The maloperation case of a gas turbine overexcitation protection was analyzed to find out the cause of the protection maloperation. The hidden danger was eliminated by adding a harmonic elimination lowvoltage potential transformer (PT).Countermeasures are proposed to prevent malfunction protection and improve the protecting function.Countermeasures can provide reference for the subsequent problems of the secondary side voltage distortion caused by the ungrounded operation mode of the primary side of the PT.