A Simplified Method to Estimate Surface Trap Distribution: Potential Decay by Charge Dissipation Along Surface

Gas-insulated equipment is indispensable in dc transmission systems. Under the dc field, the surface charge accumulation could induce insulation failure. While the surface trap characteristic is a significant factor in surface charge accumulation and transportation, the surface trap distribution is often measured by the surface potential decay (SPD) method, where surface charges dissipate through the bulk of the sample. It is however difficult to distinguish this way carriers trapped on the surface and in material bulk. In addition, it is difficult to apply the SPD method in engineering practice because of the limitation imposed by sample thickness. To overcome these difficulties a novel and simplified method of SPD along surface (SPDS) measurement is proposed in this article, which allows for estimating the surface trap distribution and conductivity characteristics of the investigated insulator. As compared with the SPD through bulk (SPDB) method, the distribution of surface traps with micrometer sized metal particles on the surface is obtained.

I. Introduction

High voltage direct current (HVdc) transmission technology is an essential technical way to solve the problem of long-distance and high-capacity power transmission [1]. At present, epoxy insulators are widely used for supporting SF₆ insulated equipment. The gas-solid interface between epoxy and SF₆ gas is the weak point of insulation in the equipment [2], [3], where flashovers frequently occur in long-time operations due to the distortion of the electric field along their surfaces by accumulated charges [4], [5]. Therefore, the accumulation and transportation of surface charge under dc voltage are believed as significant factors affecting the insulation performance of the equipment [6], [7].