Abstract: During the self-launch process after the fault outside the 6 kV induced fan area of a power plant was removed, the differential protection malfunctioned. In view of this event, analyses and calculations are carried out concerning the design selection of current transformer and its secondary load, and the conclusion is drawn that the insufficient rated output of transformer leads to the misoperation of differential protection. An idea of improving the load carrying capacity of transformer is put forward, and a scheme of replacing current transformer is adopted to solve the problem of protection misoperation.