Tapchangers for de-energized operation in natural ester fluid, mineral oil and silicone

Natural ester seed oil is a biodegradable fluid that is increasingly being used as a replacement for mineral oil and for high temperature flashpoint liquids, including silicone and RTEMP. Electrical contact coking in mineral oil and in silicone has been a common problem with many types of contact materials in tap changers for de-energized operation and also the reversing switch in load tap changers. In recent years, an accelerated aging functional life test has been developed and presented to the Transformers Committee of IEEE that has had a good correlation with field data to sort out stable versus unstable combinations. This paper presents a comparative look at natural ester fluid versus mineral oil and silicone for several popular contact pairs, and has been updated from our 2006 report to include additional materials. In addition to its biodegradability, natural ester is shown to have considerably better thermal stability with contact pairs that are unstable in both mineral oil and silicone.