A switch-mode DC/DC converter obtains the same result as an AC transformer with efficiency approaching that of a well-designed AC transformer. It is a solved problem.
HVDC is less reliable and has lower availability than alternating current (AC) systems, mainly due to the extra conversion equipment. Single-pole systems have availability of about 98.5%, with about a third of the downtime unscheduled due to faults. Fault-tolerant bipole systems provide high availability for 50% of the link capacity, but availability of the full capacity is about 97% to 98%.[19]
The required converter stations are expensive and have limited overload capacity. At smaller transmission distances, the losses in the converter stations may be bigger than in an AC transmission line. The cost of the converters may not be offset by reductions in line construction cost and lower line loss.
