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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.

This paper presents an experimental study of pre-breakdown discharges in water (usually called ldquostreamersrdquo) and transient breakdown arcs (called ldquosparksrdquo) generated by high voltage pulses, in relation with their potential application to degrade organic molecules for depollution purposes. An experimental set-up is described using semiconductor high voltage switches to produce high voltage pulses. In a point-plane gap, depending on the high voltage pulse duration, either streamers alone or streamers followed by a transient breakdown arc can be produced. Electrical measurements provide data correlated to streamer and spark processes: current, charge, power, and dissipated energy. Coupled to these electrical measurements, the degradation of 4-chlorophenol and 4-nitrophenol molecules is studied by HPLC measurements. It is reported that the energy yield for the degradation of these products is higher when sparks are used instead of streamers.

The purpose of this work is to evaluate the chemical stability of a bio-degradable insulating oil composed by a mixture of esters based on rape-seed oil. The measurements are done in comparison with a commercial mineral oil. Both oils are subjected to accelerated ageing conditions at 120degC under air (open beaker method). Dielectric properties (dissipation factor, permittivity, conductivity, AC breakdown voltage) and chemical parameters (acidity, water content) are monitored during the ageing process. In the rape-seed based oil, ageing has a negligible influence on the breakdown voltage, whereas conductivity and acid content show a large increase.