HVDC Breakeven and Projects

HVDC lines require expensive converter stations at each end that AC lines do not need. But HVDC lines lose less energy per mile. At some distance, the lower losses pay for the converter stations. That crossover distance determines when HVDC makes economic sense.

The breakeven distance is typically 300-500 miles for overhead lines and 30-50 miles for submarine cables (where AC's reactive power losses are much higher). Below the breakeven distance, the converter station cost makes HVDC uneconomic. Above it, HVDC's lower losses per mile accumulate into savings that exceed the converter cost.

Three major HVDC applications dominate globally: long-distance bulk power transfer (China's 3,300 km Ultra-HVDC lines carrying hydropower from the west to coastal cities), submarine interconnectors (the NordLink cable connecting Norway and Germany under the North Sea), and asynchronous grid ties (the back-to-back converters connecting the Eastern, Western, and Texas interconnections in the US, which cannot be linked by AC because they operate at different frequencies).

China's Ultra-HVDC. The Changji-Guquan line carries 12 GW over 3,300 km at 1,100 kV DC. Losses: roughly 5% over the full distance.

What would the losses be on an equivalent AC line?

Roughly 10-15%. AC losses include both resistive and reactive components, and reactive losses grow with distance. On a 3,300 km run, AC would lose 2-3x more energy. At 12 GW, each percentage point of avoided loss is worth hundreds of millions of dollars per year.

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