Under the call for green and low-carbon transition of new power systems, the replacement of mineral oil power transformers with ester insulating oil power transformers becomes a hot topic. In this paper, a synthetic ester retrofilling technology for 10 kV power transformers was proposed and validated by two retired transformers of T1 and T2. Subsequently, the two retrofilled transformers were put into the power grid lines with different load conditions, and their oil samples were obtained after 1 month and 3 months. Finally, the physicochemical and electrical characteristics of the retrofilled oil samples were analyzed. The experimental results indicate that water intrusion and partial discharge occurred in T1 during the operation, identified by gradually increasing indicators of H2 and CO/CO2. Mild therm aging occurred in T2, identified by the decrease in CH4 and the increase in C2H4.