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