Role of bubble and impurity dynamics in electrical breakdown of dielectric liquids

Abstract

Electrical discharges in liquids with and without gas injection was experimentally studied with two circuits (RC and single spark gap). Electrical breakdown could be initiated from the following mechanisms: electrode initiation; impurity initiation; and Taylor cone driven initiation. Discharges initiated by electrodes were observed only with the single spark gap circuit. Impurities and Taylor cones can initiate discharges when using the RC circuit. With the single spark gap circuit, bubbles were nearly stationary in the gap before and during discharges because of the limited time to respond to the electric field growth (dV/dt = 1 kV ns−1). Bubbles and impurities gain significant energy and undergo significant dynamic changes with the RC circuit under a long rising time. Charge relaxation time on bubbles or impurities in the fluids were close to 100 ms, which is comparable with the circuit rising time. Bubble dynamics observed include reduced bubble size, increased bubble speed and shape change. Taylor cones were observed on bubble surface using a high-speed camera during multiple discharge events, and usually led to breakdown. Taylor cones develop on a bubble surface through multiple states over a period of 10–500 μs. State one was typically seen on a bubble surface with a sharp tip caused by the residual charge. A single streamer was initiated on the tip of the cone at state two. State three developed multiple bush-like streamers on the cone and usually triggered a breakdown in the gap. Discharge behaviors in liquids were also investigated with the RC circuit at different pulsing frequencies. At a low pulsing frequency (1 Hz), discharge events are independent from each other. When pulsed above 10 Hz, bubbles created in the gap accumulate and participate in the next discharge event. Bubble accumulation results in a cloud where local secondary discharges are observed with a much higher frequency up to a few kHz for several milliseconds.