Selection of NGR for Industrial MV Network

NGR is to limit the ground fault current, which is to be designed for 2 times of the generator rated current.

Hence the calculation will be as below

Generator full load current x 2 / generator voltage

It is not always the case

1a i) If you have zero resistance, current in fault is high (news! MAD arc welder escapes!), -but fuses will blow quick so time is short. But breaker will take fixed time to open, even though relay detection time is short. One phase will lose voltage, this will affect all loads in system. ii) If you have very high resistance, detection relay and current transformers [CTs]will not be able to resolve the small abnormal residual current, hence not possible to trip-off only faulted circuit. e.g. 500 amp circuit breaker with 500 amp CTs and only 5 amp earth fault current due to NGR. All circuits will have to be tripped by relay on NGR. But it was only a 5 amp arc welder, and little damage was done! All circuits will be disconnected, yet again. when you close the breaker back on the faulty circuit. iii) If you have medium resistance, current is enough to detect faulted circuit reliably, only faulted load is disconnected, other loads keep going. Industry happy, except one on faulty circuit! iv) If neutral grounding is via transformer, its ratio will have great effect on actual resistance used! Question asks about "value of resistance" does it not?? 1b Strange question. Compared to what arrangement?? Not defined in Question 1a which suggests both 6k6 and 11k have own NGR. A Dyn (note the n!) has a neutral on 6k6 side, so NGR can be connected from that to ground

1a) The one and only factor is the amount of earth fault that you "want" to limit to. NGR value is: Phase Voltage / Desired earth Fault Current. If you keep the value too low, detection becomes difficult and if you keep too large, damages due to the high fault current woiuld be more. If you have rotating machines at the distribution voltage, then better to limit the earth fault current to a value less than the core damage current value (given by the rotating equipment manufacturer in the form of a core damage curve; a typicl value would be about 25A for 1 sec.).

1b) Dyn 11 means your 11kV Side is delta connected and as such you wouldn't have a neutral to earth. But, still if you want to earth the 11kV Side, you may still do so, by creating an "artificial" neutral using a "zig-zag" transformer with the NGR connected to the neutral of this zig-zag transformer. For your 6.6kV side, anyhow, you will have a neutral and you may connect your NGR, considering what is said in 1 a) above.

let me add to question.

The there are 03 voltage level in industry viz. 33 KV, 11 KV and 6.6 KV

The step-down transformers are used Dyn from 33 to 11 and from 11 to 6.6.

Now there is neutral on 11 as well as 6.6 KV level.

1. What shall be value for NGR for 11KV and 6.6 KV

2. Assuming earth fault current is limited to 30 Amps for 11 KV and 25 amps for 6.6 KV. What would be its effect on protective relay co-ordination.