Contribution of free iron particles to PD characteristics in flowing transformer oil at DC voltages

Abstract

Partial discharge (PD) characteristics of the transformer oil contaminated by metallic particles are relevant to particle movement. The motion characteristics of the particles will change when the oil flows, and hence, the PD characteristics differ from that in stationary oil. In this study, the PD characteristics were investigated at direct current voltages, and the particle trajectories were observed simultaneously. It was found that the number of discharges obviously decreased, but the discharge magnitude slightly increased as the oil flow speed increased. To explain the changes in the PD characteristics, a solid-liquid two-phase flow model was established to simulate the trajectories and the electric field distribution around particles. The experimental observations partially verified the simulated trajectories, which includes two stages, namely sedimentation and the subsequent oscillation-collision stage. With the increase of the flow speed, the sedimentation particles decreased, and the horizontal spacing between two adjacent particle-electrode collisions elongated. Therefore, the total collisions became infrequent, which reduced the number of discharges. Moreover, the higher the flow speed was, the slower the particle approached an electrode. Considering the discharge time lag, the distance between the particle and the electrode at the instant of the PD would increase, which increased the discharge magnitude.