Breakdown processes in transformer insulation under LI voltages

The work contributes to the understanding of breakdown processes in transformer oil/cellulose insulation under a LI (lightning impulse) voltage. The breakdown mechanism associated with streamer propagation, space charge distribution, material properties and insulation geometry is studied. The observed phenomena lead to a qualitative model which adds insight into the breakdown process in composite transformer insulation. Oil-impregnated pressboard, paper or polymeric material was inserted into a needle/plane electrode geometry in which oil gap distances exist between the solid insulation and the electrodes. Breakdown processes related to streamers were observed both in the oil gap and on the surface of the solid. Experiments showed that two important phenomena occurred when a streamer was initiated in oil and propagated to the surface of a solid insulation: the streamer changed its propagation from the needle-axis direction to that along the surface of the solid insulation; the streamer carried electrical charges onto the solid surface and thus redistributed the electric field in the insulation. The breakdown data were compared with those from a turn-to-turn model of transformer windings. It is found that it is the "average field" in the solid insulation after streamer development that governs puncture of the solid. A streamer can also puncture the solid insulation with no discharge pattern being observed on its surface, which happened when a large ratio exists between length of the oil gap and thickness of the solid insulation. The breakdown process is related to material type of the solid insulation.