To understand overvoltage change at a 500 kV autotransformer neutral when the neutral is grounded from directly to through a small reactance

the overvoltage at autotransformer’s neutral and entrance is calculated respectively by ATP-EMTP

and the overvoltage for adopting small reactance grounding is analyzed to determine overvoltage protection and insulation coordination.It is concluded that the insulation level of the autotransformer in 500 kV Feixi substation can keep stable after its neutral is grounded through a 5 Ω reactance under protection of MOA.This conclusion will contribute to the solution to the problem that single phase short-circuit current exceeds the switch rate breaking current in the Feixi 500 kV grid.