Contamination of paper tapes by corrosive sulfur in insulating oils may cause shorting faults between turns. Typically, this occurs at higher temperature in the upper portions of the windings of shunt reactors and power transformers. In many of the tested oils, high amounts of dibenzyl–disul?de (DBDS) were found

The nature and causes of corrosive sulfur induced failures are examined in oil-filled transformers and shunt reactors. Copper sulfide, which is formed when the corrosive sulfur in a mineral oil reacts with the copper conductors, is likely to diffuse into the paper tapes insulating the conductors. Since copper sulfide is partially conducting, the dielectric losses of the contaminated oil-impregnated-paper tapes are markedly increased; paper tapes in close proximity to the copper conductors are found to attain tan delta values > 1.0 even at room temperature. It is highly likely that thermal instabilities develop at those sites at operating temperatures, leading to increased loss currents and, ultimately, short circuits between the turns. This sequence of events is substantiated by evidence from the field, which indicates large areas of thermally degraded insulations and charred breakdown regions along the coils, the extent of which becomes more pronounced at higher operating temperatures (toward the top of the windings)

The asset management of any Transmission System Operator (TSO) cannot ignore the evaluation of the power transformers fleet. Even in the absence, to date, of any specific international guide or standard, every big electrical utility is adopting a home-made 'Health Index'. An HI is developed to summarize in numerical form the transformers' reliability for the purpose of evaluation, ranking and comparison. In the present paper the author will elucidate a new model that merge the evidences of periodic tests (DGA, furans, acidity, inductance, FDS, etc.) with the keraunic properties of a substation

The presence of corrosive sulphur in current insulating oils is discussed and typical winding failures in transformers and shunt reactors, due to the resultant formation of copper sulphide films and particulate matter, are described. Different standard test methods for the detection of corrosive sulphur in oils are compared and ASTM D1275 is shown to be the most sensitive.

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