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 the temperature dependence of the half-value period of the degree of polymerization (DP) was determined. Using identical annual load and ambienttemperature profiles, the winding temperature history and lifetime of the TUP were calculated for mineral-oil and natural-ester operation. The natural ester increased the hot spot winding temperature but markedly slowed TUP deterioration. Under the assumed loading pattern, slower ageing dominated, resulting in improved long-term DP retention with natural ester. A comparison with the IEC temperature rise limits further indicated additional loading margins when evaluated against a thermal class of 140.