0114] In the formula, The instantaneous current for depolarization (unit: A); The instantaneous current corresponds to the time (in seconds); A, C, and D are the polarization intensity coefficients of different depolarization branches; B, E, and F are the time constants of the branches, with the largest being the time constant of the third depolarization branch. .

[0115] In step S104 above, the DC conductance increment of DC conductance γ is calculated for two consecutive DC conductance intervals. The calculation method is as follows:

[0116] ,

[0117] In the formula, For changes in DC conductance, They are the first n +1 and the n The DC conductance of this test n =1,2,……

[0118] In step S106 above, after determining that the cable under test does not have water tree aging but has thermal aging or other types of aging, the asymmetry coefficient is calculated. Determine whether the asymmetry coefficient is greater than 1.0 and less than 1.1;

[0119] Therefore, the response indicates mild thermal aging, with a very low risk of breakdown.

[0120] 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.

[0121] In step S106, the DC conductance increment Whether the change is not significant, including the increase in DC conductance. If the change is within ±10% or remains zero, it indicates that the change in conductivity is small, suggesting that the cable is not aged or has thermal aging or other types of aging defects. Thermal aging or other general types of aging defects are usually caused by the cable being exposed to high temperatures or other environments for a long time, leading to the deterioration of the insulation material. The change in conductivity is not obvious or small because the aging process mainly affects changes such as the breakage of molecular chains in the material.

[0122] 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.

[0123] 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.

[0124] In summary, this invention uses the change in DC conductance calculated from multiple consecutive PDC tests as a criterion to accurately identify water tree aging and other general defects such as thermal aging; at the same time, by calculating the asymmetry coefficient, it can determine the degree of local aging of the cable. This invention can simultaneously identify the type of local aging and diagnose the degree of aging through quantifiable diagnostic values.

[0125] 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.

[0126] 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:

[0127] The PDC test unit performs PDC testing on the cable under test to obtain the polarization / depolarization current curve of the cable under test.

[0128] The conductivity calculation unit takes the steady-state value of the polarization current curve and subtracts the steady-state value of the depolarization current curve to obtain the steady-state conductance current, and calculates the DC conductance γ based on the steady-state conductance current.

[0129] 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. ;

[0130] The aging type determination unit calculates the DC conductance increment of DC conductance γ in two consecutive measurements. Determine the increase in DC conductance Is it greater than zero?

[0131] The first response unit responds by indicating that the DC conductivity increases with the number of tests, thus indicating that water treeing aging exists inside the cable under test.

[0132] The second response unit, in response to no, then the DC conductance increment. If the change is not significant or equal to zero, it indicates that the change in DC conductance 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.

[0133] The aging degree assessment unit calculates the asymmetry coefficient after determining that the cable under test exhibits water tree aging. τ as Determine whether the asymmetry coefficient is greater than 1.3 and less than 1.6;

[0134] The third response unit, upon responding, is then determined to be mild water tree aging with a low risk of breakdown.

[0135] 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.

[0136] The aforementioned PDC test unit includes a processing module, a connection module, a polarization test module, and a depolarization test module;

[0137] The processing module cleans and dries the outer surface of the cable sample under test to reduce experimental errors;

[0138] 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.

[0139] The polarization test module applies a DC voltage U to the cable to polarize it, and the polarization time is...t p Meanwhile, the curve of polarization current changing with time was recorded;

[0140] The depolarization test module performs a depolarization test after the DC voltage is turned off. The depolarization time is... t d Simultaneously, the depolarization current decay curve over time was collected.

[0141] Example 3: 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.

[0142] The 12m three-phase cable was stripped into three single-phase samples.

[0143] 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 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.

[0144] 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.

[0145] The third phase cable was kept intact as a defect-free control sample.

[0146] 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;

[0147] 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. Figure 6 As shown.

[0148] In the process of identifying cable aging types, the local aging type of the cable is identified by measuring the change in DC conductance multiple times; the statistical results of DC conductance are shown in Table 1, and the statistical results of the time constant and unbalance coefficient of the third branch are shown in Table 2.

[0149] Table 1 shows that the DC conductance of the unaged and heat-aged cables did not change significantly in multiple tests, while the DC conductance of the water-tree-aged cable continuously increased in multiple tests. Table 2 shows the asymmetry coefficient of the unaged cable. The asymmetry coefficient of thermally aged cables is 1.12, while that of water-tree aged cables is... Within the specified range, and through the analysis of the asymmetry coefficient of cables with different aging characteristics, the severity of aging defects can be quantitatively analyzed, providing data support for line inspection and maintenance. Furthermore, this method significantly improves the accuracy of determining local aging types by using threshold parameters for different aging types.

[0150] Example 4: To verify the effectiveness of the method of the present invention, a section of cable undergoing power outage maintenance was tested in a substation. The cable has a rated voltage of 10kV, a length of 1100m, and a conductor cross-section of 3×185mm².

[0151] During PDC testing, 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 for 90 seconds. The curve of polarization current changing over time is recorded. Figure 8 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 9 As shown.

[0152] In the process of identifying the local aging type of the cable, the aging type of the cable is identified by the change in DC conductance measured multiple times. The statistical results of DC conductance in this embodiment are shown in Table 3, and the statistical results of the time constant and unbalance coefficient of the third branch are shown in Table 4.

[0153] The statistical results of DC conductance obtained from multiple tests in Table 3 show that the DC conductance of the three-phase cable does not change much after multiple tests, indicating that there is no water tree aging defect. It is determined that the aging defects of the three-phase cable are all thermal aging or other general defect types.

[0154] Table 4 shows the asymmetry coefficient of phase A cable. The asymmetry coefficient of phase B cable The asymmetry coefficient of C-phase cable The asymmetry coefficients of the three-phase cables are all within the thermal aging range, with phase C exhibiting the largest asymmetry coefficient. According to the aging degree judgment formula, phase A cable shows mild thermal aging, while phases B and C show moderate thermal aging. Therefore, during maintenance and repair, the operating status of phases B and C should be the primary focus. Furthermore, among these three phases, phase C also shows the most severe localized aging. Based on analysis of local personnel and the geographical environment, the cable indeed only exhibits general defects such as thermal aging, and there are no water treeing defects. This demonstrates the effectiveness of the method. This method, by using threshold parameters for different aging types and judging the degree of aging, significantly improves the accuracy of the judgment.

[0155] Table 1 DC conductivity of cables with different local aging types in Example 3 Example 1

[0156]

[0157] Table 2. Time constants of cable depolarization in the third branch under different local aging types in Example 3.

[0158]

[0159] Table 3. Results of multiple tests on DC conductance of different phase cables in Example 4.

[0160]

[0161] Table 4. Time constants and asymmetry coefficients of the third branch of different phase cable polarizations in Example 4

[0162]

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, and the DC conductance γ is 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 the DC conductance increment of DC conductance γ in two consecutive calculations. Determine the increase in DC conductance Whether the changes are significant; Therefore, if the DC conductivity increases with the number of tests, it is determined that water treeing aging exists inside the cable under test. The DC conductance increment is either positive or negative. The change is not significant, meaning the increase in DC conductance is not significant. If the change is within ±10%, it indicates that the change in DC conductance 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... t p 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]. t d 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 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 conductance γ is then calculated based on the steady-state conductance current, including: The formula for calculating DC conductance is: ， Where γ is the DC conductance, 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 the DC conductance increment γ between two consecutive DC conductance measurements. The calculation method is as follows: ， In the formula, For changes in DC conductance, They are the first n +1 and the n The DC conductance 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 takes the steady-state value of the polarization current curve and subtracts the steady-state value of the depolarization current curve to obtain the steady-state conductance current, and calculates the DC conductance γ based on the steady-state conductance 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 conductance increment of DC conductance γ in two consecutive measurements. Determine the increase in DC conductance Whether the changes are significant; The first response unit responds by indicating that the DC conductivity increases with the number of tests, thus indicating that water treeing aging exists inside the cable under test. The second response unit, in response to no, then the DC conductance increment. The change is not significant, meaning the increase in DC conductance is not significant. If the change is within ±10%, it indicates that the change in DC conductance 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... t p 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... t d Simultaneously, the depolarization current decay curve over time was collected.

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