Investigation of charge conduction and self-breakdown in transformer oil

With a fast coaxial setup using a needle/plane geometry and a high sensitivity electrometer, conduction mechanisms in transformer oil at varying temperature and hydrostatic pressure are quantified. There are 3 stages in the conduction process prior to breakdown for highly nonuniform field geometries. Stage I is characterized by a resistive current at low fields. Stage II consists of a rapid rise in the injection current associated with increasing field due to a "tunneling" mechanism through the metal/dielectric interface. The transition from the resistive to tunneling stage occurs when the applied field reduces the barrier at the metal/insulator interface to a point where tunneling of charge carriers through the barrier begins. This transition point is polarity dependent. In stage III, at high fields the current reaches space charge saturation at electron mobilities >100 cm²/V*s prior to breakdown. The processes of final breakdown show distinct polarity dependence. Data for the negative needle exhibits strong pressure dependence of the breakdown voltage, which is reduced by 50% if the hydrostatic pressure is lowered from atmospheric pressure to hundreds of mtorr. Such a strong pressure dependence, at reduced hydrostatic pressure, indicates breakdown is gaseous in nature. This is supported by images of bubble/low density regions forming at the current injection point. Positive needle discharges show a reduction of only about 10% in breakdown voltage for the reduced pressure case. A 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.