Discharge phenomena in positive rod to plane air gaps under lightning impulse and DC voltages have been investigated. Under impulse voltages, the 50% flashover voltage
V50 agrees with the 50% corona inception voltage for the gap shorter than
dc which depends upon the rod diameter φ.
V50 at
dc rises with
dc at a rate of about 14kV/cm for
dc<2.8cm(φ<1.5cm), whereas, it rises at a rate of about 5kV/cm for
dc>2.8cm(φ>2cm). Experimental results show that the flashover process in the former region (
Gp process) is different from that in the latter region (
Lp process). Under DC voltages, the critical gap length, for which the first corona may lead to flashover with no successing corona, corresponds to
dc. In the gap shorter than
dc flashover occures through
Gp or
Lp process, but in the gap longer than
dc, flashover may occur through another process which can not be seen under impulse voltages.
The present paper describes these flashover characteristics and processes with experimental results on the basis of photographic observation with an image converter camera.
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