AbstractA perfluorocarbon (PFC)‐immersed 275‐kV transformer with compressed SF6, gas has been developed. This paper describes the basic cooling and insulation characteristics of PFC, the ac partial discharge voltage, and the aging of PFC‐immersed insulation. The results demonstrate that a prototype 275‐kV 100‐MVA three‐phase transformer can be operated without any difficulties for an extended period of overvoltage. This prototype has an ac partial discharge initiation strength which is 1.5 times that of the ac test voltage and a lightning impulse breakdown strength which is 1.5 times that of the test voltage. A 275‐kV 250‐MVA three‐phase transformer was built and is being operated at the Abe substation of Chubu Electric Power Co., Inc. The transformer has been operating satisfactorily.