Partial Discharge and Deterioration Characteristics of Oil-Immersed Pressboard Under Bubble Defects

Bubbles are typical defects in the oil-paper insulation system of transformers. The partial discharge (PD) caused by them significantly reduces the insulation performance of oil-paper insulation, posing a great threat to the insulation of transformers. However, there is currently limited research on the evolution process of bubble PD and the deterioration characteristics of oil-immersed pressboard (OIPB) caused by bubble PD. This article first constructs a bubble PD test platform, using air bubbles in stagnant oil as the research object. In the research object, the shape, volume, and gas composition inside the bubbles will change with the variation of voltage and PD, which can fully reflect the accumulative effect of long-term PD in bubbles. Then, this article conducts experimental research on the evolution and breakdown process of bubble PD. The results show that the evolution of PD inside bubbles may greatly change the phase-resolved PD (PRPD) spectrum, and this article proposes a possible explanation for the evolution of bubble PD: the gas composition inside the bubble dominates the evolution of PD. Finally, based on the inhibitory or promoting effects of different fault gases on the evolution of PD, this article establishes a connection between bubble PD and the deterioration of OIPB. Scanning electron microscopy (SEM) is used to analyze the microstructure of OIPB in different damage states, and the micromechanism of bubble deformation and insulation failure is discussed. The work helps to understand the PD characteristics of bubble defects and the mechanism of causing insulation failure in OIPB, providing a reference for risk assessment of transformers.