Effect of Thermal Bubbles on Partial Discharge and Breakdown Characteristics of Oil-Immersed Pressboard

The risk of bubble formation is one of the main factors limiting transformer overload operation and moisture management. We already have some knowledge of the temperature and moisture conditions for thermally induced bubble inception, but there is a lack of understanding about how it affects the discharge characteristics of oil-paper insulation systems. In this paper, we investigate the formation characteristics of thermally induced bubbles and their effect on the partial discharge and breakdown characteristics of oil-immersed pressboard. The results show that temperature and moisture are the main factors affecting bubble formation. The higher the temperature or moisture content, the more easily bubbles are formed. The formation of thermally induced bubbles significantly reduces the partial discharge inception voltage and breakdown voltage of oil-immersed pressboard. It is noteworthy that the insulation properties of oil-immersed pressboard begin to deteriorate before the bubble inception temperature. We conjecture that a large number of microbubbles have already started to form inside it before the visible bubbles form on the surface. When the microcavities and holes inside the pressboard are filled with gas, the insulation properties are significantly degraded. The conjecture about the mechanism is subject to further experimental verification. The research in this paper helps to provide a reference for risk assessment and fault diagnosis during transformer overload operation.