Improvement of subharmonic injection schemes for huge hydro-generator stator ground fault protection

The stator winding of huge hydrogenerator unit has large leakage capacitance to earth. Traditional subharmonic injection schemes for generator stator grounding fault protection have poor performance in detecting high impedance grounding faults (HIGF). In order to improve them, a reactor added in parallel with the resistance in the secondary of a neutral grounding distribution transformer is applied to compensate the leakage capacitance in the paper. By measuring the voltage and current of the resonant signal injected from the additional reactor, grounding fault resistance can be calculated directly and HIGF protection for 100% generator stator windings is proposed. Simulation results show that the improved subharmonic injection scheme can detect HIGF with fault resistance higher than 8 kΩ.