Effect of electrode surface roughness on breakdown in SF6-N2and SF6-CO2gas mixtures

The effect of electrode surface roughness on the breakdown potential in SF 6 , SF 6 -N 2 , and SF 6 -CO 2 has been measured and calculated using two surface-roughness models (a multi-ridge model and a single-needle-tip-protrusion model). The calculations are in reasonable agreement with the experimental results, indicating that although the addition of N 2 or CO 2 to SF 6 reduces the sensitivity of its dielectric strength to electrode surface imperfections, the breakdown strength of SF 6 gas remains higher than that of SF 6 -N 2 and SF 6 -CO 2 (50%-50%) even when pR for the multi-ridge model is as great as 40 kPa mm representing an exceptional surface imperfection in engineering application conditions. Only in the case of a single needle-tip protrusion can the dielectric strength of the SF 6 -CO 2 be slightly higher than that of SF 6 gas, and in no case will the dielectric strength of SF 6 -N 2 exceed that of pure SF 6 as has been reported by some previous investigators.