Conduction and Electric Fields in Dielectric Liquids in Needle-to-Plane Gaps

This article describes the measurements of conduction currents in both neat and commercial insulating liquids. Conduction in thermodynamic equilibrium is measured by applying a low ac voltage in a uniform field. High-field pre-discharge currents are measured in a needle-to-plane configuration stressed with high-voltage ac and steep step high-voltage pulses. There is a significantly increased conduction for high field compared to low-field conditions. Notably, measurements at high fields in needle-plane configurations indicate space charge limitation of currents with polarity differences. The high-field conduction exhibits considerable variation across different liquids. At high applied fields, electronic processes are probably governing the charge formation. Simulations indicate that space charges may reduce the local field in front of needle tips at time scales relevant for propagating streamers.