Re: Interesting possibilities
>> AC is better as it can transfer higher power over longer distances
>I know that is true of overhead power lines, so curious why subsea interconnects are usually DC- DC..
>I can partly guess as it is unlikely that UK/France are synchronised grids (!) and there is some fancy interaction between AC cables laid close to each other (you have to de-rate them)
AC is not better over long distances. The frequency limits the maximum length of power lines, Light (and electrons) is slow in this context. And the constant change in current direction and strength generates a magnetic field that adds to the power loss of the resistance of the lines and being an inductor further restricts the maximum length of a line. Add to this the long lines being (bad) big capacitors that constantly have to be charged and discharged. This makes very long AC lines not a viable option.
Long distance high power lines are preferred DC lines. And very long distance must be DC, because the would be no power coming out of the end of the line.
It depends, what you call long distance. Above several hundred km the technical burden / cost of generating high voltage DC and regenerating AC from that is less than the losses on the AC line. And if you connect grids, you avoid possible long distance harmonics.
Even middle distance AC networks are an art to prevent blackouts from harmonics. See the Spanish blackout this year, that started with failing production capacity and then two generators in different locations going into a pendulum catastrophe, after that all dominoes fell.