On 27/07/14 22:33, Nightjar "cpb"@ insert my surname here wrote:
On 27/07/2014 22:24, newshound wrote:
On 27/07/2014 21:40, wrote:
Nightjar wrote:
AC for simple long-distance transmission...
Except for underwater cables, where it can cause unacceptable
transmission losses.

How does immersing a 11kV AC cable in water increase transmission
losses? This isn't a joke question, I can't see how the medium
surrounding a cable changes the action of the cable itself, other
than cooling effects.

jgh

It's losses in the dielectric, so it applies to underground cables too.
Dielectric losses in air are low.

Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, which is
why our links to the continent are HVDC.

I thought it was more the issue that you cannot phase lock France and
the UK? So DC is a natural choice if you have to re-invert it. And if
you have to do that, might as well transmit in DC too.

Not saying your reason is not a good reason - I just thought it was a
secondary reason to a fairly immutable primary problem.

Even then, the cables are very
specialised and there are IIRC only three manufacturers of HVDC
underwater cables in the world. That is a limiting factor on how many
offshore wind farms can be built, as they need HVDC underwater cable,
the manufacturers have a limited capacity and well filled order books,
while increasing capacity is not something that can be done overnight.