Streamer and breakdown phenomena under step and lightning impulses in various hydrocarbon liquids

In this study, measurements of breakdown voltages and time to breakdown are carried out over a large range of gap distances (from 2 to 20 cm) versus liquid nature. Measurements were obtained in divergent field, with either step or lightning impulse voltages. Non-polar hydrocarbon liquids with different chemical structures are studied: saturated, aromatic, polyaromatic, mineral oils. Large effects of liquid nature, gap distance, and voltage waveform, are seen. In very long gaps with lightning impulse, streamers with a low propagation velocity (a few km/s) are quenched, due to the rapidly decaying voltage. Breakdown can be only due to fast streamers, which appear in very different conditions according to the liquid nature. Measurements under step voltage provide a useful basis to interpret the complex variations obtained under lightning impulse. These results also show that testing and comparing liquids under lightning impulse in short gaps (a few cm, such as in standard tests) does not provides data relevant for the actual behavior of liquids under very high voltage.