Four protection principles for the stator ground fault are introduced:the fundamental frequency zero-sequence voltage principle, the third harmonic voltage amplitude ratio principle, the fault component of third harmonic voltage amplitude ratio principle and the third harmonic voltage vector comparison principle. The sensitivity of the four principles is analyzed based on the parameters of three turbine generators. Analysis shows that the best combination on sensitivity is the fundamental frequency zero-sequence voltage principle and the third harmonic voltage vector comparison principle. However, their sensitivity may be still insufficient in the case of large generator connected with the grounding transformer. In order to improve the sensitivity and the reliability of the dual frequency stator ground fault protection while considering the disturbance of the high side third harmonic voltage, an improved design is introduced. In this design, the braking coefficient is reduced and the threshold criterion is adaptive, which guarantees both the sensitivity and reliability. The conclusion of this paper can provide reference for the research and development of the stator ground fault protection.