Abstract: In a 220kV substation, a short circuit fault of a capacitor set in low voltage side of the main transformer was occurred. Great amount of conductive smoke dust, which was produced by the short circuit fault, whisked into the switch cabinet near the main transformer low voltage side, and the cable of the CT difference current circuit in main transformer low voltage side was burned away, and the differential protection of the main transformer was tripped. The reasons of the whole accident were analyzed, the preventive measures were put forward, and the power supply reliability of the main transformer was raised effectively. 

Key words: power system, fault record, short circuit, protection trip, preventive measure