A method for evaluating the status of distribution cables based on the combined weight theory

Abstract

The present invention discloses a distribution cable status evaluation method based on combined weight theory, which comprehensively considers the comprehensive status of the three components of the distribution cable body, the intermediate joint and the terminal joint. First, the degradation degree of the indicators of different components is calculated, and the weight coefficients of each indicator in the distribution cable degradation evaluation are calculated based on the hierarchical analysis method and the entropy weight method, and then the combined weight coefficient is determined by the weight of the hierarchical analysis method and the entropy weight method. Finally, the operation status evaluation model is obtained by combining the distribution cable reliability analysis sub-model and an embodiment is given based on the data collected on site, verifying the effectiveness of the model. The present invention constructs a distribution cable operation status evaluation method, which can provide support for differentiated maintenance according to the evaluation results, avoid the problems of insufficient maintenance and excessive maintenance in traditional preventive maintenance, and effectively improve the reliability and economy of the operation of power distribution cables.

Description

Technical Field

[0001] The present invention belongs to the field of power cable engineering, and particularly relates to a method for evaluating the state of distribution cables based on the combined weight theory. Background Art

[0002] Distribution cables are one of the most important devices in the process of electric energy transmission, with advantages such as high power supply reliability and low possibility of electric shock. Underground distribution cables have gradually replaced overhead distribution lines and have become the mainstream of future cable development. Due to cable manufacturing, installation anomalies, or long-term insulation aging and deterioration, which ultimately lead to insulation breakdown, it is a major hidden danger to the safe operation of the power grid. Therefore, it is necessary to evaluate and analyze the state of distribution cable lines to provide guidance for the operation and maintenance of power cables.

[0003] Currently, the research on the state evaluation of distribution cables mostly uses the analytic hierarchy process model based on the point deduction method for feature fusion. This method has the disadvantages of strong subjectivity and difficulty in accurate quantification. The present invention combines the advantages of the analytic hierarchy process and the entropy weight method based on the combined weight theory, avoiding the disadvantage of strong subjectivity of the model weight and overcoming the problem that the accuracy of the objective model completely depends on the data quality. At the same time, the present invention combines the statistical model of the reliability of distribution cables, and introduces variables such as laying method, operation mode, and operation years through this method to further improve and correct the accuracy of the model. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for evaluating the state of distribution cables based on the combined weight theory, and propose a distribution cable state evaluation index system and its deterioration degree that comprehensively consider the distribution cable body, intermediate joints, and terminal joints. The combined weight is comprehensively determined by using the analytic hierarchy process and the entropy weight method, a reliability analysis model of the distribution cable is established, and the health index of the comprehensive operation state of the distribution cable is obtained.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A method for evaluating the state of distribution cables based on the combined weight theory, characterized by comprising the following steps:

[0007] Step 1: Establish a distribution cable state evaluation index system and calculate the index deterioration degree;

[0008] Step 2: Solve the combined weight of each index based on the combined weight theory;

[0009] Step 3: Establish a reliability model of the distribution cable;

[0010] Step 4: Combine the cable state evaluation and the reliability analysis results to obtain the comprehensive operation state evaluation of the distribution cable.

[0011] Furthermore, the described method for evaluating the status of distribution cables based on the combined weight theory is characterized in that the establishment of the distribution cable status evaluation index system and the calculation of the index deterioration degree in step one include the following steps:

[0012] Referring to the standard of "DL / T 2106-2020 Guide for the Status Evaluation of Distribution Network Equipment", evaluate the health status of the main body, intermediate joints and terminals of the distribution cable respectively. First, establish the index systems of the three components to obtain the distribution cable operation status evaluation index system. The distribution cable operation status evaluation indexes include:

[0013] 1) The indexes of the distribution cable main body include: insulation resistance, partial discharge, appearance, fire protection and dielectric loss;

[0014] 2) The indexes of the distribution cable intermediate joints include: temperature, fire prevention and retardation, appearance and partial discharge;

[0015] 3) The indexes of the distribution cable terminals include: pollution, appearance, fire prevention and retardation, and partial discharge;

[0016] Then, solve the distribution cable index deterioration degree. The calculation method of the index deterioration degree is:

[0017] Normalize the distribution cable operation status evaluation indexes, and their value range is 0-1. When the deterioration degree is 1, it indicates that the index seriously deviates from the normal state, that is, the equipment is in a fault state; when the deterioration degree is 0, it indicates that the equipment is in the best state; the specific calculation expression is as follows:

[0018]

[0019] In the formula, z n is the deterioration degree of the corresponding index. When z n <0, let z n =0; when z n >0, let z n =1; z' is the value of the current test to be evaluated for this index; z s is the warning value of this index, and z f is the initial value of this index. Furthermore, the described method for evaluating the status of distribution cables based on the combined weight theory is characterized in that the solution of the combined weight of each index based on the combined weight theory in step two includes the following steps:

[0020] Use the analytic hierarchy process to determine the subjective weight W and the entropy weight method to determine the objective weight V respectively, and then jointly determine the combined weight by the corresponding subjective and objective weights of each index. Specifically as follows:

[0021] First, use the analytic hierarchy process to analyze historical data to obtain the subjective weights of each index. During the evaluation process, use the entropy weight method with the experimental data itself to calculate the objective weights of each state variable, so as to correct the initial subjective weights obtained by the analytic hierarchy process, and then determine the objective weights. Finally, obtain the combined weight q.

[0022] Furthermore, for a power distribution cable condition evaluation method based on the combined weight theory, it is characterized in that the determination process of the subjective weight is as follows:

[0023] When using the analytic hierarchy process to determine the subjective weight, it is also necessary to determine the importance degree of each index. Let the importance degree of a certain index be represented by Im(X), where X represents each index;

[0024] ① Subjective weights of each index of the power distribution cable body:

[0025] Use W 1j (j = 1, 2, 3, 4, 5) to represent the subjective weights of the insulation resistance, partial discharge, appearance, fire protection, and dielectric loss of the power distribution cable body index respectively; Im(partial discharge) > Im(dielectric loss) > Im(insulation resistance) > Im(appearance) > Im(fire protection);

[0026] ② Subjective weights of each index of the power distribution cable intermediate joint:

[0027] Use W 2j (j = 1, 2, 3, 4) to represent the subjective weights of the temperature, fire protection and flame retardancy, appearance, and partial discharge of the intermediate joint index respectively; Im(partial discharge) > Im(temperature) > Im(appearance) > Im(fire protection);

[0028] ③ Subjective weights of each index of the power distribution cable terminal joint:

[0029] Use W 3j (j = 1, 2, 3, 4) to represent the subjective weights of the pollution, appearance, fire protection and flame retardancy, and partial discharge of the terminal joint index respectively; Im(partial discharge) > Im(appearance) > Im(pollution) > Im(fire protection);

[0030] ④ Subjective weights of the secondary indexes (body, intermediate joint, terminal joint)

[0031] Use W i (i = 1, 2, 3) to represent the subjective weights of the body, intermediate joint, and terminal joint respectively; Im(intermediate joint) > Im(terminal joint) > Im(body).

[0032] Furthermore, for a power distribution cable condition evaluation method based on the combined weight theory, it is characterized in that the determination process of the objective weight is as follows:

[0033] Determine the objective weight using the entropy weight method. According to the cable data collected on-site, combine the deterioration degree calculation method to calculate the normalized deterioration degree data, form a decision matrix, and determine the entropy weight of each index.

[0034] V 1j (j = 1, 2, 3, 4, 5) respectively represent the objective weights of the insulation resistance, partial discharge, appearance, fire protection, and dielectric loss of the distribution cable body index.

[0035] V 2j (j = 1, 2, 3, 4) respectively represent the objective weights of the temperature, fire protection and flame retardancy, appearance, and partial discharge of the intermediate joint index.

[0036] V 3j (j = 1, 2, 3, 4) respectively represent the objective weights of the pollution, appearance, fire protection and flame retardancy, and partial discharge of the terminal joint index.

[0037] V i (i = 1, 2, 3) respectively represent the objective weights of the cable body, intermediate joint, and terminal joint.

[0038] Furthermore, the described method for evaluating the state of a distribution cable based on the combined weight theory is characterized in that the determination process of the combined weight is as follows:

[0039] q ij = αW ij +(1 - α)V ij

[0040] In the formula, q ij is the combined weight, α is the combination coefficient, and its value ranges from 0 to 1, ensuring that ∑(q ij - W ij ) 2 +(q ij - V ij ) 2 has the minimum value.

[0041] Furthermore, the described method for evaluating the state of a distribution cable based on the combined weight theory is characterized in that the establishment of the distribution cable reliability model in step three includes the following steps:

[0042] By establishing a Weibull proportional hazards model, combining the prior knowledge and failure data of high-voltage cables, applying Bayesian inference to estimate the parameters in the model and the characteristic quantities reflecting the cable reliability, and analyzing the reliability of high-voltage cables under different conditions:

[0043] The reliability R(t) model of the distribution cable is as follows:

[0044] R(t) = exp[-exp(γ 0 + γ1 X 1 +γ 2 X 2 )t β

[0045] Where: X 1 is the line average load rate; X 2 is the dummy variable for direct burial laying; γ 0 is the basic regression parameter, representing the effects of other factors other than the covariates to be studied; γ 1 , γ 2 are the regression parameters corresponding to X 1 , X 2 , characterizing the effects of these 2 covariates on the cable reliability; β is the shape parameter; t is the operation life in days.

[0046] Furthermore, for the above-mentioned method for evaluating the distribution cable status based on the combined weight theory, it is characterized in that, for the step of obtaining the comprehensive operation status evaluation of the distribution cable by combining the cable status evaluation and the reliability analysis result in step four, it includes the following steps:

[0047] Solve the health index of the distribution cable, aggregate multiple index systems using the weight coefficients, and realize the comprehensive judgment of the overall operation status and health of the distribution cable to obtain the overall deterioration degree z of the distribution cable; combine with the reliability analysis result R(t) obtained in step three to obtain the overall health index HI of the distribution cable. The calculation formula of HI is as follows:

[0048] HI(t) = R(t) × z

[0049] The finally calculated health index has a full score of 100, and the larger the value, the better the health status.

[0050] The advantages of the present invention are:

[0051] Currently, the research on the status evaluation of distribution cables mostly uses the analytic hierarchy process model based on the deduction method for feature fusion. This method has the disadvantages of strong subjectivity and difficulty in accurate quantification. The present invention combines the advantages of the analytic hierarchy process and the entropy weight method based on the combined weight theory, avoiding both the disadvantage of strong subjectivity of the model weight and overcoming the problem that the accuracy of the objective model completely depends on the data quality. At the same time, the present invention combines the statistical model of the reliability of the distribution cable, and introduces variables such as laying method, operation method, and operation life through this method, further improving and correcting the accuracy of the model. Description of the Drawings

[0052] Figure 1 It is the evaluation index system for the deterioration degree of the distribution cable.

[0053] Figure 2 ​It is a calculation method for the combined weight of the operation status evaluation index of the distribution cable.

[0054] Figure 3 It is a calculation method for the health index of the comprehensive operation status evaluation of the distribution cable. Specific implementation mode

[0055] Select 9 old cables over 10 years old, and describe the technical solutions in the embodiments of the present invention clearly and completely. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Embodiment 1:

[0056] As Figure 1 、 2 、shown in Figure 3, a distribution cable status evaluation method based on the combined weight theory includes the following steps:

[0057] Step 1: Establish a distribution cable status evaluation index system and index deterioration degree;

[0058] Step 2: Solve the combined weight of each index based on the combined weight theory;

[0059] Step 3: Establish a distribution cable reliability model;

[0060] Step 4: Combine the cable status evaluation and reliability analysis results to obtain the comprehensive operation status evaluation of the distribution cable.

[0061] Specifically, establishing a distribution cable status evaluation index system and index deterioration degree includes the following steps:

[0062] Combined with the actual data of 9 old cables collected on site, the deterioration degree results of each first-level index are obtained according to the index deterioration degree and the solution method of each index deterioration degree in Step 1, as shown in the following table:

[0063]

[0064] Specifically, the deterioration degree calculation method:

[0065] The present invention uses the method of deterioration degree to normalize each index of the cable. Its value range is 0 to 1. When the deterioration degree is 1, it indicates that the index seriously deviates from the normal state, that is, the equipment is in a fault state; when the deterioration degree is 0, it indicates that the equipment is in the best state. The specific calculation expression is as follows:

[0066]

[0067] In the formula, z n is the deterioration degree of the fault symptom. When z n <0, let z n = 0; when z n > 0, let zn = 1; z' is the value of the current test to be evaluated under the fault symptom; z s is the warning value of the fault symptom, z f The initial value of the fault symptom.

[0068] 1.1 Degradation degree of each index of the distribution cable body

[0069] The distribution cable body includes indicators: insulation resistance, partial discharge, appearance, fire protection, and dielectric loss, which are represented by z 1j (j = 1, 2, 3, 4, 5) respectively, and z 1 represents the degradation degree of the cable body.

[0070] ① Degradation degree of insulation resistance

[0071] After the cable body has been operating for a long time, the insulation resistance will decrease. According to the test regulations, after the insulation resistance test, the insulation resistance value should not change significantly, otherwise it is unqualified. Since the existing literature does not specify the specific value of the significant change, this paper selects half of the measured value before measurement as the abnormal value, and takes 0 when the degradation after measurement is more than 80% of the measurement before. The degradation degree change percentage is a data that the larger the better, and takes the minimum value among the three phases, using z 11 ' to represent the degradation degree of this index. When it is between 0 and 1, it is obtained.

[0072]

[0073] ② Degradation degree of partial discharge

[0074] The discharge amount of the distribution cable body should be less than 100 pC. More than 100 pC is an abnormal state, and less than 10 pC is considered noise. The partial discharge degradation degree z 12 :

[0075]

[0076] ③ Degradation degree of appearance

[0077] Appearance condition No damage Slight damage Obvious damage Severe damage <![CDATA[Degree of deterioration z 13 > 0 0.3 0.6 1

[0078] ④ Degradation degree of fire protection

[0079]

[0080] ⑤ Degradation degree of dielectric loss

[0081] The dielectric loss of the distribution cable is detected by an ultra-low frequency sine wave voltage, and 1.0U is detected 0Standard deviation of the lower dielectric loss value. If the standard deviation is less than 0.1, the deterioration degree of the dielectric loss is 0. If it is greater than 0.5, the deterioration degree is 1. If it is between 0.1 and 0.5, the deterioration degree of the dielectric loss is:

[0082]

[0083] 1.2 Deterioration degree of each index of the intermediate joint of the distribution cable

[0084] The intermediate joint of the distribution cable includes indicators: temperature, fire prevention and flame retardancy, appearance, and partial discharge, which are represented by z 2j (j = 1, 2, 3, 4) respectively, and z 2 represents the deterioration degree of the cable body.

[0085] ① Deterioration degree of the intermediate joint temperature

[0086] The temperature of the intermediate joint of the cable is normal when it is less than 75°C and abnormal when it is greater than 100°C. The deterioration degree z 21 :

[0087]

[0088] ② Deterioration degree of the intermediate joint fire prevention and flame retardancy

[0089] Generally, it is required that the cable joint be treated with a fireproof coating for surface flame retardancy; and a flame retardant tape be wrapped around the adjacent cable or fireproof paint be brushed.

[0090] After the above treatment, the deterioration degree z 22 is 0; when the fire prevention and flame retardancy treatment is not carried out, the deterioration degree is 1; in the case between the two, the deterioration degree is 0.5.

[0091] ③ Deterioration degree of the intermediate joint appearance

[0092]

[0093] ④ Deterioration degree of the intermediate joint partial discharge

[0094] The discharge amount of the intermediate joint is considered noise when it is below 10 pC; the severity of the intermediate joint of the cable has a great relationship with its operating time. When the operating years are within 5 years, the discharge amount greater than 300 pC is in an abnormal state, and when the operating years are more than 5 years, the discharge amount greater than 500 pC is in an abnormal state;

[0095]

[0096] 1.3 Deterioration degree of each index of the distribution cable terminal

[0097] The distribution cable terminal includes indicators: pollution, appearance, fire prevention and flame retardancy, and partial discharge, which are represented by z 3j(j = 1, 2, 3, 4) indicates z 3 indicates the deterioration degree of the cable body.

[0098] ① Deterioration degree of the pollution condition of the terminal joint

[0099]

[0100] ② Deterioration degree of the appearance condition of the terminal joint

[0101] Appearance condition No damage Slight damage Obvious damage Severe damage <![CDATA[Degradation level z 32 > 0 0.3 0.6 1

[0102] ② Deterioration degree of the fire prevention and flame retardancy of the terminal joint

[0103] The switchgear of the distribution cable terminal needs to have fire prevention, flame retardancy and anti-small animal measures. If there are fire prevention, flame retardancy and anti-small animal entry measures, the deterioration degree z 33 is 0, and in other cases it is 1.

[0104] ④ Deterioration degree of partial discharge of the terminal joint

[0105] The discharge amount of the terminal joint below 10 pC is considered noise; the severity of the cable terminal joint has a great relationship with its operation time. Within 5 years of operation, if the discharge amount is greater than 3000 pC, it is in an abnormal state. If the operation life is more than 5 years, the discharge amount greater than 5000 pC is in an abnormal state;

[0106]

[0107] Specifically, solving the combined weights of each index based on the combined weight theory includes the following steps:

[0108] I. Solving the subjective weight:

[0109] ① Subjective weights of each index of the distribution cable body:

[0110] Importance degree of each index: Im(partial discharge) > Im(dielectric loss) > Im(insulation resistance) > Im(appearance) > Im(fire prevention). According to the principle of the AHP algorithm, the following comparison matrix can be determined.

[0111]

[0112] Index Insulation resistance Partial discharge Appearance Fire protection Dielectric loss <![CDATA[Subjective weight (W 1j )]]> 0.1145 0.4534 0.1341 0.0546 0.2434

[0113] ② Subjective weights of each index of the distribution cable intermediate joint:

[0114] Importance degree of each index: Im(partial discharge) > Im(temperature) > Im(appearance) > Im(fire prevention). According to the principle of the AHP algorithm, the following comparison matrix can be determined.

[0115]

[0116] Index Temperature Fire protection Appearance Partial discharge <![CDATA[Subjective weight (W 2j )]]> 0.2289 0.0699 0.1932 0.5081

[0117] ③Subjective weights of each index of the distribution cable terminal joint:

[0118] Importance degree of each index: Im(Partial discharge) > Im(Appearance) > Im(Fouling) > Im(Fire protection). According to the principle of the AHP algorithm, the following comparison matrix can be determined.

[0119]

[0120] Index Contamination Appearance Fire protection Partial discharge <![CDATA[Subjective weight (W 3j )]]> 0.1125 0.2399 0.0716 0.5761

[0121] ④Subjective weights of the secondary indexes (cable body, intermediate joint, terminal joint)

[0122] Importance degree of each index: Im(Intermediate joint) > Im(Terminal joint) > Im(Cable body). According to the principle of the AHP algorithm, the following comparison matrix can be determined.

[0123]

[0124] Index Body Intermediate joint Terminal joint <![CDATA[Subjective weight (W i )]]> 0.1125 0.2399 0.0716

[0125] II. Determine the objective weights

[0126] Steps to determine the objective weights using the entropy weight method are as Figure 2 shown.

[0127] Based on the cable data of the actual data collected on site, combined with the deterioration degree calculation method, calculate the normalized deterioration degree data, form a decision matrix, and determine the entropy weights of each index.

[0128] ①Objective weights of each index of the cable body:

[0129]

[0130]

[0131] ②Objective weights of each index of the cable intermediate joint:

[0132] Index Temperature Fire protection Appearance Partial discharge <![CDATA[Objective weight (V 2j )]]> 0.1227 0.2603 0.2317 0.3853

[0133] ③Objective weights of each index of the cable terminal joint:

[0134] Index Contamination Appearance Fire protection Partial discharge <![CDATA[Objective weight (V 3j )]]> 0.2163 0.2186 0.2178 0.3473

[0135] ④Objective weights of the secondary indexes (cable body, intermediate joint, terminal joint):

[0136] Index Body Intermediate joint Terminal joint <![CDATA[Objective weight (V i )]]> 0.1097 0.4844 0.4059

[0137] III. Determine the combined weight

[0138] The combined weight q ij is jointly determined by the corresponding subjective and objective weights of each index;

[0139] q ij = αW ij +(1 - α)vV ij

[0140] where α is the combination coefficient, and its value ranges from 0 to 1, ensuring that ∑(q ij - W ij ) 2 +(q ij - V ij ) 2 has the minimum value; choosing α = 0.5 as the combined weight coefficient is relatively reasonable.

[0141] ① The combined weights of each index of the cable body:

[0142] Index Insulation resistance Partial discharge Appearance Fire protection Dielectric loss <![CDATA[Combined weight (q 1j )]]> 0.1777 0.2836 0.1328 0.1167 0.2892

[0143] ② The combined weights of each index of the cable intermediate joint:

[0144] Index Temperature Fire protection Appearance Partial discharge <![CDATA[Combined weight (q 2j )]]> 0.1758 0.1651 0.2124 0.4467

[0145] ③ The combined weights of each index of the cable terminal joint:

[0146] Index Contamination Appearance Fire protection Partial discharge <![CDATA[Combined weight (q 3j )]]> 0.1644 0.2293 0.1447 0.4616

[0147] ④ The combined weights of the secondary indicators (cable body, intermediate joint, terminal joint):

[0148]

[0149]

[0150] Specifically, establishing a reliability model for distribution cables involves the following steps:

[0151] Establish a Weibull proportional hazards model, combine the prior knowledge and failure data of high-voltage cables, and use Bayesian inference to estimate the parameters in the model and the characteristic quantities reflecting the cable reliability, and analyze the reliability of high-voltage cables under different conditions.

[0152] The reliability R(t) model of the distribution cable is as follows:

[0153] R(t) = exp[-exp(γ 0 - γ 1 X 1 + γ 2 X2 )t β

[0154] where: γ0 = -4.69, γ 1 = -50.07, γ 2 = 11.03, β = 1.98; t is the operating life in days. X 1 is the average load factor, taking the values 0.105, 0.35, and 0.7 under low load, normal load, and high load respectively; X 2 is a dummy variable for direct burial laying. In a dry environment, it refers to non-direct burial laying, X 2 = 0; in a humid environment, it refers to direct burial laying, X 2 = 1;

[0155] In this example, the operating loads of the 9 cables are all normal loads, indicating X 1 = 0.35. At the same time, the cable laying environment is humid, referring to the direct burial laying situation, X 2 = 1. The operating life is 15 years, so t = 15 × 365 days 。

[0156] Substituting into the calculation, the reliability R(t) can be obtained as 0.9792.

[0157] Specifically, solving the comprehensive operation status evaluation of the distribution cable involves the following steps:

[0158] Comprehensive operation status evaluation of the distribution cable:

[0159] Calculation of the deterioration degree of secondary indicators:

[0160] Deterioration degree z of the distribution cable body:

[0161] Deterioration degree z of the intermediate joint of the distribution cable 2 :

[0162] Deterioration degree z of the terminal joint of the distribution cable 3 :

[0163] ③ Comprehensive operation status evaluation of the distribution cable:

[0164] Overall deterioration degree z of the distribution cable:

[0165] Overall health index HI(%) of the distribution cable: HI(t) = R(t)·(1 - z).

[0166] ① Overall deterioration degree z of the distribution cable:

[0167] ​

[0168] The overall deterioration degrees of 9 distribution cables are respectively:

[0169]

[0170]

[0171] ② The overall health index HI (%) of the distribution cable: HI = R(t)·(1 - z)

[0172] The health indexes HI of 9 distribution cables are respectively:

[0173] Cable serial number 0 1 2 3 4 5 6 7 8 HI(%) 86.01 63.60 80.70 86.39 83.26 80.32 79.11 97.58 90.30

[0174] The full score of the health index HI of the distribution cable is 100. The larger the value, the better the health state.

[0175] As mentioned above, it is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and all should be covered within the protection scope of the present invention.

Claims

1. A method for evaluating the state of distribution cables based on the combined weight theory, characterized in that, it includes the following steps: Step 1: Establish an evaluation index system for the state of distribution cables and calculate the deterioration degree of the indexes; Step 2: Solve the combined weights of each index based on the combined weight theory; Step 3: Establish a reliability model for distribution cables; Step 4: Combine the cable state evaluation and the reliability analysis results to obtain an evaluation of the comprehensive operating state of the distribution cables; The establishment of the reliability model for distribution cables in Step 3 includes the following steps: By establishing a Weibull proportional hazards model, combining the prior knowledge and failure data of high-voltage cables, and applying Bayesian inference to estimate the parameters in the model and the characteristic quantities reflecting the cable reliability, analyze the reliability of high-voltage cables under different conditions: The reliability R(t) model of the distribution cable is as follows: In the formula: X 1 is the average line load rate; X 2 is the dummy variable for direct burial laying; γ 0 is the basic regression parameter, representing the effects of other factors other than the covariates to be studied; γ 1 and γ 2 are X 1 and X 2 corresponding regression parameters, characterizing the effects of these two covariates on the cable reliability; β is the shape parameter; t is the operation years, with the unit of days; The combination of the cable state evaluation and the reliability analysis results in Step 4 to obtain an evaluation of the comprehensive operating state of the distribution cables includes the following steps: Solve the health index of distribution cables, aggregate multiple indicator systems using weight coefficients, and achieve a comprehensive judgment of the overall operating status and health of distribution cables to obtain the overall deterioration degree of distribution cables z ; Combined with the reliability analysis results obtained in Step 3 R ( t ), the overall health index of the distribution cable is obtained HI , HI and the calculation formula is as follows: The final calculated health index has a full score of 100. The larger the value, the better the health status.

2. A method for evaluating the state of distribution cables based on the combined weight theory according to claim 1, characterized in that, The establishment of the evaluation index system for the state of distribution cables and the calculation of the deterioration degree of the indexes in Step 1 include the following steps: Referring to the standard of "DL / T 2106-2020 Guide for the State Evaluation of Distribution Network Equipment", evaluate the health status of the main body, intermediate joints and terminals of the distribution cables respectively. First, establish an index system for the three components to obtain an evaluation index system for the operating state of the distribution cables. The evaluation indexes for the operating state of the distribution cables include: 1) The indexes of the main body of the distribution cable include: insulation resistance, partial discharge, appearance, fire protection and dielectric loss; 2) The indexes of the intermediate joints of the distribution cable include: temperature, fire protection and flame retardancy, appearance and partial discharge; 3) The indexes of the terminals of the distribution cable include: pollution, appearance, fire protection and flame retardancy, and partial discharge; Then solve the deterioration degree of the distribution cable indexes. The calculation method of the index deterioration degree is: Normalize the evaluation indicators of the operating status of the distribution cable. The value range is 0 to 1. When the degradation degree is 1, it indicates that the indicator seriously deviates from the normal state, that is, the equipment is in a fault state; when the degradation degree is 0, it indicates that the equipment is in the best state. The specific calculation expression is as follows In the formula, is the degradation degree of the corresponding indicator. When , let ; when , let ; is the test value of this evaluation for this indicator; is the warning value of this indicator, is the initial value of this indicator.

3. A method for evaluating the state of distribution cables based on the combined weight theory according to claim 1, characterized in that, The solution of the combined weights of each index based on the combined weight theory in Step 2 includes the following steps: The analytic hierarchy process is used to determine the subjective weight respectively W and the entropy weight method is used to determine the objective weight V , and then the combined weight is jointly determined by the corresponding subjective and objective weights of each index, as follows: First, use the Analytic Hierarchy Process (AHP) to analyze historical data to obtain the subjective weights of each index. During the evaluation process, use the entropy weight method with the experimental data itself to calculate the objective weights of each state variable, and then correct the initial subjective weights obtained by the Analytic Hierarchy Process to finally obtain the combined weights q .

4. A method for evaluating the state of distribution cables based on the combined weight theory according to claim 3, characterized in that, The determination process of the subjective weight is as follows: To determine the subjective weight using the analytic hierarchy process, it is also necessary to determine the importance degree of each index. Let the importance degree of a certain index be represented by Im( X ), X representing each index; The subjective weights of each index of the main body of the distribution cable: Use W 1j (j = 1, 2, 3, 4, 5) respectively represent the subjective weights of the insulation resistance, partial discharge, appearance, fire protection, and dielectric loss of the distribution cable body; Im (partial discharge) > Im (dielectric loss) > Im (insulation resistance) > Im (appearance) > Im (fire protection); The subjective weights of each index of the intermediate joints of the distribution cable: Use W 2j (j = 1, 2, 3, 4) represent the subjective weights of the intermediate joint's target temperature, fire prevention and retardancy, appearance, and partial discharge respectively; Im(partial discharge) > Im(temperature) > Im(appearance) > Im(fire prevention); The subjective weights of each index of the terminal joints of the distribution cable: Use W 3j (j = 1, 2, 3, 4) respectively represent the subjective weights of the terminal joint indicators of pollution, appearance, fire prevention and flame retardancy, and partial discharge; Im (partial discharge) > Im (appearance) > Im (pollution) > Im (fire prevention); The subjective weights of the secondary indexes Use W i (i = 1, 2, 3) respectively represent the subjective weights of the main body, the intermediate joint, and the terminal joint; Im(intermediate joint) > Im(terminal joint) > Im(main body).

5. A method for evaluating the state of distribution cables based on the combined weight theory according to claim 4, characterized in that, The determination process of the objective weight is as follows: Use the entropy weight method to determine the objective weight. According to the cable data collected on site, combine the deterioration degree calculation method to calculate the normalized deterioration degree data, form a decision matrix, and determine the entropy weight of each index; V 1j (j = 1, 2, 3, 4, 5) respectively represent the objective weights of the insulation resistance, partial discharge, appearance, fire protection, and dielectric loss of the distribution cable body; V 2j (j = 1, 2, 3, 4) respectively represent the objective weights of the intermediate joint's target temperature, fire prevention and retardance, appearance, and partial discharge; V 3j (j = 1, 2, 3, 4) respectively represent the objective weights of the terminal joint index for dirtiness, appearance, fire prevention and flame retardancy, and partial discharge; V i (i = 1, 2, 3) respectively represent the objective weights of the main body, the intermediate joint, and the terminal joint.

6. A method for evaluating the state of distribution cables based on the combined weight theory according to claim 5, characterized in that, The determination process of the combined weight is as follows: In the formula, q ij is the combined weight, is the combined coefficient, whose value ranges from 0 to 1, ensuring that the value is the smallest.

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