When engineers look at power distribution in a modern data center, we see something silly: AC converted to DC (in the UPS), then converted back to AC (again in the UPS), then converted back to DC (in the equipment power supplies). Why not distribute DC, and avoid the inefficiencies of all these backandforth conversions?
Seveal highly respected companies from the U.S. and Japan are jointly considering a single DC voltage for powering data center equipment, probably be in the 350 Vdc ~ 400 Vdc range. There is a huge amount of experience in the world with 48 Vdc power distribution in telecommunications, but the power requirements are much higher in data centers. So a higher voltage, such as 380 Vdc, is needed to keep the currents and copper requirements in a reasonable range.
The arguments in favor of DC are:
But there are problems, too: What kinds of connectors should be used? What about interrupting devices like fuses and circuit breakers? Sustained arcs that would have selfextinguished during an AC zerocrossing? Magnetic fields and forces?
Power quality in DC systems People who are not familiar with power problems seem to think that DC will provide perfect power quality. I have been trying to correct this misunderstanding. There will be faults on any DC system, and loose connections, and capacitive loads turning on and inductive loads turning off. Also, I've pointed out that the data about power quality on 48 Vdc systems, which is very good, is probably not applicable because for the same number of watts, at 400V, we're talking about a source impedance that is probably one or two orders of magnitude higher.
If you would like to be connected with the people who are working on this project, please send me an email at Alex@PowerStandards.com, and I will see what I can do.
Will future data centers be powered from DC?
For some reason, recently I've been getting a lot of questions from all over the world about power quality and source impedance. Here's a quick tutorial.
Sometimes it's useful to think of any AC power source as a perfect source of voltage that has an impedance inserted between the source and wherever you are. Think of this impedance as a series resistor and a series inductor. Sometimes the impedance can be mostly inductive (especially if the conductors are far apart, as on transmission and overhead distribution lines), or it can be mostly resistive (if the conductors are bundled closely together, as they usually are inside buildings).
When your load draws current, there is a voltage drop across the impedance, so your local voltage becomes lower. You can see this in the RMS voltage  turn on a big load, and the local RMS voltage goes down. And you can see it in the waveform  draw a pulse of current, and you'll see a notch in the voltage waveform. The bigger the current pulse, and the bigger the impedance, the deeper the notch (or, for singlephase electronic load currents, the flatter the peak of the voltage waveform).
Now imagine what happens when you try to draw a really large amount of current. Maybe you have a short circuit, or maybe you're placing a discharged capacitor across the conductors. What happens? Plenty of current flows, but how much? For resistive impedances, the amount of current is simply the source voltage divided by the source impedance. I've seen source impedances on a 120volt 20amp branch circuit in a building in the 4ohm range, so you get 30 amps into a short circuit. And I've also seen source impedances on a 200volt 20amp branch circuit in the 0.02ohm range, which gives roughly 1000 amps into a short circuit. It all depends on the source impedance.
So what happens if the impedance is too low? Well, you can get very large current flows  perhaps higher than the designer of the equipment expected (if the designer even thought about the problem). You see this problem when your first turn on a piece of equipment: fuses open, or circuit breakers trip, The initial inrush current in some carelesslydesigned equipment is limited primarily by the source impedance, and if the source impedance is too low then the overcurrent devices will operate. The same problem occurs more frequently at the conclusion of a voltage sag, if the source impedance is sufficiently low.
But it's far more common to find a source impedance that is too high.
For an interesting, practical paper on source impedance, see J.M. Russell's paper Optimizing Mains Impedance: Real World Examples.
Bad connections and protective devices increase source impedance.
(Photo courtesy of Bob Dettore)
Here's the general outline of most of my power quality seminars: McEachern Seminar Outline. If you would like to arrange a seminar, please let me know.
Alex McEachern's power quality seminars.
Can you help me organize a series of 1day Power Quality seminars in India, for 2008?
P.Chandrasekar kindly suggested that the following organizations might be interesting sponsors for these seminars:
2008 Power Quality Seminars in India  can you help organize?
With best wishes 
Alex McEachern
