Effect of silicone grease soaking on electrical tree degradation of silicone rubber

Silicone rubber (SR) insulating materials have been widely used in the high voltage cable accessories due to their excellent mechanical and electrical properties. During the installation of cable accessory, silicone grease is used to enhance the sealing of the interface between cable accessory insulation and cable insulation. However, during the long-term operation of cable, silicone grease will gradually soak into the SR material and make it swell, resulting in deteriorated performance of SR material to resist electrical tree. Therefore, the initiation voltages and patterns of electrical tree in SR with varied soaking time were studied in this paper. The results showed that the electrical tree initiation voltage gradually decreased, and the pattern of the electrical tree gradually changed from the branch-type to the bush-type with the development of silicone grease soaking time. Combining the experiment results of isothermal surface potential decay (ISPD) and crosslinking density, it can be seen that the break of crosslinking network in SR caused by silicone grease decreased the crosslinking density and the charge density of deep trap. Afterwards, the free volume in SR expanded and the ability of the molecular structure to bound the charge was weaken, which results in the changes of electrical tree properties.