Pulsed Dielectric Surface Flashover at Atmospheric Conditions

Abstract

Dielectric flashover along insulators in vacuum has been sufficiently researched in the past. Less studied, but of similar importance, is surface flashover at atmospheric pressures and the impact of various electrode geometries, humidity, and type of gas present. Previous research has shown distinct arc behavior in air and nitrogen for an electrode geometry in which the electric field lines curve above the dielectric surface. Specifically, flashover experiments in nitrogen have shown that the arc path will follow the electric field lines, not the dielectric surface. As a result, it was concluded that the arc development path, whether along the electric field line or the surface of the dielectric, is related to the oxygen content in the atmospheric background. It is believed that this dependence is due to the arc's production of UV radiation in an oxygen rich environment. Further testing, in a pure nitrogen environment with UV illumination of the surface prior to the pulse application, has shown that UV plays a significant role in the arc development path. There is a near linear relationship between the percentage of liftoffs and the time delay between UV application and flashover. Additional studies have also shown a relationship between the UV intensity and the percentage of liftoffs. Based on these results we will discuss the physical mechanisms primarily involved in unipolar flashover at atmospheric pressure. Additional experimental results regarding the effects of humidity on the liftoff phenomenon will be presented as well.