Overload Duration Analysis of Mineral Oil and Natural Ester in Retrofilled Distribution Transformers

Abstract

Natural ester fluids such as FR3 provide higher fire safety, better biodegradability, and extended insulation life compared with mineral oil. Retrofilling existing transformers with FR3 offers a practical means to enhance safety and environmental performance without redesigning the unit. This study evaluates the thermal performance and overload endurance capability of a 160 kVA distribution transformer before and after retrofilling from mineral oil to FR3. Results show that steady-state temperature rises increased modestly after retrofilling (+1.3–4 K for top oil and windings) but remained within IEC limits. Under 1.5 p.u. normal cyclic loading, the mineral-oil unit reached its 120 °C hot-spot limit after 5 hours and 20 minutes, whereas the FR3 unit stabilized near 126 °C and did not reach its 140 °C limit within 10 hours. Under 1.8 p.u. long-time emergency loading, FR3 extended the time-to-limit to 5 hours and 20 minutes, compared with 4 hours and 30 minutes for mineral oil (18.5% increase). Overall, FR3 trades slightly higher temperature rises for a wider operational safety envelope and longer overload endurance, supporting safer peak-shaving and contingency operations in retrofilled assets. These findings provide actionable insights for utilities considering FR3 retrofilling as a cost-effective transformer upgrade strategy.