Space-Charge Effects on Streamer Propagation and Acceleration Voltage in Transformer Insulating Liquids

Voltage-time (V-t) curves for breakdown are important in insulation coordination for design of transformers. The time to breakdown for a liquid insulated gap is determined by the velocity of a breakdown streamer and the distance it must propagate to short circuit the insulation. Streamer velocities vary with applied voltage, and for commercial insulation liquids the voltage at which the streamer accelerate to very high velocities vary a lot. The processes behind this acceleration are not yet determined. It seems clear that streamer propagation is governed by the high field within the liquid phase in front of the propagating plasma channel. In the context of new results on measured space charge limited currents for several liquids, this paper will discuss how the electric field in front of a streamer may be limited by space charges, suppressing the possibility of the field to increase to a level where propagation will accelerate. Lack of space-charge limiting the field in front of the streamer may explain the early acceleration voltage that esters show compared to mineral oils.