Financial Analysis
An HVDC transmission line costs less than an AC line for the same transmission capacity. However, it is also true that HVDC terminal stations are more expensive due to the fact that they must perform the conversion from AC to DC, and DC to AC. But over a certain distance, the so called "break-even distance" (approx. 600 – 800 km), the HVDC alternative will always provide the lowest cost.
The break-even-distance is much smaller for subsea cables (typically about 50 km) than for an overhead line transmission. The distance depends on several factors (both for lines and cables) and an analysis must be made for each individual case.
The break-even distance concept is important, but only one of a number of factors, such as controllability, that are important to consider in choosing an AC or HVDC transmission system.