On the correlation between propagation mode, charge and shape of streamers in mineral oil

This paper presents a study of prebreakdown phenomena in mineral oil under impulse voltage, in point-plane gaps up to 10 cm. In highly overvolted gaps, the measurements show that above a critical voltage, a large increase in propagation velocity is recorded (from 2 to 100 km/s), correlated to large modifications in the streamer shape and charge. This phenomenon is analysed by modelling streamers as conducting objects of various shapes, calculating the charges and fields by a charge simulation method, and comparing them to measurements. The good agreement obtained shows that streamers behave as good conductors, and that the transition between streamer modes strongly depends on the "electrostatic" properties of streamers.