Electrical tree initiation of silicone rubber after thermal aging

Power cables operate at high temperatures over long periods of time, and the electrical behavior of silicone rubber (SIR) in the new types of extra-high-voltage prefabricated cable accessories would change as a result of thermal aging. In this study, tests were conducted to reveal the effects of thermal aging (1000 h at 60-180°C) on the electrical treeing behavior. It was found that with increasing thermal aging time, the average electrical tree initiation voltage (ATIV) of SIR initially increases to a peak value and then decreases, finally becoming stable within 1000 h. Meanwhile, the probability of pine-like trees decreases at first and then increases, whereas the probability of bush-like trees initially increases and then decreases. The thermal aging temperature affects the rate of ATIV following the Arrhenius equation. These results strongly imply the existence of a thermal aging process that greatly influences the treeing degradation process. The results obtained using differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy (XPS) indicate that thermal oxidation plays a major role in the initial thermal aging process and facilitates additional crosslinking, which enhances ATIV. With increasing thermal aging time, thermal degradation and thermal crack reactions play leading roles, resulting in decreased crystallinity and ATIV. Microcracks are present after long-term thermal aging, and they are the dominant factor in ATIV stability. ATIV stability also provides a theoretical basis for the electrical strength design margin for insulating materials.