Generator feeding transformer feeding Transfer Switch OCPD & Grounding 1

[Skogs]

This is my first thread. I'm a fairly new electrical engineer at a consulting firm. I have a scenario that seems a bit tricky to me, if anyone has any tips, please chime in.

We have a client that has an existing trailer mounted generator (120/208) & transformer (208-240/120) feeding a transfer switch in a control/pump building which feeds one MCC. Not sure what grounding/bonding is done at the moment.

They are converting their electrical service from 240 open delta to 480/277 as they are increasing hp (3 new 40 hp motors on vfds) by more than the utility allows on the 240 open delta. The client wants to keep the existing generator, meaning change out the transformer to a 208-480/277.

Some questions.
1. The transformer (208-480/277) is a separately derived system, correct? Does this mean a 4 pole transfer switch is required?
2. Is the generator a separately derived system?
3. Does the generator need it's own grounding electrode?
4. Are there any codes against (not allowing) this type of installation?
5. The existing generator is 100kw and has a 400A Breaker. The new automatic transfer switch will be service entrance rated with a utility breaker. Will it also need a breaker on the generator/transformer line side?

After writing this I'm wondering if looking into a 120/208v service would be better. The bulk of the load are those 3 pump motors. The utility was going to install 150 kva transformer bank.

 

[waross]

Time to pull in a new trailer. 100 kW won't handle 3 40 HP pumps.
Some guesses:
A 40 HP pump will draw about 52 Amps at 480 Volts.
3 pumps X 52 Amps x 277 Volts x 3 phases / 1000 = 130 KVA added load.
130 KVA / 0.8 PF = 162 kW
I would be looking at about 275 kW prime rated or 300 kW standby rated.
The simplest, most dependable grounding system is to use a three pole transfer switch and run the neutral solidly to the generator.
However, protection issues may force you to use a four pole ATS and switch the neutral.
I would use a 480 Volt transfer switch, and a 480 Volt generator. Instal a suitable transformer to feed any low voltage loads.
Grounding. The trailer and the generator must have an equipment ground. The grounding at the main panel should be adequate but I would go the extra mile and add ground electrodes near the entrance to the trailer. The simplest but not the only method, (nor necessarily the best) may be two electrodes, ten feet apart, one on each side of the entrance to the trailer.
Generator size: VFD's and generators don't work and play well together. The issue is poor Distortion power factor caused by the rectifiers in the front ends of the VFDs. The distorted current wave form, and the accompanying distorted voltage wave form may cause voltage instability. The Automatic Voltage Regulator can't cope with the distortion. Generators are frequently over sized about 2:1 for VFD service.
BUT some VFDs are better than others. It would be a good idea to post the make and models of the proposed VFDs and wait for jraef to comment.

On an entirely different note; I was a little confused when I saw your handle. You have not done anything wrong, but you have inadvertantly chosen the nickname of one of the most knowledgeable and active gurus in the electrical fora. (Skogsgurra, Skoggs for short.) If you will consider changing your handle, site management will probably be glad to assist you.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

When did we start dividing kVA by PF to get kW?


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If we learn from our mistakes I'm getting a great education!

 

[waross]

I'm losing it Scotty. I guess "A little confused" was an understatement.
Starting over (in the middle) A gen set rated at 130 KVA will be rated at 104 kW.
But if the actual power factor is better than 0.8 the prime mover may not pull the load.
A 40 HP pump will draw about 52 Amps at 480 Volts.
3 pumps X 52 Amps x 277 Volts x 3 phases / 1000 = 130 KVA added load.
Your 100 kW set should be rated at 125 KVA.
So the demand in KVA will be 130 KVA plus 125 KVA = 255 KVA That would be 204 kW. However it may be safer to use an assumed power factor of 0.9 giving a set rated at 230 kW. But that does not yet make any allowance for distortion power factor. The actual size of the set will probably have to be somewhat greater for VFD service.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogs]

Thanks for the replies waross. Would Skogy be a ok or still too similar to Skogsgurra?

Pumps - I should've said 2 not 3. There will be 3 but only 2 ran at any time.

Waross - In your first reply, I would use a 480v generator also if the client didn't have an existing 208v generator they'd like to get by with. In your last reply, why did you add the demand of the 3 pumps to the size of the generator to come up with 255 kva?

On the VFD/distortion issue, we aren't sole sourcing, but some approved manufacturers would be AB, Eaton/CH, Sq D...

On the grounding and bonding, I added a link/attachment to a pdf. If you can view it, I'd appreciate any critiques. Thanks.

 

[Skogs]

Does it take any time for the uploaded file (to engineering.com) to be viewable?

Here's another try without the spaces in the filename.

 

[Skogs]

Third try!

 

[Skogs]

Has anyone seen a distribution transformer mounted on a trailer? I see there is a code section 250.34(B) for grounding Vehicle-Mounted Generators, but how about Vehicle-Mounted Transformers?

Would there be a parallel ground path because there are two separate systems (the generator or primary side and the secondary side of generator transformer) that have their own grounding electrodes yet they are tied together because of them both being bonded to the trailer?

Here is my latest grounding and bonding schematic. If anyone has seen anything like this or has any input as to what might not be to code, feedback is appreciated. Thanks.

 

[waross]

Two suggestions.
I would install a ground bus (or jumper) in the trailer and connect the trailer frame, generator frame, generator neutral and transformer neutral all to the ground bus. I would consider leaving the four ground rods as a generator trailer presents added touch and step potential hazards.
I would consider a neutral switching transfer switch. You have more than one connection between neutral and ground. switching the neutral will avoid this issue.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogs]

Thanks Waross,
Why would you ground neutral of generator if it's not used?

Now, plans are changing and the generator and transformer are no longer going to be on a trailer. They will be moved to/permanently installed on a new concrete pad.

 

[waross]

I work under the Canadian Electrical Code which requires the neutral to be grounded if by so doing the phase voltages will be limited to 150 Volts to ground or less. 120/208 neutrals must be grounded in Canada.
Can someone comment on NEC regulations?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogs]

I think it's 250.20(B)(1) that would require this generator neutral to be grounded.