Effect of silicone grease on electrical tree degradation of silicon rubber under AC electric field

Silicone grease (SG) is usually applied on the interface between the insulation of cable and cable joint to enhance the sealing of interface. However, in long-term operating process, SG will gradually diffuse into silicone rubber (SiR) matrix and cause the electrical performance degradation of SiR, which can even result in the failure of cable joints. The commercial HTV SiR with functional silica filler was used in this study, and the electrical tree characteristics SiR samples with different SG coating times were investigated. The crosslinking density and trap distribution were analyzed to explain the transformation of electrical tree with different coating times. It was found that the electrical tree initiation voltage dropped from 13.35 to 10.46 kV and the fractal dimension increased from 1.25 to 1.52, while the electrical tree length showed an almost constant trend. The morphology of the electrical trees gradually changed from branch-type to bush-type. The decline of crosslinking density from 1.92 × 10⁻⁴ to 1.76 × 10⁻⁴ mol/g is attributed to the breaking of crosslinking structure between silica and SiR matrix, which can lead to the expanding free volume and decreased deep trap density. More high-energy electrons can be generated, which aggravates the degradation of molecular chains and further results in the decreased electrical tree initiation voltage. In addition, the denser morphology of electrical tree is considered to be caused by the more growth direction for electrical tree provided by the expanding free volume.