Grease-oil compatibility in tribological applications - supergrease

Abstract: A common reason to malfunctioning of tap-changers is wear of tribological contacts. In order to reduce wear, it would be desirable to lubricate the contacts. Since the tap-changer is working in transformer oil, it is necessary that whatever lubricant is added is compatible with transformer oil. In this work, the compatibility of lubricating grease in transformer oil has been investigated. A wide variety of greases have been studied in order to understand what chemical properties that is desirable for this type of application. Compatibility tests of grease in transformer oil at 80°C have been performed. A ball-on-plate tribological test was performed on the successful grease candidates from the compatibility test. The successful candidates were also exposed to a vapour-phase dehydration process, in order to guarantee that the lubricant could be added to the transformer before delivering the tap-changer to clients. Finally, the transformer oil samples were analysed with FTIR, tan delta and surface tension measurements to evaluate if the greases had, by any means affected the oil. One polytetrafluoroethylene (PTFE) grease was successful in all tests and proved that introducing lubricating grease in the mechanical contacts could decrease the coefficient of friction.