- Stator Earth Fault Protection Definition: Stator earth fault protection limits ground fault current to minimize damage to the stator core and winding.
- Ground Impedance Role: Grounding the stator with high impedance can reduce fault current but may lower relay sensitivity, requiring additional sensitive relays.
- Resistance Neutral Earthing: In this method, the stator’s neutral point is grounded through a resistor, which is connected with a current transformer and protective relay.
- Relay Types: Depending on the connection method, either an inverse time relay (for direct substation connection) or an instantaneous armature attracted relay (for star-delta transformer connection) is used.
- Alternative Grounding Method: A distribution transformer and resistor setup can also ground the stator, with an overvoltage relay ensuring protection.
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 such cases, a sensitive ground/earth fault relay is used alongside differential protection of alternator. The type of relay arrangement for stator earth fault protection depends on the stator neutral earthing method. For resistance neutral earthing, the stator’s neutral point is connected to the ground through a resistor.
A current transformer is connected across the neutral and earth connection of the alternator, with a protective relay connected to the transformer’s secondary. The alternator can feed the power system either directly to the substation bus bar or through a star delta transformer. If connected directly to the substation bus bars, an inverse time relay is used to coordinate with other system relays. If connected via a star-delta transformer, an instantaneous armature attracted relay is used as there is no need for coordination with other earth fault relays.
That is why; in this case instantaneous armature attracted type relay is preferable to be connected across the CT secondary.
It should be noted that 100% of the stator winding cannot be protected in a resistance neutral earthing system.
The percentage of the stator winding protected against earth faults depends on the earthing resistance value and the relay setting. Resistance grounding of the stator winding can also be achieved using a distribution transformer instead of a direct resistor. In this setup, the transformer’s primary is connected across the earth and the stator winding’s neutral point.
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.
