Observation and Simulation for the Movement of Metallic Particles in Flowing Transformer Oil under AC/DC and Combined Voltages

The movement of metallic particles in flowing transformer oil is observed under AC, DC and AC/DC combined voltages. It is found that the particles execute sedimentation followed by repeated up-down motions in the vertical direction, and meanwhile move along with the oil flow in the horizontal direction. The up-down motion is affected by applied voltage. With the application of AC voltage, the bounce height from the grounded electrode does not exceed the half of electrode separation. After the DC component is introduced, the particles can arrive at the high-voltage electrode. When the ratio of combined voltage between AC and DC components is 1:1, the particles drop slightly in the upward motion and rise slightly in the downward motion. As the ratio is reduced, no such slightly dropping or rising is observed. In order to analyze the experimental results, a simulation model about solid-liquid two-phase flow subjected to electric field is constructed. Simulated trajectories of particles are in good agreement with the experiments. Based on the model, the evolution of force condition exerted on a particle during its motion is analyzed, and the relationship between partial discharge (PD) frequency and particle movement is briefly discussed.