Characteristics of streamer discharges in air and along insulating surfaces

Abstract: Power quality is dependent on the dielectric properties of the insulators used throughout the power generation and power transmission utilities. Knowledge of the pre-discharge stage associated with the electrical breakdown across outdoor insulators is essential if one is to improve the performance of these insulators under various electrical and environmentalconditions. The streamer discharge is an important sub-process in the development of partial and disruptive discharges. Parameters concerning the propagation of streamer discharges in air and along insulator surfaces are thus of importance for understanding the way in which the streamer characteristics can be influenced. The studies reported in this thesis were made on streamer discharges in air and over insulating surfaces at atmospheric pressure.Experiments on positive streamer discharges in air are presented. Currents associated with streamer discharges have been measured in a three-electrode gap system that provides a quasi-uniform electric field. Parameters for the streamer discharges such as the electric field required for the stable streamer propagation, speed of the streamer propagation and the chargeinvolved in the streamer discharge have been investigated. A simplified streamer model, assuming the streamer as a finitely conducting channel has been studied through computer simulation. The agreement between experiments and simulations has shown that this model can predict some properties of the streamer discharges.Experiments on streamer propagation over insulating surfaces and the comparison of breakdown channels in air and along insulator surfaces have been reported. Differences between streamer discharges in air alone and streamer discharges over insulating surfaces have been discussed. It is suggested that the charge deposited on the insulator surface plays asignificant role in determining whether the spark channel follows the insulator surface or takes a path in air.In an attempt to develop diagnostic tools for investigation of the charge density and the amount of energy required to initiate a discharge in an ambient electric field, the effect of absorbed laser pulse energy has been studied for laser triggered electrical discharges.