Abstract: This case analysis focuses on an incident that occurred during the power transmission process of a 110kV substation after the completion of the renovation project for the 10kV III section busbar PT cabinet. During this process, there was a single-phase breakdown of the harmonic suppression unit, damage to the secondary microcomputer harmonic suppression device, abnormal 10kV III section busbar voltage, and burning and peeling of the N600 wiring on the voltage common panel at the terminal block. The article starts from aspects such as the equipment structure, changes in the three-phase-to-ground voltages before and after the fault, and the current flow direction, and conducts an analysis based on the fault phenomenon. It also proposes solutions and reflections.