BEHAVIOR OF STREAMERS UNDER DIVERGENT AC FIELDS IN TRANSFORMER OILS AT LARGE GAPS

In this paper we present the results of investigations on pre-breakdown phenomena in transformer oils under various divergent ac fields at large gaps (less-than-or-equal-to 25 cm). The direct visualization of the light emitted by streamers was performed with a highly sensitive video device. Streamers of both polarities can be observed, but it is found that positive streamers play a dominant role since their ability to propagate is much greater than that of the negative ones. Positive streamers can stop before they reach the opposite electrode, at a length determined mainly by their initiation voltage. Moreover, geometrical parameters such as the gap distance, the shape and the size of both electrodes, are shown to have a significant influence, which is discussed in terms of capacitive coupling between the streamer filament and the electrodes.