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

The primary difference between HVDC XLPE cable and AC cable is the electric field in insulation and influence factors. For AC cable, the electric field distributes as relative permittivity in insulation regardless of temperature. For HVDC cable, the electric field distributes as conductivity in insulation which is the function of temperature and electric field, so that electric field would be influenced by temperature and itself.Because of the complicated electro-thermal coupling in insulation, the computation of steady-state rating in HVDC cable is more difficult than AC cable. If the load current is excessive, not only the conductor temperature would be over the limit of long-time operation, but also the electric field strength would be over the endurance limit in insulation due to the change of conductivity and the space charge accumulation, and then the insulation breakdown. Therefore, the conductor temperature and electric field should be included in computation of steady-state rating.This paper deduces the analytic formula of conductor temperature, temperature distribution and temperature difference of insulation based on the establishing of steady-state thermal circuit model. FEM model is built for the computation of temperature distribution. The computed results of analytic formula and FEM model are compared. This paper analyzes the influence rule of load current and surface temperature of cable to temperature distribution. Electro-thermal coupling FEM model also be built and the electric field is be computed. This paper analyzes the influence rule of load current and surface temperature of cable to electric field, the maximum electric field strength change with temperature difference of insulation, leakage current change with load current, and influence rule of temperature to electric field.Based on the research of temperature distribution and electro-thermal coupling, this paper regards the limit temperature of long-time operation and limit temperature difference of insulation as constraint conditions for steady-state rating computation, and deduces the analytic formula of steady-state rating of HVDC cable. The steady-state rating FEM 3D model is built. The computed results of analytic formula and FEM model are compared. This paper analyzes the influence rule of ambient temperature and thermal resistance to steady-state rating. The analytic formula of steady-state rating is expanded as control temperature of water and control load current computation method of HVDC cable load cycle test system. FEM model is used for verification.This paper demonstrates the test of cable heat cycle test of 110 kV XLPE AC cable, states the rationality of verifying conductor temperature and temperature difference of insulation of HVDC cable used AC cable test. The computed results of analytic formula and test are compared to verify the veracity of analytic formula.