Method for charge analysis and insulation state evaluation of active-service cable based on potential test

Abstract

The invention relates to an on-service cable charge analysis and insulation state evaluation method based on a potential test. The method is suitable for non-destructive detection and state evaluation of in-service cables with different voltage grades. 1) applying a direct-current voltage to a cable terminal to insulate and charge the cable; (2) after charging is finished, the high-voltage end and the grounding end of the terminal are shorted temporarily; 3) measuring the open-circuit voltage of the cable terminal to obtain a voltage change curve of the cable; 4) extracting data in a rising stage in the voltage change curve, and processing the data to obtain a potential attenuation curve of the cable; 5) analyzing and calculating charge characteristics and trap parameters of cable insulation based on an isothermal relaxation theory; according to the method, a cable body structure and a wiring mode do not need to be damaged, the charge characteristics and the trap parameters of cable insulation can be obtained, field implementation is facilitated, and an effective means is provided for insulation degradation diagnosis and service life prediction of the cable in active service.

Description

Technical Field

[0001] The present invention belongs to the field of cable insulation detection, and particularly relates to a method for analyzing the charge of in-service cables and evaluating the insulation status based on potential testing. Background Art

[0002] Cross-linked polyethylene (XLPE) cables are key equipment in power transmission and distribution projects. Due to advantages such as small volume, light weight, and flexible laying, XLPE cables have gradually become the first choice for urban underground pipe corridors and dense power grids. During long-term operation of the cables, space charge accumulation is likely to occur in XLPE insulation under the action of electrical, thermal stresses, and the environment, which will further lead to electric field distortion and insulation degradation. Insulation defects inside the cables may cause partial discharge or even breakdown, resulting in regional power outages and economic losses. At present, there is a lack of effective detection means for the charge characteristics of in-service cable insulation. Therefore, an evaluation technology for the insulation status of in-service cables based on charge and trap analysis cannot be formed.

[0003] Currently, the common measurement methods for the charge and trap characteristics of polymer materials mainly include: pulsed electroacoustic (PEA) method, laser-induced pressure pulse (LIPP) method, thermally stimulated current (TSC) method, isothermal surface potential decay (ISPD) method, etc. The above measurement methods are mainly applied to the testing of laboratory insulation samples, and there are certain limitations for the testing of coaxial cables. Among them, when the PEA method is applied to the testing of coaxial cables, the outer sheath and metal sheath of the cable need to be damaged; when the LIPP method is used for on-site testing, the multi-layer structure of the cable needs to be considered, and it is very difficult to ensure the feasibility and accuracy of the measurement; due to the need for a rapid cooling and uniform heating process, the TSC method cannot be realized in on-site testing. The ISPD method is convenient for measurement and low in cost, and is widely used in the characterization of trap distribution and charge transport characteristics of polymer materials. However, at present, the potential decay test for cable insulation is mainly based on the corona charging method, and the test objects are mainly cable insulation slice samples, and there are few potential decay tests for coaxial cables.

[0004] CN 118914769 A discloses a method for calculating the breakdown field strength of cable insulation with different thicknesses at different temperatures. Trap parameters are obtained through surface potential decay testing, and the breakdown field strength is predicted by combining with the bipolar carrier model. However, the research object only focuses on cable insulation slice samples, which is applicable to the performance research of insulation materials in the laboratory environment and is not applicable to the detection and analysis of in-service cables.

[0005] CN 116482208 A discloses a method and device for evaluating the compatibility between silicone rubber and impregnating agent for capacitors. The trap energy level and density are calculated through the surface potential decay curve to evaluate the compatibility between silicone rubber and impregnating agent. This method uses the corona charging method and is aimed at the testing of material-level samples.

[0006] CN 119087147 A discloses a method for identifying equivalent circuit parameters of a transformer insulation system. Based on the measured results of the recovery voltage of the transformer, the characteristic quantities of the recovery voltage, the characteristic quantities of the frequency-domain dielectric spectrum, and the parameters of the extended Debye model are obtained, so as to carry out the diagnosis of transformer insulation aging. This method does not analyze the charge and trap characteristics of the transformer insulation and does not propose an insulation state evaluation method based on charges and traps.

[0007] CN 106199194 A discloses a method for extracting the surface tracking state characteristics of ethylene-propylene rubber insulation of mine cables. Multiple time-domain characteristic quantities are extracted from signals such as polarization current and recovery voltage, and the surface tracking state of the mine cable insulation is characterized based on the characteristic quantities. This patent cannot analyze the charge and trap characteristics of the cable insulation. Similarly, the research object only targets the surface tracking corrosion of ethylene-propylene rubber insulation samples and does not target the entire cable.

[0008] CN 113899997 A discloses a method for diagnosing the insulation state of a transformer based on an improved support vector machine. The recovery voltage method is used to measure the characteristic quantities of the existing transformer oil-paper insulation system, and the degree of polymerization of unknown oil-impregnated paperboard is predicted, so as to achieve the purpose of diagnosing the insulation state of unknown transformers. This method uses a machine learning algorithm and relies on multi-source data fusion, but it cannot analyze the charge and trap characteristics of the test object and cannot diagnose the state of the test object based on charge and trap parameters.

[0009] Most of the existing technologies are aimed at the charge and trap tests of insulation samples, and cannot realize the analysis of the charge and trap characteristics of in-service cables. There is a lack of an evaluation method for the insulation aging state of in-service cables based on charge and trap analysis. At present, most charge measurement technologies rely on equivalent models or destructive tests in the laboratory environment, which cannot meet the rapid detection requirements of in-service cables and have insufficient on-site applicability. In addition, the traditional potential decay test takes up to several hours and is difficult to apply on-site. Summary of the Invention

[0010] Based on the above problems, the present invention proposes a method for analyzing the charge of in-service cables and evaluating the insulation state based on potential testing, which can achieve the efficient testing of the cable potential without damaging the cable body structure and without changing the wiring of the in-service cable, and realize the charge analysis of the in-service cable insulation and the evaluation of the insulation state based on the potential results.

[0011] The specific technical solution is: a method for analyzing the charge of in-service cables and evaluating the insulation state based on potential testing, the method comprising:

[0012] 1) Apply a DC voltage to the cable terminal to charge the cable insulation;

[0013] 2) After the charging is completed, short-circuit the high-voltage end and the grounding end of the cable terminal;

[0014] 3) Disconnect the high-voltage terminal and the grounding terminal, measure the open-circuit voltage of the cable terminal, and obtain the voltage change curve of the cable;

[0015] 4) Extract the data in the rising stage of the voltage change curve, and obtain the potential decay curve of the cable after processing;

[0016] 5) Calculate the charge characteristics and trap parameters of the cable insulation;

[0017] 6) Evaluate the aging state of the cable insulation based on the potential characteristic parameters and trap parameters.

[0018] The present invention has the following beneficial effects:

[0019] The present invention can realize non-destructive and rapid detection of the potential curve of in-service cables, obtain the charge characteristics and trap parameters of in-service cable insulation, and comprehensively evaluate the insulation defects and aging state of in-service cables from voltage parameters, charge characteristics and trap parameters, which can provide an effective means for cable insulation degradation diagnosis and life prediction. Description of the Drawings

[0020] Figure 1 is the schematic diagram of the potential test for in-service cables;

[0021] Figure 2 is the flow chart of the method of the present invention;

[0022] Figure 3 is the potential curve diagram of the measured 220 kV in-service cable;

[0023] Figure 4 is the trap distribution diagram of the 220 kV in-service cable. Detailed Embodiments

[0024] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other. To achieve the above object, the present invention adopts the following technical solutions.

[0025] The present invention provides a method for analyzing the charge of in-service cables and evaluating the insulation state based on potential testing, which is applicable to non-destructive testing and state evaluation of in-service cables with different voltage levels.

[0026] The implementation steps are as follows:

[0027] 1) Apply a DC voltage to the cable insulation for charging at the cable terminal 1, and the charging electric field ( 1) 300 - 1000 V / mm, where U is the charging voltage, D is the cable insulation thickness, and the charging time is 5 - 30 min;

[0028] 2) After charging, short - circuit the high - voltage end and the grounding end of the cable terminal, and the short - circuit time is 10 - 120 s;

[0029] 3) Disconnect the high - voltage end and the grounding end, measure the open - circuit voltage of the cable terminal, and the measurement time is at least up to the time when the voltage curve appears at its peak, about 5 - 120 min, to obtain the voltage change curve of the cable;

[0030] 4) Extract the data in the rising stage of the voltage change curve, and after processing, obtain the potential decay curve of the cable;

[0031] 5) Analyze and calculate the charge characteristics and trap parameters of the cable insulation based on the isothermal relaxation theory;

[0032] 6) Evaluate the aging state of the cable insulation based on the potential characteristic parameters and trap parameters.

[0033] Figure 1 is a schematic diagram of potential measurement. Among them, 1 is the cable terminal, 2 is the high - voltage DC power supply, 3 is the voltage measurement unit, 4 is the relay, and 5 is the parallel resistor. Among them, S1, S2, and S3 of the relay 4 are the connection terminals of the high - voltage DC power supply, the grounding end, and the voltage measurement unit connection terminal respectively.

[0034] Furthermore, Figure 2 is a flow chart of the method for analyzing the charges of in - service cables and evaluating the insulation state based on potential measurement; the specific steps are as follows:

[0035] Before testing the cable terminal 1, pretreatment is required. Connect the high - voltage end and the grounding end of the cable terminal 1 to release the residual charges in the cable.

[0036] Apply a DC voltage to the cable terminal 1 through the high - voltage DC power supply 2 to charge the cable insulation.

[0037] After charging, control the relay 4 to the S2 state, and briefly short - circuit the high - voltage end and the grounding end of the cable terminal 1 to release the residual charges at the high - voltage end.

[0038] Control the relay 4 to the S3 state, and measure the open - circuit voltage of the cable terminal through the voltage measurement unit 3 to obtain the curve of the cable voltage changing with time; the parallel resistor 5 is used to adjust the arrival time of the cable voltage peak, and its resistance value is generally 1 - 3 orders of magnitude smaller than the cable insulation resistance.

[0039] Extract the voltage peak and peak - time parameters in the voltage curve, and based on the data in the voltage rising stage, obtain the equivalent potential decay curve of the cable after processing.

[0040] Analyze the charge characteristics of cable insulation based on the isothermal relaxation theory, and calculate the trap parameters of cable insulation, such as trap energy level and trap charge density.

[0041] Based on the potential decay curve of the cable, that is, the curve of the cable potential changing with time , the formula for calculating the trap parameters is as follows:

[0042] ;

[0043] ;

[0044] Among them, is the trap energy level, is the Boltzmann constant, is the cable temperature, is the trap charge escape frequency, is the current density, D is the cable insulation thickness, is the electron charge, is the initial occupancy of the trap, is the trap energy distribution function, is the vacuum permittivity, is the relative permittivity of the cable insulation, is the cable potential value, is the trap charge density, is the time.

[0045] Comprehensively evaluate the aging state of the cable insulation according to the obtained voltage peak, peak time, charge characteristics, trap energy level and trap charge density. The higher the voltage peak, the longer the peak time, the deeper the trap energy level and the larger the trap density correspond to the poorer insulation state.

[0046] Example 1: On-site cable potential test

[0047] Perform a potential test on a 220 kV in-service XLPE cable. The cable length is about 140 m. The charging voltage is 8 kV and the charging time is 600 s; after charging is completed, the short-circuit time between the high-voltage terminal and the grounding terminal is 30 s, the voltage measurement time is 500 s, and the parallel resistance is 10 GΩ. The obtained potential curve is as Figure 3 shown. It can be seen from the figure that the voltage peak of the cable is about 3.2 V and the peak time is about 180 s. Based on the potential change in the voltage rising stage, the calculated trap parameters of the cable insulation are as Figure 4 shown. There is a trap energy level center in the cable insulation, about 0.92 eV, and the peak value of the trap charge density is about 1.7×10 11 m -2 . According to the cable voltage peak, peak time and trap parameters, combined with other parameters of the cable insulation, the aging state of the cable can be evaluated.

[0048] Figure 3 It is the potential curve of the existing 220 kV XLPE cable, which reflects the variation characteristics of the cable insulation voltage with time. The abscissa is time and the ordinate is the voltage value. The voltage peak and the corresponding time are marked in the figure, which are the voltage characteristic parameters. The data between zero and the voltage peak are the cable potential for trap parameter calculation. .

[0049] Figure 4 It is the trap parameter distribution of the existing 220 kV XLPE cable, where the abscissa is the trap energy level and the ordinate is the trap charge density, that is, the cable insulation trap parameters calculated according to Figure 3 the obtained cable potential The aging state of the cable insulation can be judged according to the trap energy level and the trap charge density. Generally speaking, the deeper the trap energy level and the larger the trap charge density, the more serious the cable aging.

[0050] In the present invention, the test object can be other capacitive power equipment, such as transformers, bushings, etc.

[0051] In the present invention, the measurement object can be an existing device or an insulation sample.

[0052] In the present invention, the charging voltage, charging time, short - circuit time and measurement time can be adjusted according to the test object.

Claims

1. A method for analyzing the charge of in-service cables and evaluating the insulation status based on potential testing, characterized in that The method includes the following steps: 1) Applying a DC voltage to the cable terminal to charge the cable insulation; 2) After charging, short-circuiting the high-voltage terminal and the grounding terminal of the cable terminal; 3) Disconnecting the high-voltage terminal and the grounding terminal, measuring the open-circuit voltage of the cable terminal, and obtaining the voltage change curve of the cable; 4) Extracting the data in the rising stage of the voltage change curve, and obtaining the potential decay curve of the cable after processing; 5) Calculating the charge characteristics and trap parameters of the cable insulation based on the potential decay curve; 6) Evaluating the aging state of the cable insulation based on the potential characteristic parameters and trap parameters.

2. The method for analyzing the charge of in-service cables and evaluating the insulation status based on potential testing according to claim 1, wherein In step 1), the charging electric field E is 300 - 1000 V / mm, where, , U is the charging voltage, D is the cable insulation thickness, and the charging time is 5 - 30 min.

3. A method for analyzing the charge of an in-service cable and evaluating the insulation status based on potential measurement according to claim 1, characterized in that In step 2), the short-circuiting time is 10 - 120 s.

4. The method for analyzing the charge of an in-service cable and evaluating the insulation status based on potential testing according to claim 1, characterized in that, In step 3), the measurement time is at least up to the time when the voltage change curve appears at the peak.

5. The method for analyzing the charge of an in-service cable and evaluating the insulation status based on potential measurement according to claim 1, wherein Based on the potential decay curve, the voltage peak and the peak time are obtained.

6. The method for analyzing the charge of an in-service cable and evaluating the insulation status based on potential testing according to claim 1, characterized in that The formula for calculating the trap parameters is as follows: ； ； Among them, is the trap energy level, is the Boltzmann constant, is the cable temperature, is the trap charge escape frequency, is the current density, D is the cable insulation thickness, is the electron charge, is the initial trap occupancy, is the trap energy distribution function, is the vacuum permittivity, is the relative permittivity of the cable insulation, is the cable potential value, is the trap charge density, is the time.

7. A method for analyzing the charge of in-service cables and evaluating the insulation status based on potential testing according to claim 1, characterized in that The test object can also be other capacitive power equipment, such as transformers and bushings.

8. A method for analyzing the charge of an in-service cable and evaluating the insulation status based on potential testing according to claim 1, characterized in that The charging voltage, charging time, short-circuiting time, and measurement time can be adjusted according to the test object.

9. A method for analyzing the charge of in-service cables and evaluating the insulation status based on potential testing according to claim 1, characterized in that, The measurement object can be an in-service device or an insulation sample.

10. A method for analyzing the charge of in-service cables and evaluating the insulation status based on potential testing according to claim 1, characterized in that Based on the obtained voltage peak, peak time, charge transport characteristics, trap energy level, and trap charge density, the aging state of the cable insulation is comprehensively evaluated.

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