Brendan Diggin

The presence of elemental sulphur in transformer mineral oil has caused major problems in transformers over the past 15 years. It has been reported widely that the formation of elemental sulphur in oil has been closely associated with and caused by the regeneration process of the oil in various regeneration plants throughout the world. This presentation will illustrate how elemental sulphur is produced in mineral oil, the problems that this sulphur can create and how it can be removed from transformer oils. While oil regeneration is an environmentally preferred and cost-effective solution, as well as being a tool for transformer life extension, it can also have detrimental effects on assets. In cases where elemental sulphur has been introduced into the oil, a decision must be made by the asset owner whether they continue to run the transformer with additional gas-in-oil monitors and control measures such as reducing the load, or whether to carry out a load tap changer replacement and/or elemental sulphur removal with a proven oil regeneration plant. This decision is largely influenced by the age of the transformer, the condition of the insulation in the transformer and its position on the network. The process of removing elemental sulphur has been implemented in the field by ESB on several large GSUs and auxiliary transformers. However, one of the main influencers on the processing time and how effective it can be is down to the type of activated alumina clay used as there is a wide variation in how quickly various clays can remove elemental sulphur. Ideally, any elemental sulphur removal should be carried out while the transformer is energised and on load. The formation of copper sulphide in transformer windings has caused several failures in generator transformers, HVDC converters and in HV reactors over the last 25 years. The root cause of these failures has been attributed to the tendency of certain oils to contain sulphur compounds and which can react with copper to form copper sulphide. Some of the units which have failed contain dibenzyl disulphide (DBDS) which is one of several sulphur compounds known to cause copper corrosion in transformers under certain conditions. This presentation will also discuss an investigation into abnormal gassing in a 220kV shunt reactor on the ESB Network in Ireland which had a corrosive uninhibited oil, high DBDS levels and which also had a high stray gassing tendency according to ASTM D7150. The potential for copper sulphide formation as well as the production of hydrogen was a major concern, and coupled with the fact that the unit was a reactor heightened the risk of failure due to the corrosive oil. The presentation will explain the decision to regenerate the oil on-line and add inhibitor which made the oil non-corrosive and dramatically reduced the stray gassing problem. Finally, some case studies on oils which are non-corrosive to ASTM-1275 and which have silver sulphide built up on the oltc contacts will be discussed.

Кључне речи: Elemental sulphur, DBDS, Silver Corrosion, Oil Regeneration