Comment by philipkglass | Hacker News Reader

Comment by philipkglass | Hacker News Reader

I swoop in on something like this looking for the first obvious error in units/arithmetic/materials that renders the whole thing ludicrous, but the author has a spreadsheet and it looks like the units are about right. It's an absurdly cheap cable in terms of materials to transmit 10 GW across an ocean. The main things that render it dubious as a practical matter:

- I don't know if operating at 14 million volts is achievable in terms of converter stations. Today's highest voltage HVDC projects operate at 1.1 megavolts and it took years of development to get there from 0.6 megavolts.

- The mechanical practicality of thousands of kilometers of silica clad aluminum. There's a big mismatch in coefficients of thermal expansion and silica is brittle.

Still, this appears to be facially valid in scientific terms if not in engineering terms. That's impressive! It's a really thin intercontinental cable carrying a lot of power.

The whole thing reminded me of this discussion here from 3 years ago:

https://news.ycombinator.com/item?id=31971039

It has rough numbers for a globe-spanning HVDC cable on the order of a meter in diameter (assumes voltages more like present day commercial HVDC, much thicker conductor to compensate).

There's more to glass than simple silica soda lime formulations.

Glass chemistry is still a dark arcane art on the fringes with discoveries made all the time.

I'm not suggesting either of these are better suited or even equivalent insulaters but they are more flexible than what many think of as glass:

https://cen.acs.org/materials/inorganic-chemistry/glass-isnt...

https://www.corning.com/au/en/innovation/the-glass-age/desig...

Not to forget Pyrex (the original formulation, not the trademark)

> There's a big mismatch in coefficients of thermal expansion and silica is brittle.

The way these are manufactured together means the silica with the lower CTE solidifies first - giving a tube filled with molten aluminium. Next the aluminium solidifies. Then the whole thing cools down and the aluminium probably delaminated from the walls of the tube, leaving a gap of a few hundred micrometers. The aluminium also ends up stretching slightly (one time).

During use, the inner core will heat up and cool down, fairly substantially (perhaps by 100C), using that gap that formed as the cable was manufactured.

Building a circuit breaker that can handle 14 megavolts of DC seems improbable to me.

14MV would be capable of sustaining an arc 1400 feet long in normal atmosphere. I struggle to imagine how you'd build such a thing. You could maybe have a high volume sf6 pump system that would cool and quench the arc on breaker trip with a constantly replenished sf6 supply.

Isn't sf6 on the way out due to it being an extremely potent GHG?

Not sure what the alternative would be for really high voltages? Vacuum insulated switchgear seems to be a hot topic at the moment, but not sure how it'd work with such extreme voltages?

GE has some replacement gas, I'm not sure of the composition, but it isn't as good as SF6 unfortunately.

Even 1.1GV systems use semiconductor breakers. Basically, stacks and stacks of transistors. The actual physical breakers are only operated when the voltage is safely off.

1.1MV?

D'Oh. Of course.

I considered that. Considering the cheap cost of the cable, the best solution appears to simply be 'dont have a breaker'. In either over current or over voltage conditions, simply sacrifice the cable.

Obviously you engineer the convertor stations to minimize the chances of that happening - stopping the convertors automatically if anything looks abnormal. The cable has sufficient capacitance that you have multiple milliseconds to respond, so automated systems should have no difficulty.

> simply sacrifice the cable

How is that different from a fuse?

If I said to build a 3000 kilometer fuse and quench it with the entire Atlantic ocean, people would tell me I was being silly.

But given how expensive the wire will be to lay, what about an actual fuse that's cheaper than laying a whole new wire?

How much cheaper, and what are the odds of needing the fuse?

If it's cost effective then go for it. But the specific thing they're skeptical about is whether a 14MV 750A fuse will be cheap enough.

I believe resistive losses are the primary limiting factor, not insulation.

The higher your voltage, the lower your resistive losses.

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