Bubble Motion Characteristics in the Transformer Oil Gap at the Top of HV Winding

The bubbles generated in the oil-paper insulation and their movement will reduce the dielectric strength and lead to insulation failure in power transformers. To explore bubble motion characteristics and the possible accumulation location, a bubble motion simulating model is established coupling with the electric field and the oil flow. The motion and deformation of the bubble in the oil gap at the top of high-voltage winding are simulated under ac electric field. The results show that the bubble motion trajectory presents an S-shape during the rising period near the T-cross oil gap driven by the electric field force and pressure from the oil flow. During the motion process, the bubble undergoes periodic deformation with a frequency of 100 Hz. The presence of the bubble changes the electric field distribution, bringing about a more significant distortion of the electric field. In the L-shaped oil gap near the electrostatic ring, the bubble motion is mainly driven by pressure from the oil flow. Moreover, if the oil flow velocity is very slow, the bubbles may accumulate on the inner wall of the angle ring, which will give rise to further deterioration of insulation. A test platform is built to observe the bubble trajectory in the oil gap and the results are consistent with simulated in the model. The results obtained are of great significance to present a better understanding of the bubble dynamics in the transformer oil gaps. Insulation failure risk caused by bubble motion and accumulation in oil gap needs to be paid attention to and further explored by researchers.