Optical observation of pre-breakdown and breakdown phenomena in transformer oil

High-speed time-resolved Schlieren photographs are taken between impulse voltage application and breakdown in transformer oil using an image convertor camera at various applied voltages and framing speeds. Needle-to-plate electrodes are used and the oil is under vacuum. The pre-breakdown disturbance accompanied by shock waves emanates from the needle electrode for both polarities of the needle point. The disturbance from the negative point propagates step by step like a lightning discharge in air, while that from the positive point moves radially and reaches the plate electrode in one step. The pre-breakdown disturbance seems to be a gaseous phase which is generated in the liquid.