Compared with the usual high and low ground resistance setting scheme of injecting 20 Hz source based ground fault protection， self-adaptive setting scheme can automatically adjust the ground resistance setting value in accordance with allowable value of ground current and fault location， and can execute protection matching the fault level. However， the impact of third harmonic of fault current is not considered for existing self-adaptive ground fault protection， so that its low setting value and refuse action occur due to the high third harmonic of fault current while the ground fault is near the neutral point of generator. Considering the fundamental and third harmonic of fault current， the injecting 20 Hz source based ground fault protection is adaptively set according to the allowable root-mean-square（RMS） value of ground current. The improved self-adaptive setting scheme has no protection dead-zone and is in favor of safe operation of generator.