The influence of surface roughness and coating on the impulse breakdown voltage in SF6

The surface roughness of SF₆ insulated systems has a significant influence on the breakdown voltage, especially at high gas pressures. Small surface irregularities can reduce the breakdown voltage. A thin dielectric coating could be a possible remedy. To evaluate the influence of rough surfaces, with and without dielectric coating, the distribution function of the breakdown voltage is of interest. Low probability quantiles and the influence of the first breakdown are relevant for designing GIS systems. This paper deals with the probability function of the impulse breakdown voltage of aluminum electrodes with different surface roughness, with and without dielectric coating, in SF₆. A hemisphere-plane arrangement at an SF₆ pressure of 0.5 MPa was investigated. The half sphere was covered with a thin epoxy resin coating. The plane electrode is polished and uncoated. The surface roughness and coating thickness were varied and the cumulative frequency functions of the breakdown voltage for positive and negative lightning impulse were measured.