Protection Riddle No.15 – Generator Protection

When the 59GN is applied in a GFN scheme amplification of the 60 Hz zero-sequence voltage may require a much higher setting on the 59GN relay. Generators can produce significant zero-sequence harmonic voltages during normal operations. These harmonic voltages will appear across the secondary resistor and voltage relay. The most significant is the third harmonic, which can be from 1 to 10% of the generator terminal voltage. If the 59GN relay is to be set near 6 V to achieve 95% coverage of the stator winding, the relay used must be tuned to fundamental frequency (60 Hz) rendering it insensitive to third and other zero-sequence harmonic voltages. So the neutral overvoltage and broken-delta schemes can detect faults over 95% of the winding, but the 5% near the neutral is unprotected. If a second ground occurs in the same phase near the generator terminal it will be undetectable and unrestricted by the grounding impedance. The generator differential relay would not operate for this fault, because current at each end of the winding would be the same. This extremely high magnitude fault would persist until it evolved into a phase-to-phase fault that is detectable by the differential relay. As a result of these considerations and the development of electronic relay technology, several 100% winding protection schemes are now available. They fall into two categories: third-harmonic voltage schemes and neutral injection schemes.