Research Of Electric Field Analysis And Interface Defect In Cable Joints

Cable joints play a role of connection and transition in power transmission,and it is an important part to ensure the safe and reliable operation of power system.However,the requirements for the laying level of the cable joints are higher.In the construction process,it is affected by installation technology,surrounding environment and external force damage,it will lead to defects such as conductive impurities,scratches,moisture and so on at the composite interface between the joint and the cable body.After the cable joints are put into operation,the electric field distortion will appear at the defect position.It will cause the partial discharge even the breakdown of joints after long-term operation,which will affect the safe operation of power system seriously.Therefore,it is of great significance to study the distribution of electric field and the influence of interface defects on the distribution of electric field in the cable joints.So as to reveal the real reasons for the decline and even failure of the electrical strength of the cable joints.In this paper,the macro model of 10 k V cable joint and the meso model of two-dimensional RVE were built by the simulation software of COMSOL Multiphysics.The effects of typical interface defects such as conductive impurities,scratches and moisture,and different expansion ratio on the electric field distribution of the cable intermediate joint were studied.At the same time,the composite samples of XLPE/SR with three kinds of interface defects were prepared,and the breakdown strength test was carried out to verify the consistency of the simulation and experimental results.The simulation results show that the three kinds of defects will lead to the obvious electric field distortion at the place of them in the composite interface,and the aging of insulation will increase the electric field strength of interface,and increase the effect of defects on the electric field distribution at the same time.The electric field distortion at the end of the water tree is serious after it formed.With the growth and development of water branches,the electric field strength in the area close to each other decreases,but the local electric field concentration still exists.The greatest influence is on the end of stress cone after the expansion of cable joints.And the larger the expanding ratio is,the more obvious the stress and electric field concentration are near the end.In the tensile state,there will be "stress concentration band" and obvious "delamination" phenomenon between the transverse arrangement of carbon black particles.The debonding damage in the interface between the carbon black particles and the matrix will occur when the tensile strength is too large.With the increasing of diameter expansion ratio,the distance decreases between arranged carbon black particles vertically,and the particles close to each other will form conductive channels.However,with the increasing of the distance between arranged carbon black particles horizontally,it is difficult for the particles to cross the potential barrier and make the electronic transition.The two aspects change the conductive network of the stress cone and affect its conductivity.The results of breakdown test show that the breakdown strength of composite sample of XLPE/SR is lower than the non-compound samples of XLPE and silicone rubber.When there are three kinds of defects in the composite interface,the breakdown strength of the samples decreases obviously,and the dispersion increases.The variation range of the conductive impurity sample is larger than other two kinds of defects.Compared with before aging,the breakdown strength of all samples decreased and the dispersion increased after aging,but the variation range was lesser,except for the samples with water defects at the interface.While the breakdown strength of the samples with water defects at the interface increased,but not more than the samples without defects.And the data dispersion decreased,but still greater than the samples without defects.The above results verify the correctness of the simulation test.