Why AC Power Distribution in Data Centers

[GenSwgrEng]

For a long time, I've wondered why data centers don't use DC distribution instead of AC. After all, it takes a lot to bring in AC power from the drip, rectify it to DC for the UPS batteries, invert it back to AC, synchronize AC outputs from multiple UPSs, and distribute AC power to the computers, which are just going to rectify it to DC again. It seems to me that this add unnecessary complexity and inefficiency into the system. Why can't they just distribute electrical power as DC within the data center? The telecom industry uses 48VDC. I'm sure there must be a reason, or they would be doing it. Anyone know why?

Engineers are always honest in matters of technology and human relationships. That's why it's a good idea to keep engineers away from customers, romantic interests, and other people who can't handle the truth.

 

[rasevskii]

Well, in ancient times, DC was the flavor of choice. The standard in the USA was 125/250V or 250/500V 3-wire, center grounded. From a gentleman called Edison...

Nowadays they like to pick standard systems out of catalogs and combine together, to Hell with the heat and losses.

600 and 750 volt DC systems are still used for subways and tramways and no reason not to have in your data center, it is standard equipment off the shelf from many suppliers. But real juicy arcs when there is a fault...

125VDC is still the standard for battery systems in power stations and substations. Off the shelf equipment.

rasevskii

 

[86ed]

It's not a matter of DC vs. AC distribution. It's the generation of your power source. Do you expect to have the Data Center equipped with its own dedicated DC generation systems that would continuously feed the loads as well as the back up DC battery banks? No. This would be highly impractical.

AC power from the utility would be your primary(and maybe secondary)source. As a backup you would have an emergency diesel generator, AC of course, and then finally your DC batter banks and associated DC/AC converters. These batteries are continuously charged by your AC primary source.

 

[rcw retired EE]

In the July 2007 IEEE Transactions on Industrial Applications, Swedish engineers analyzed using DC for commercial buildings, doing exactly what you suggested. They looked at voltage drop, equipment losses, and other factors and concluded DC would be better. They thought 368 VDC was best, because they could reuse most of the existing wiring, which is not applicable to new server rooms.

DC power should be viable. The telephone central offices ran that way for a few decades- utility power backed up by diesel generators all feeding banks of rectifiers charging large battery banks.

Maybe the problem is getting server manufacturer's to use a 48 VDC power source? Is an AC-DC power supply much cheaper than a DC-DC?

 

[umrpwr]

Currently working for a company that specializes in data center design we looked at using DC distribution several times in the past and proposed it for use on different projects, but it has never been accepteded for any of the projects I have worked on. There is still some debate about the efficiencies as well. The newer UPS systems are much more efficient with some using a transfomrerless design. Also the number of voltage transformations in the design are being reduced and higher utilization voltages are being used. In my experience one reason that DC isn't being used is not because of technical reasons, but rather costs. Although DC would be more efficient there are also other tradeoffs including fewer equipment manufacturers which drives the cost. At the voltages that would used for a DC distribution system in a data center it would be comparable to the tranaction systems for trains. AC systems are still the standard. I suspect that you will continue to see the use of higher voltages for disgtribution systems within data centers. The use of 600V systems are now more common as is the elimination of step-down transformers and PDUs. The server loads are now more commonly feed at 400/230Vac. I also have heard of some discussion about using 5kV disctribution to feed directly to a PDU for step-down to 400/230Vac. Anyone who has experience with data centers either as a designer or from the utility perspective as a consumer knows that the power desnities just continue to increase.

Below are a couple of articles by my colleagues which discuss DC distribution for data centers.

http://www.datacenterknowledge.com/archives/2006/07/10/dc-powered-data-centers-sooner-or-later/

 

[Skogsgurra]

Yes Curt - and I was involved in some of those installations. Especially the one in Toreboda where I lectured for Japanese telecom guys that were studying this alternative power distribution technique.

It is a friend of mine, John Akerlund, that pushes this technology. He was head of a power supply research department at Ericsson Systems and is now doing the same work in his own company UPN. A paper can be found here:

http://www.upn.se/html-files/Archive/DC and AC UPS comparison IEEE paper 071220_F.pdf

Annabelle Pratt of Intel has shown interest in the technique. UPN has developed sockets and distribution components - including RCDs for DC - for 300 - 400 V DC systems.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

The NTT sent a large group to study the installation in Toreboda. John to extreme right in picture and me third from right.

http://gke.org/pub/files/DC distribution ELICOM japaner 1.jpg

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

Just to be clear, you cannot use 48VDC for a data center, they use FAR too much power. I was working on SMALL server rack designs for one of the big search engine companies recently. Each rack of 12 blade servers is using 90A at 380V 3 phase, or 19.75kW. If you wanted to do that with 48VDC, that would be 411Amps. That is a lot of current to distribute and switch around inside of a 19 inch rack, and that is just ONE rack. The racks go into a shipping container-like room where the total load is 400kVA, which at 48VDC would be 8,333A!

Data centers use a lot more power than most people think they do.

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