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Nuisance ground fault.

Nuisance ground fault.

Nuisance ground fault.

(OP)
Dear All,
I have a design paralleling 6 gensets (2180 kW) of 400 V through6 GSU of 3.75 MVA (there will be breakers before the GSU), 0.4/20kV Delta Wye connections.
The Gensets are in wyes and solidly grounded in each gensets (the neutrals will be bonded here). The transformers will solidly grounded too in each of them (the neutrals will be bonded here). They will be feeding 24 kV switchgear and will be supplying load of MCCs in various location. There will be 4 feeders on the switchgear.
Will there be any nuisance tripping on ground fault with this configuration since the shync will on the HV side and the faults will be going back to its source (Transformers)and they all adjacent to each other in the transformer yard.
The question is really if this design is workable(Single line is attached).
Thanks for any feedback.

RE: Nuisance ground fault.

Looks good, should have no issues during synchronizing. Minor unbalance during 3-pole closing due to phasing, half cycle duration max but 50BN can be set above that, 51BN will too slow and 87B will be unaffected.

Drawing has switch symbol in breaker location.

Cheers

RE: Nuisance ground fault.

What breakers are going to have ground fault protection and what type of ground fault detection will be used?

RE: Nuisance ground fault.

(OP)
Hi Sebeen,
All the berakers of 24 kV will be equipped with ground fault protection, any suggestion on the type? since we haven't decide yet.
Thanks.

RE: Nuisance ground fault.

Standard feeder O/C relays, the likes of Schweitzer SEL-751A.

With the currrnt configuration bus differential can also be incorporated using SEL-87E.

Just my own view.

Cheers

RE: Nuisance ground fault.

Lot of small step-up transformers.
I would prefer few generators on the common 400kV bus with connection to one or two GSU with bus-tie on the 24kV side.
Synchronization on the 400V breakers.

RE: Nuisance ground fault.

Why not 24 kV generators?

RE: Nuisance ground fault.

20kV is max for very big generators ( > 200MVA ).
You can ask why not 6.6kV or 3.3kV , or 4.4kV
6300A of 400V is very big GCB and lot of copper bars

RE: Nuisance ground fault.

slavag, that's why I was asking. These are reasonably small generators - 2.2 MVA. I just looked at the generator downstream circuit breakers and used the rule of thumb - "if your breaker is above 4000A then you are at the wrong voltage".

So I'd be using the 24 kV and saving on transformers.

RE: Nuisance ground fault.

If you use distribution class breakers then be very careful with the high X/R ratio and the long time to first current zero. Most distribution breakers won't withstand a heavy fault. Proper generator breakers are expensive and bulky.

sibeen,

You do not want a 20kV generator. Air-cooled machines almost all have PD problems, and I doubt you want the complication of a hydrogen machine. wink

Run the generators at 3.3kV / 6.6kV and you won't see many problems. PD starts appearing above about 10kV.

RE: Nuisance ground fault.

We have 20MVA and larger at 250MVA units where breaker tripping is delayed interrupting symmetrical currents only, so lower rated breaker can be used. These are very small generators on this design, distribution breakers would be already at 250MVA interrupting capability or typically around 40-50 KA, may work.

RE: Nuisance ground fault.

(OP)
Thanks for all responds,

The first intention of using small generators is to have an ease to take each generator from the system once in a time while the need of overhaul is came. Shync all in the 400 V side is resulting in high kA rating, In this design I was wondering weather the grounding system applied could really visibly worked, since I slodly ground all the transformers neutral seperately in each transformer, so i don't have one common bonding of neutral and ground (the same things applied for generators).
any comments?
I make one option for 6.6 kV and get rid the transformers, looks good so far.
Thanks

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