Currents in AC stressed liquid insulated needle plane gap

Currents and space charge phenomena in 8 different dielectric liquids have been investigated for an ac stressed needle plane gap. Applied frequencies ranged from 0.1 Hz to 100 Hz. A high resolution analogue-digital converter and an active suppression of capacitive currents were used to extract the small conductive currents. Tip radii and voltages were varied. The currents varied non-linearly with voltage. At low frequencies the perfluorpolyeter and mineral oils had almost symmetrical current in both polarities, while cyclohexane, white oil and esters had asymmetrical current with lower peak current for positive polarity. Above a certain instantaneous voltage level - within the power cycle - the currents increased with the voltage squared, fitting a space charge limited current model. Clear indications of heterocharge space charges were revealed by varying tip radii and frequencies. The heterocharge will result in an increase of the electric field. The results show that at higher voltages a purely resistive model for the liquids becomes invalid. It is also evident that the presence of space charges will influence partial discharge behavior.