Transformer Over Passivation to Prevent On-Load Tap Changer Silver Sulphide Corrosion

On-load tap changer (OLTC) is designed to change transformer turn ratio while transformer is under load. According to many surveys, majority of transformer failures were occurred due to OLTC failures. An understanding of OLTC failure mechanisms can help to ensure OLTC reliability and develop appropriate failure mitigation techniques. The formation of a thick layer of silver sulphide on the contacts is one such failure mode. It is caused by the corrosive Sulphur compounds in transformer insulating oil. Silver sulphide is a semi-conductive particle. Therefore, it can enhance the tap selector contact resistivity. Also, dielectric properties of insulating oil reduces when it mixes with oil. Although transformer oil passivation has been used to mitigate copper sulphide corrosion in transformer windings, it has not been effective for silver sulphide. In this paper, the results of a laboratory-based experiment on passivator effectiveness for silver sulphide is presented.