Effect of Large Mechanical Stress on Electrical Tree Characteristics of Silicone Rubber

Silicone rubber (SIR) is widely used in the field of high-voltage cable accessories insulation due to its excellent electrical properties. However, according to statistics, in 2016 alone, there were many cable accidents at home and abroad, among which cable accessories failure accounted for 85.5%. In order to fit closely with the cable, the high-voltage cable accessories adopt the installation method of pre-expanding and maintain the expanding state in the long-term operation of the cable. The silicone rubber is deteriorated by the circumferential mechanical stress, which is easy to lead to electrical tree and finally cause breakdown fault. At present, because the steel needle electrode is easy to form gas gap with the insulating material which results in the error of electrical tree experiment under the large deformation, the research on mechanical stress is mainly focused under 30% deformation, which is far less than the actual diameter expansion ratio of 45%. In this paper, in order to approach the actual working state of the insulation material of accessories, the initial voltage and morphological characteristics of silicone rubber electrical tree under large tensile ratio of 0%, 25%, 45% and 60% were studied. In order to avoid the stress concentration at the tip of the traditional steel needle electrode, a new semi conducting needle electrode was designed and successfully manufactured, and a real-time observation system of electrical tree under power frequency was built. The results showed that the initial voltage of semi conducting needle electrode was higher than that of traditional needle electrode under the same tree initiation condition. With the increase of tensile mechanical stress, the initial voltage decreased from 15.30kV to 14.33kV, and the electrical tree tended to grow in the direction of stress application. Combined with the test results of Differential scanning calorimetry and Thermogravimetric Analysis, it can be seen that the "melting point" and the temperature of maximum thermogravimetric rate increased first and then decreased with the increase of tensile ratio. At low tensile ratio, small-scale fracture of SIR molecular chain occurred, and the amorphous region increased. At high tensile ratio, the number of broken molecular chains increased. On the whole, the physical crosslinking of silicone rubber was destroyed, some molecular segments cracked, the free volume inside the material became larger, the electrical tree was more easily induced, and the electrical tree resistance of the material decreased.