Mapping of discharge clusters in void based on surface resistivity

In this paper a novel approach of mapping discharge clusters inside a void in high voltage insulating material, is presented. Operating conditions in the high electric field, with long-lasting exposition, promote formation of local defects being sources of partial discharges, resulting in surface erosion and origination of locally eroded hotspots. Certain class refers to surface effects in the discharge sources, denoted as voids and following surface recombination processes. Discharges occur in localized spots creating so called effective discharge surface. Research was focused on assessment of the changes of surface properties of investigated dielectric material in long term test in flat gaseous inclusions with a diameter much bigger than thickness of the void. The presented experiments were performed on Nomex paper, designed for high voltage applications, including motor conductor coil wrap and transformer layer insulation. Partial discharges cause thermal effects and temperature rise around discharge clusters leading to dielectric material degradation and local variations of surface resistivity. The electrode setup created homogenous electric field distribution. The aging test consisted of the sequence of substages, with total duration equal to 240 hours.The goal of the experiment was to identify discharge channels by mapping of surface resistivity inside voids, using multipoint method.

I. Introduction

Partial discharge exposure in gaseous voids within solid dielectrics leads to changes in surface conditions, such as roughness or morphology, as well as in physical properties, such as charge accumulation and transport, surface and bulk conductivity [1–4, 9, 11, 16]. For example surface conductivity influences the decay of deposited charges and hence the long term inception conditions in a void [5, 10, 13, 15]. A new method is presented, for determining the effective discharge area and mapping discharge channels in the void during subsequent aging periods.