Streamer propagation in composite oil/cellulose insulation under LI voltages

A needle/plane geometry was used to study fundamental properties of streamer propagation in composite transformer oil/cellulose insulation under LI (lightning impulse: 1.2/50 /spl mu/s) voltages. The plane electrode was either an aluminium plate (bare oil gap) or covered with laminated oil-impregnated paper of different thickness. Breakdown voltage and streamer propagation speed were seen to differ at different polarities and dependent on the gap length of the needle/plane geometry. A ratio of oil-gap length to electrode cover thickness seems to exist, below which, the cover thickness dominates the breakdown of the composite insulation. Above that, length of the oil gap dominates the breakdown.