The rise and fall of the internal temperature of power transformer will cause the moisture to change its distribution and aggregation position, and the local high moisture content will seriously affect the electrical strength of oil-paper insulation. Therefore, it is necessary to study the adsorption and desorption of moisture in cellulose insulation at different temperatures. In this paper, three oil-cellulose mixed systems (OCS) of 105 atoms with different moisture contents were established by molecular dynamics method, and temperature rise and temperature drop simulations were conducted respectively. The changes of the water molecule number (NW) in the interface domain and oil domain were obtained. By analyzing the solvent accessible surface area (SASA) and the microscopic scanning electron microscope (SEM) images of cellulose insulation, the effect of temperature changes and the deterioration of cellulose molecules on the moisture adsorption and desorption in cellulose insulation were studied. The results show that after the high-temperature system is reversely cooled, water molecules in the oil domain will migrate rapidly to the cellulose domain, while the irreversible deterioration of cellulose after high temperature leads to the weakening of its adsorption capacity. As a result, a large amount of water is retained at the interface. For the simulation of temperature rise, the higher the temperature is, the more water molecules accumulate in the oil domain and interface domain, and the stronger the desorption effect of cellulose on moisture. Notably, NW in the interface domain is not a simple increment trend, but an oscillatory increase and decrease trend that decreases first and then increases. The higher the temperature is, the more obvious the trend is. The research results have important theoretical value for the real-time monitoring of moisture in oil-immersed power equipment and the evaluation of its insulation performance

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.