Investigations of silver sulfide formation on transformer OLTC tap selectors and its influence on oil properties

The on-load tap changer is a vital component of a power transformer. Recently, several utilities have reported the failure of on-load tap changers, suspected to have been caused by silver sulfide corrosion. However, the mechanism of silver sulfide formation has not been well documented in the literature. The silver sulfide influence on transformer oil properties has not been effectively studied either. This paper presents an experimental study on silver sulfide in a transformer, with the view of developing a future test. The reaction between corrosive sulfur in mineral oil, and the silver coating of the tap selector and the resultant silver sulfide film, have been investigated. Silver sulfide is semi-conductive and can reduce the dielectric properties of the oil. Thus, this paper presents breakdown voltage, conduction and permittivity measurements on oil samples with different quantities of silver sulfide. Frequency domain spectroscopy (FDS) technique was used to analyze the transformer oil, in order to determine the influence of silver sulfide on the dielectric losses of oil.