The Effect of Electrode Geometry on the Lightning Impulse Breakdown in SF₆ / N₂ Mixtures

Abstract

Investigations were undertaken to focus on the effect of methodical variation of the rod-diameter in a rod-plane geometry under lightning impulse voltages in SF₆ / N2 mixtures (1 to 20 % of SF₆), over a pressure range of 0.1 MPa to 0.5 MPa for gaps of 5 mm to 80 mm with rod diameter of 0.1 (sharp needle), 0.8, 1.5, 3.0, 5.0 and 10.0 mm and a 230 mm dia. plane electrode of Rogowski profile. the experimental results indicate that the breakdown voltages are sensitive to small amounts of SF₆ gas impurities in N₂ under lightning impulse voltage conditions for diffirent non-uniformities investigated. the highlight of this paper describes V₅₀-rod diameter characteristics, which shows maxima in their V₅₀ at all gap spacings and gas pressures. These maxima (V₅₀ min.) do not appear to occur at any fixed value of rod-diameter, but vary depending on the gap pressura and gap spacing.

The significance of V₅₀ min. in breakdown characteristics imply for a given gap length a safe voltage level exists (generally referred to as “Forbiden-Zone”) below which no breakdown can ocur irrespective of the rod-diameter and a gas pressure. the practical interpretation of the results suggested that the irregularities on the electrode surfaces such as roughness, scratches, etc., do not necessarily diminish the dielectric strength of either SF₆ or SF₆ gas mixtures, below certain voltage level.