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Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?

jgh

All AC circuits have resistance, inductance and capacitance.
With long cables, the resisatance and capacitance effects become more
important.

Capacitance depends on the nature of the insulation, the surface area of the
conductors and the distance apart they are.
With overhead cable, the distance apart is much greater than with
underground cables.
So capacitance is much more with the latter.
Also the permitivity of air is much less than plastic insulation.