[shdudns]

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 need for niche or custom parts.

- 3 phase @ 400Hz is x6 = 2.4kHz. Six diodes will rectify it with almost no relevant amount of ripple (Vmin is 87% of Vmax) and very small caps will smooth it.

As an aside, with three (or more) phase you can use multi-tap transformers and get an arbitrary number of poles. 7 phases at 400Hz -> 5.6kHz. Your PSU is now 14 diodes and a ceramic cap.

- you still get to use step up/down transformers, but at 400Hz they're very small.

- merging power sources is a lot easier (but for the phase angle)

- DC-DC converters are great, but you're not going to beat a transformer in efficiency or reliability

[adamking]

> no significant skin effect at 400Hz -> use speaker wire, lol

now run that unshielded wire 50 meters past racks of GPUs and enjoy your EMI

> The 400Hz distribution industry is massive; the entire aerospace industry runs on it

nothing in that catalog is rated for 100kW–1MW rack loads at 800Vrms

> 3 phase @ 400Hz is x6 = 2.4kHz... Your PSU is now 14 diodes and a ceramic cap

you still need an inverter-based UPS upstream, which is the exact conversion stage DC eliminates

> large voltage/current DC breakers are.. gnarly, and expensive. DC does not like to stop flowing

SiC solid-state DC breakers are shipping today from every major vendor

> DC-DC converters are great, but you're not going to beat a transformer in efficiency or reliability

wide-bandgap converters are at 95%+ with no moving parts

[shdudns]

"now run that unshielded wire 50 meters past racks of GPUs and enjoy your EMI"

Multipole expansion scales faster than r^2.

Also, im not in the field (clearly) but GPUs cant handle 2.4 kHz? The quarter wavelength is 30km.

"nothing in that catalog is rated for 100kW–1MW rack loads at 800Vrms"

Current wise, the catalog covers this track just fine. As to the voltages, well that's the whole point of AC! The voltage you need is but a few loops of wire away.

"you still need an inverter-based UPS upstream, which is the exact conversion stage DC eliminates"

So keep it? To clarify, this is the "we're too good for plebeian power, so we'll transform it AC->DC->AC", right?

"SiC solid-state DC breakers are shipping today from every major vendor"

Of course they do. They're also pricey, have limited current capability (both capital costs and therefore irrelevant when the industry is awash with GCC money) and lower conduction, and therefore higher heat.

They're really nice though.

"wide-bandgap converters are at 95%+ with no moving parts"

transformers have no moving parts. Loaded they can do 97%+ efficiency, or 2MW of heat eliminated on a 100MW center.

[prezk]

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.

[bigiain]

> I realized I may be talking to a serious audiophile, didn't mean to disrespect your Monster cables.

I am a recovering audiophool.

I do own a pair of 2m long Monster Cable speaker cables (with locking gold plated banana plugs). I am fairly certain I've used welders with smaller cables.

(In my defence, I bought those as a teenager in the late 80s. I am not so easily marketed to with snake oil these days. I hope.)

(On the other hand, I really like the idea of a reliably stable plus and minus 70V or maybe 100V DC power supply to my house. That'd make audio power amplifiers much easier and lighter...)

[shiroiuma]

>- 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.

[shdudns]

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.

[bigiain]

If you could get that 100,000Amps flowing through your speaker wire, the vaporised copper and the plasma channel would probably keep your 100MW flowing, at least until your building caught fire.

[jacquesm]

Even your monster cable? ;)

[bigiain]

Well, it'd still vaporise, but it'd sound smoother and more musical as it did it, and the soundstage from the plasma arc would be _stunning!_

[jacquesm]

It's almost worth the experiment and your cables are a sacrifice I'm willing to make. For science, of course.