Effect of electrode surface roughness on breakdown in nitrogen/SF6 mixtures

Breakdown voltages in uniform-field and coaxial-electrode systems have been calculated for nitrogen/SF6 mixtures at pressures up to 5 bar for smooth electrodes, and for electrodes whose surface roughness is represented by a hemispherical protrusion. The calculations show that the addition of nitrogen to SF6 reduces sensitivity to electrode surface condition. For practical systems, having values of the product (pressure × roughness) of 50¿500 bar ¿m, this tends to offset the loss of dielectric strength predicted for smooth electrodes with the addition of nitrogen. With a 30¿m protrusion, for example, the breakdown voltage at 4 bar of a 70 mm/220 mm coaxial system containing 70% nitrogen by volume exceeds that for pure SF6. The results of the calculations are compared with uniform-field measurements with an artificial cathode protrusion and with published data for a.c. and impulse breakdown in a coaxial-electrode system.