Effect of dissolved sulphur hexafluoride on electrical breakdown in mineral oil

The effect of dissolved sulphur hexafluoride (SF6) on the breakdown strength of mineral oil has been investigated. Variations of the breakdown strength of the oil with the partial pressure of SF6 in solution, i.e. Clark´s curves, followed a double-peak curve. It was found that stress conditioning an oil sample containing SF6 resulted in a reduction in the breakdown strength of the liquid. Photographic studies of prebreakdown and breakdown phenomena showed that, when SF6 was dissolved in the liquid at relatively high partial pressures, breakdowns were preceded by luminous events at anode sites, and the breakdown sparks started at the anode and proceeded towards the cathode. A model of the events leading to the breakdown of the liquid is suggested, which explains most of the observed phenomena. According to this model, the cathode and anode space charges are interdependent, and there exists the possibility of ionisation taking place at anode sites, owing to the high field intensification produced by SF6¿ ions. Breakdown is finally triggered by a gaseous phase in the oil, caused by dissociation of the oil molecules and the evolution of hydrogen bubbles.