DC Comeback

Edison lost the War of Currents in the 1890s because DC could not be efficiently transformed to high voltage. A century later, power electronics solved that problem, and DC is returning for specific applications where it outperforms AC.

High-voltage direct current (HVDC) transmission eliminates two problems that plague long AC lines. First, AC lines have reactive power losses: the oscillating current creates electromagnetic fields that store and release energy each cycle without doing useful work. DC has no oscillation, so no reactive losses. Second, AC systems must be synchronized in frequency and phase across the entire interconnected grid. DC links between regions transfer power without requiring synchronization, acting as controllable valves between asynchronous grids.

Identify the crossover. HVDC converter stations cost more than AC substations (roughly $200-300 million per terminal). But DC lines are cheaper per mile (two conductors instead of three) and lose less power per mile. For distances above roughly 600-800 km overland (or 50-80 km undersea), HVDC is cheaper overall.

Apply to undersea cables. AC cables longer than about 60 km cannot function because the cable's capacitance absorbs all the reactive power before it reaches the other end. Every undersea power cable longer than this uses HVDC.

Why is DC preferred for connecting solar farms to distant cities?

Long distance plus no synchronization. A 1,000 km HVDC line from a desert solar farm to a coastal city avoids the reactive losses and stability issues of an equivalent AC line, and it can connect grids that operate at different frequencies.

The future grid will likely be a hybrid: AC for local distribution and short-range transmission, DC for long-distance bulk power and undersea connections.

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