Numerical Modeling of Atmospheric Air Breakdown in a Weakly Nonuniform Electric Field Due to a Positive Lightning Impulse

Assessing the breakdown voltage (BDV) of an insulation system typically requires extensive and expensive testing. This article examines the application of numerical methods to estimate the BDV of sphere gaps (SGs) in weakly nonuniform electric fields due to a positive lightning impulse (LI) voltage. A set of electron and ion continuity equations that describe the physics of air discharge, coupled with Poisson’s equation, are used to model the air breakdown (BD) phenomenon. The study investigates the formation of primary and secondary avalanches that lead to self-sustaining discharge in the air gap. The role of the cathode in producing electrons due to secondary ionization, which is responsible for the onset of a self-sustaining discharge, is also explored. BD is achieved through a cathode-directed streamer initiating at the anode, accompanied by a steady stream of electrons produced at the cathode. A high-speed camera is used to capture the BD, which revealed the formation of a bright spot at the cathode during the BD. The predicted BDV is found to closely match the experimentally determined BDV, and the velocity of the cathode-directed streamer is computed to be
$1 \times 10^{8}$
cm/s.