Oil Colour Change as a Non-Intrusive Field Indicator of Internal Arc Faults in Ester-Filled Transformers

Power transformers are among the most critical assets in renewable energy projects, substations and modern grid infrastructure. Their operational reliability has a direct impact on grid stability, outage prevention and the safe integration of inverter-based resources (IBRs).
In recent years, ester insulating liquids — both natural and synthetic — have been increasingly adopted because of their environmental compatibility, high fire-point and good thermal stability [1][2]. However, for hermetically sealed ester-filled transformers, identifying internal faults in the field remains challenging, particularly when high-energy arc discharge has occurred inside the tank.