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A new criterion to detect the stator single phase-to-ground fault for Powerformer is proposed in this paper, which is based on the direction of zero-sequence compositive power flow. By virtue of the analysis of new fault characteristics of Powerformer and the comparison with conventional methods, a novel identification scheme is put forward. The proposed approach detects the ground fault by analyzing the power direction. Only zero-sequence current from the terminal of Powerformer and zero-sequence voltage from the busbar are used in this scheme. Combined with third-harmonic principle, it can provide a 100% coverage of fault detection for the stator windings of Powerformer. The effectiveness of this scheme is validated with the Matlab simulation results.