Long-term Performance of Natural Ester Liquids

After testing several dozen transformers in the factory, one of the first commercial natural ester-filled transformers was installed in a large amusement park in 1997. This 1500 kVA, 12.47 kV - 480/277 V transformer has remained in service under continuous operation without any maintenance intervention since installation. The utility responsible for the energy supply to the amusement park adopted the natural ester fluid in all distribution transformers in 2005 and has had great success with its application. A sample of the insulating liquid was provided to Cargill in 2019, after 22 years in service, allowing a detailed screening of its properties. The results confirmed the robustness of the natural ester, since only one property was found to slightly exceed the limits for a new unused liquid. All properties still are within the IEEE specification of a new fluid filled into a new transformer (prior to energization), far from requiring any action. This article also includes another case of a 145kV / 11.1 MVAr shunt reactor which, unintentionally, operated at free breathing conditions for +7 years. As expected, the continuous contact with ambient air in a region of high average ambient temperature and moisture led to changes of fluid properties. During a scheduled outage, a maintenance intervention was performed, and the most economical alternative was to replace the fluid. Nine years later, the shunt reactor remains in continuous operation, without any further intervention. This paper will present the remarkably positive results of both cases, comparing normal and abnormal aging processes of natural ester liquids after decades of continuous operation.