Neutral Voltage Protection -59GN:
Neutral Voltage Protection is used to protect the alternator/generator stator winding from stator earth fault. Most commonly, the generator’s neutral point is connected to ground through a resistor (NGR or NGT) to limit the fault current. during Normal condition, the secondary voltage across neutral grounding transformer is less and approximately zero. If the earth fault occurs on the generator stator winding means the voltage across the neutral point to ground will be elevated. Therefore, there must be some voltage present in the NGT secondary. That neutral voltage is monitored by neutral voltage relays 59N. if the voltage in the NGT secondary reaches the allowable tolerance, then the neutral voltage protection operates the generator circuit breaker.
Neutral Voltage Protection -59GN is also called 95% stator earth fault protection. The above relay setting protects the generator/alternator’s bus duct, the low-voltage winding of the step-up transformer associated with the generator and other interconnection with the stator winding.
Note: If the earth-fault nearer to the neutral point of the generator will not give sufficient neutral point voltage to activate the neutral point voltage (95 %) relay 59GN. Therefore we use 100% stator protection relays which means the stator’s 95% of the winding is protected by neutral point voltage and the remaining 5% is protected by another relay is called 27TN. Exactly, (59GN + 27TN)