In NZ the "DC link was upgraded about 10 years ago from +/- 500kV to
+/-750kV IIRC.
This link runs from the Southern Area of the South Island to the South end
of the North Island  - a distance of maybe 600km (?) including an undersea
section. The link is bi-directional but, due to the population & energy
requirements, is pretty much entirely used in the South - North direction.

Corona Discharge is visible at times - as is a slight "hair raising"
feeling when putting out on one of the local golf courses - with the
transmission line appearing to be far too close overhead! (Could be an
imagined effect however).

Voltage conversion is carried out by solid stae devices (thyristors (?)) at
both ends.

The link also carries fibre optic communication cable - used by the power
transmission people & with additional capacity sold/rented to other users.

All the above subject to correction.

Richard P









>> I don't think any country uses DC for power transmission.
>
>Except for a couple 0.5MV systems, you're right.

DC is used for very long transmission lines. The wavelength at 50Hz is
6000km and if any line segment approaches (even by far) one quarter of
this (1500km) then a significant part of the energy will be radiated into
space. Also dielectric loss in air becomes very high over such distances.
So people who have really long overland line segments use very high DC
voltages. What I was intrigued by was, how does one convert 0.5MV DC
*down* (up is relatively easy) to something reasonable, like 110kV. Are
they using valves ? If they do, are they buried in lead ? (500kV + vacuum
electrons = hard x-rays).  What semiconductors would work at 500kV and
several amps ? (2A at 500kV is 1MW.  - wow). What happens to birds that
sit on the wire ? (corona at 500kV is fierce). What about lightning. The
wires must be really high up. 500kV would jump 0.5 meter gaps w/o corona
but much more with ? Uhh.

Peter

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