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.
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.
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.
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
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).
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).
