11 ms·

I might misremember, but aren't some of the submerged sea power cables (Norway, Germany, UK) DC? If so, why? And how does that rhyme with AC being more efficien

I might misremember, but aren't some of the submerged sea power cables (Norway, Germany, UK) DC? If so, why? And how does that rhyme with AC being more efficient?

junon 3y ago
Disclaimer that I'm not an expert. As far as I understand, AC runs on the outside of the cable rather than through the conductor. DC does not. That's one of several reasons why.
- deleted 3y ago
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cfn 3y ago
I don't know if it is the same issue but AC lines longer than a certain distance will approach the wave length of an antena and start to radiate energy (like a radio broadcast antena). In that case you use DC. When I studied this back in school I was told there was such a line in South Africa (if I recall correctly).
joneholland 3y ago
There’s a dedicated DC trunk line running down the entire west coast. It’s how hydropower generated in Washington state is sold to California. High voltage DC to bridge grids is really common. https://en.m.wikipedia.org/wiki/Pacific_DC_Intertie https://en.m.wikipedia.org/wiki/Pacific_DC_Intertie
wesleyd 3y ago
My understanding here is that the saving comes from needing fewer cables, although I’m sure there’s a lot more to it than that.
ben_w 3y ago
A whole bunch of different effects. AC/DC have different costs for voltage converters and per-distance efficiency, so there are some distances where AC makes more sense and others where DC makes more sense; the distance changes as tech improves. AC mostly conducts on the outer surface of the wire, while DC conducts with the whole cross section, giving you different scaling issues as the current changes. Under water, AC suffers from significant capacitive loss — the wire acts as one side of a capacitor and the entire ocean as the other. At certain frequencies and wire lengths you also get inductive losses, though IIRC that affects only RF cables in practice and and the design of the trans-Siberian railway in theory as no other place even seriously considered having a sufficiently long conductor for the frequency used.
tuetuopay 3y ago
As others mentioned, undersea cables suffer from capacitive losses. But another big factor is grid synchronisation: two AC grids cannot be joined if they are not perfectly synced. Their phase must match. DC having no such phase, it’s very useful to connect independent grids (e.g. UK to the rest of the EU, or the three main grids in the US)

I might misremember, but aren't some of the submerged sea power cables (Norway, Germany, UK) DC? If so, why? And how does that rhyme with AC being more efficient?

junon 3y ago

Disclaimer that I'm not an expert. As far as I understand, AC runs on the outside of the cable rather than through the conductor. DC does not. That's one of several reasons why.

deleted 3y ago
[deleted]

cfn 3y ago

I don't know if it is the same issue but AC lines longer than a certain distance will approach the wave length of an antena and start to radiate energy (like a radio broadcast antena). In that case you use DC. When I studied this back in school I was told there was such a line in South Africa (if I recall correctly).

joneholland 3y ago

There’s a dedicated DC trunk line running down the entire west coast. It’s how hydropower generated in Washington state is sold to California. High voltage DC to bridge grids is really common. https://en.m.wikipedia.org/wiki/Pacific_DC_Intertie https://en.m.wikipedia.org/wiki/Pacific_DC_Intertie

wesleyd 3y ago

My understanding here is that the saving comes from needing fewer cables, although I’m sure there’s a lot more to it than that.

ben_w 3y ago

A whole bunch of different effects. AC/DC have different costs for voltage converters and per-distance efficiency, so there are some distances where AC makes more sense and others where DC makes more sense; the distance changes as tech improves. AC mostly conducts on the outer surface of the wire, while DC conducts with the whole cross section, giving you different scaling issues as the current changes. Under water, AC suffers from significant capacitive loss — the wire acts as one side of a capacitor and the entire ocean as the other. At certain frequencies and wire lengths you also get inductive losses, though IIRC that affects only RF cables in practice and and the design of the trans-Siberian railway in theory as no other place even seriously considered having a sufficiently long conductor for the frequency used.

tuetuopay 3y ago

As others mentioned, undersea cables suffer from capacitive losses. But another big factor is grid synchronisation: two AC grids cannot be joined if they are not perfectly synced. Their phase must match. DC having no such phase, it’s very useful to connect independent grids (e.g. UK to the rest of the EU, or the three main grids in the US)