In power plants, the Generator neutral side is earthed through Neutral Grounding Transformer (NGT) ? Why? Also across the secondary side of the NGT one resistor is connected. why? How to calculate the rating of the NGT and also the resistor?
I've never heard of the the Neutral Grounding Transformer here in the UK, where traditionally the generator neutral is connected to earth by an LNER Liquid Neutral Earth Resistor. The main reason for this is a fault limiter device where fault current was dissipated by heating water based solution in the tanks.
I assume that the NGT is impedance which replaces the LNER and thereby the reason for the resistor.
This whole issue was discussed at length in this forum not long ago - try searching it.
There is a need to ground the system and the currently used method for large generators is to ground them through a combination of a distribution transformer and a resistance across its secondary terminals.
The resistance at the secondary side will be reflected to the primary side multiplied by the square of the distribution transformer ratio, thus limiting the phase to ground fault, what is very much desirable.
IEC 34 and IEEE reccomends limting this value to something like maximum 15A primary.
Then a voltage relay will be connected to measure the voltage drop across the resistor and, therefore, define if a stator fault to ground exists.
But providing the proximity to neutral leeds to uncertaity, there is not possible to detect a ground fault in the last 10% of the stator winding.
For this purpose, a special protection must be employed in order to detect faults in this region.
Normally generator grounding through resistance is done to protect the system from over current in case of fault.this used when the fault current level above the withstand-able value of your equipment/protection relay and the neutral earthing resistance used to minimize the fault current to a value at which your protection relay can operate
Generator
Re: Generator
