Prebreakdown and breakdown phenomena in large oil gaps under AC

This paper presents a study of prebreakdown and breakdown phenomena under AC voltage in mineral oil in large gaps to 60 cm. The investigations presented concern the study of streamers and the measurement of breakdown voltages in rod-plane and sphere-plane gaps. Also, the influence of a contamination by solid particles in the oil has been considered. A specific breakdown mode under AC voltage is evidenced, where “bursts” of streamers lead to the lowest breakdown fields recorded. Numerical values of the mean field in oil required for “direct” or “burst” breakdown modes are derived from the experiments. As a consequence, the great sensitivity to the presence of particles of EHV transformers insulation with large oil gaps is pointed out