Evaluation Methodology, Numerical Verification and Retrofilling Procedure for Power Transformers to Ensure K‐Class Flame Retardancy and Environmentally Friendly

ABSTRACT With increasing demands for fire safety and environmental friendliness, retrofilling transformers with alternative insulating liquids has become a growing trend. However, retrofilling power transformers remains uncertain, as more residual mineral oil compromises K‐class flame retardancy, and its impact on environmental performance has yet to be clarified. Meanwhile, there is a lack of evaluation methodology and retrofilling procedure for power transformers. This paper presents a synthetic ester retrofilling conducted on a 20 MVA/110 kV transformer. Kerosene vapour‐phase drying was implemented to supplement traditional oil draining, and the insulating liquid achieved a fire point of 304°C, meeting K‐class flame retardancy after retrofilling. The estimated residual mineral oil content was less than 0.8 wt%, significantly lower than the typically reported value (e.g., 3 wt%–5 wt%). Standardised testing confirmed that the synthetic ester containing 3.5% mineral oil maintained its environmental performance, achieving 82.6% biodegradability. Additionally, the evaluation methodology and recommended procedure for retrofilling power transformers are proposed in this study. The performance changes of the retrofilled transformer were discussed in detail. A significant decrease in lightning impulse insulation margin and insulation resistance was observed, which should be given particular attention.