Charge and correlated light emission of streamers in insulating oil

This paper presents results from the experimental study of the charge and light emission behaviour of prebreakdown phenomena in insulating oil. The investigations were carried out in the strong non-uniform field of a 85 mm point-plane geometry with square-wave voltages up to 600 kV. The pre-breakdown phenomena were evaluated by recordings of charge and correlated light emission, as well as high-speed photographs. Previous investigations have evidenced a strong temporal correlation between light pulses and current pulses or charge steps, resp., at positive voltages with amplitudes close to the 50%-breakdown voltage. Now, this correlation is also proven for higher voltages for polarities. While at low voltages, primary streamers show a linear rise of charge which changes subsequently into a step-wise growth, high overvoltages result in the occurence of faster secondary streamers with an exponential charge increase. The initiation of a tertiary streamer leads to a dramatic rise of the charge, i.e. during some nanoseconds the output range (4000 nC) of the charge sensor is saturated.