Investigation of surface charges on polymeric insulators and the influence of sessile water droplets

Abstract

High-voltage line and post insulators belong to the most important components in electric power transmission and distribution. The insulators ensure insulation between e.g. the high-voltage transmission line conductors and the crossarms. Outdoor insulators are exposed to different weather conditions like rain, ice, snow, UV-radiation, temperature variation and pollution [1]. Nevertheless, the surface has to withstand the environmental conditions and stresses with minimal possible surface ageing or degradation [2]. Depending on the application various materials are used for the insulators. The most common types of insulators are ceramic, glass and composite insulators. Nowadays, composite insulators are increasingly used as outdoor insulators because of their several advantages in comparison to glass or ceramic insulators, like reduced weight, high robustness or hydrophobic surface properties [2]. The hydrophobic surface properties of silicone rubber prohibit the development of a liquid layer on the surface. An occurring liquid layer (impurity layer) could increase the surface conductivity and may lead to an insulation failure. Instead of a liquid layer individual droplets are developed on a silicone rubber surface. The surface hydrophobicity causes high contact angles between the silicone rubber and sessile water droplets (typically θ ≈ 90°), which lead to the formation of electrically critical points. The contact line of the water droplets (e r ≈ 81) forms a triple zone with the surface of the insulator (e r ≈ 3) and the surrounding air (e r = 1). Due to the different permittivities of the materials, the applied electric field is locally enhanced at the triple zone of the droplets [3]. The electric field strength depends on several factors like the contact angle, type of applied voltage and droplet volume. The locally existing electric field strength can exceed the dielectric strength of the surrounding of the insulator, typically air. This leads to the formation of electrical partial discharges, which possibly deteriorate the surface properties, and therefore can enhance the ageing of the insulator [3].