Understanding generator stator ground faults and their protection schemes

Because of stator winding construction, an insulation failure is more likely to result in a winding-to-ground fault than in a phase-to-phase fault. Other failures, such as a broken conductor or turn-to-turn fault, may not be detected until the fault develops into a winding-to-ground fault. Consequently, stator ground fault protection is one of the indispensable schemes for protecting a generator stator winding. Dedicated stator ground fault protection is required for high-impedance-grounded machines. Neutral overvoltage protection is the simplest method for detecting a stator winding-to-ground fault, but this method does not detect faults over the entire winding. Therefore, additional protection is required in the form of third-harmonic undervoltage, third-harmonic voltage balance, or subharmonic injection. Each of these protection schemes is available in several alternatives.