Study On Composite Insulation Defects And Electric Field Simulation Of Distribution Cable Accessories

With the conversion of overhead lines to underground cable projects,the number of power cables in distribution networks is increasing,and distribution cable accessories,which are indispensable for distribution lines,are increasingly used in urban distribution networks.Distribution cable accessories as the key connection part of distribution system,its security and stability is very important.The research of distribution cable accessories involves defect simulation,fault detection,electric field simulation and so on,and there are complex interface structures in the accessories.The research of distribution cable accessories involves defect simulation,fault detection,electric field simulation and so on.The composite insulation defect analysis and electric field simulation research of AC distribution cable accessories can provide guidance and suggestions for cable accessories fault warning and post-analysis,which is of great significance to the safe and stable operation of distribution network.In this study,the method of combining experimental law exploration and composite insulation defect simulation of cable accessories are adopted.The physical and chemical properties and electrical insulation properties of composite insulation（XLPE-SIR）for distribution cable accessories under damp conditions were tested and analyzed.Further study the influence law and influence mechanism of acid-base environment on insulation performance of distribution cable accessories.On this basis,a simulation model of composite insulation defects of distribution cable accessories was established to study the electric field distribution characteristics of composite insulation interface moisture defects and typical gas-solid defects.Firstly,the experimental study on the moisture defects of composite insulation of distribution cable accessories is carried out.The results show that XLPE and SIR are hydrophobic and hydrophilic respectively.The longer the immersion time of XLPE and SIR insulation materials,the more O-H groups.After immersion,the electrical performance of XLPE deteriorates more than that of SIR.The resistivity of XLPE decreases by an order of magnitude after immersion,while that of SIR decreases by48.3%.The relative permittivity of XLPE and SIR increased to 2.24 and 3.03.The breakdown field strength of XLPE and SIR showed a decreasing trend.Secondly,the influence of acid-base environment on insulation performance of distribution cable accessories is studied.The results show that,relatively,the acid base size has a great influence on the resistance and breakdown performance of SIR sample,and the ion concentration has a great influence on XLPE sample.The volumetric resistivity of the specimens decreased to different degrees after soaking in different acid and alkaline solutions.The breakdown field strengths of both XLPE and SIR specimens were influenced by strong acid and strong alkaline solutions.Similar to the breakdown voltage,strong acid and strong alkaline solutions had significant effects on the flashover voltage of XLPE-SIR specimens.Thirdly,the electric field distribution characteristics caused by typical moisture defects of distribution cable accessories are explored.The electric field distribution of the attachments with moisture defects such as water drop,water film and water tree was calculated,and the internal electric field distortion caused by the three defects was compared.When the defect is located at the junction of three,the law of electric field distortion is water tree defectd>water film defect>water drop defect.The maximum electric field distortion is 4.91 k V/mm when the water drop defect is located at the interface 3.5 mm away from the three joint points.When the water film defect is 12 mm away from the three joint point,the maximum local electric field is 5.89 k V/mm.The maximum electric field of water tree defect is 20.3 k V/mm at the three junction point.Combined with the moisture test and simulation data,the weight analysis of the damp position and shape of cable accessories was carried out.Finally,the electric field distribution characteristics caused by typical gas-solid defects of distribution cable accessories are studied.A simulation model of interface defects of double-layer plate structure was established to simulate and analyze the electric field distortion caused by solid defects,and the electric field distortion caused by metal defects and semi-conductive defects was larger.Insulation defects have less effect on electric field than the first two.Considering the actual structure of cable accessories,the double-layer plate dielectric defect model was extended to gas-solid defect model of distribution cable accessories,and the electric field distribution characteristics of cable accessories caused by typical gas-solid defects were studied.The results show that the maximum field strength distortion caused by bubble defect and air gap defect is 13 k V/mm and 4.58 k V/mm,respectively,which both exceed the air breakdown threshold.The maximum distortion electric fields caused by metal impurities and semi-conductive impurities are about 22 mm away from the semi-conductive fault,which is about 3.67 k V/mm.In contrast,the maximum electric field distortion caused by the insulation defects occurs at the root of the stress cone,and the electric field distortion decreases with the defects away from the root of the stress cone.