AC-DC Crossover Distance
Prerequisites
AC is cheaper to build at each end (transformers are simpler than converter stations). DC loses less power per mile (no reactive losses, no skin effect). At some distance, DC's lower line losses overcome its higher terminal costs. This crossover distance determines which technology wins for any given project.
The economics split into two components. Terminal costs: AC substations cost far less than HVDC converter stations ($50-100M vs. $200-500M per terminal). Line costs per mile: HVDC lines lose roughly 3% per 1,000 km versus 6-8% for equivalent AC lines, and HVDC requires fewer conductors (2 vs. 3) and narrower rights-of-way.
Find the crossover. For overhead lines, the breakeven distance is roughly 500-800 km (300-500 miles). Below that distance, AC's lower terminal costs dominate. Above it, DC's cumulative line savings exceed the terminal cost premium.
Why is the crossover distance much shorter for submarine cables?
Submarine cables change the math. AC submarine cables suffer extreme capacitive charging: the cable's insulation acts as a capacitor, absorbing reactive current that leaves less capacity for real power. Beyond about 50-80 km, so much capacity is consumed by reactive charging that AC submarine cables become impractical. Every major undersea interconnector (UK-France, NorNed, Basslink) uses HVDC.
The crossover distance is why short transmission lines are AC, long-haul bulk power corridors are increasingly HVDC, and all ocean crossings are DC.
HVDC converter stations cost $200-500M per terminal versus $50-100M for AC substations. Despite this, HVDC is preferred for lines longer than 500-800 km because:
The decision is purely economic. Below the crossover, AC's cheaper terminals win. Above it, DC's lower losses per mile save enough to offset the expensive converter stations. The crossover is the distance where cumulative line savings equal the terminal cost premium.
The answer is DLesson complete
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