Conduction and breakdown mechanisms in transformer oil

Michael Butcher, Andreas A. Neuber, Michael D. Cevallos, James C. Dickens, Hermann Krompholz

Research output: Contribution to journalArticlepeer-review

65 Scopus citations


With a fast coaxial test setup using high speed electrical and optical diagnostics, prebreakdown current pulses and shadowgraphy images are measured for direct current (dc) breakdown in Univolt 61 transformer oil. Also, dc currents across the gap are measured using a high sensitivity electrometer. The conduction and breakdown mechanisms in transformer oil as function of applied hydrostatic pressures are quantified. Together, this information provides data on the development of current flow in the system. We have identified three stages in the conduction process prior to breakdown for highly nonuniform fields. Stage 1 is characterized by a resistive current at low fields. Increasing the applied electric field lowers the effective barrier at the metal/dielectric interface allowing a "tunneling" mechanism to begin, leading to the rapid rise in the injection current observed in stage 2. 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. The processes of final breakdown show a distinct polarity dependence. 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. We will discuss possible links between conduction current and dc breakdown.

Original languageEnglish
Pages (from-to)467-475
Number of pages9
JournalIEEE Transactions on Plasma Science
Issue number2 III
StatePublished - Apr 2006


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  • Conduction mechanisms
  • Liquid breakdown
  • Mobility
  • Pressure effects
  • Temperature effects


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