Breakdown Failure Analysis Of 220 Kv Cable Joint With Large Expanding Rate Under Closing Overvoltage

The silicone rubber is subjected to either the mechanical stress or the pulse voltage, making it is easier for initiation and development of electrical tree, which is the main reason for breakdown failure of cable joint. Studying the effect of electrical tree for breakdown failure of cable joint is the valid way to ensure the safety and stability of the cable system. In this paper, a breakdown failure case of 220 kV integral prefabricated silicone rubber cable joint with large expanding rate under closing overvoltage is discussed. Through dissection, it was found that there were electric trees inside the cable joint produced by factory C. Based on the failure characteristics, the mechanical stress model of the cable joint and the closing overvoltage model of the system were established to analyze the influencing factors of electric trees. Furthermore, some sample tests were carried out to discuss the relationship between material performance and electrical tree, including carbon black test, gel content test, thermogravimetric test, tensile test and volume resistivity. The results showed that the large circumferential stress would destroy the physical crosslinking of silicone rubber, leading to a decrease in tensile strength and volume resistivity. This was the main factor affecting the growth of electrical trees. Meanwhile the overvoltage and the poor material characteristics of cable joint would accelerate the growth rate of electrical trees. Eventually the breakdown failure of cable joint occurred when the electric trees running through both polarities. The analytical method and conclusions proposed in this paper can provide suggestions and guidance for the analysis of similar breakdown accidents in the future.