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The sparkover behaviour of sphere-rod gaps under standard impulse voltages is investigated by assessing the influence of gap spacing, impulse voltage waveshape and polarity, and atmospheric conditions on the distribution of the sparkover voltage. At sparkover the growth of streamers of both polarities can be estimated since a value of 2 MV/m is found to be characteristic for the average negative streamer gradient. The IEC atmospheric conditions correction is discussed and compared with a new optimised correction procedure. The feasibility of using the sphere-rod gap as substandard for measuring impulse voltages, replacing the conventional more expensive sphere gap, is examined. [ABSTRACT FROM AUTHOR]

The positive impulse breakdown behaviour of conductor-rod gaps 50 cm in length is investigated while negative DC voltages pre-stress the gap. Breakdown voltage and time to breakdown variations are discussed in terms of the parameters influencing the discharge mechanism such as the gap geometry, the amplitude of the pre-stress negative DC voltage applied at the rod, the waveshape of the applied impulse voltage at the conductor and the variation of absolute humidity. It is shown that pre-stressing, by producing sufficient DC coronas at both electrodes, results in a significant increase in the breakdown voltage. There is a threshold value of the pre-stress DC voltage above which the gap factor against breakdown may double its value. Both absolute humidity and pre-stressing affect, in a combined way, the dielectric strength of the gaps. [ABSTRACT FROM AUTHOR]