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A 240-MVA, 165-GRDY/20-kV generator step-up transformer was insulated with Envirotemp FR3 fluid. The transformer was energized during a cold temperature period in mid-December 2010. This is believed to be the largest generator step-up transformer filled with a natural ester fluid that was energized at low temperatures. The lack of cold start experience of natural ester-filled power transformers created questions and at least some uncertainty as to the outcome. This paper describes the instrumentation used and data collected during the cold startup procedure. In addition, a computational fluid dynamics analysis of this transformer design was performed that provided calculated temperatures and elapsed times until steady-state temperatures were achieved. Comparisons were made between the actual and calculated values. The results indicate that the cold startup procedures for a power transformer are the same whether filled with mineral oil or natural ester fluid of similar characteristics as used for this paper. [ABSTRACT FROM PUBLISHER]