Streamer inception in mineral oil under ac voltage

This paper describes an experimental study of streamer inception in mineral oil under ac voltage, with rod and point electrodes. Positive and negative streamer inception frequencies versus voltage are investigated in gaps up to 40 cm with different electrode shapes and different conditioning of the oil (filtered oil, addition of cellulose particles, water). Streamer inception probability increases exponentially versus field, and it is not possible to simply define an “inception voltage”. A voltage (or field) value correlated to an inception probability must be used to properly compare different experiments (comparison between liquids, influence of pollution, etc.). Under ac, several effects are superposed to reduce dielectric strength: “scale effects”, influence of pollution, long time duration. With sharp points, injected space charges considerably influence experiments, and the results obtained cannot be extrapolated to practical applications in which the effect of space charge is mostly absent.