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Data centers are transitioning from AC to DC

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Re: Data centers are transitioning from AC to DC

#21

Post by cogman10 »

sghiassy wrote: post #13

I’ve always wondered about these new High-Voltage DC (HVDC) transmission lines. I always thought AC’s primary benefit was its transmission efficiency?? Would love to learn if anyone knows more about this

The primary benefit of AC is it's really easy to change the voltage of AC up or down.

The transmission efficiency of AC comes from the fact that you can pretty trivially make a 1 megavolt AC line. The higher the voltage, the lower the current has to be to provide the same amount of power. And lower current means less power in line loss due to how electricity be.

But that really is the only advantage of AC. DC at the same voltage as AC will ultimately be more efficient, especially if it's humid or the line is underwater. Due to how electricy be, a change in the current of a line will induce a current into conductive materials. A portion of AC power is being drained simply by the fact that the current on the line is constantly alternating. DC doesn't alternate, so it doesn't ever lose power from that alternation.

Another key benefit of DC is can work to bridge grids. The thing causing a problem with grids being interconnected is entirely due to the nature of AC power. AC has a frequency and a phase. If two grids don't share a frequency (happens in the EU) or a phase (happens everywhere, particularly the grids in the US) they cannot be connected. Otherwise the power generators end up fighting each other rather than providing power to a load.

In short, AC won because it it was cheap and easy to make high voltage AC. DC is comming back because it's only somewhat recently been affordable to make similar transformations on DC from High to low and low to high voltages. DC carries further benefits that AC does not.

Re: Data centers are transitioning from AC to DC

#22

Post by throw0101d »

umvi wrote: post #17

I don't understand why new houses don't just have one high quality AC/DC converter so you can just use LED lighting without every bulb needing its own AC/DC converter. I imagine the light bulb cartel wouldn't really like that.

> I don't understand why new houses don't just have one high quality AC/DC converter so you can just use LED lighting without every bulb needing its own AC/DC converter.

IEEE 802.3bt can deliver up to 71W at the destination: just pull Cat 5/6 everywhere.

* https://en.wikipedia.org/wiki/Power_over_Ethernet#Standard_i...

* https://www.usailighting.com/poe-lighting

Re: Data centers are transitioning from AC to DC

#24

Post by stego-tech »

otterley wrote: post #10
stego-tech wrote: post #8

I've been hearing this line for over a decade, now. "Immersion cooling will make data centers scale!" "Converting to DC at the perimeter increases density!" Yes, of course both of those things are true, and yes, some data centers do engage in those processes for their unique advantages. The issue is that aside from specialty kit designed for that use (like the AWS Outposts with their DC conversion), the rank-and-file…

Those vendors all have DC power supply options, to my knowledge. It’s hardly new; early telco datacenters had DC power rails, since Western Electric switching equipment ran on 48VDC. https://www.nokia.com/bell-labs/publications-and-media/publi...

That’s just it though, telco DCs != Compute DCs. Telcos had a vested interest in DC adoption because their wireline networks used it anyway, and the fewer conversions being done the more efficient their deployments were.

Every single DC I’ve worked in, from two racks to hundreds, has been AC-driven. It’s just cheaper to go after inefficiencies in consumption first with standard kit than to optimize for AC-DC conversion loss. I’m not saying DC isn’t the future so much as I’ve been hearing it’s the future for about as long as Elmo’s promised FSD is coming “next year”.

Re: Data centers are transitioning from AC to DC

#25

Post by shdudns »

shiroiuma wrote: post #6
shdudns wrote: post #3

How is DC better than a three phase delta 800Vrms, at 400Hz? - Three conductors vs two, but they can be the next gauge up since the current flows on three conductors - no significant skin effect at 400Hz -> use speaker wire, lol. - large voltage/current DC brakers are.. gnarly, and expensive. DC does not like to stop flowing - The 400Hz distribution industry is massive; the entire aerospace industry runs on it. No ne…

>- no significant skin effect at 400Hz -> use speaker wire, lol. What are you talking about? There's a very significant skin effect at 400Hz. Skin effect goes up with frequency. These datacenters use copper busbars, not cable, so skin effect is an important consideration.

At 100 000 A for a 100 MW data center at 1000 V, speaker wire is a joke.

You obviously need at least a dozen stands in parallel!!

Clearly skin effect scales with frequency but, 400 Hz is still low, only 2.5x lines frequency (the scale is by the root); so the skin depth is 3mm. 3mm on each side makes for a pretty hefty rectangular cross-section.

Re: Data centers are transitioning from AC to DC

#26

Post by bandrami »

adrr wrote: post #23

Our houses should be DC. So wasteful to have all these bricks to change to AC to DC.

I've worked overseas a lot and one thing that's really different from 2 decades ago is that I simply don't need a step-down transformer anymore because every single thing I plug in converts to DC (or otherwise accepts dual-voltage) anyways. So I have a giant collection of physical plug adapters because every device I use just needs to fit into the socket and takes care of it from there.

(My stand mixer is the lone sad exception)

Re: Data centers are transitioning from AC to DC

#27

Post by fortran77 »

umvi wrote: post #17

I don't understand why new houses don't just have one high quality AC/DC converter so you can just use LED lighting without every bulb needing its own AC/DC converter. I imagine the light bulb cartel wouldn't really like that.

Do you want your house to burn down? Lower voltages for LED lights mean higher current.

Re: Data centers are transitioning from AC to DC

#28

Post by ericd »

adrr wrote: post #23

Our houses should be DC. So wasteful to have all these bricks to change to AC to DC.

Something to consider, and something I got a vivid demonstration of while playing with solar panels, DC arcs aren't self-extinguishing, unlike AC arcs. At one point I stuck a voltage probe in, and the arc stuck with it as I pulled the probe away. It also vaporized the metal tip of the probe.

My understanding is that DC breakers are somewhat prone to fires for this reason, too.

Re: Data centers are transitioning from AC to DC

#29

Post by kccqzy »

adrr wrote: post #23

Our houses should be DC. So wasteful to have all these bricks to change to AC to DC.

That’s actually a recent phenomenon. Before the age of electronics most household appliances either worked with AC or DC equally well (like incandescent bulbs) or worked well with AC only given the technology at the time (think anything with a motor, fans, HVAC compressors etc).

Re: Data centers are transitioning from AC to DC

#30

Post by Majromax »

umvi wrote: post #17

I don't understand why new houses don't just have one high quality AC/DC converter so you can just use LED lighting without every bulb needing its own AC/DC converter. I imagine the light bulb cartel wouldn't really like that.

With modern technologies, that's power over ethernet or USB-C. Other comments in this thread point out that the telephone service also routinely used 48V for the ring signal.

However, higher DC voltage is riskier, and it's not at all standard for electrical and building code reasons. In particular, breaking DC circuits is more difficult because there's no zero-crossing point to naturally extinguish an arc, and 170V (US/120VAC) or 340V (Europe/240VAC) is enough to start a substantial arc under the right circumstances.

Unfortunately for your lighting, it's also both simple and efficient to stack enough LEDs together such that their forward voltage drop is approximately the rectified peak (i.e. targeting that 170/340V peak). That means that the bulb needs only one serial string of LEDs without parallel balancing, making the rest of the circuitry (including voltage regulation, which would still be necessary in DC world) simpler.

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