The Research On The Electric Field Distribution In HVDC Cable Under Temperature Gradient Effect

HVDC electricity transmission has become an important direction of developing high-voltage electricity transmission because of its low loss, high reliability, large carrying capacity, little communication and ecological impacts, suitable for long-distance transmission. Compared to overhead lines, HVDC power cable lines have a unique advantage when crossing the channels, tunnels and cities. Electric field distribution is the key factor determining the short-time voltage withstanding ability and the long-term running reliability of HVDC cable insulation. Under stable operation condition, the electric field distribution of HVDC cables is mainly determined by the conductivity of insulating materials. The phenomenon of electric field distribution reversion will happen under the higher temperature gradient. Because the nonlinear conductivity of insulation materials is related with temperature and electric field, the transient electric field distribution in HVDC cable at the moment of putting into operation and voltage polarity reversion will be complicated.For this reason, the effect of the nonlinearity of insulating materials on the electric field distribution in HVDC cables under different temperature gradients, different voltage types and different nonlinear conductivity were analyzed by using the software of COMSOL Multiphysics.The simulation results show that: for the determined insulating material with the certain nonlinear properties, the phenomenon of electric field distribution reversion in the cable insulation will be more serious under the higher temperature gradient. The maximum transient electric field during the voltage reversal process is closely related to the voltage polarity inversion time, and the maximum transient electric field will increase and the location of the maximum electric field transient will move to the conductor shield as the voltage polarity inversion time decreases. Lightning impulse and switching impulse will produce very high electric field in cable insulation, and the maximum electric field of DC voltage superimposing negative impulse is far lower than the maximum electric field of DC voltage superposing positive impulse. The simulation results also show that the steady-state and the transient electric field in cable insulation under certain temperature gradient can be uniformed through reducing the conductivity activation energy and increasing the electric field dependent coefficient of conductivity of the insulating materials.According to the simulation results, it is recommended that the conductivity activation energy should be reduced and the electric field dependent coefficient of conductivity of the insulating materials should be increased by the effective control means in research and development of insulating materials, and that the temperature gradient effect and the transient electric field distribution problems during the voltage polarity reversal and impulsing should be concerned in the design of HVDC cable.