Mechanisms of charge conduction and breakdown in liquid dielectrics
Butcher, Michael D.
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With a fast coaxial test setup using high speed electrical and optical diagnostics, pre-breakdown current pulses and shadowgraphy images are measured of DC breakdown in Univolt 61 transformer oil as well as luminosity in liquid nitrogen. DC leakage currents are measured using a high sensitivity electrometer. The conduction and breakdown mechanisms as a function of applied hydrostatic pressures and temperatures are quantified. Together this information provides data on the development of current flow in the system. Three stages in the conduction process prior to breakdown have been identified for highly non-uniform fields in transformer oil. Stage 1 is characterized by a resistive current at low fields. Increasing the applied electric field alters the effective barrier at the metal/dielectric interface allowing a "tunneling" mechanism to begin, which causes an increasing rise in the injection current observed in stage 2 for increasing fields. In stage 3, at high fields the current reaches space charge saturation with an apparent mobility of 3*10-3 cm2/V*s prior to breakdown. A descriptive model of conduction in liquid dielectrics is presented. The processes of final breakdown show distinct polarity dependence for both transformer oil and liquid nitrogen. In transformer oil, a strong pressure dependence of the breakdown voltage is recorded for negative needle/plane breakdown; a 50% reduction in breakdown voltage is observed when the hydrostatic pressure is lowered from atmospheric pressure to hundreds of mtorr. Positive needle discharges show a reduction of only about 10% in breakdown voltage for the reduced pressure case. Weak pressure dependence indicates the breakdown mechanism does not have a strong gaseous component. Similar results due to changing hydrostatic pressure have been observed in liquid nitrogen. Images of bubbles/low density regions forming at the current injection point indicate breakdown is gaseous in nature. Electron avalanches in the low density region of the vapor bubble form a thick, “bushy” channel. Shadowgraphy images of positive needle breakdown show no localized “bubbles” and correlate to a weak dependence on pressure. A thin filamentary “streamer-like” appearance characterizes positive breakdown. Comparisons of transformer oil and liquid nitrogen show some similarities in the temporal development of final breakdown but not in all stages of conduction leading to breakdown. Possible links between conduction current and DC breakdown are discussed.