Generator Grounding!!!

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faresos

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I have a situation where there is an existing 3000A,480/277V, 3 phase, 4-wire service switchboard installed in a 5 story building. The owner decided to use one of those floor as a data center, so he rquested a seperate switchboard and a seperate generator to power this floor. The basis of design is we have added another section to the existing switchboard to house a new 2000A breaker to feed a new 2000A/480V/3 phase/3-wire switchboard that is dedicated for the fifth floor. Now my question is: I'm specifying a 1250KW generator with 2000A breaker (with GFP) that will power that new 2000A switchboard via 2000A ATS. The new switchboard will be used to power (2)500kva ups's (480V input & ouput) and some lighting, receptacles and mechanical panels. Is there any special grounding I need to do for the generator since we are specifying a 3-wire system? never done 3-wire generator system. Is that a delta system (since it's 3-wire) that I'm dealing with? Any thoughts will be appreciated, thanks

Sam
 
faresos,

Need a few more 'givens'.

Is the new 2000a c.b. tapped onthe load side of the 3000a?

Is the 2000a ATS part of the new switch gear ? (built at the factory ) as a

unit?

There are no 277vac loads, lights???

Is the gen. labeled 277/480 , and your just not using the neutral ??
 
More information would be good, I agree.

Are you planning on letting Phase C go away, and just hang on to phases A & B with the generator?

I guess what I'm getting at is, can you name the three wires for us?

As far as grounding goes, if you switch the neutral with the ATS then it is a seperately derived system, like a transformer. It would need to be connected to the building's Grounding Electrode System per 250.30.

If you do not switch the neutral with the ATS, then it is not a seperately derived system. See FPN #1 to 250.20(D). You would need to specify a generator where the bonding jumper between neutral and ground is removable or nonexistent.
 
Really need some more info. I do a lot of IT designs and smell a problem already 1.2 Meg genset with 2-500 UPS, lighting, and mechanical load sounds like a overloaded generator to me. Are you a PE or had a PE review the design? How are the UPS interfaced with the generators? Are there rectifier walk-in circuits? What about single phase loads?
 
Thank you all for replying, here is some answers to your questions



benaround said:
faresos,


Is the new 2000a c.b. tapped onthe load side of the 3000a? YES

Is the 2000a ATS part of the new switch gear ? (built at the factory ) as a

unit? The ATS will be bought seperatlly from the switchboard (individual unit)

There are no 277vac loads, lights??? There will be 277V loads but we are providing a transformer: 480V Delta pri- 480Y/277V secondary

Is the gen. labeled 277/480 , and your just not using the neutral ?? We havn't bought the Generator yet but I'm specifying 480V/3 phase/ 3 wire. Not sure this is the right spec or not!!!!

Thanks again,
Click to expand...
 
georgestolz said:
More information would be good, I agree.

Are you planning on letting Phase C go away, and just hang on to phases A & B with the generator?

I guess what I'm getting at is, can you name the three wires for us?

As far as grounding goes, if you switch the neutral with the ATS then it is a seperately derived system, like a transformer. It would need to be connected to the building's Grounding Electrode System per 250.30.

If you do not switch the neutral with the ATS, then it is not a seperately derived system. See FPN #1 to 250.20(D). You would need to specify a generator where the bonding jumper between neutral and ground is removable or nonexistent.
Click to expand...

George:

I'm specifying all phase conductors (3-wire) but not the neatral since we are not using it. Does all generators comes with neatral bar terminal and is up to use to use it or leave it?
 
Farsos, can't say I have ever seen a 3-wire generator per se, but have done a lot of work simular to what you are doing.

For the genset I use 480 wye set, bond the Xo to the GES, then just run your 3-phase conductors out and no neutral.

For the ATS you are going to want a break-before-make 3-pole type.

If by chance you are trying to use a 3-wire generator to obtain the benefit of no outage in the event of a L-G fault, you can use a delta/delta transformer between the genset and ATS to achieve this.

The only problem I see facing you is the 2-500KVA UPS systems. If commercial power was lost and the generator takes more than say 30-seconds to come on line, I don't think your generator is going to handle the load because of battery recharge current. Two ways around this. 1 Rectifier Walk-In capability of the UPS. This requires control signals between the genset and UPS. 2. Oversize the genset by a factor of 2X.

Even with Recifier Walk-In capability, I think you are pushing it too far with a 1.2MW Genset and 1MW UPS, not to mention you other loads on top of that. As general rule of thumb I would never push a genset no more than 75% capacity, with the UPS alone you are execeeding 85%, add recharge current and you are over budget.

Just my 2-cents
 
faresos,

A 480vac 3ph 3wire gen will be a SDS.

GEC 250.30(B)(2) use 250.66

GE 250.30(A)(7)

Bonding 250.104(D)

By not being sure of exactly what type of gen. ,makes it tuff to give exact

answer. This should get you started. One more thing,listen to dereckbc's advise!!
 
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If the UPSs are more than maybe %50 of the genset load, make sure that the genset has an electronic speed governor (AFAIK, most of that size do). The harmonics that a UPS can throw back up the line can play hob with the regulation.

What goes in must come out
What's supplying the HVAC? If you're supplying 1MW to the UPSs, you need something like 280 tons of AC to remove that heat load, and that's gotta be on the genset, too. (I hope the UPS aren't supplying HVAC...)
 
Thank you all for replying!!!

just need to clarify that the UPS system is redundant and only 500KVA(400KW) will be the total load on the GEn plus other loads( lighting, receptacles & HVAC) which will bring the total load approx. 1500KVA (connected) and I'm sure we could do diversity on that load.
If I understand this correctly, I need to specify 4-wire system but I need to bond the neautral to the GES, Is that mean it's a SDS?. also, what I don't understand Dereck statment about the ATS "for the ATS you are going to want a break before-make 3-pole type."

Thanks again,
 
A word of caution, with GFP you must use a transfer switch that transfers the neutral, at least for a service, I am not sure on a feeder. The GFP if used with a generator with a system bonding jumper won't work correctly.
I've never done a service or feeder with GFP, but I have seen this requirement in the IEEE books
 
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