The effect of surface charge on the performance of cast resin insulation for gas insulated substations

The maximum charge densities which can exist on the surface of a conical barrier are calculated, and it is shown how these vary with voltage and location. An experimental rig is constructed which allows conical barriers, of similar dimensions to those of many manufacturers, to be stressed by a DC voltage. The maximum voltage available is approximately equal to 1 p.u for a 420 kV system, thus reproducing the trapped charge conditions found after switching operations on isolated sections of busbar in a 420 kV GIS. It is shown that charge present on the concave surface of the barrier significantly lowers the barrier flashover voltage. The charging conditions required for surface flashover vary across the barrier and this is attributed to differences in the surface conductivity. The track marks generated are similar in appearance to those previously reported and occur across the convex surface of the barrier. They originate at the outer triple junction. The direction and location of the flashovers are explained by analysing the field distribution across the barrier with charge present on its concave surface