Electric Charges on Insulator Surfaces and their Influence on Insulator Performance

Licentiate thesis, 2008

Abstract Polymeric materials used in high voltage insulation systems are subjected to electric fields which can be strong enough to initiate electrical discharges in the surrounding gas environment. Generated electrical charges may be deposited on polymer surfaces and alter electric field distributions, thus affecting insulation performance and their corresponding breakdown characteristics. In this thesis, analyses are presented on charge dynamics and its relaxation on corona treated thick samples of technically important polymeric insulating materials, such as polydimethylsiloxane based rubber, popularly called silicone rubber (SIR), and ethylene-propylene-diene-monomer (EPDM) based rubber. Measurements of surface potential, by means of a vibrating capacitive probe, were used in this work as a convenient method to obtain different material characteristics. It was observed that the charged area of the samples as well as its surface potential magnitude were both increasing with increasing number of corona pulses and their voltage level. After charging by a few impulses the surface charge distributions were similar to the distributions resulting from long DC charging. This means that the charged surface is limited in size independently of charging method. The time derivative of the surface potential decay may be used to estimate charge carrier mobility as well as conductivity of an insulating material. The experimentally deduced conductivities are compared with conductivities measured by standard method and the results are within reasonable margins. By using a surface demarcation energy model, material characteristics, in particular densities of surface states (traps) and their energy distributions could be estimated from the surface potential measurements.