Electrical Charges on Polymeric Insulator Surfaces and their Impact on Flashover Performance

Doktorsavhandling, 2012

Utilization of remotely located sustainable energy resources such as solar, wind and hydro calls for further developments of long-distance electric power transmissions operating at ultra-high voltage (UHV) levels which may exceed 1000 kV ac and 800 kV dc utilized today. It is foreseen that insulation of such transmission systems is to be based on polymeric materials that can provide a number of technical benefits over traditionally used glass and porcelain based insulation. As several studies have provided evidences of possible deterioration of withstand properties of polymeric insulators when electric charges are accumulated on their surfaces, especially under dc and impulse stresses, there is a need for acquiring knowledge on the mechanism lying behind such influences for improving design of polymeric insulators as well as related testing methodology. The research presented in the thesis focuses on three main subjects, i.e. surface charging of polymeric insulation materials by external corona discharges, surface charge decay on thick material layers and impact of surface charges on dc and impulse flashover performance of model insulators. Charging of polymeric surfaces was analyzed by utilizing a computer model describing development of corona discharges in air. Validity of the model was verified by comparing the calculated and measured corona currents in a needle-plane electrode system with a dielectric barrier. The simulations showed, depending on time regimes of the applied voltage, formation of either positive glow or burst current pulses that led to different conditions of charge deposition on the polymeric surface. The distributions of deposited charges were further investigated by measuring potential distributions along surfaces of flat and cylindrical samples of silicone rubbers, typically used for insulator manufacturing. The obtained results demonstrated existence of significant differences in the surface charge patterns for different charging conditions. Dynamic behavior of the deposited surface charges was studied on fresh and aged samples of the silicone rubbers. The developed experimental procedure allowed for distinguishing between the effects of surface charge neutralization through the bulk of polymeric material or by so-called gas neutralization, the latter being conditioned by presence of free ions in the surrounding air. It was found that increased amount of free ions in the air as well as aging of the material accelerated strongly the charge decay process. For the aged materials the decay became sensitive to air wetness. At the same time, the characteristics measured under bulk neutralization regime were utilized for estimating the conductivities of the studied materials. The impact of surface charging on flashover performance of polymeric insulator models was analyzed experimentally and theoretically. In the experimental study, a cylindrical composite insulator model was used, which consist of a fiberglass-epoxy rod covered with a layer of silicone rubber. The surface of the insulator was charged to various magnitudes and negative dc flashover voltage levels were determined. The obtained results demonstrated that deposition of a negative surface charge enhanced the dc withstand level, whereas deposition of positive charge reduced it. The dc flashover voltages of the charged insulator model were also calculated by means of a model utilizing streamer breakdown criteria. The measured and the calculated dc flashover voltages remained in good agreement for the utilized charging conditions. This further allowed using the model for simulating the influence of surface charging on impulse flashover performance of insulators. A parametric study was performed to investigate the effects of charge magnitude, its polarity and location on the insulator surface. The simulations shown that the dependence of impulse flashover voltage on charge polarity and concentration was similar to that of dc flashover voltage for the cases when the insulator was located far away from the ground (symmetry with regard to voltage polarity). However, when the insulator was standing on a grounded plane, the effect of surface charging was dependent on the polarity of the applied impulse voltage. In the case of positive impulse, presence of positive charge increased the flashover voltage, whereas negative charge yielded their reduction and vice versa.