PD characteristics and deterioration mechanism of oil-impregnated pressboard with bubble defect

The bubbles are typical defects of oil-paper insulation systems, and partial discharge (PD) initiated by them can significantly degrade the insulating performance of oil-paper insulation, which is a great threat to the insulation state of power transformers. However, the evolution process of bubble PD is less studied, and the deterioration relationship between bubble PD and pressboard insulation is still unclear. In this paper, the evolution characteristics of PD and breakdown of oil-impregnated pressboard under bubble defects are studied. The results show that bubble PD inception voltage (PDIV) and breakdown voltage (BV) are not only related to bubble size, but also affected by oil temperature. As the oil temperature increases, the bubble PDIV and BV tend to decrease and then increase. Continuous observations of the bubble radius and gas composition under the effect of PD show that the evolution of the bubble PD is attributed to the change in the gas composition inside the bubble. Then, the bubble deformation characteristics under the influence of electric field were analyzed, and it was found that the bubbles were stretched along the direction of electric field and deformed pulsatingly at a frequency of 100 Hz. Finally, the microscopic morphology, elemental content and distribution of oil-impregnated pressboard under different damage states were analyzed by scanning electron microscopy (SEM) and X-ray energy spectrometry (EDS), and the microscopic mechanisms of bubble deformation and insulation failure were discussed. This work contributes to the understanding of the mechanism of pressboard insulation failure due to bubble defects and provides a reference for risk assessment of power transformers.