Multiple Discharges Before Leader Inception in Long Air Gaps Under Positive Switching Impulses
There are multiple corona bursts before leader inception when the rising rate of the applied voltage or electric field is not sufficiently high enough in long positive sparks. In existing studies, no attention has been paid to whether these corona bursts occur in the same location, and they are mostly considered directly as belonging to the same discharge. However, this paper presents that in a typical rod-plate long air gap, the multiple corona bursts before leader inception are distributed in at least two different locations, and the highest probability of three discharges occurs. Also, the discharge occurs with the highest probability in the time sequence ‘tip-tip-side-tip-other side’ of the electrode in the first five corona bursts. For each discharge, the first corona current is a single, double exponential pulse, while the following corona currents are mostly a superposition of multiple pulses. The above findings are mainly based on experiments in a 1.4 m air gap under positive switching impulses, in which the voltage, current, and high-speed images were recorded simultaneously. Finally, based on the experimental results, this paper discusses the effects brought by ignoring the multiple discharges on key parameters of leader inception and makes some suggestions to optimise long spark experiments.