Positive streamer discharges in air and along insulating surfaces: experiment and simulation

University dissertation from Uppsala : Acta Universitatis Upsaliensis

Abstract: The power quality of modern society relies on the electrical properties of the dielectric insulators used in the power industry. Much research work has been conducted with an aim to understand and predict the insulating behaviour of such materials under different kinds of atmospheric conditions, but still there are many unsolved problems. In particular, there is a lack of knowledge concerning the electrohydrodynamic and electrophysical processes at the insulator surface and the surrounding medium. No detailed knowledge exists at present of the processes governing the development of electrical discharges along the surface of insulators.With an aim to enhance the knowledge in this field in general and on the electrical performance of outdoor insulators in particular a detailed study of the positive streamer discharges in air and along dielectric surfaces was conducted. The study was also extended to gain more knowledge on the water drop initiated electrical discharges in air and the attachment of natural lightning flashes to a Franklin conductor.In the first phase, the study was focused on positive streamer discharges propagating in air. The spatial distribution of the charge of a branched streamer discharge was obtained and the charge contained in a single streamer branch was quantified. In the second phase measurements and simulations of streamer discharges propagating along insulating surfaces were conducted with an aim to understand how the insulating surfaces interact with streamer discharges. In addition to quantifying the parameters of streamer discharges propagating along insulating surfaces, the results of these studies made it possible to separate and quantify the effects of the dielectric constant and the surface properties on the streamer discharges. In the third phase a comprehensive computer algorithm was developed to simulate 3-dimensional propagation of positive streamer discharges in air and along dielectric surfaces taking into account the branching effect. The conditions necessary for the initiation of streamer discharges were applied to obtain the minimum strength of the background electric field required to initiate electrical discharges in the presence of water drops. In particular the study provided an explanation of how lightning flashes are initiated in thunderclouds in background electric fields as low as 200 kV/m. Finally, the study was extended to understand the performance of lightning conductors paying special attention to the influence of conductor radius and the streamer inception criterion.