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The breakdown mechanism of dielectric–covered rod/plane air gaps under positive lightning impulse voltages is investigated. Several gap lengths, varying between 2.5 and 15 cm, were employed and the simple air gaps were regarded as reference. The basic characteristics of coronas, namely inception time and voltage, were measured at applied voltages just sufficient for their inception up to voltages causing breakdown. Multiple level tests were conducted; thus, corona inception and breakdown probability distributions were obtained and time to breakdown was measured at several voltage levels. Interpretation of the results was made based on oscillograms of the electric field strength at the earthed plane, which was monitored through a capacitive probe, geometric field calculations and still photographs of the discharge at breakdown. In the dielectric-covered rod/plane gaps, breakdown occurs at higher applied voltages as a result of increase in both corona inception voltages and discharge path. The development of the discharge depends on the electric field distribution as modified by both space and surface charges associated with coronas preceding breakdown. Surface charge accumulation may affect the discharge path at breakdown. [ABSTRACT FROM AUTHOR]

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]