Abstract: The power supply of the cooler is extremely important in the forced oil circulation transformer system, and the loss of power may cause transformer tripping failure. This paper analyzes the specific causes of the failure caused by the N-phase line in the power circuit of the cooler of a forced oil circulation transformer during the AC system transformation of a 220kV substation in Yunnan Province, and proposes preventive measures to avoid the complete shutdown of the cooler during the AC system transformation through the detailed analysis of the power circuit of the cooler. To avoid similar incidents in the future.