Air breakdown in the conductor-rod gap under composite slow-front impulse/alternating voltages

The dielectric strength of a conductor-rod gap under combined alternating and impulse voltage is discussed. The effect of the alternating voltage on the impulse profile, under withstand and under the breakdown condition, is described in terms of the angle in the cycle at which the impulse is triggered. Sparkover data are presented for composite voltages using three switching impulses, which have times to peak of 119, 241 and 342 μs in the absence of pre-stress voltage. Results are given for triggering angles 0°, 90°, 135°, 180° and 270° and it is shown that significant variations occur in the composite sparkover voltage depending on both the amplitude of alternating voltage and the triggering angle. The results are discussed in terms of the influence of the corona set up by the alternating voltage