NGR Closing of Multiple Generator

Each individual GTG should be protected by its own NGR, at all times. Is this a new installation. Obviously, the system had performed in the past. The unbalanced GTG is not able to clear unbalance effectively, fast enough.

If Both NGR's are closed fault current will flow in both GTG's and we will have to face blackout.

Don't do it, then.

Voltage of what in reference to what?

Generator Voltage which is 11 KV L-L

How does the 11kV L-L vary when the trip has operated?

When one generator has trippped , voltage of other generator whose NGR is opened varies

A Bakar,

Assuming your generator rated full load current is higher than the maximum allowable earth-fault current (100A in your case), can you kindly explain why the need to trip the generator immediately? What anticipated damage are you preventing here?

Generator is notbeing tripped immediately we have 51GN protection . i need what is recommended pratice in such type of power systems and what are pros and cons of both schemes.

We have to trip generator on earth fault to avoid core damage

Pl. go for bus earthing with Zig-Zag Transformer & NGR. Once both the generators are synchronised, bring bus earthing on-line & open both the NGRs. Thus, the system will be grounded and the generators wouldn't be. This would solve many problems.

It is a normal practice also to earth (in this case via NGR) all generators in operation. However, the requiremnt with this method is that you have to set the GF setting at the 11kV downstream consumer reference to the sum of all the NGR rated current. If one day you decide to run with only one generator, while the other generator on maintenance, due to safety you have to disconnect the NGR for that off-service generator, and re-configure the GF current setting again at the consumers.

Also if one day your company decides to expand by adding more generators, you have to reconfigure them yet again.

One way to avoid this is by introducing neutral-earthing transformer to your 11kV system, and reduce the NGR rating connected to your genset to say 5A (which is more forgiving to your core insulation)

It sounds to me from your posting that this earth-fault occurs within the core windings. If it occurs elsewhere downstream gen terminal, it should have been cleared by other protection e.g. differential prot, and 51G at consumers.

Having said that, I can't understand why both generators will trip when both are grounded, because it's very unlikely both genset to have internal faults simultaneously (unless you set them on instantaneous trip - which you already said no).

Not sure what your generator winding configuration is: "WYE" or "DELTA" ?

If "WYE" configured and you lift/unground the star point (wye point);

The "B" phase voltage will definitely become unstable. ("Wild-leg" condition)