Understanding the Forming Mechanism of “Metallic Particle Bridges” in Flowing Transformer Oil Under DC Voltage: Experiments and Simulations

“The suspended solid particle mechanism” is often used to explain the breakdown in contaminated liquid dielectrics. However, the bridging behaviors of metallic particles will be influenced by the oil flow, ultimately affecting the breakdown characteristics. In this article, experimental observations of metallic particle bridging in flowing transformer oil are carried out under dc voltage first. It is revealed that particles will form dynamic bridges in the direction of the electric field lines, wherein they exhibit constant motion and frequent collisions. As time progresses, multiple bridges gradually converge and merge. In addition, the formation of particle bridges will be hindered by increased oil flow velocity, consequently prolonging the time requested for the bridge breakdown. Moreover, metallic particle bridges are formed most quickly at electric field strengths of 1 and 2 kV/mm, while they fail to be built at 4 kV/mm. To elucidate the bridging mechanism, a solid–liquid two-phase flow model is further established and the bridging behaviors are simulated under various conditions. The changes in force and velocity components during collisions reveal that the local electric field gradient force plays a critical role in bridging. Finally, the impact of oil velocity and electric field strength is explored.