This is to be noted that, the star point or neutral point of stator winding of an alternator is grounded through an impedance to limit the ground fault current. Reduced ground fault current causes less damage to the stator core and winding during ground or earth fault. If the ground impedance is made quite high, the ground fault current may become even less than normal rated current of the generator. If so, the sensitivity of phase relays becomes low, even they may fail to trip during fault. For example, a current lower than rated current makes it difficult to operate differential relays for ground fault.
In that case, a sensitive ground/earth fault relay is used in addition to the differential protection of alternator. What type of relaying arrangement will be engaged in stator earth fault protection of alternator depends upon the methods of stator neutral earthing. In the case of resistance neutral earthing the neutral point of stator winding is connected to the ground through a resistor.
Here, one current transformer is connected across the neutral and earth connection of the alternator. Now one protective relay is connected across the current transformer secondary. The alternator can feed the power system in two ways, either it is directly connected to the substation bus bar or it is connected to substation via one star delta transformer. If the generator is connected directly to the substation bus bars, the relay connected across the CT secondary, would be an inverse time relay because here, relay coordination is required with other fault relays in the system. But when the stator of the alternator is connected to the primary of a star Delta transformer, the fault is restricted in between stator winding and transformer primary winding, therefore no coordination or discrimination is required with other earth fault relays of the system.
That is why; in this case instantaneous armature attracted type relay is preferable to be connected across the CT secondary.
It is should be noted that, 100 % of the stator winding cannot be protected in resistance neutral earthing system.
How much percentage of stator winding would be protected against earth fault, depends upon the value of earthing resistance and the setting of relay. The resistance grounding of stator winding can also be made by using a distribution transformer instead of connecting a resistor directly to the neutral path of the winding. Here, primary of a distribution transformer is connected across earth and neutral point of the stator winding.
Secondary of the transformer is loaded by a suitable resistor and one over voltage relay is also connected across the secondary of the transformer. The maximum allowable earth fault current is determined by the size of the transformer and the value of loading register R.
This resistance is connected with the secondary, reflects to the primary of the transformer by the square of the turns ratio, thereby adding resistance to the neutral to ground path of the stator winding.