Streamer-to-leader transition in a sphere-to-plane air gap tested with switching impulses

Atmospheric air has been used during decades as main insulating media in the development of larger electric energy systems. The complexity of the many physical processes involved in the breakdown of air as insulating material, include different stages: the corona inception, streamer-to-leader transition, and leader propagation. These stages are accompanied with different levels of optical radiation and changes in the measured voltage and current across the air gap. During the first stages, low currents and transient events like the fast-current impulses occurring at the corona inception and at the streamer-to-leader transition, can be detected. In this paper we aim to analyze the results of a measurement campaign done at a high voltage laboratory while testing a sphere-to-plane electrode configuration with positive switching voltage impulses, recording voltage, current and fast/still camera images. The measured current impulses were compared with numerical simulations done for the estimation of electric charge produced at the leader-corona region, required to achieve the streamer-to-leader transition.