Impact of Oil Velocity on Partial Discharge Characteristics Induced by Bubbles in Transformer Oil

Bubble is a typical insulation defect in transformer oil, which can easily initiate partial discharge (PD) during the operation of transformer. Moreover, it migrates along with the oil flow and may be deformed due to the forced oil circulation and temperature difference within transformer bulk, leading to the complexity of PD characteristics and PD mechanism. However, the understanding of to this phenomenon is not clear. In order to identify it, finite element method was employed to simulate the movement process and deformation of bubbles, as well as electric field distribution under the conditions with different oil flow rates. In addition, numerous experiments were performed, mainly including the change of PD phi-q and phi-n patterns with oil flow rates. By comparing the simulation and experimental results, it is found that PDs are mainly concentrated in the negative half-cycle. Electric force causes bubbles to stretch along the electric field direction. At the stationary condition, bubbles eventually exhibit horizontal stretching and cause maximum electric field distortion. However, bubbles eventually exhibit vertical stretching in flowing oil. Flowing oil can significantly reduce PD intensity. The PD intensity declines rapidly with the increase of the flow velocity, and then increases slowly.