Resistance characteristics and electrification characteristics of GIS epoxy insulators under DC voltage

Now that gas insulated switchgear (GIS) for ac systems are becoming increasingly compact as specifications are rationalized, more consideration of their insulation characteristics for residual dc voltage is required. Furthermore, with dc power transmission technology drawing more and more global attention, clarifying the insulation characteristics of GIS for dc voltage is increasingly important. In this paper, to understand the insulation characteristics of epoxy resin, which is widely used for GIS insulating spacers, factors determining the resistivity of the epoxy insulator surface layer under dc voltage were initially investigated on an experimental basis. Consequently, it emerged that the bulk resistance was more dominant than the surface resistance for the dc resistance of epoxy resin due to the dependency of the test sample resistance value on their radius. Since the electric field might be concentrated if some part of this insulator surface layer showed non-uniform resistivity, the influence of the curing agent, one of the potential causes of this non-uniformity, was subsequently investigated with its content as a parameter. As a result, the volume resistivity in the long-term region was likely to decline or vary for epoxy resin containing less curing agent due to the presence of numerous polarized components unreacted with curing agent. In addition, the presence of micro protrusions or similar, if any, on the insulator surface or electrode is considered to cause electrification due to the concentration of electric field on the surface layer. Accordingly, the relationship between their surface roughness and electrification level was investigated using gaps between insulators or an electrode and an insulator facing each other, respectively. Consequently, where the surface roughness of the insulator or electrode was high, a current component with a large damping time constant, considered attributable to electrical charges moving across the gap, appeared after the charging current components and an electrification condition was observed.