Assessment of dibenzyl disulfide and other oxidation inhibitors in transformer mineral oils

In recent years some unexpected operational failures of power transformers were attributed to the presence of the undeclared dibenzyl disulfide (DBDS) antioxidant in mineral insulating oils. This study examines the characteristics of DBDS and other antioxidants dissolved in a mineral oil. Acidities as neutralization numbers, interfacial tension values and depletion rates were obtained at 100°C for solutions of the antioxidants 2,6-di-tert-butyl-p-cresol (DBPC), 2,6-di-tert-butyl-phenol (DBP), dibenzyl disulfide (DBDS), 2-tert-butyl-p-cresol (2-t-BPC), Nphenyl- 1-naphthylamine, 1,2,3-Benzotriazol (BTA) and methylated-BTA in transformer mineral oil that have been exposed to an accelerated aging process involving flow of oxygen through them at a rate of 1.0 L h^-1 over intervals of 24, 48, 72, 96 and 120 hours. The results show that the excellent antioxidant characteristics obtained for DBDS are lost when a copper plate is placed in the mineral oil sample. This is attributable to sulfur corrosion. The results also suggest that DBPC is most suited for use as an antioxidant in transformer mineral oil with DBP a close second. The higher depletion rates encountered for 2-t-BPC and N-phenyl-1- naphthylamine make them less suitable antioxidants. The lower effectiveness of 2-t-BPC as an antioxidant relative to DBPC and DBP may be attributed to the lesser steric hindrance of its - OH group. The results for BTA and methylated-BTA and their higher depletion rates indicate that they would not be suitable antioxidants.