Effect of electrode surface roughness on electrical breakdown in high voltage apparatus

This paper considers the statistical theory of electrical breakdowns in high voltage (HV) devices. The extended probability distribution of breakdown is deduced for nonuniform gaps. The breakdown of high voltage apparatus is sensitive to local irregularity of the electric field which may result from the presence of defects such as surface roughness. The surface roughness leads to existence of localized microscopic projection with local electric field strength at the projection top larger than the average field at the electrode surface. The link between parameters determining the probability of electrical breakdown and the parameters determining microscopic geometry of electrode surface is obtained. In this paper a simplified breakdown model is used as a basis for statistical treatment. According to the model the breakdown proceeds if the electric field strength at the projection top exceeds the critical value. The method of dielectric strength calculation for HV devices using a simplified model of breakdown is presented. The paper gives an example of calculation of the breakdown voltage for vacuum switch TVS-43. Results of calculations are compared with available experimental data and Weibull distribution

I. Introduction

The insulation strength of discharge gaps is one of the main factors determining the reliability of HV devices. The electrical breakdown is induced either by microscopic projection on electrode surface or by free conducting particles and occurs as a result of self-sustaining avalanche process of ionization of gas molecules by free electrons. In this paper our attention is focused on the electrical breakdown induced by microscopic projection on the electrode surface. Conditions at which the breakdown of a discharge gap takes place are determined by rates of gas ionization and electron emission at the cathode [1], [2]. The rates of these processes rise sharply with increase in electric field strength. Therefore breakdown occurs at the place of local amplification of electric field caused by sharp microscopic projections on the electrode surface arising on the surface due to random factors such as machining of a surface, erosion under the action of environment or electrical discharge, etc. Existence of inevitable microscopic pollutions on the surface, oil films, and absorption of gas molecules may also effect the dielectric strength. Emission surface properties and electric field at electrodes are random variables which can differ in various sites of the surface and change accidentally. The breakdown voltage level of a high voltage device can therefore be predicted in a statistical sense [3]. Also it is necessary to note, that the surface of electrodes can be significantly changed as a result of tests. In high power switching devices erosion of the electrodes, caused by arcing, substantially changes the electrode surface state.