Impedance grounding will limit the current in the event of a ground fault. The reduced fault current may avoid a destroyed generator core in the event of an internal fault to ground. The generator protection must be able to detect internal faults and and in most cases be able to shut down the prime mover. The fault current for ground faults external to the generator will be limited by the grounding impedance.
That said, if there are any line to neutral loads, the neutral must be solidly grounded.
Bill
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"Why not the best?"
Jimmy Carter
The stator core on a large set is a high value asset and the lower the fault current the better. Big machines generally use NGTs because they make it simpler to employ a physically rugged low resistance resistor in an arrangement which keeps the primary fault current to a few amps. The secondary current is generally large enough to drive a protection CT.
Just add to ScottyUK's view; conventionally the relay connected to the NGT secondary side is voltage operrated. This makes the earth fault sensing more sensitive.
You can do whatever you want. It's just that people who have forgotten more about generators than you'll ever know have already figured out that a neutral earthing transformer is a better way to solve the problem. Plain resistors are really limited to LV applications.
The OP didn't give a hint about the voltage of the generator. If it's LV, then a resistor. If it's at the low end of MV, then either is commonly used. For higher voltages than a few kV, then it's into the territory of NGT.
