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Answer Posted / avinash singh
Both NGR and NGT are used to for restricting earth fault current. The standard practice is to have a single grounding in a system with same voltage. If we generate 11kV through the generator and there is no step up transformer before connecting the generator to the generator Circuit Breaker than we will have to use NGR and restrict current at 100A instead of restricting it at 10A as the relay at the feeder side will not be able to detect such small fault current as the relay is getting its input reading from a CT which might be above 1000/1A rating. The relay will not be able to sense small value current as low as 10A as it is quite a low value. So we need to consider NGR considering the sensitivity of relay and CT.
When there is a transformer between generator and generator circuit breaker than you can have the secondary of the transformer grounded and you will get the path for earth fault at feeder end through that grounded system. Now in this case you need not protect the feeder from the generator side and you only need to protect the generator now. So, you will keep the current because of earth fault as low as possible which enters the generator. In this case you can use an NGT and lower the earth fault current to 10A. The CT for sensing earth fault current will be connected at secondary i.e. LT side of transformer and as the voltage is low there, the current will be more and CT can easily sense that much current and the relay will trip the generator as soon as the fault occurs. The resistance will also be connected on LT side of transformer as we can connect low value resistor there and it will have a higher value resistance effect on the primary side of NGT which is HT side which is connected to the main neutral line of generator. Hope this answer helps.
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