Pressure Dependent Propagation of Positive Streamers in a long Point-Plane Gap in Transformer Oil

The propagation of positive streamers in a naphtenic transformer oil in an 80 mm point-plane gap has been investigated under an impulse voltage being close to a step voltage and under pressures ranging from 0.1 to 1.7 MPa. As has previously been found in short gap experiments of 1 – 7 mm in various liquids, increasing voltage leads to shorter stopping length of non-breakdown streamers and higher breakdown voltages while the velocity is close to independent of pressure. The “acceleration” voltage from which streamer velocity rapidly increase with increasing voltage is also close to pressure independent. It is argued that this indicates that the processes determining velocity must take place in the liquid phase at the streamer head while conditions in the channel determines the stopping length of non-breakdown streamers.