The “Corrosive sulfur without DBDS – C2” criticality is caused by the presence in oil of some corrosive sulfur compounds such as polysulfides and disulfides. The latter can be used as antioxidant additives in some types of insulating oils and can produce effects equivalent to more well-known DBDS. The sulfur compounds present in corrosive oils react with copper and silver surfaces inside the transformer. The result is the formation of copper sulfide or silver sulfide.
Copper sulfide increases as the temperature rises, reaching its peak in the presence of localised hot spots. The result is the formation of deposits and macroparticles that can circulate dangerously in the oil, causing partial discharges of energy and power arcs.
However, copper sulfide can also form on 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. Copper sulfide crystals push against the layers of paper, gradually arriving at the outermost layer until causing the loss of its insulating properties. This circumstance may also cause partial energy discharges and power arcs (without any specific signs or symptoms) eventually resulting in a catastrophic failure.
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Causes of corrosive sulfur without DBDS – C2 criticality
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When it can occur (life cycle phases)
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| Lack of requisites for purchase of oils (new or recycled) | Requisites and purchase |
| Defciencies in quality control for individual lots or individual supplies of insulating oil | Acceptance of insulating oils |
| Deficiencies in analytical procedures for the verification of corrosive sulfur compounds | Oil acceptance, factory test, installation and pre-energisation, operation, old age, post-mortem |
| Cross-contamination through use of oil, plants, tanks or containers contaminated by corrosive sulfur compounds (for toppings up, impregnations, fillings or treatments) | Factory test, installation and pre-energisation, operation, old age, post-mortem (oil recycling) |
Surprisingly it has been found that some types of oils (especially with paraffinic base) containing initially non-corrosive sulfur compounds have demonstrated corrosive behaviour during the life cycle of the transformer; this is due to oxidation, or degradation in general, under particular electrical or thermal stress conditions (e. g. dibenzothiophene – DBT).
