Influence of electrode surface roughness on the impulse breakdown probabilities of SF6

The influence of the electrode surface roughness on the impulse breakdown probabilities of pressurized SF6 was examined in a coaxial cylinder electrode geometry (cylinder diameters 70/30 mm) for both polarities at a pressure p of 6 bara. The results show that (a) for both positive and negative polarity, an increase of the maximum height of irregularities Rmax of the inner electrode leads to a reduction of the breakdown electric field E prevailing at the inner electrode; (b) for negative polarity, the 1% breakdown field on the inner electrode is lower than for positive polarity; (c) for negative polarity, the standard deviation is smaller resulting in a bigger difference between the 50% breakdown probabilities for the two voltage polarities; (d) the experimentally obtained plots of E/p versus pRmax under constant pressure (6 bara) and Rmax variable (between 10 and 50 µm) are appreciably deviating from these obtained from a one protrusion theoretical model, especially at low values of pRmax, and the E/p shows a weaker dependence on pRmax than does the theoretical model. However, plots of E/p versus pRmax obtained under variable pressure (2–6 bara) and Rmax constant (about 15 µm) are in better agreement with the theoretical model.