HVDC Converter Technology

The newer, more flexible converter technology cannot handle the largest HVDC projects on earth. That job still belongs to 1970s thyristor technology.

HVDC links require converter stations to translate between AC and DC at each end. Two semiconductor technologies do the job with fundamentally different tradeoffs. Line-commutated converters (LCC) use thyristors: older, cheaper per megawatt, handling up to 12 GW on a single link, but they depend on the connected AC grid to switch off their thyristors and cannot operate into a weak or islanded grid. Voltage-source converters (VSC) use IGBTs: newer, self-commutating, able to work with weak grids and renewable sources, but more expensive and lower maximum power.

Choosing a converter for the context. Two projects: a 3,000-mile HVDC backbone linking a mature Chinese coal grid to a major load center, and a 200-mile offshore wind farm connection to a coastal grid with limited AC strength.

Which technology fits which project, and why?

It depends on what the grid looks like at each end. The backbone uses LCC: enormous power volumes, strong synchronous grids at both ends, cost-per-megawatt advantage at scale. The offshore wind connection uses VSC: the wind farm cannot provide the short-circuit current LCC needs, and the coastal interconnection may also be electrically weak. VSC's self-commutating switches work regardless of grid strength.

As grids add more inverter-based generation (which weakens short-circuit strength), VSC becomes the default for new projects, a policy-relevant shift because VSC links cost more.

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