HVDC transmission

HVDC is the surprising comeback of direct current for the one job everyone thought AC had won forever: hauling vast amounts of power across enormous distances. Edison's DC lost the 'war of the currents' because AC transforms voltage so easily; but for a 2,000-kilometre line, AC has a fatal flaw — its alternating field forces the cable to charge and discharge like a giant capacitor, wasting power, especially underground or undersea where AC simply cannot go beyond about 80 km. HVDC sidesteps all of this by sending steady, non-alternating power down the wire.

The catch is the price of admission: you need converter stations at each end — enormous halls of power electronics that turn AC into high-voltage DC and back again — costing hundreds of millions of dollars. So HVDC only wins past a 'break-even distance' of roughly 600 km overhead or 50 km undersea, beyond which its lower line losses repay the converter cost. This is why HVDC threads China's deserts to its coastal cities, links the UK to Norway under the North Sea, and stitches together grids running at different frequencies that could never be directly joined with AC.