Breakdown Characteristics of Flowing Transformer Oil Contaminated with Cellulose Particles under Slightly Non-Uniform DC Field

The insulating oil in the oil channels of large oilimmersed transformers is always in flowing state due to the cooling system. The oil flow has an undeniable influence on insulating property of transformer oil. The breakdown characteristics of transformer oil in flowing state were carried out in this paper. The characteristic breakdown voltages at different flow rates were extracted, and the images of breakdown process were recorded using a high-speed camera. The results showed that the oil flow with low velocity had little effect on insulating property, and breakdown occurred along the bridge across the gap. The oil flow with high velocity significantly improved the insulating property, and no bridge across the gap could be formed. The distortion of the electric field at the end of single particle or partial bridges is severe, leading to a partial streamer, which eventually develops into the breakdown.