Parallel conductor ground

[pademama]

I have an duct bank installation from 15KV gear
to a GSU. I'm running 9 #750MCM per phase in 9
PVC conduits. These are connected to the gear bus
with protection provided with the generator 3000A breaker. Per art. 250.122 there is a 400kcml ground conductor run in each of the 9 conduits.
The contractor asked if a bare copper conductor could be run instead of insulated cable. He is suggesting running #500MCM bare copper. I can't
find anything in the code disallowing this, but Iwanted to check with you guys.
Thanks

 

[bennie]

Re: Parallel conductor ground

What's a GSU?

 

[pademama]

Re: Parallel conductor ground

GSU stands for Generator Step Up transformer
in this case 45MVA

 

[bennie]

Re: Parallel conductor ground

I assume the duct bank is from the generator to the transformer. The generator will be wye wound. The star point will be grounded within the stator. There may be a solid, or an impedance ground to the earth.

An equipment ground is not necessary. The wye source will feed a delta primary, with a wye secondary. The secondary will be the medium voltage and will derive a multi-ground neutral conductor.

The generator controls can sense a fault and dump the exciter much faster than any other fault sensing equipment.

 

[pademama]

Re: Parallel conductor ground

Glad to have a response
Actually the 15kv side of the xfmr is delta
and high side 34.5kv is wye. There is ground fault
protection from a Ct on the HO of the high side that is wired back to 50/51 in the relay panel.
Sensing a fault, it will trip the generator breaker and lock it out. You got me thinking though about weather a ground conductor is require to be run in each conduit between the xfmr and the switchgear. We have a ground grid of #500MCM to which the gear and transformer are connected and of course the generator is also connected to this grid.

 

[don_resqcapt19]

Re: Parallel conductor ground

Bennie,
All systems, grounded or ungrounded require equipment grounding conductors per 250.4.
Don

 

[bennie]

Re: Parallel conductor ground

For technical correctness 15 KV is not a standard voltage for line to line. Is this a 13.2/7.6 KV Generator voltage? There is a standard voltage of 14.4 KV but is a delta source. Most all generators are wye wound.

Also, is there an impedance ground system at the generator?

Fault detection on the secondary, and dumping the primary is normal, actually that is the only way it can be done.

Isolated bus ducts are often used on the generator phases. All grounds should be kept from any possible contact with the isolated bus. Ground fault current on a generator winding will create a disaster.

A generator is the opposite of a motor, a fault on a motor can be detected. A fault in a generator winding will have to burn clear.

 

[pademama]

Re: Parallel conductor ground

The generator is 71MVA, 13.8kv. and there is an impedance ground system on the generator.
Thanks for the help. You guy's are great and have
been a valuable resource to me.