Simulation Study On Electro-thermal Coupling And Steady-state Rating Computation Of HVDC XLPE Insulation Cable Joint

Cable joint is the weak link where faults frequently occur during operation of HV cable line. The design of DC cable joint becomes more complex than AC cable joint, due to the conductivity characteristics and some problems such as multilayer interface of dielectric, space charge, and so on. Temperature and electric field are important parameters reflecting the insulation performance, moreover, the interaction between the electric and thermal field has a direct connection with long-term operation performance of cable system. Therefore, the study of electric and thermal field’s regularities of distribution has great significance on monitoring the performance of insulation and preventing accidents.The distribution of electric field and the accumulation of space charge are affected by the temperature changes in the insulation layer of HVDC cable joint, which is different from AC cable joint. So the insulation temperature and temperature difference changes in the insulation layer need to be paid attention to. A simplified model of the HVDC XLPE cable joint was established, and an iterative calculation method of steady-state thermal field based on changing of dc resistance of conductor was put forward. The temperature of joint conductor, XLPE insulation and SIR reinforced insulation, and temperature difference of insulation layer were affected by current and ambient temperature, which was studied with iterative calculation method. And the influence of the iterative calculation was also analyzed. Based on the characteristics of the interaction between electric and thermal field, a direct coupling simulation method based on the finite element was provided, compared with the calculative results of indirect coupling. Besides, the direct coupling field was calculated under different current and insulation conductivity. On the basis of simulating calculation of electro-thermal field, a method of the maximum allowable operating temperature of conductor and the maximum allowable temperature difference of insulating layer was provided to determine steady-state current-carrying capacity of HVDC cable joint as two constraint condition. And the results of current-carrying capacity in this article were compared with the results of IEC60287 standard, influence of ambient temperature on the current-carrying capacity of cable joint was also studied. Finally, feasible scheme of optical cable built in cable joint and temperature measurement system were provided. Combined with simulating database, by entering the current and maximum temperature of implanting position of optical cable, the data of electric and temperature in cable joint can be monitored in real time.In this paper, the research methods and conclusions can provide reference for safe operation, evaluation of current-carrying capacity, selection and optimization design of insulation materials and online monitoring of electrical and thermal field of dc cable system.