Dynamic Behaviour of Bubbles in Oil Under Electrothermal Ageing

Abstract

During long-term operation of converter transformers, the internal oil–pressboard insulation undergoes irreversible degradation. Partial discharge in the weakened insulation will cause the formation of a large number of bubbles in the insulating oil. The dynamic behaviour of bubbles in the strong electric field region will threaten the safe operation of the transformer. To clarify the dynamic behaviour of bubbles in oil under electrothermal ageing, the migration trajectories and deformation process of bubbles in oil under partial discharge at different ageing stages are investigated experimentally, and the influence of the initial bubble radius on migration trajectory, velocity and morphological changes are examined through numerical simulation. The experimental results indicate that as the ageing time increases, the influence of the kinematic viscosity of the insulating oil on bubble migration increases, and the influence of the electric field strength distribution on bubble migration increases and then decreases. It also becomes more difficult for bubbles to return to a spherical shape. The simulation results indicate that as the initial bubble radius increases, bubbles move more easily away from the strong electric field region and more easily enter the rising stage. The maximum degree of longitudinal stretching of bubbles increases.