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#184660 - 02/17/09 01:38 AM BB transformer for 277 to 240
pdh Offline
Member

Registered: 01/20/05
Posts: 354
Using a 240:32 transformer configured for 272:240, fed on the 272 side with 277 volts nominal, should get about 244.5 volts nominal out on the 240 side. That's 244.5 volts L-N.

272 volts is close enough to 277 volts that I'm assuming the transformer should be OK with it.

How often is this kind of thing done?

I'm just pondering the possibilities for running large numbers of computers directly on 240 volts. This is not a current project or such ... just "armchair thinking".

I'd think 244.5 should be OK with the 100-240 volt computer power supplies. The idea with the autotransformer is how to get close to the 240 volt level with minimal transformers (hence the buck-boost autotransformer) in cases where the utility won't provide anything else between 208/120 and 480/277 (which is probably most of them), such as the ideal 416/240.

Using a 240:48 transformer configured for 288:240 would step 277 volts down to 230 volts. But it would be somewhat less optimal than the previous configuration (5.6% more current on conductors and 50% more transformer capacity).

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#184662 - 02/17/09 02:52 AM Re: BB transformer for 277 to 240 [Re: pdh]
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
You are better off using an Isolated Transformer for this scenario.

The Auto Transformer derived from Buck/Boost arrangements are better suited for "Non-Critical Loads", like small Motors, Resistance Heating elements, etc.

A "Buck / Boost" Auto Transformer has poor regulation (+/- 20% at best), with an unstable output Voltage.

For a Project like you have described - driving large numbers of Computers at 240V, using an Isolated Transformer of either Single Phase or 3 Phase would be the better choice.

If Single Phase, go with a 480VAC Primary, to 120/240VAC Secondary.
Ground the Secondary's "Center Tap", but do not use it as an active Circuit Conductor (AKA "Neutral"). Just bond the Grounding Electrode System (GES) to the Center Tap, along with Equipment Grounding Conductors (EGC).
Bring an EGC to the first Panelboard, and use only 2 Pole devices.

If 3 Phase, go with a 480VAC Delta Primary to 240VAC Delta Secondary. "Corner Ground" the Secondary side, or "Center Tap Ground" one winding - but not both!!!

To get really tricked out, use a 480VAC Delta Primary, 240VAC Wye Secondary (Ground the common point), with an "Auxiliary" Delta Secondary for "killing" circulating currents.

Just my 2˘

Scott
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#184668 - 02/17/09 10:29 AM Re: BB transformer for 277 to 240 [Re: pdh]
LarryC Offline
Member

Registered: 07/05/04
Posts: 776
Loc: Winchester, NH, US
I think the issue that will bite you, is using 240V 2 pole breakers will fill up you panels quick. Plus can you get breakers with LOW enough current ratings? 240V x 5A = 1200 VA.

Larry C

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#184670 - 02/17/09 11:39 AM Re: BB transformer for 277 to 240 [Re: Scott35]
pdh Offline
Member

Registered: 01/20/05
Posts: 354
Originally Posted By: Scott35
You are better off using an Isolated Transformer for this scenario.

The Auto Transformer derived from Buck/Boost arrangements are better suited for "Non-Critical Loads", like small Motors, Resistance Heating elements, etc.

A "Buck / Boost" Auto Transformer has poor regulation (+/- 20% at best), with an unstable output Voltage.

Are you sure this reputation is not the result of typical uses involving small transformers that have high impedance with widely varying loads? If a small BB transformer is driving a single device that goes on and off, I'd expect to see a significant voltage change because of the significant load change. One thing about a large data center is there is rarely such significant load changes.

Originally Posted By: Scott35
For a Project like you have described - driving large numbers of Computers at 240V, using an Isolated Transformer of either Single Phase or 3 Phase would be the better choice.

This would mean much larger transformers. For example if the data center loads needed 375 kVA in an isolation transformer, a 50 kVA 240:32 transformer would do where its secondary were supplying 1/7.5 of the voltage.

Another option I had thought of was using 120/240 autotransformers to step 208/120 up to 416/240. But this would be larger transformers and larger service conductors.

Originally Posted By: Scott35
If Single Phase, go with a 480VAC Primary, to 120/240VAC Secondary.
Ground the Secondary's "Center Tap", but do not use it as an active Circuit Conductor (AKA "Neutral"). Just bond the Grounding Electrode System (GES) to the Center Tap, along with Equipment Grounding Conductors (EGC).
Bring an EGC to the first Panelboard, and use only 2 Pole devices.

These are disadvantages I'm trying to work around. I suspect one of the reasons (of many) that data centers still use 120 volts (in USA) to computers is because of the "two hots" problem of 240 volts. One of the goals is to avoid 2 pole OCPDs. This would be a lot of circuits of probably 20 amps. I'm also ready envisioning 3 pole OCPDs providing 20 amps of 416Y/240 to each rack. The densities of some of these data centers can get very high.

Originally Posted By: Scott35
If 3 Phase, go with a 480VAC Delta Primary to 240VAC Delta Secondary. "Corner Ground" the Secondary side, or "Center Tap Ground" one winding - but not both!!!

Well, obviously you can't ground both grin

This would present a lopsided load to the utility, since the loads would all be on 2 phases that are only 60 degrees apart. On the size scale where 3 phase is needed, this approach would have to be replicated 3 times at three different phase angles. When all grounds are bonded together, that triple system would effectively be a 6-star 480/416/240 system (a pair of 416Y/240 systems overlaid on each other at 180 degrees).

Originally Posted By: Scott35
To get really tricked out, use a 480VAC Delta Primary, 240VAC Wye Secondary (Ground the common point), with an "Auxiliary" Delta Secondary for "killing" circulating currents.

That would get the 416Y/240 system which is desired. But why not a WYE-WYE transformer (277 to 240 times 3)? If it were the case that computer power supplies can be had to run directly on 277 volts (I've actually seen them, but not in the numbers that would make it practical to plan a whole data center around them), then the circulating currents would be pushed upstream to the utility, assuming their transformer was DELTA-WYE.

Supposedly, switch-mode power supplies these days ... at least the better quality ones ... do not present a significant harmonic issue. That would, of course, still be something to be studied when building a large data center.

If the utility would provide 416Y/240 directly, that seems to be the way to go. I'm sure they would be concerned with the harmonics. If this were a data center in Europe, everything would be powered with 400Y/230 and nothing more would be done (unless harmonics is an issue).

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#184671 - 02/17/09 11:42 AM Re: BB transformer for 277 to 240 [Re: LarryC]
pdh Offline
Member

Registered: 01/20/05
Posts: 354
Originally Posted By: LarryC
I think the issue that will bite you, is using 240V 2 pole breakers will fill up you panels quick. Plus can you get breakers with LOW enough current ratings? 240V x 5A = 1200 VA.

Larry C

Why would LOW current breakers be needed?

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#184672 - 02/17/09 12:44 PM Re: BB transformer for 277 to 240 [Re: pdh]
LarryC Offline
Member

Registered: 07/05/04
Posts: 776
Loc: Winchester, NH, US
I obviously made an incorrect assumption. I was thinking individual breakers for individual power supplies. My mistake.

Larry C

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#184673 - 02/17/09 03:02 PM Re: BB transformer for 277 to 240 [Re: LarryC]
pdh Offline
Member

Registered: 01/20/05
Posts: 354
Even with individual breakers, why would they need to be less then 15 amps? Nothing would need OCPDs any less. You couldn't use smaller than AWG 14 on any of the wiring.

I'm thinking about data centers ranging in size from 24 to 1200 rack cabinets, with 18 to 108 computers per cabinet (and lots and lots of air conditioning). Maybe ONE 240 volt 20 amp circuit would be enough per rack.

I'm assuming the air conditioning would generally be powered at 480.

One idea (if the UPS makers would do it) is for a UPS that takes 480 or 277 volts in on the AC-DC side, and produces 240 volts out on the DC-AC side. This would be a continuous online dual conversion UPS producing a separately derived system just for that one cabinet. If they can fit it in 4U to 6U of rack cabinet space, one of these could be put in each cabinet at the bottom.

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#184677 - 02/17/09 08:04 PM Re: BB transformer for 277 to 240 [Re: pdh]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
They use 120v equipment because that is what they make for the consumer market. When a data center uses 240v it is to avoid having a neutral and the associated noise in the room. They want it all to be line to line. In fact we didn't even bring a neutral into the data center panels before 120v PCs became so common. Everything, including the monitors and small controllers was 240/208. There was a "convenience" transformer, 208/240 to 120v in most of the frames but they didn't ground either leg of the output.
When small switcher supplies came into the market most of those concerns went away and we started seeing 120v stuff all over the place.

The design assumes you will have a branch circuit with O/C protection based on the line plug and they have overload protection in the power supply so you won't need unusual breakers.
_________________________
Greg Fretwell

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#184684 - 02/18/09 12:09 AM Re: BB transformer for 277 to 240 [Re: gfretwell]
pdh Offline
Member

Registered: 01/20/05
Posts: 354
Every switching power supply I have seen can do 220-240 (setting the voltage selector) or 100-240 (autoranging). I can believe the desire for less noise in L-L connections, although I have never encountered it with computer equipment. I have encountered it when I worked in TV. So when commodity PCs came around, why the shift from 240 L-L to 120 L-N? Maybe because it was the only L-N choice below 277 and they wanted to do L-N instead of L-L?

240 L-L is what I'm planning to do at home once I find the right surge protection and UPS equipment that is compatible. But for 240 L-N all the European stuff becomes usable.

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#184688 - 02/18/09 07:34 AM Re: BB transformer for 277 to 240 [Re: pdh]
LarryC Offline
Member

Registered: 07/05/04
Posts: 776
Loc: Winchester, NH, US
My guess as to why not many people make 277 compatible equipment is that for UL safety ratings. The spacing and clearance requirements for line voltage carrying components become larger above 265 VAC.

Larry C

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