Bubble-generating Temperature Properties of Transformer Winding Based on Moisture Equilibrium Curves for Liquid and Insulating Paper

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.