Tracking copper sulfide formation in corrosive transformer oil

Abstract

Corrosive sulfur in the last decade has been recognized as a major risk to high voltage transformers. An initial study undertaken by CIGRE on copper sulfide in transformer insulation investigated possible sources, i.e. Dibenzyl Disulfide (DBDS), it also looked at possible mitigating techniques, i.e. passivation, environmental influences, i.e. temperature, and improvement to oil corrosion standards, i.e. EN 62535. Although there was significant volume of research undertaken the high voltage industry still felt there is a lack of understanding in the process of copper sulfide formation and long-term effects of mitigation techniques. Hence a new CIGRE transformer working group, A2.40, was created. The identification of techniques with the ability to track the mechanism of copper sulfide (Cu2S) formation is fundamental to the current research objectives. The technique suggested in this paper involves the use of X-ray fluorescence (XRF), which is used for elemental analysis, to track the quantity of sulfur in the oil. As the deposits of Cu2S form in the conductors and paper insulation the amount of sulfur in the oil decreases. By using a series of laboratory experiments is possible to investigate how the rate of formation of Cu2S changes with time, suggesting that there is an initial chemical reaction needed for the Cu2S deposits to form. Scanning Electron Microscopy with Energy-dispersive X-ray spectroscopy (SEM-EDX) has been used to investigate the surface of copper at several different stages of the copper sulfide formation. By manipulating variables, i.e. temperature, it is possible to map their influence and provide a more precise risk assessment of transformer with corrosive oil to electrical utility companies. The experimental results suggest the possibility of developing a method to indirectly measure Cu2S deposition on conductors and insulation paper by tracking sulfur concentration changes in the transformer oil.