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 a nd impulse voltage is discussed. The effect of the alternating voltage on t he impulse profile, under withstand and under the breakdown condition, is d escribed in terms of the angle in the cycle at which the impulse is trigger ed. Sparkover data is presented for composite voltages using three switchin g impulses, which have times to peak of 119, 241 and 342 mu s in the absenc e of pre-stress voltage. Results are given for triggering angles 0 degrees, 90 degrees, 135 degrees, 180 degrees and 270 degrees and it is shown that significant variations occur in the composite sparkover voltage depending o n both the amplitude of alternating voltage and the triggering angle. The r esults are discussed in terms of the influence of the corona set up by the alternating voltage.