Experimental study of the thermal stability of irgamet 39 and dibenzyl disulfide in the laboratory and in transformers in service

In recent years, several documented cases of transformer failures have been attributed to the presence of corrosive sulfur species in the insulating oil [1], [2]. However, all sources of corrosive sulfur in oil have not been completely identified, yet. They can be residuals from the refining process, or they are formed under favorable operating conditions, such as high ambient and operating temperature and low oxygen concentration. It has been recently shown that noncorrosive sulfur compounds can become corrosive in oil with dissolved oxygen in the range of a few hundreds to a few thousands of parts per million, producing, for example, metal sulfides, after being exposed to high temperatures, electrical stresses, transient phenomena, DC fields, and hot metal surfaces. This has been quite commonly found in closed-type transformers (rubber bag or nitrogen blanketed) but can also occur in open breathing transformers, that work under more-or-less constant load, and in this case they breath very little [1], [3].