Recently., to align with the global environment and promote life extension., the use of vegetable oil-derived esters with extremely high saturated moisture quantities instead of mineral oil in the insulating and cooling systems of transformers has increased. In this study., we investigate the bubble-generating temperature of transformer windings in a steep overload operation and their mechanism in liquid-filled transformers. Subsequently., we derive a basic formula based on moisture equilibrium data of various insulating oils and insulating paper. An analysis and a comparison of the bubble-generating properties of winding models of both conservator and nitrogen-sealed types in mineral oil presented in previous publications and this study reveal that both models are almost identical; in addition., bubble generation in the winding model is caused by the vaporization of adsorbed moisture in the insulating paper. These results facilitate the acquisition of bubble-generating properties between the winding surface temperature and moisture content in insulating paper using a similar experimental technique that utilizes esters.