Investigation of Cellulose Insulation Ageing in Transformers Retrofilled with Ester Fluids at Different Service Years

Retro-filling transformers with ester fluids have been increasingly practiced. Transformers may be retrofilled at different service years. In this paper, the ageing behaviour of the cellulose insulation of transformers retrofilled at different service years (with different conditions of paper insulation) are studied. Since the temperature may rise after the retrofill due to the high viscosity of the ester liquids, a temperature increase of 10 °C and 20 °C in cellulose-liquid insulation systems after retrofill is considered. The degree of polymerization (DP) of the cellulose insulation and the moisture in both cellulose and liquid insulation are measured before and after retrofilling. The measurements are used for modelling the remaining useful life of the cellulose-liquid insulation system. The process of insulation systems reaching the impregnation equilibrium after retrofilling is investigated through the fire and flash point measurements of the liquid insulation and frequency domain spectroscopy (FDS) measurements of the cellulose-liquid insulation system.