Actions recommended by IEC 60422 ed. 4-2013 (table 5, p. 31) in the presence of “corrosive sulfur” are:
to perform a risk assessment and then decide either:
A. to reduce the corrosiveness of the oil adding a copper passivator, or
[NOTE – following oil passivation, regular monitoring of the concentration of passivator is required. In the event continuous depletion of the passivator, remove the cause of the corrosion as below]
B. to remove the source of corrosion by changing the oil, or
C. by removing the source of corrosion by removing corrosive compounds with appropriate oil treatments.
NOTE – in the presence of DBDS, follow the recommendations in CIGRE brochure 378:2009 [3]
A. Passivation
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 breakdown, the first 33 days after passivation and the last 590 after passivation.
B. Oil replacement
Despite changing the oil, 10-15% of the old contaminated oil remains impregnated, ie absorbed in the transformer papers, which release it over time (the time it takes to integrate is about 90 days). So the old oil contaminates the new oil, and consequently it is impossible to completely remove the DBDS with a single oil change. (more)
C. Removal of corrosive compounds, depolarisation
The countermeasure devised and employed by Sea Marconi is included in this category. This is a selective DBDS depolarisation process implemented on site, whilst the transformer remains in service (and under load), with no requirement to drain it. The process is carried out with a Modular Decontamination Unit (DMU) especially produced by Sea Marconi. The transformer is connected to the DMU by flexible hoses; the oil contaminated with DBDS is sucked from the lower part of the transformer and transferred into the DMU, 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 circuit, and the DBDS is gradually removed through a number of stages (< 10mg/kg) (more)
NOTE – Since approximately 1,000,000 tons of mineral insulating oil are produced each year, and DBDS was used from 1989 to 2007, there are around 18 million tons of oil in circulation potentially contaminated with DBDS. An average distribution transformer contains 250kg of oil, which means that 72,000,000 transformers in the world could contain DBDS, and of these, by default, it is reasonable to assume that at least 50% actually do.
The incidence of DBDS-contaminated oils is certainly even more widespread, since it is common practice to mix oils of different bases (“blended” oils), for example naphtha-based with paraffin-based, in order to improve its viscosity for use in cold climates, effectively doubling or even tripling the DBDS content.
