Influence of a metallic particle at a metal/insulator/gas triple junction in air and SF6

In gas insulated switchgears (GIS), high voltage electrodes are supported by solid insulators. A "triple junction" (TJ) exists at the interface between metal, insulating solid, and gas. This area constitutes a preferential zone were partial discharges (PDs) can appear. A further degradation of insulation performance can occur if solid particles accumulate at that place. This paper presents experiments carried out to obtain a quantitative account of the influence of a metallic particle located close to a triple junction area, both in dry air and SF6. PDs are detected and measured by conventional electrical measurements, and observed optically with a high-sensitivity intensified camera. Measurements under AC voltage show the influence of the particle on PD inception and flash-over voltage, in comparison with the same geometry without particle. In SF6, the particle induces a large decrease of PD inception and breakdown voltages, which become nearly independent of the gas pressure.