Why use hvdc system?

Answer 1

In New Zealand we have a HVDC system to carry power from generators in the lower South Island, and across Cook Strait (c40km) to the North Island where the main demand is. Two 'wires' one +250kV and the other -250kV. Earth as a backup conductor. Three cables across Cook Strait, to allow for a spare, which has indeed been needed. The power transferred is about 1200MW.

The rectifiers used to be Mercury-arc, but these have now been almost completely replaced by Thyristors.

HVDC is very important for the underwater leg, as an AC voltage needed for the same power would need to be over 350kV, a more serious insulation demand. And of course, the same comment applies to the porcelain insulators needed on the overland leg - some 570km.

There are accessory problems of course - radiated noise for example, for the conversion is not to completely smooth DC. (Imagine the Capacitors that would require!) So the 'DC' contains some of the original sine wave, and this waveform is re-assembled at the North island terminal.

Answer 2

Although EHV reduces resistive losses, it still suffers from inductive and capacitive losses. There are no inductive and capacitive transmission losses with HVDC. However, HVDC requires special equipment to step up and rectify the power at the generator and invert and step down the power at the load end substation.

Answer 3

HVDC systems are used for long-distance power transmission because they are more energy-efficient and can transmit larger amounts of power over longer distances with lower losses compared to AC systems. HVDC also allows for better control of power flow, improves grid stability, and makes it easier to interconnect power systems that operate at different frequencies or voltages.