With conventional E/F relays 100% of the stator winding can not be protected due to less driving voltage when earth fault occurs near to the neutral point of the stator winding. Over voltage relay is used for this purpose which can take care of 95% of the stator winding only. the rest 5% shall be protected through some other means. There is a method of using 3rd harmonic voltage generated by the generator to effect this protection.
Any synchronous generator generates 3rd harmonic voltage which is present in the winding. Usually the generator neutral is connected to ground through a resistance. When an earth fault occurs near the neutral point of the winding, the 3rd harmonic voltage produced will be decreased. an under voltage relay tuned to 3rd harmonic frequency (ie:150Hz in case of 50Hz fundamental frequency) will drop off and cause tripping thus effecting protection. Voltage supervision relays are used to bypass this protection during start up of the generator.
There is another method by injecting subharmonic voltage in the neutral circuit and detecting earth fault by sensing the change in the impedance. This method is more expensive due to additional equipments for subharmonic voltage injection.
