Effect of temperature and pressure on DC pre-breakdown current in transformer oil

Summary form only given. Any attempt to model the complex interaction of hydrodynamic and electronic processes leading to breakdown in transformer oil suffers from the lack of microscopic transport data. Also, interface processes, such as electron emission from metal electrodes immersed in liquid, are poorly understood. As a first step toward the understanding of breakdown phenomenology, the voltage-current characteristics for pre-discharge conditions are measured. An experimental setup was constructed which allows temperature variations between 10/spl deg/C and 50/spl deg/C, at pressures between 0.5 and 3 bar. DC currents ranging from a few nA with a few kV of applied voltage, to a few /spl mu/A prior to full breakdown are measured using an electrometer. Preliminary results at NTP with a tip-plane geometry indicate Ohmic behavior at low voltages, Schottky emission at intermediate voltages, and saturation due to space charge at high voltages, and allow estimates on the physical parameters governing these effects. The indicated temperature range of the measurements is associated with a variation of the viscosity of a factor of 3, where we anticipate similar relative changes for the transport of electrons. The intermediate voltage range where a Schottky emission process is assumed will be emphasized. For instance, the electron mobility, derived from experimental data to about 0.06 cm/sup 2//Vs at NTP in the intermediate voltage range should distinctly vary with changing temperature and pressure.