Due to the advantages of electrical performance and light structure, cross-linked polyethylene (XLPE) is widely applied in the insulation field of high-voltage AC/DC cable. However, in recent years, the number of power outages caused by cable faults has been increasing year by year. Aiming at the gap discharge problem of 500 kV XLPE cable sheath to clamp during the no-load energization of main transformer in a hydropower plant, firstly, ATP-EMTP software is used to establish the electromagnetic transient simulation analysis model of primary equipment in the whole plant. Then, the fault causes of discharge phenomenon of the cable aluminum sheath are analyzed, and the different kinds of fault treatment measures are compared. Finally, the corresponding solution is put forward.