Treatments

Passivation consists of adding a substance to the oil that should protect the copper inside the transformer from the corrosive action of DBDS. Analyses carried out on passivated oils in machinery have shown a decrease in passivator content in the first few days after it is added. In other cases, instead, the protective action of the passivator in relation to copper has been shown to be uneven, thus allowing the formation of copper sulfide in some areas.

The case of the Brazilian electricity grid in August 2005, reported in the CIGRE 378:2009 brochure, shows that 50% of passivated reactors suffered a failure – the first 33 days after passivation and the last 590 days after passivation. (read more)

Despite changing the oil, 10-15% of the old contaminated oil load remains absorbed in the transformer papers, which release it over time (the time it takes to integrate is about 90 days). The old oil thus contaminates the new oil, and consequently it is impossible to completely remove the DBDS with a single oil change. (read more)

The countermeasure devised and employed by Sea Marconi is included in this category. This is a selective DBDS depolarisation process implemented on site while the transformer remains in service (and under load), with no need to empty it. The operation is carried out using a Modular Decontamination Unit (MDU) specifically created by Sea Marconi. The transformer is connected to the MDU by flexible tubes; the oil contaminated with DBDS is sucked from the lower part of the transformer and ends up in the MDU, which heats it, filters it, degasses it, dehumidifies it and decontaminates it before pumping it back into the upper part of the transformer. This creates a closed loop and every time the oil is circulated the corrosive sulfur compounds are removed (< 10 mg/kg expressed as DBDS equivalent) (read more)