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Every Utility has "Never Clarified Transformer Failure" cases in their statistics.
Due to the diversity of copper sulphide appearance, it is highly likely that certain cases were not properly identified. A reason for such failures could be due to arcing between adjacent disks or conductors of a winding, due to the formation of deposits of copper sulphide on the cellulosic insulating paper.
Various forms of sulphur can be present in transformer oil at sub–% levels, such as mercaptans, thiols, thiopens, disulphides and polysulphides, with a high number of these sulphur species being highly reactive.
Although these organo sulphur compounds are antioxidants, under certain conditions such as lack of oxygen, high load and high operating temperature, they may change to corrosive sulphur and react with copper. This is potentially corrosive sulphur and standard tests such as ASTM D1275/B, DIN 51353 and ISO 5662 are unable to detect it.
Up until now expensive standard laboratory testing such as ASTM D1275/B and more recently IEC 62535 were the only route to determine the presence of potentially corrosive sulphur in the oil.