Evaluation of Temperature Rise and Insulating Paper Lifetime in Retrofilling Transformer with Natural Ester

This study investigates the thermal ageing behaviour of thermally upgraded paper (TUP) in a mineral-oil-designed transformer retrofilled with natural ester. Although natural esters offer environmental advantages and slow ageing of the insulating paper, their physical properties can reduce transformer cooling performance, resulting in a higher hot spot temperature after retrofilling. A thermal circuit model was constructed using factory acceptance test data from a 66 kV, 15 MVA transformer. The temperature rise characteristics were adapted for natural-ester operation through empirical corrections, reflecting the differences in the physical properties of the insulating liquids. The deterioration characteristics of the TUP in natural ester were obtained from accelerated ageing tests in which…