Oxidative stability of mineral naphthenic insulating oils: Optimization of commercial antioxidants and metal passivators

Mineral naphthenic oil is normally used in transformers, circuit breakers and other electrical equipment for insulating and cooling functions. Due to their usage conditions, it may be subjected to oxidation reactions caused by the presence of oxygen, water and metals, affecting the oil performance. Using the ASTM D2440 method, oxidation studies were carried out on insulating oil, using commercial antioxidants and metal passivators, to assess and compare their thermal oxidative stability, monitored by FTIR analysis, total acid number and sludge content on the oxidized samples. An experimental sequence for combinations of antioxidants and metal passivators was devised to seek the best concentrations that could be used to improve the stability of the insulating naphthenic oil. The combination of antioxidant Irganox L115 and metal passivator Irgamet 039 achieved the best performance in inhibiting the oil oxidation among the studied samples.