Outdoor Polymeric Insulators: Role of Corona in Performance of Silicone Rubber Housings

In the struggle to accept outdoor polymeric insulators as alternatives to the traditionally used porcelain and glass insulators, researchers and engineers have tested numerous ideas. Insulator housings made of silicone rubber (SIR) currently dominate the component design, thanks to their hydrophobic surface properties with excellent long-term durability. Despite of five decades of lasting development, some issues related to the performance of this material in various working conditions still remain to be resolved, including an understanding of the impact of corona discharges on the material deterioration as well as on insulator durability although these are extensively studied questions. The impact on insulator performance of high voltage direct current stress especially needs elucidation. An overview on present use of SIR composite insulators is first provided in this paper. Thereafter the results of recent investigations on the mechanism of corona damage are summarized with an emphasis on the differences between the effects of alternating (AC) and direct current (DC) stresses, demonstrating by evaluating the electrical, mechanical and structural characteristics of SIR-based materials, that the damage is less severe under a DC stress. The effect is attributed to reduction in corona intensity in the vicinity of the material surface by stored electric charge. The same charge has however a positive or negative impact on the insulator voltage withstand under impulse conditions, depending on their polarity.