Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Data centers are transitioning from AC to DC

spectrum.ieee.org

Post reply on HN
Print view View original

31–40 of 432 posts

Re: Data centers are transitioning from AC to DC

#31

Post by torginus »

adrr wrote: post #23

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

I've had discussed with people familiar with the matter, and they convinced me its really not worth it for many reasons, the main one being safety - DC arcs are self sustaining - AC voltage constantly goes to zero, so if an arc were to form, it gets auto extinguished when the voltage drops. With DC this never happens, meaing every switch or plug socket can create this nice long arcs and is a potential fire hazard.

Re: Data centers are transitioning from AC to DC

#32

Post by prezk »

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…

An advanced AI rack might use 100kW = 800V 125A, requiring gauge 2, quarter inch diameter---this isn't your lol speaker wire. Actually, I apologize, I realized I may be talking to a serious audiophile, didn't mean to disrespect your Monster cables.

The skin depth by the way is sqrt(2 1.7e-8 ohm m / (2 pi 400Hz mu0))=~3mm for copper---OK for single rack, but starts to be significant for the type of bus bars that an aisle of racks might want.

As for efficiency, both 400Hz transformers AND fancy DC-DC converters are around 95% efficient, except that AC requires electronics to rectify it to DC, losing another few percent, so the slight advantage goes to DC, actually.

As for merging power, remember that DC DC converter uses an internal AC stage, so it's the same---you can have multiple primary windings, just like for plain AC.

Re: Data centers are transitioning from AC to DC

#33

Post by fragmede »

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.

because shorts and voltage loss are a real issue at that scale.

Re: Data centers are transitioning from AC to DC

#34

Post by analog31 »

kccqzy wrote: post #29
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).

Taking it to an extreme, the house I lived in while in grad school had wall lamp fixtures that doubled as electric and gas lamps. At some point I imagine it would have been possible to choose between using electric or gas by either flipping the switch or turning a valve. They said "Edison Patent" on them. We could have lit the house on AC, DC, or gas.

Thinking about the failure modes gave me the heebie jeebies, but the gas had been disconnected ages prior.

Re: Data centers are transitioning from AC to DC

#35

Post by mjuarez »

bluGill wrote: post #16
otterley wrote: post #7

DC power has been an option for datacenter equipment since I was a young lad racking and stacking hardware. Cisco, Dell, HPE, IBM, and countless others all had DC supply options. Same with PDUs. What’s old is new again. See e.g. https://www.dell.com/support/kbdoc/en-us/000221234/wiring-in...

48vdc was common in phone exchanges. They filled the basement with lead-acid batteries and to could run without the grid for a couple weeks. In turn the phone was 99.999% reliable for decades.

Not to be _that_ guy, but it was technically -48V DC.

Honestly, that was pretty surprising to me when I had to work with some telco equipment a couple of decades ago. To this day, I don't think I've encountered anything else that requires negative voltage relative to ground.

Re: Data centers are transitioning from AC to DC

#36

Post by divbzero »

bluGill wrote: post #16
otterley wrote: post #7

DC power has been an option for datacenter equipment since I was a young lad racking and stacking hardware. Cisco, Dell, HPE, IBM, and countless others all had DC supply options. Same with PDUs. What’s old is new again. See e.g. https://www.dell.com/support/kbdoc/en-us/000221234/wiring-in...

48vdc was common in phone exchanges. They filled the basement with lead-acid batteries and to could run without the grid for a couple weeks. In turn the phone was 99.999% reliable for decades.

Interesting, so this is why the phone line still worked when power was out across the whole town.

Re: Data centers are transitioning from AC to DC

#37

Post by bigiain »

adrr wrote: post #23

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

Sure, maybe?

If your house gets 800V DC you're still gonna need "bricks" to convert that to 5VDC of 12VDC (or maybe 19VDC) that most of the things that currently have "bricks" need.

And if your house gets lower voltage DC, you're gonna have the problem of worth-stealing sized wiring to run your stove, water heater, or car charger.

I reckon it'd be nice to have USB C PD ports everywhere I have a 220VAC power point, but 5 years ago that'd have been a USB type A port - and even now those'd be getting close to useless. We use a Type I (AS/NZS 2112) power point plug here - and that hasn't needed to change in probably a century. I doubt there's ever been a low voltage DC plug/socket standard that's lasted in use for anything like that long - probably the old "car cigarette lighter" 12DC thing? I'm glad I don't have a house full of those.

Re: Data centers are transitioning from AC to DC

#38

Post by arijun »

adgjlsfhk1 wrote: post #19
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

AC is less efficient than DC at a given voltage. The advantage of AC is that voltage switching is cheap, easy and efficient. Switching DC voltage is way harder, more expensive, and less efficient. However the switching costs are O(1) and the transmission losses are O(n) so for some distance (currently somewhere around 500 km) it's worth paying the switching cost to get super high voltage DC. The big thing that's chan…

> AC is less efficient than DC at a given voltage

To expand on this, a given power line can only take a set maximum current and voltage before it becomes a problem. DC can stay at this maximum voltage constantly, while AC spends time going to zero voltage and back, so it's delivering less power on the same line.

Re: Data centers are transitioning from AC to DC

#39

Post by bigiain »

ericd wrote: post #28
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.

Heh - I vaporised a fairly large soldering iron tip (probably 4mm copper cylindrical bar?), when I fucked up soldering a connector to a big 7 cell ~6000mAHr LiPo battery and shorted the terminals. Quite how I didn't end up blind or in hospital I don't know. It reinforced just how much respect you need to pay to even low-ish voltage DC when the available current was likely able to exceed 700A by a fair margin momentarily. I think those cells were rated at 60C continuous and 120C for 5 seconds.

Re: Data centers are transitioning from AC to DC

#40

Post by prezk »

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

Important factor is that AC at given nominal voltage V swings between 1.41V and -1.41V, so it requires let's say 40% better/thicker insulation than the equivalent V volts DC line. This is OK for overhead lines (just space the wires more) but is a pain for buried or undersea transmission lines; for that reason, they tend to use DC nowadays.

BTW, megavolt DC DC converters are a sign to behold: https://en.wikipedia.org/wiki/File:Pole_2_Thyristor_Valve.jp...

Post reply on HN