Pre-Breakdown Discharges in Rod-Plane Gaps in SF6 under Positive Switchihg Impulses

Pre-breakdown discharge currents in SF6 rod-plane gaps are measured under positive switching impulses and at different gas pressures. Four types of discharge currents could be distinguished, namely, single pulse discharges, multiple-pulse discharges, incomplete breakdown pulse, and corona-free direct breakdown. The effect of impulse magnitude, gas pressure, and gap geometry on the relative frequencies of occurrence of the four types of discharges are examined. The random dispersion in discharge pulse delays is related to the rate of production of initiatory electrons near the anode. A relationship between this rate of production and the electric field in the gap is derived, and its dependence on gas pressure and impulse front duration is investigated.