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At line-frequency skin effect is not significant for almost any practical conductor (ie. reasonably conductive and with reasonable cross-section). What gets significant for long distance transmission is that few thousand kilometer long power transmission line starts behaving like, well, transmission line even on frequencies as low as 50/60Hz. Also HVDC elegantly sidesteps problem of grid synchronization.

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Oh, that is completely not true for commercial AC power transmission -- just look up at a big power pylon and see the multiple power conductors per phase.

See the pretty blue picture: http://en.wikipedia.org/wiki/Skin_effect

Skin effect in copper is about 9mm at 60Hz

That's part of the reason those power lines are steel-cored with copper jackets; the other reason being strength, of course.

They're Aluminium over a steel core, not copper. A copper over steel version would be significantly heavier.

headsmack yes, of course.

I assumed that skin depth at 50/60Hz is on the order of 20mm and that 40mm diameter conductor is not entirely practical to install on pylons (or generally in longer lengths than few meters), obviously I was slightly off in that estimate.

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