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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.

This paper presents a study of prebreakdown and breakdown phenomena under ac voltage in mineral oil in very large gaps up to 80 cm. The investigations concern the measurement of breakdown voltages together with the recording of streamers. Experiments were conducted with rod-plane and sphere-plane electrode systems at different distances. The results describe breakdown mechanisms for moderately divergent fields and large gaps. A breakdown mode specific to the ac voltage is evidenced, in which "bursts" of streamers initiated at each halfwave can appear and lead to the lowest breakdown fields recorded under ac. Values of the average fields required for "burst" or "direct" breakdown modes are deduced from the experiments. The influence of particles is also considered.

This paper presents a summary of the introductory lecture to be presented at the 1996 ICDL on the particular problem of prebreakdown phenomena in long liquid gaps (centimeters to decimeters). The specific topics developed concern the characterisation of the propagation of positive discharges in mineral oil, including a description of the various propagation modes, together with the study and modelling of their gaseous nature and electrical properties.