Simulation of the electrical field distribution influenced by the defects in 220 kV silicone rubber cable joint

In recent years, several cases of 220 kV silicone rubber cable joint failures have been reported in China. The faults were hypothetically attributed to air gap at insulation-semicon interface and electrical tree in insulation. It is thus important to gain a better understanding on the influence of these types of defects on the electric field distribution. In this paper, we present the simulation of 220kV cable joints under different defected cable joints. The parameters set in the simulation were based on the experimental results obtained by testing the relative permittivity, dissipation factor, volume resistivity of the silicone rubber of the defected cable. The breakdown strength of the silicone rubber was also tested to investigate whether or not the theoretical electric field obtained in the simulation is higher than the breakdown strength. The results show that the electrical field concentrates at the surface of high voltage shield and the interface between the semi-conducting stress cone and cable insulation. Air gap and electrical tree distort the electric field distribution. The strength of the electric field at the defect is much higher than the breakdown strength of silicone rubber. Therefore, defects present in cable joint are supposed to accelerate the thermal ageing of the cable insulation and hence promote the growth of electric tree which will finally lead to unexpected cable failure.