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The oxidation of mineral oils is the most important cause of poor power transformer performance. High voltages, high temperatures, and the presence of oxygen and metallic surfaces are some of the factors that may trigger oxidation reactions. The use of antioxidant additives and metal passivators in insulating oils has the purpose of improving their performance, by increasing their stability to oxidation so as to ensure a more reliable operation of the transformer. This work presents a kinetic experimental study of the oxidation reactions of a naphthenic mineral oil (with and without additives) as a function of temperature. Oil samples were doped with phenolic antioxidants (AOX), a metal passivator (MPA), and combinations of these (AOX and MPA) in order to verify the efficiency of such additives in increasing thermo-oxidative stability. All doped and nondoped oil samples were submitted to an accelerated oxidation test according to modified ASTM D-2440 method. The level of degradation was assessed by monitoring the area under the carbonyl band (around 1713 cm -1 ) in the FTIR spectrum and comparing it to that obtained for the new nonoxidized oil. The results obtained for the oxidation of nondoped oil samples showed that the kinetic behavior may be well represented by a first-order homogeneous reaction rate law, with energy of activation around 10 kcal/mol. Our results also show that hindered phenolic antioxidants significantly reduce the formation of oxidation products. The addition of a metal passivator, even at very low levels (8 ppm), causes a synergetic effect with all antioxidants tested, leading to a significant improvement in the resistance of the oil against oxidation.

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