DC Breakdown Characteristics At XLPE/SR Interface And Effect Factors

Cross-linked polyethylene(XLPE) power cables and its accessories have a broad application prospect in power transmission projects as the development of voltage sourced converter high voltage direct current(VSC-HVDC) technologies.The service statistics have shown that the interface between cable insulation and accessory insulation is easily to be breakdown. In response, the researches mainly focus on nano-modification of both XLPE and silicone rubber(SR). The amount of space charge will be reduced if the electrical conductivities of XLPE and SR are about equal. However, the impacts of interfacial pressure, temperature, space charge and installation defects on the breakdown characteristics at the interface of XLPE/SR have not been studied.Firstly, the electric field distribution of HVDC cable joint with temperature gradient and installation defects was simulated. The results show that the electric field at the end of high voltage screen decreased and stress cone increased as the increasing of temperature. Besides, the field will be reversed at the same time. The semi-conductive particles and air gap on the interface of cable insulation have great influence on the distribution of electric field. The field strength at the end of high voltage screen was weakened by the particles and air gap, but the field intensity at the position of defects increased largely, which become more seriously as the distance between defects and stress cone decreasing.Secondly, the impacts of pressure, temperature and space charge on the DC breakdown voltage at the interface of XLPE/SR were researched. The results show that the interfacial breakdown voltage decreased first and then increased as the increase of pressure at different temperatures. The breakdown voltage decreased as the temperature increasing under lower interfacial pressures. On the contrast, the breakdown voltage increased as the temperature increasing. The breakdown voltage can be reduced largely by the influence of space charge.Finally, the stress relaxation characteristics of SR under thermal-mechanical aging conditions were studied. The results show that the elasticity modulus of aged SR keeps constant as the testing temperature increasing. The elasticity modulus and deformation rate of aged samples increased largely as the increase of aging time,which follows linear rule. The deformation rate of inner/outer radius of cylindrical insulation model was obtained under the assumption that the height and volume of cylindrical model remain unchanged in the process of aging, which makes it possible to simulate the pressure distribution of cylindrical insulation model under different aging conditions.