Current measurement of predischarges in insulating oil under impulse conditions

This paper presents results from the experimental study of the current and charge behavior of predischarges occuring in insulating oil under impulse voltages. In order to minimize the influence of stray capacitances on the current measurement a specific device was developed and constructed. The streamer inception and propagation was visualized using high speed shadowgraph photography technique. To permit a chronological relation between voltage, camera monitor signals as well as the current, these characteristic quantities were simultaneously recorded with a digital storage oscilloscope. For both predischarge polarities the propagation must be differentiated in three successive stages: primary, secondary and tertiary streamer. During positive primary streamer propagation a continuously increasing charging could be obeserved. However, beyond a critical voltage, short current impulses could be detected discharging and charging the streamer structure in less than 200 ns. With the inception of the secondary streamer the charge increases dramatically. For the first negative predischarge stage a similar behavior could be observed. However, the charge curve changes from the linear to a step-wise increase leading to a step-wise streamer growth.