Jaques, ON5MJ wrote:
"I am not aware of this but it makes sense that efficiency of AC/DC +
DC/AC conversion must be higher than the use of pure AC transmission."

Yes. It makes no sense to lose more in conversion than on the
transmission line.

The problem with extreme high voltage power transmission is insulator
flashover and corona. Alternating current and voltage have effective
values which are their peak values divided by the square root of two.

This means that peak volts times peak amps divided by two is the same
average power as rms volts times rms amps.

The same average power transmission requires peak values 1.414 times the
rms value, effective value, or d-c value.

In d-c tramnsmission, peak and effective values are the same 100% of the
tiime, so the required d-c voltage is only 0.707 times the a-c voltage
peak for the same power transmission.

Jaques also wrote:
"By definition loads vary all of the time but voltage must not vary
accordingly."

Use of d-c eliminates reactance as a cause of voltage variation. It also
eliminates "skin effect" as an impefance so that the entire
cross-section of the line is used.

The case for extremely high voltage d-c transmission is pretty good.

Best regards, Richard Harrison, KB5WZI