“Streamers” in liquids: Relation with practical high voltage insulation and testing of liquids

This paper discuss the relations between laboratory studies of breakdown precursors in liquids (usually called "streamers"), and practical applications such as high voltage insulation of power transformers and testing of liquids. The conditions existing within high voltage power transformers (ac voltage, moderately divergent electric field, presence of insulating solids, particles in the oil, etc.) are rather different from those frequently used in the laboratory to study streamers (very divergent field -point-plane geometry-, and impulse voltage). Parameters such as field geometry, voltage waveform, liquid conditioning, can influence either streamer inception or propagation. After a short presentation of streamer phenomena in liquids, the most important effects of these parameters are recalled in this paper. Many features previously observed empirically by the measurement of breakdown voltage can be explained by the behaviour of streamers in conditions close to those actually present in transformers. It remains rather difficult today to model and predict streamer properties (initiation fields, propagation length, velocity, ...) when the liquid nature is changed. The large variety of physical processes involved during initiation and propagation, and the non-linearity of many processes make the study of breakdown in liquids complex. However, knowledge about streamers is helpful to define and measure parameters of practical importance (initiation and propagation conditions), providing a more scientific basis to test and compare the properties of liquids.