We have 02 generator 26MVA each. Normally both are in operation and synchronized. 100 A, 63.5 Ω NGR's are installed in each generator.

Our scheme is:
NGR of one GTG is closed.
In case of ground fault when GTG (whose NGR is closed) is tripped NGR is opened.
system becomes ungrounded.
In above scheme problem is when system is ungrounded, voltage does not remain stable and other GTG also trips on Over-voltage.

We are proposing to Close both NGR's when both GTG's are in operation. both This will create a problem that both GTG's will be tripped in case of ground fault.

which scheme we should adopt? what is recommended practice? what are pros and cons of both schemes?

 

Abu… providing each generator with a NGR is the positive corrective-action for the following reasons:

o Any third-harmonic present caused by having unlike alternators, becomes miniscule.

o Eliminates switching gear to isolate the individual NGR circuit and its complication to operation.

o Removal of one generator from service does not require concern about inserting the other NGR.

You pointed out the major negative reason against individual NGRs... coordination is impossible! But, since the NGR (or even contribution from two sources) results in relatively little stator damage, i.e., winding-insulation vs iron-damage, you should consider Differential Ground-Fault or Restricted Earth-Fault protection!

No doubt that DGF or REF is certainly more expensive than individual NGR! But, it is a major contributor to asset safety, reliability, and availability!

Let us know if you would like additional information.

Regards,
Phil Corso