Introduction

In the introduction to technical standard IEC 62697-1 “Test methods for quantitative determination of corrosive sulfur compounds in unused and used insulating liquids” 2012 (p. 7) states that:
 Sulfur may be present in insulating liquids (used in transformers and other electrical equipment) in various forms
 The total sulfur concentration depends on the origin of the liquid, the refining process, the formulation and the presence of additives
 Additives include antioxidants containing sulfur, such as the DBDS
 Some sulfur compounds are non-corrosive and others are highly corrosive to metal surfaces inside transformers, for example
 The presence of these types of corrosive sulfur has been the direct cause of breakdowns in equipment used for the generation, transmission and distribution of energy for several decades
 The IEC standard has therefore ruled that both new insulating oils and oils already in use must be free of these corrosive sulfur compounds
 The impact of corrosive sulfur is mainly related to the presence of a specific corrosive compound of sulfur: DBDS

DBDS was identified as the main cause of sulfur corrosion in July 2005 at the Sea Marconi laboratories. The discovery was made whilst analysing oils used in shunt reactors in Brazil, which had suffered catastrophic failures a few days earlier. Sea Marconi, in conjunction with Terna and the University of Missouri, confirmed the discovery after extending the analysis to include other European and South American oils. It was first published on 20 June 2006 in Turin, at the IEC WG 35 meeting on corrosive sulfur. Subsequent publications contained both the methods for detecting DBDS and countermeasures taken by Sea Marconi in Europe and South America to tackle the problem of “DBDS corrosive sulfur – C1” using a process of depolarisation.

Along with the discovery of DBDS as the principle cause of the corrosive sulfur phenomenon (July 2005), Sea Marconi also studied the corrosive action of both the sulfur compounds that are normally found in oil, and degradation by product additives.

Thanks to Sea Marconi’s professional expertise, DBDS was discovered during an intervention that we carried out ten years ago for a transformer construction company in South America; the investigation was requested following a series of catastrophic failures of shunt reactors in their first 18-24 months of operation. Studies failed to establish the cause of the catastrophes with any degree of certainty, but served to increase our understanding of various phenomena (eg the part that refining processes play and the tendency of certain oils to dissolve metals), which subsequently led to better understanding of the role that DBDS plays and the reasons why it is used in oils.