Bridging Characteristics of Cellulosic Particles in Flowing Transformer Oil

The flow of contaminated transformer oil has a significant influence on the formation of bridges composed of impurity particles and in turn affects its breakdown progress, but the relevant phenomenon may be not well interpreted by the classical suspended solid particle mechanism. In this article, an oil circulation device was built, and the development of cellulosic bridges in slightly nonuniform electric field at different flow velocities was recorded by a high-speed camera. The results show that, when dc voltage was applied, the time required to form cellulosic bridges across the oil gap (named complete bridge) first decreased and then increased with the rise of flow velocity. It indicates that the low-speed oil flow could accelerate the formation of complete bridge in the dc field, while the high-speed oil flow delays or prevents it. When ac voltage was applied, the length of the longest partial bridge (it did not bridge two electrodes) and the number of cellulose particles accumulated in the gap both increased first and then decreased with the rise of flow velocity. Finally, the results were analyzed according to the force conditions of particles. This article contributes to understand the effect of oil flow on bridging characteristics of cellulose particles and lays the foundation for the subsequent studies of breakdown mechanism about flowing contaminated liquid dielectrics.