To reduce the risk of transformer damage, increase capacity, and extend the life of transformers, plant-based esters have been increasingly substituted for conventional mineral oils derived from fossil resources. The bubble-generating temperatures in overloaded transformer windings, which cause severe insulation performance, are reported to be higher in esters than in mineral oils. In this study, the mechanism of bubble generation in an ester-filled transformer winding model for simulating overload was investigated using both conservator and nitrogen-sealed types as insulating oil degradation prevention systems. In addition, two types of esters with similar saturated water contents were used, and the experimental and analytical results were compared. The bubble-generating condition in the analysis was that the sum of the partial pressures of water vapor and dissolved gas in the winding insulation paper was larger than the sum of the pressure above the oil surface and the differential pressure between the oil surface and winding. Consequently, it was confirmed that the temperature characteristics of the bubble-generating ester-filled winding model for simulating overload were higher than those obtained using mineral oil. This phenomenon resulted from the adsorption of water by the surrounding polar ester molecules during the vaporization of water in the insulation paper in the winding due to overloading. Therefore, a bubble generation mechanism was proposed, in which the water content of the paper after overloading was reduced compared to the water equilibrium state between the insulation paper and oil before overloading.