Experimental study and simulation of positive predischarge phenomena in insulating oil

This paper presents the results obtained both from computer simulations and experimental studies of the positive predischarge propagation in strongly non-uniform fields in insulating oil. For the positive predischarge three different consecutive propagation patterns could be found. At low voltages the predischarge consists of filigran channels forming a fan-shaped structure. With rising voltages the number of thin channels increases and the structure appears in an umbrella-type pattern. At even higher voltages a secondary streamer originates from the primary structure. This secondary streamer shows high similarity to a coniferous branch, propagates with a higher velocity and causes significantly reduced breakdown times. The computer simulations based on these experimental results were carried out using a Dielectric Breakdown Model (DBM) which was modified by introducing the channel conductivity and its variation during discharge development. This modification allows one to simulate the main spatial and temporal features of the various predischarge structures and consider the transition from primary to secondary streamer including their different velocities.