Study of electric breakdown of liquid dielectrics using Schlieren optical techniques

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Abstract
The work is a contribution to the understanding of the mechanism of electric breakdown in liquid dielectrics. The liquid dielectric chosen throughout the experiments was n-hexane, because an extensive literature exists on the breakdown of this material. The applied voltage was a rectangular pulse of different durations supplied from a 125 kv, five-stage Marx-Goodlet impulse generator. A novel rotary multiple electrode containing eight pairs of electrodes was incorporated in a simple and efficient fractionation-filtration unit. A protection circuit, incorporating an inverter and a trigatron, limited the duration of breakdown current. A series of photographs of the state of n-hexane under the applied electric field was taken by Schlieren technique, using a spark light-source and a still camera. A statistical survey of the results shows the formation of a region of very low refractive index at the cathode. It is suggested that this is due to streamers with a very large number of branches, which propagate towards the earthed electrode. The rate of growth increases with voltage and above a certain voltage, if enough time is allowed, the stepped streamers bridge the electrode gap and breakdown of the liquid results.

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