Development of a New Method for the Assessment of Mineral Insulating Oil Corrosivity Against Silver

Corrosive sulfur compounds in mineral insulating oils can easily react with metal surfaces in power transformers (PTs) and form deposits, which can further cause failures of the electrical equipment. Copper sulfide formation is well studied over the last couple of decades. However, silver sulfide formation also requires attention, due to a significant number of failures of on-load tap changers (OLTCs) caused by silver corrosion reported worldwide from different utilities. The conventional silver corrosion tests (DIN 51353 and ASTM D1275-15) do not provide an adequate assessment of oil corrosivity against silver, even if the oil contains dibenzyl disulfide (DBDS) and other reactive sulfur compounds (disulfides and mercaptans) in high concentrations. The new testing method for silver corrosion is performed on a significant portion of oils containing DBDS, as a major reactive sulfur compound, which remained “noncorrosive” following the ASTM D1275-15 silver strip test. The method is demonstrated on five oils from free-breathing PTs in service, containing DBDS in a wide range of concentrations (from approximately 40 to 200 mg/kg of DBDS). The new method comprises a modification to the ASTM D 1275–15 test setup, by introducing longer test duration times and continuous oxygen ingress, which is found to be the most crucial parameter for promoting the formation of silver sulfide deposits. This is confirmed by energy-dispersive X-ray (EDX) results, showing an increased sulfur weight percentage on the silver strip after the test. The application of the modified ASTM silver corrosion test will contribute to improving the assessment of sulfur corrosion risk.