A new principle of stator single phase to ground fault protection for Powerformer is proposed based on the energy function of the fault components. Different from the traditional protection schemes， which needs the measurements of the zero-sequence fundamental frequency voltage and the zero-sequence third-harmonic voltage at both the neutral and the terminals， the proposed approach detects the ground fault by analyzing the energy direction. Zero-sequence current from the terminal of Powerformer and zero-sequence voltage from the busbar are used in this scheme， which can distinguish the internal faults from external faults. Simulation results using MATLAB have shown that， under different fault conditions， the new scheme can obtain high sensitivity and reliability that a large generator Powerformer requires.