Formation of corrosive sulphur in oil and subsequent copper sulphide deposition in paper has led to a number of large transformer failures worldwide. It was the cause of the failure of Lackenby SGT4 and other transformers will be removed from the system early because the problem is believed to be advanced.

Part of the complex process leading to transformer failure involves the mobilisation of copper containing material into the paper insulation surrounding the windings, which is known to be influenced by the presence of corrosive sulphur species in the oil, and extreme operating conditions.

Although there have been many attempts to better understand the mechanism by which formation of copper sulphide occurs none have yet been conclusive. They have not led to sufficient understanding to allow diagnosis of the problem without inspection and better mitigation methods may still arise if the mechanism is better understood.

Objectives

The key objective of this project is to reduce the risk of transformer failure and unreliability resulting from corrosive sulphur in oil.

This key objective will be met by:

• Better understanding of the mechanism by which copper sulphide failures occur and the effectiveness of passivation

• Fully understanding the effects, both chemical and electrical, of passivation on transformer insulation performance

• Investigating the reasons for silver corrosion in tap changers and to formulate monitoring/assessment strategies in order to provide a measure of asset health.