Temperature-Dependent Electrical Tree in Silicone Rubber under Repetitive Pulse Voltage

Silicone rubber (SiR), which is employed as the main insulation materials in the cable accessories, faces the challenge of high temperature during the overload running process. Pulse voltage is inevitable during the switch operation process of the converters or lighting, which aggravates the operation state of the insulation materials. To understand the electrical treeing process at the high temperature with repetitive pulse voltage considering the actual operating state, the needle-plate electrode system was stressed in the experiment. In the test, the ambient temperature (T-amb) was set to 30, 60, 90, 120 and 150 degrees C, while the pulse frequency was 5, 20, 100, 200 and 1000 Hz. Electrical tree shapes, tree length, fractal dimension (FD), tree inception and tree breakdown probability were studied to investigate electrical tree growth characteristics. The results indicate that the tree shapes are depended not only the temperature environment but also the pulse frequency. Higher pulse frequency and higher temperature facilitate bush tree shape to be generated. With the pulse frequency increasing, the tree length becomes longer, while FD tends to be larger. However, the tree length decreases as the FD increases with the temperature elevating at the same pulse amplitude. As more hot electrons to collide the molecular chain with pulse frequency increasing, the tree inception and breakdown probability become higher. Nevertheless, the tree inception probability tends to be larger as the mechanical property of silicone rubber drops with the temperature elevating, while breakdown probability becomes lower as the shorter tree length grows for a given pulse number.