Condition Evaluation and Replacement Method of Damp Cable

Cable lines are now being widely used. According to statistics data, cable dampness is the main factor affecting the operation of cable lines at present. If the cables are damp, evaluate the degree of damp and replace the cables that are severely damp in sequence. In this paper, the physical and chemical properties, mechanical properties and dielectric properties of the cable are used to evaluate the status of the cable. By comparing the length and carrying capacity of cables in operation, the degree of damp was sorted by skyline algorithm, and the most serious cables were replaced first.


Introduction
With the development of transforming overhead lines into underground cables, the number of power cables in distribution network is increasing day by day.Compared with overhead lines, cable lines are widely used in urban distribution networks because of their advantages such as relatively low operation failure rate, good safety, high power supply reliability and adaptability to various adverse weather conditions [1].In recent years, with the increasing of the construction of the distribution network, the cabling rate of 10kV lines has increased rapidly.The statistical analysis shows that cable faults are mostly the result of multi-factor coupling such as overvoltage and bad weather environment.Most of the faults caused by cable accessories are damp and waterlogged.The fault of cable accessories is one of the key factors that restrict the further improvement of power supply reliability of cable lines.At present, whether directly buried cable, or channel laying cable, the biggest threat of damp.The buried cable is directly in contact with the soil.After rain, the water and air will enter the cable body along the cable joints and cable sheath damage.Trench cables will also form water because of rain and other factors, and then enter the cable body.After water enters the cable body, it will form a water tree under the action of high voltage electric field.If the water tree runs through the insulation layer, it will form a discharge, which will burn through the insulation layer of the cable and cause a fault [2].In this paper, a mathematical model is established on the basis of cable condition evaluation, and a method of data analysis is proposed to comprehensively evaluate the running condition of cable, make corresponding adjustments on the basis of the original inspection period, and give the replacement sequence of damp cable.

Performance Evaluation Model of Damp Cable
The basic idea of selecting the evaluation model and establishing the fuzzy comprehensive evaluation model is to transform each index in the system into a membership vector through fuzzy processing [3].Membership matrix is formed by combining the membership degree vectors of all indexes in the system.Select the weight vector expression to get the weight vector, select the fuzzy operator to get the final state evaluation value.
Step 1: Establish evaluation index set E. The cable status evaluation system is regarded as a multifactor set, the set of different influencing factors e1, e2, e3... em is called evaluation index set E, denoted as: E={e1, e2, e3... , em} (m indicates the number of indicators).The index set of comments in this paper is: E={elongation at break, breakdown field strength, thermal decomposition temperature, carbonyl index, melting temperature}.Among them, elongation at break represents the degree of mechanical aging, breakdown field strength represents the degree of dielectric aging, thermal decomposition temperature represents the degree of thermal cracking, carbonyl index represents the degree of thermal oxygen aging, and melting temperature represents the degree of thermal oxygen aging.
Step 2: Create comment set S. The comment set is used to judge the level of each indicator in the indicator set, which can be expressed as: S={s1, s2, s3... , sn} (n indicates the number of comments).According to the evaluation of cable status, it can be divided into s1-s6, a total of 6 grades.
Step 3: Establish the evaluation matrix A. The evaluation matrix can be obtained by calculating the membership function between the index set E and the comment set S. When there are m×n evaluation indexes, the matrix is: A= a11 a12 ... a1n a21 a22 ... a2n ... ... ... ... am1 am2 ... amn Step 4: Determine the weight vector W. Different methods are used to analyze the influence of various factors on the evaluation results of cable status, and the weight distribution of each evaluation index is obtained.The weight vector W can be expressed as: W={w1, w2, w3, …, wn}.
Step 5: Calculate the evaluation result vector C. The fuzzy evaluation result is: C=W^A.Where, "^" is a fuzzy operator.Fuzzy operators include salient fuzzy operators and weighted average fuzzy operators.Considering that the cable state is not static, the results of each evaluation index will affect the comprehensive evaluation results, so the weighted average fuzzy operator is selected, and the formula is: Where, ci is the evaluation result, wi is the weight coefficient, and aij is the membership degree.The weighted average fuzzy operator is used to preserve the information as much as possible, which can evaluate the cable condition more comprehensively.

Cable Comprehensive Evaluation and State Analysis
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Cable Physical And Chemical Properties Test
Compared with polyethylene, crosslinked polyethylene improves heat resistance, mechanical and electrical properties.These properties are closely related to the degree of crosslinking of crosslinked polyethylene [4].If the cross-link is too low, its heat resistance may be poor, and it may melt deformation when used at high temperature; If the cross-linking path is too high, the insulation will harden and the elasticity is insufficient and easy to age, so insufficient and excessive cross-linking will lead to the decline of insulation performance.So in order to make crosslinked polyethylene meet the requirements of use, we must ensure the appropriate crosslinking degree.It can be concluded that the crosslinking degree is a very important parameter for the insulation of crosslinked polyethylene cable.The crosslinking degree is usually characterized by gel content.
The gel content can be used to reflect the degree of crosslinking of XLPE cable insulation.The gel content was measured by JB/T10437-2004.The main principle of this method is to use xylene to dissolve the uncrosslinked part of XLPE insulation, and the insoluble part, namely the crosslinked part, is called gel, so that the gel content can be calculated to reflect the crosslinking degree of XLPE insulation.

Mechanical and dielectric properties testing
Tensile strength and elongation at break is two of the insulating material mechanical properties of commonly used indicators, according to the GB/T2951.11 -2008 / IEC 608 11-1-1:2001 (cable and cable insulation and sheath material should suit the general test methods part 11: General test methods -Measurement of thickness and overall dimension -mechanical properties test) The aged cable samples at each stage and the new cable samples are made into dumbbell plates [5].Before the test, two lines were marked in the middle of each dumbbell specimen.The spacing of the marked lines is 20mm.During the test, only the specimen whose breaking point occurs between two marks is included in the test result.Tensile strength refers to the ratio of the maximum tensile stress endured by the sample until fracture to the initial test cross-sectional area of the material (between the marks) in the tensile test, and the formula is: ɸ=P/A ɸ is tensile strength, P is the maximum tensile stress endured by the specimen when it is stretched to fracture, A is the initial cross-sectional area of the material.Elongation at break refers to the ratio between the increase of the distance between the marking lines when the sample is broken and the initial marking distance when it is not stretched under the action of tensile force of the insulating material, expressed in percentage.The formula is as follows: δ =Δ L/L0 δ is the elongation at break, expressed in percentage.Δ L is the increase of marking distance when the sample breaks.L0 is the initial distance before the stretch.

Formatting Author Affiliations
Dielectric polarization and loss will occur under the action of external electric field [6].Dielectric constant ε and dielectric loss factor tan δ are two important parameters describing dielectric polarization and loss under the action of external electric field.The response of the two external electric field frequency and temperature can explain some laws of dielectric structure, the polarization and relaxation mechanism of internal microstructure and the law of their interaction.Before the test, the sample was cut into a circle with a diameter of 30mm.In order to eliminate the influence of incomplete contact surface caused by uneven material surface on the test results, the upper and lower surfaces of the sample were symmetrically sprayed with gold electrodes with a diameter of 20mm.Considering the differences in cross-linking between the inner and outer layers during cable production and the differences in electrical stress on the inner and outer layers of insulation during electrical aging, all samples were taken from the middle of the insulation layer.The test results of physical and chemical properties, mechanical properties and dielectric properties of the cable are input into the evaluation matrix A to obtain the evaluation matrix of the cable.In the fuzzy comprehensive evaluation system, a membership vector corresponding to each index factor of the membership function can be obtained, and the fuzzy evaluation matrix of the system can be obtained by calculating the membership vector corresponding to all index factors.

Comprehensive Evaluation and Result Analysis
Table 1 shows the gel content data of the section sample after cable aging at different aging stages at 75℃, 82℃ and 96℃.The test temperature is selected closest to the operating temperature of the cable.A: Characteristic fuzzy variable B: Function type C: Characteristic fuzzy variable D: weight After the fuzzy comprehensive evaluation results are further analyzed and obtained, the evaluation grade corresponding to the largest index factor can be selected as the final evaluation result.R is used in this paper to represent the final evaluation result.

Evaluation Method Of Replacement Sequence Of Damp Cable
After the cable status evaluation is completed, the replacement sequence cannot be determined by a single state quantity.Because the operation and maintenance of the cable also consider the capacity and the importance of the degree.The damped cable condition evaluation method proposed in this paper is based on the following cable operation and maintenance rules: RULE1: the lower the cable status evaluation score, the more easily the cable will be broken down, the sooner the cable will be replaced; RULE2: the more the cable load, the larger the load cable, the more important the cable, the sooner the cable to be replaced; RULE3: the longer the cable, the more water trees produced in the middle, the more prone to failure, the sooner the cable should be replaced.Therefore, this paper divides the evaluation of damp cable into three dimensions: cable state T1, cable load T2, and cable length T3.The state of damp cable is divided into three dimensions for evaluation.
RULE1: The smaller the T1 is, the worse the cable status is, and the sooner the cable is replaced.RULE2: The larger T2 is, the worse the state of the cable is, and the sooner the cable will be replaced.RULE3: The longer T3 is, the worse the state of the cable is, and the sooner the cable is replaced.
In order to make the rule consistent, rule 1 is changed to the reciprocal of T1, then rule 1 is changed to, the larger T1 is, the worse the cable state is, and the sooner the cable will be replaced.
Through the above analysis, the cable can be represented by the three-dimensional state quantity.L= (1/T1, T2, T3) The three-dimensional state quantity of 10 damp cables is as follows: L1(2.It is difficult to evaluate the three-dimensional state quantity.Therefore, the invention firstly reduces it to two dimensions for evaluation.Since the length of the power cable is the easiest to obtain, the T3 dimension is eliminated in the invention, and T1 and T2 dimensions are preferred for evaluation.Put 1/T1 and T2 into the evaluation coordinate system in the form of horizontal and vertical coordinates.
The abscissa is the reciprocal of the cable status evaluation score, and the ordinate is the cable load current.This paper uses skyline as a tool to select the optimal solution [7].As can be seen in Figure .1,compared with other points, L2, L5, and L4, the cable water tree is longer and the cable load is higher, so they should be treated first.The three selected points, L2, L5 and L4, are compared through the third dimension, the cable length T3.The L2 cable is 3,254 meters long, the L4 cable is 4,518 meters long, and the L5 cable is 1,684 meters long.Because the L4 cable is the longest and the L4 state volume is the worst, it is replaced first.

Conclusion
In this paper, the insulation aging state of damp cable is evaluated based on fuzzy clustering method.
According to the results of linear regression correlation analysis, several characteristic parameters such as elongation at break, breakdown field strength and so on are selected as the comprehensive evaluation parameters of cable insulation aging state from multiple insulation aging influence factors.Using skyline as an evaluation tool, the damp cable replacement was prioritized and the worst cable was selected to be replaced first

Acknowledgment
This work was supported by Shandong Youth Innovation Team Program (No.2019KJN025).

Figure 1 .
Figure 1.Skyline of evaluation of damp cable

Table 1 .
Gel content data at different stages of aging at different temperature In this paper, the average value of physical and chemical properties of cable at a certain temperature is taken as the parameter.For example, at 82℃, the test value of physical and chemical properties of cable is 0.81397.Based on the test results of physical and chemical properties, mechanical properties and dielectric properties of the cable, the fuzzy matrix A of the member ship degree of each index is obtained.The membership function is established according to the clustering center obtained by fuzzy clustering according to the cable insulation state detection data under different field aging states.The membership function of each characteristic fuzzy variable is shown in Table2.

Table 2 .
Membership function of each fuzzy set