THE DYNAMICS OF DC PREDISRUPTION IN LIQUID INSULATING MEDIA

Results of predisruptive current, light and electrical charge measurements are presented for the liquids tetraester, transformer oil and cyclohexane, under point-plane electrode arrangements with negative and positive high dc voltages. Observed current and light pulses existed both in the kHz and the MHz frequency ranges for both polarities of the point electrode. The electrical charge for the current pulses either increased or decreased sharply whenever these pulses occurred. In general, three pulse regimes were evident: (1) current pulses with no associated light pulses, (2) current pulses correlated with light pulses and (3) light pulses with no associated current pulses. Analysis of the results obtained suggests the occurrence of charge injection followed by processes of bubble formation and electronic avalanche multiplication in the liquid phase, and partial discharges in localized expanding bubbles. The steplike propagation of streamers is explained and streamer velocities were estimated. Involvement of both electronic and gaseous phenomena in predisruptive processes is also suggested.