The physical process of microsecond pulsed discharge in transformer oil is the foundation for studying the initial mechanics of faults in ultra-high voltage transformers. However

the microsecond-scale electrohydraulic effect in transformer oil has not been investigated yet. In this paper

the shock wave and bubble generated by microsecond pulsed discharge in transformer oil between needle-needle electrodes are studied using a Schlieren system with a high-speed camera. The results show that the discharge channel in the 0.1 mm oil gap forms in 9.50 µs when the streamer and the first shock wave are observed. The bubble between electrodes expands in the subsequent 218.50 µs. Then

the bubble collapses when the second shock wave is observed. The velocities of the first and the second shock wave are 1388.16 and 1465.46 m/s

respectively. With the gap distance increase

all the breakdown voltage

the discharge energy

and the ratio of the mechanical energy to total energy increase

the breakdown current

the acceleration of the container wall

and the velocity of the shock wave decrease. It can be concluded that the first shock wave is generated by the streamer of microsecond pulsed discharge and the second shock wave is induced by the rapid density change due to the bubble collapsing.