Generators & NGR Switching

A ground fault most likely occurs elsewhere in the whole system than can be pinned down to generator 1, 2, or 3.

Nothing happens except that you get an alarm for an earth fault somewhere in the system. If this alarm is ignored and the cause is never corrected then the instant a second earth fault occurs large fault currents will flow and hopefully the protective relaying will isolate fault location, which will be easily evident from the sparks and smoke emanating from it. Never ignore an alarm!

Ram Consult:

You have well explained the first part of my question & its cleared but the second part still confusing/not cleared, that is.

If fault/Over current happened on Generator# 1 or 2 while their Isolator to NGR are OFF/Open then, how the fault current for G1 OR G2 will pass through the NGR ??

Thanks

Because of the parallel generator connection, all three generators are connected to the same neutral conductor therefore the current flowing through the neutral is a sum of all neutral current returning to all three generators windings.

The active NGR is monitoring the neutral and will sense the difference between the normal neutral current and the ground fault current which does not return to the generators.

The sensing and protective tripping will occur regardless of where the ground fault is physically located in the system resulting in isolation of all three generators from the grid.

Individual Generator winding protection such as instantaneous, differential, and time over current relaying is not affected by the active neutral NGR location and will still function normally.

Please understand that ground fault current does not return to the generators via the neutral conductor which is the basis of NGR operation.

I hope this helps.

SHOCKHISCAN,

Thanks for your explanation.

It won't, and that's why there will be no appreciable short circuit current contribution from those ungrounded machines. It's also the reason why Earth Faults should be alarmed, not tripped.

Depending upon the exact location of the fault you could have some appreciable arcing and subsequent damage though, due to the current flow due to the capacitive coupling between the windings and the iron, that's why you should also have a differential zone across the generator leads for a tripping function.

That's the meaning of "Protection in Depth", but the protective relaying philosophy is dependent upon the importance of the asset being protected and its influence on the overall process/plant operations. This is not a "one size fits all" proposition so your exact criteria may be different..