POSITIVE DC CORONA AND SPARKOVER IN SHORT AND LONG ROD-PLANE GAPS UNDER VARIABLE HUMIDITY CONDITIONS

The variations, with humidity, of the positive DC corona-current voltage characteristics and of sparkover stresses have been studied in rod-plane gaps of spacings in the range 10 less-than-or-equal-to d less-than-or-equal-to 150 cm. The steady component of the current in the glow regime is described by the square law equation over the whole range of gaps, requiring only adjustment of a coefficient for each combination of gap spacing and humidity employed. The rate of change of current with humidity, at any constant voltage, is the same for all gap spacings. The mean stress for sparkover is the same for gaps 50 less-than-or-equal-to d less-than-or-equal-to 150 cm, irrespective of rod profile, and is linear with humidity variation, but anomalous low sparkover stresses occur above 19 g/m3 in the largest gaps. In shorter gaps, the rod profile plays a larger role, and the transition from glow-controlled to streamer-controlled breakdown is demonstrated in the 20 cm gap.