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Corrosive Sulphur

The customer site is a combined heat and power, gas-fired power plant on the east coast of England. Its original generating capacity of 730 megawatts was expanded to 1,240 MW in 2009 making it one of Europe’s largest CHP plant.

The station operators had come under pressure from their insurance company to address the problem of “potentially corrosive sulphur” a phenomenon affecting unused oil supplied for filling new transformers in the 1990s, the “bad” oil was dosed with an additive called DBDS which, given sufficiently high operating temperatures or local high temperature overheating can lead to the formation of corrosive sulphur with the risk of transformer insulation failure. Having worked on this site before EOS were called in to discuss the options:

1. Do nothing and continue to monitor

2. Add a synthetic metal deactivator (passivator) to protect the copper surfaces from corrosive attack

3. Replace the oil

4. Employ on-site mobile regeneration to remove the DBDS wich has been identified as the primary cause for corrosive sulphur production in transformers

EOS has now successfully regenerated the oil in two generator transformers each containing approx 115,000 litres of insulating oil and around 100ppm DBDS. Treatment took approx 25 days per transformer and was carried out with the transformers energised during part of the process as the available outage time was not long enough to complete the work otherwise.

DBDS and corrosive sulphur tests were carried out after each nominal “pass” of the oil – every 3-4 days, and DBDS fell steadily to end up <5ppm with the CCD corrosive sulphur test changing from “potentially corrosive” to “non-corrosive”.

After one year results for DBDS have not changed and the oil remains “non-corrosive”.