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This paper proposes an improved stator ground fault protection scheme for the generator through injecting 20Hz voltage to realize the selectivity of the protection. The insulation resistance value of the generator is firstly calculated to determine whether or not ground fault occurring. When the ground fault happens, the 20Hz differential current between the neutral and terminal of each generator will be obvious. The phase relationship between the 20Hz differential current and injected 20Hz voltage is derived and analyzed with the variation of insulation resistance. The new protection scheme can judge the fault generator according to the phase relationship, so that it can fulfill a selective shutdown. The simulated results verify the accuracy of calculating the grounding transition resistance and selecting the fault generator, which achieves the stator ground fault protection by injecting 20Hz voltage in the generators connected to expanding units.