Study on Abnormal Behavior of Bulk and Surface Breakdown in Transformer Oil Under Nanosecond Pulse

In the vacuum insulation experiments, the ldquoworm-holerdquo phenomenon f_q been found. A worm-hole is a discharge that passes through the bulk of the material, as opposed to a surface trace. The same phenomenon was found in our experiments of transformer oil bulk breakdown and flashover. The output voltage of pulse generator is in the range of 50 kV to 300 kV. The pulse width is 40 ns. The capacitive divider and the self-integral Rogowski coil are used for measuring voltage and current waveforms respectively. The plate-plate electrode system is used. Nylon 6 and organic glass are used as solid insulation materials. There are two sample shapes adopted: column and trapezoid. Sample thickness is from 1 mm to 5 mm. The experiments indicate that when sample´s thickness is less than 1mm, bulk breakdown almost happens. The sample is thicker, the flashovers happen more. There is the probability of bulk breakdown when sample´s thickness is 5 mm.