Causes

The problem of “DBDS corrosive sulfur – C1” is caused by the presence of a specific additive in oil: DBDS (dibenzyl disulfide).

“Presence of this compound has been shown to result in copper sulfide formation the surfaces of  copper conductors under normal operating conditions of transformer” [from IEC 62697-1 of 2012, p. 7].

The DBDS present in corrosive oils reacts with the copper (conductors) and silver (contacts) inside transformers, to form copper sulphide and silver sulphide. The conversion of DBDS into copper sulfide increases with higher temperatures (and over time), reaching its peak in the presence of localised hot spots. The result is the formation of deposits and macro particles that can circulate dangerously in the oil, causing partial discharges of energy and power arcs.

However, copper sulfide can also form on the windings, which are also made of copper. In this case there is a progressive migration of copper sulfide from the conductors on the windings to the layers of paper around them. The copper sulfide crystals push against the layers of paper, gradually arriving at the outermost layer and causing the loss of its insulating properties. This circumstance may also cause partial energy discharges and power arcs (without any specific signs or symptoms) until it eventually results in a catastrophic failure.