Charges on polymeric insulators and their effect on flashover characteristics

Nowadays polymeric materials are widely applied in outdoor high voltage insulation systems gradually replacing traditionally used porcelain and glass. It is believed that polymer based insulation will become dominant at ultra high voltages, e.g. in 1000 kV ac and 800 kV dc overhead transmission lines, which are presently being developed. Although withstand performance of polymeric insulating materials has been studied over the years, there is still a lack of complete physical understanding of all important aspects related to practical situations.Thus, the behavior and performance of polymeric materials for high-voltage applications in presence of accumulated surface charges, which can be generated in practice due to different sources (e.g. corona, surface discharges, space charge build-up, etc), are not completely understood today. This phenomenon needs to be considered when designing insulation systems and when performing insulator testing, especially for high-voltage dc (HVDC) applications where presence of surface charges is the inherent property of the system. The research conducted within the thesis project focused on experimental investigations of the influence of pre-deposited surface charges on impulse flashover characteristics of polymeric model insulators in air. The study also involved development of surface charge deposition system based on dc corona discharge as well as measurements of charge dynamics and its relaxation on cylindrical polymeric insulators made of poly-dienethylsiloxane based rubber, so called Silicon Rubber (SIR) and Room Temperature Vulcanized Silicon Rubber (RTVSIR) based rubber, which are widely used in outdoor high voltage insulation systems. The results presented and discussed in the report indicate that the presence of charges on surfaces of polymeric insulators can lead to a decrease or an increase of insulator withstand voltages depending on the polarities of the deposited surface charges and applied impulse voltages