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DC Power Saves 15% Energy and Cost @ Data Center
Krishna Dagli writes "Engineers at the Lawrence Berkeley National Laboratory and about 20 technology vendors this month will wrap up a demonstration that they said shows DC power distribution in the data center can save up to 15 percent or more on energy consumption and cost. The proof-of-concept program, set up at Sun Microsystems' Newark, Calif., facility, offered a side-by-side comparison of a traditional AC power system and a 380-volt DC distribution system, running on both Intel-based servers and Sun systems."
I always thought the opposite was true. Here is a wiki quote that also supports that:
Taken from http://en.wikipedia.org/wiki/War_of_Currents/
You can also store DC whereas you cannot store AC, meaning UPS always need an AC-DC followed by a DC-AC stage. Since we have had large FET power transistors it has been possible to make DC/DC conversion very efficient - especially since, if you were beginning again, you would not choose 50 or 60 Hz for best efficiency.
In fact, already the PC is using a DC bus to power small peripherals (USB) and it works surprisingly well.
I may be wrong about this, but it was Edison who accused DC power of being more dangerous ("Westinghoused") only to have AC adopted for the pleasant US custom of humanely frying criminals.
Pining for the fjords
In a properly designed DC system, your no more/less safe than your already are.
(Sorry for the repost - I finally remembered my login)
Depending on where you are in the world 3 phase AC is 415V or 480V, and in industry we have no problem handling that. 380 VDC doesn't seem much of an issue to me with regards to insulation safety etc and I have dealt with control panels that have operator controls running at 240VDC (and grabbed them accidently and lived to tell the story) Though now days operator controls are being specced as 24VDC.
.. Bzzt .. Nope. grounding yourself is always an issue with ground referenced power systems. And I would never rely on any power system being perfectly isolated from ground. That sort of misguided thinking leads to nasty surprises.
But as for DC killing you quicker, I would disagree that its the type of system that kills you, it will depend on the type of damage that the shock causes. You can use a 9VDC battery to kill yourself if you apply it in a manner that a small current (mA level) flows to your heart and I would guess that the same level of AC current would also do the trick. On the other hand if you pass a large current through your body that causes physical damage (major burns etc) then it won't matter if its AC or DC if the so much of the body is destroyed as you will die eventually.
As for not worrying about grounding yourself with DC
I am Slashdot. Are you Slashdot as well?
From the article:
(emphasis mine)
The power lost in the cables varies as the resistance of the cable and the current in the cable.
The power delivered to the equipment varies as the current in the cable and the voltage on the cable.
A 380 volt DC system can deliver as much power per unit current in the cables as a 380 volt AC system (assuming a near-unity power factor).
Ergo, the size of the cables for a 380VDC system will be the same as the size of cables for a 380VAC system.
So, if the comparison is against a 240VAC system, then a 380VDC system will have SMALLER cables, not larger. Only if the system being compared against is a 440VAC system will the cables be larger.
Also - a 380VRMS AC system will have a peak voltage of about 540 volts (two significant digits in, two significant digits out), and thus will require MORE insulation than a 380VDC system.
Also - the first things a switching power supply does is rectify the AC into DC and dump it into a capacitor (and usually do power factor correction): so a power supply designed to run from DC needs neither the power factor correction nor the big capacitor (a smaller cap will still be needed, but not one that can carry the system through the bulk of the AC cycle when the voltage is below peak). This makes the power supply simpler, and removes switching losses from the rectifier (granted, a modern synchronous rectifier based on IGBJTs will have a very low loss - but it still is a loss.)
Also - creating a backup for 380VDC is pretty easy - you use a battery bank floated at the 380VDC level. No need to "switch" from mains power to battery - you are ALWAYS running on battery, and the mains power is just charging the battery. This is how the phone company does it - the central office has a bank of batteries providing 48VDC, which is float charged from the mains. Lose mains power, and the system doesn't even blink.
(Yes, you need to have fusing to prevent those batteries from going nuclear if shorted, but that is a much simpler problem to solve than the issues of switching to backup power for an AC system.).
Yes, you have to design the equipment to run off the 380VDC - so you need different power supply front ends: most power supplies are split into 2 parts - the front end that takes mains power and makes about 300VDC on a cap, and the back end that makes the lower voltages from that - so the back end of the power supply does not need to be redesigned. Moreover, most power supplies use an off-the-shelf front end module, and any "magic" is in the back end - so this is NOT a major issue.
www.eFax.com are spammers
277/480VAC power distribution involves 3-phases of current which are 120 degrees out of sync with each other and a forth wire for neutral. In order to get 120VAC, you just need to connect between one of the phases and neutral; you don't need a step-down transformer. The wikipedia article here has a decent discussion:
http://en.wikipedia.org/wiki/Three-phase_power
"The human race's favorite method for being in control of the facts is to ignore them." -Celia Green
So you also failed electrical theory, as well.
DC is harmless unless it has a path to carry it. You can grab a 380 volt DC line and not feel a thing. now if you then touch a grounded object, or the return path you are dead. But you have to make the connection. AC is lethal at 220v. As others posted it does have the advantage of forcing the mucsles to spasm so you can let go of the wire, But still zaps you every time you touch the cable.
Go look up the history of Edison vs Westinghouse. Edison wanted DC power all around because it is inherently safer. a Broken AC wire can zap you, were as a broken DC wire can be touched with bare hands.
i thought once I was found, but it was only a dream.
Copper losses are created by current and are described by the equation I^2 * R. So as you double your current, you quadruple your power losses.
Conversely, if you halve your current by boosting the voltage, you can reduce your transmission losses by 75%. Thats a pretty good reason to go with higher voltage. And since this is in the datacenter, you can train your people not to pee on the red wire.
Remember, You are unique...just like everyone else.
Speaking of conductor sizes, the article said this:
A DC system also would mean having to bring in larger cables than now exist with AC power.
I challenge this notion. Conductor size is not related to whether the power is AC or DC or what frequency of AC it might be; it is related to current.
Larger cables are needed when more current is passed. Traditionally, you need larger cables for DC, because traditionally, DC power systems were lower voltages (12, 24, 48) than AC systems, and these lower voltages required larger currents for same power (e.g. 100W= 830mA at 120V, but 8.3A at 12V). Running at 380V, however, you get to lower the current (excluding the reduced current caused by the 15% power savings) versus a 120V system.
Expanding on that, the reduced conductor size is proportional to the square of the reduced current. Simply by going from 120V to 380V (a factor of 3.17), you change the current flow downward by a factor of .32. This means you can change the cable cross-section area to by a factor of .1; you reduce the cable to one-tenth its original size; one tenth the copper.
www.wavefront-av.com
Talk about common misconception. AC is what grabs you, DC will blow you clear. Haven't you heard the stories about the people with wet hands grabbing something electric and getting electrocuted and not being able to let go etc? All the stories of not being able to let go all occurred with 120V AC current. DC is what they use in lighting systems at TV studios because it is easier and safer to work with "live". It only shocks you if you become a part of the circuit. You have to connect positive to negative. With DC if you only touch positive without being close to the negative nothing happens even at 380V. It doesn't "ground" the same way AC does. With DC you have to actually complete the circuit!
DC blows you free, AC grabs you plain as that and the parent is spreading misinformation I've seen here 100X before.
From http://www.andamooka.org/reader.pl?pgid=liecDCDC_
Note to the wise: Wherever possible, always approach a circuit with the back of your hand. If it is DC, the muscle reaction in case of contact/shock will tend to pull your arm away. If it is AC, same thing will happen. Depending on the voltage present on the conductor, you may even feel the hairs on the back of your hand react to the field produced, i.e., they will 'stand up'.
CPD.
I recommend this website especially the section on Health and Safety before someone gets killed from following electrical safety advice from Slashdot. Some really good advice about lockouts, measuring supposedly dead points 3 times (once to see if its live, once against a known source, and once to make sure your meter wasn't faulty the first time) and making first contact using the back of your hand.
No trees were harmed in the posting of this message. However, a great number of electrons were terribly inconvenienced.
That 48vdc comes from the central office, where thousands of amps of it are used to power the switch, all the transport gear, and most of the auxilliary equipment. (Air conditioning is all AC powered, but everything else runs from the central DC plant.)
The power conductors in central offices are oversized out of paranoia, and because sometimes you have a foot-thick pileup of power cables leaving a fuse bay and you want to make really sure resistive heating is negligible. Also, most equipment has redundant power feeds, A and B, but either feed is large enough to handle the entire load. During normal operation when both sides are sharing the load, the resistive drop in the wires is absurdly low.
The other advantage of 48v is that it's below the 50v "low voltage" standard in the NEC, which means it's easier, legally, to work with. The 300-plus voltage they're using in this study loses that advantage.
Also consider this: AC voltage and power are measured RMS, but the insulation has to withstand P-P voltage. So to deliever the same power on the same conductors, the DC system's insulation has a greater margin of safety.
It's really easy to create DC. Just take a DC motor and spin the axle.
A datacenter takes 3-phase 440VAC in, which goes directly into the backup power system. This converts the the AC into DC to be fed into the batteries, then the batteries are fed into a DC-AC converter to put out 60Hz sine wave AC. The AC from that converter then gets distributed to each computer. Each computer in turn takes that AC and converts it into 12/5/3.3VDC. Unfortunately all those AC-DC converters sitting in each computer are unnecessarily inefficient. By eliminating the DC-AC-DC steps, it's possible to make things much more efficient. Simply take the 380VDC from the batteries, and convert it to lower voltage at the computers. Of course there's loss in the DC-DC converter, but it's much less than the standard DC-AC-DC because it uses a high-frequency square wave instead of low-frequency sine wave AC.
dom
Ummm....No.
1) 480 3 phase can be 3 wire or 4 wire. 3 wire is called Delta (floating ground or one of the legs can be tied to ground). 4 wire is called Y (typically the 4th wire is the "center" of the Y and is grounded.
2) You get 277 VAC reference to ground with Y. You get nothing stable with Delta floating. And you get 480 VAC or 0 VAC with Delta one leg grounded. I'm not advocating one way or the other, it depends on a lot of things which configuration you choose.
3) Either way, to get 120 VAC, you have to use a transformer to reduce the voltage. Phasing is not adusted. If you have 480 VAC 3 phase, you'll get 120 VAC 3 phase, though that's misleading because you always connect 120 VAC 3 phase in the Y configuration and measure 120 reference to ground, not phase to phase. The actual number phase to phase is some weird number I can never remember like 177 or something.
4) Corrolary to #3, in a home system, you have 240 VAC which is really only two 180 degree phased 120 VAC lines. To get 240 VAC 3 phase you need a specific transformer which will have seperate taps to provide 120 VAC if you so desire (or just use two different transformers to achieve it).