Abstract: This article analyzes the 110 kV busbar differential protection action accident of a 220 kV substation. Based on the operation mode of the system before the accident, the action message of the protection device, and the waveform of the main transformer fault recorder, it is comprehensively determined that the cause of the accident is a floating balloon that caused a ground short circuit fault in the A phase switch of the busbar. Then trigger the action of the bus differential protection device. Finally, measures were proposed to eliminate the fault. And active and effective prevention and control measures have been formulated for such incidents, creating a safe and stable operating environment for the substation.