Finite Element Analysis on On-load Tap Changer (OLTC) Tap Selector Electrical Breakdown Mechanism Caused by Silver Sulphide Corrosion

An on-load tap changer (OLTC) is the main moving part of a power transformer, and is designed to maintain the voltage without interruption. OLTCs are experienced to many stresses during operation. Therefore, it is important to have an understanding on failures mechanisms to protect assets from catastrophic failures. This paper discusses one particular failure mode taking place in an OLTC selector's switch, which is when the silver used to coat the contacts reacts with corrosive sulphur compounds dissolved in the insulating oil. As a result, a thin layer of silver sulphide is formed on the selector's contacts. Due to mechanical forces applied on this layer it flakes off and forms conductive particles suspended in the oil. Consequently, the dielectric strength of the oil can be reduced due to these particles, and the flashover can destroy the transformer. In this paper finite element modeling of a tap selector is used to study the impact of silver sulphide on the electric field in between tap selector contacts. Breakdown voltage test results are used to validate the simulation outputs.