"The Natural Philosopher" wrote in message
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Ah. Yes that makes sense. 20 miles of overhead line would be fine (*) but
20 miles of coaxial cable will have significant capacitance.

Its not coaxial I think, well only in the sense that the sea forms the
outer conductor, together with the armor..

http://www.caledonian-cables.com/pro...es/XLPE-Dc.jpg

is my understanding of a link. I am not sure whether a return cable is
provided or whether the sea does that job

According to https://en.wikipedia.org/wiki/HVDC_Cross-Channel, " there is no
provision to permit neutral current to flow through the sea" and the cable
is bipolar which I interpret as being two wires at +/- half the total
voltage wrt to earth. And "although each station includes an earth
electrode, this is used only to provide a neutral reference, and only one of
the two electrodes is connected at a given time so that there can be no
current flow between them."

I presume if the cables were transmitting AC and were to be widely
separated, the capacitive effects between them could be made negligible but
the capacitive effect wrt earth would not be - and I imagine that water
makes a better dielectric than air, which is why wires strung between pylons
in the air don't suffer the same losses as cables buried in(and insulated
from) water.

Interesting. I'd always thought that the purpose of the DC link was to
isolate the frequency/phase of the UK system from the European system, but
it seems that this is a by-product of the main purpose of avoiding
capacitive losses.

So that's two things I've learned in the last few days:

- why the UK is not AC-linked to Europe

- that even modern clocks may still use the mains frequency as a time source
and that it's not only synchronous motors in mechanical clocks that use
this, and that not all mains digital clocks use their own quartz crystal.