Prebreakdown Phenomena in Liquid Dielectrics

A new set-up, using a video camera, allowing easy and economical access to the velocity distribution of streamers is presented. Our study focuses on the behavior of negative streamers in a variety of liquids, either pure or containing ionic or non-ionic additives, in point-plane geometry. The propagation velocity distribution has always a more or less marked minimum, whatever the polarity and the shape of the streamer (bush-like or filamentary). Such a velocity distribution is similar to that of the field on a growing conducting sphere. There is generally a very close correlation between the shape and the velocity: the higher the velocity the more filamentary the streamer. This is still the case even when in the same liquid we have either subsonic or supersonic velocity. It is shown that the positive transient currents are also in favor of the model of the conducting sphere and not of a unique conducting filament even though there is evidence that the streamer is filamentary. Some of the theoretical models of streamer propagation already proposed are appraised, in particular that of Devins et al. which is thought to be the most accurate in describing experimental results. As concerns the initiation phase, the time lag until the appearance nce of the streamer dec, reases sharply when voltage is increased. This time lag also depends on space charge phenomena. Initiation voltages are always higher with positive polarity than for negative polarity for a series of tested simple or industrial liquids.