HVDC for long distance and B2B significance

For long-distance bulk power transmission, HVDC is preferred because beyond the break-even distance it is cheaper, has lower losses and has no stability limit.

Why HVDC for long-distance bulk power

1. Lower line cost: a bipolar DC line needs 2 conductors instead of 3, uses lighter and cheaper towers, and needs narrower right-of-way. For the same insulation, DC working voltage can be close to the AC peak, so more power per conductor.
2. No stability limit: AC power $P = \frac{V_sV_r}{X}\sin\delta$ falls as $X$ grows with length, and the line needs series compensation. DC has no reactance or power angle, so any distance is possible.
3. No reactive power or charging current in the line: there is no Ferranti effect and no need of shunt reactors along the line.
4. Lower losses: no skin effect, no reactive current, lower corona loss. For long lines the saving in losses pays for the converters.
5. Break-even distance: terminal cost is high for HVDC but line cost per km is lower. The total costs cross at about 500–800 km for overhead lines (30–50 km for cables); beyond this HVDC is cheaper.

 Cost |        / AC
      |      /
      |    /   ___-- DC
      |  / __--
      |/--
      +---|------------> Length
          break-even

6. Fast power control: converters change power in milliseconds, which helps stability of both AC systems and allows exact scheduling.
7. No increase in fault level of the connected AC systems.
8. Long cables possible: for routes with sea crossings.

Examples: Three Gorges–Shanghai (±500 kV, about 1000 km), Xiangjiaba–Shanghai (±800 kV, about 2000 km), Rihand–Delhi (±500 kV) in India.

Significance of back-to-back (B2B) HVDC

In a B2B link the rectifier and inverter are in the same station with no DC line. Its significance:

• Connects two asynchronous grids (different frequency, or not in step), e.g. 50/60 Hz systems in Japan, and earlier the regional grids of India.
• Gives controlled power exchange for trading.
• Prevents transfer of faults, frequency disturbances and oscillations between grids; does not raise fault level.
• Low DC voltage and high current lower the valve cost.