HVDC vs HVAC and HVDC scope in Nepal

HVAC transmits power as three-phase alternating current, while HVDC converts AC to DC at a rectifier, sends it on a DC line and converts back at an inverter. HVAC is cheaper for short and medium distances and easy to tap; HVDC is better for long-distance bulk power, cables and asynchronous links.

Comparison with justification

So HVAC is preferred when distance is below the break-even distance (about 500–800 km for overhead lines, 30–50 km for cables) or when many tappings are needed; HVDC is preferred beyond it.

 Cost
  |          / AC total
  |        /
  |      /    __-- DC total
  |    / __--
  |  /--   (DC starts high:
  |/ |      converter cost)
  +--+---------------------> Distance
   break-even distance

Scope of HVDC in Nepal

1. Power export to India and Bangladesh: Nepal will have large wet-season surplus from big hydropower (Upper Karnali, Arun-3, West Seti, Pancheshwar, Koshi basin). Bulk transfer of several GW to distant Indian load centres (Delhi, Bihar) or to Bangladesh can be cheaper by HVDC.
2. Asynchronous link with India or China: a B2B HVDC station allows controlled power trading without joining the two grids in synchronism, so disturbances in the large Indian grid do not pass to Nepal's weak grid. A link with China (different grid) would need B2B HVDC.
3. Controlled power flow: power scheduling for trade can be set exactly, which suits cross-border contracts.
4. Narrow right-of-way in hills and forests, where land acquisition is hard.
5. Stability of a weak grid: VSC-HVDC can support voltage and give black-start ability.

Limitations in Nepal: high cost of converter stations, technical expertise, the need for large enough power (above about 1000 MW) to justify HVDC, and the fact that most domestic lines are short. So in the near term Nepal's internal grid will remain HVAC (132/220/400 kV), with HVDC most suitable for large export corridors and asynchronous cross-border links.