When moisture increases in a transformer, hydrolysis becomes one of the main aging mechanisms of the cellulose insulation. Water reacts with the polymer chains in paper and gradually breaks them down, reducing the degree of polymerization and mechanical strength. This process is irreversible and accelerates with temperature. Even moderate moisture can significantly shorten insulation life over time, which is why controlling moisture is more critical for long-term reliability than its immediate effect on oil dielectric strength alone.