Effect of insulator configuration on ageing deterioration of polymer insulators

According to our accelerated ageing test results of 275-kV full scale insulator strings coated with the same silicone rubber, conventional porcelain long-rod insulators showed better anti-ageing performance compared with transmission line polymer insulators. As the reason for such results two factors are considered. One is difference in core diameter, and the other is difference in shed shape. In order to clarify the reason for such results, we started comparative accelerated ageing tests on five different specimens, coated with the same room temperature vulcanized silicone rubber, with almost the same shed shapes but three different core diameters and with almost the same core diameters but three different shed shapes. As of 380 cycles of accelerated ageing test, any discernible deterioration was not observed on any specimens, although partial discharges had been observed at the energized end core surface of the transmission line polymer insulator after 200 cycles. It takes so long to get any significant deterioration by the accelerated ageing test. Since it is considered that the main cause of surface deterioration of polymer insulators is partial discharges, we investigated the partial discharge inception voltages of these specimens under contaminated and wet conditions. The transmission line polymer insulator showed the lowest partial discharge inception voltage. Very large ratio of shed to core diameter is considered to be the cause of easy occurrence of partial discharges due to concentrated voltages allotted at the core surfaces. We prepared three specimens to improve the voltage distribution of the transmission line polymer insulator and evaluated. Promising results were obtained.