Beyond the ‘break-even’ distance, HVDC transmission systems cost less, even with the added expense of terminal stations. Meanwhile, an HVDC line has lower power losses than an HVAC of the same capacity in practically all cases, which means more power is reaching its final destination.
HVDC systems also have a lower environmental impact because they require fewer overhead lines to deliver the same amount of power as HVAC systems. And HVDC interconnections enable power systems to use generating plants more efficiently, for example substituting thermal generation with available hydropower resources.
The technology is a key component in the future energy system based on renewable energy sources, such as wind and solar power which are often both volatile and remotely located.
Positive effects on the power systems
Many HVDC transmissions have been built to interconnect different power systems. The links help existing generating plants tied into a power system operate more effectively, so new power station builds can be deferred. This makes economic as well as environmental sense.
The obvious environmental benefit is not having to build a new power station, but there are even greater gains coming from the operation of an interconnected power system that uses its available generating plants more efficiently. There are great environmental advantages to linking a power system with large hydroelectric resources to a system with mostly thermal generation. You can reduce thermal generation (predominately at peak demand) by tapping the hydro generation, which also helps to run the thermal generation more efficiently at constant output, without having to follow load variations.
One bipolar HVDC overhead line is comparable to a double circuit AC line from a reliability point of view. Therefore, a single HVDC line with two conductor bundles has less environmental impact than a double circuit AC line with six conductor bundles - it requires less space and has less visual impact.
With HVDC Light it is possible to use extruded polymer cables for DC transmission. This has made the use of buried land cables an interesting alternative to traditional overhead lines.
HVDC transmission losses are lower than AC transmission losses in practically all cases. An optimized HVDC power transmission line has lower losses than AC lines of the same capacity. Losses in the converter stations must also be added and they are about 0.6 percent for HVDC Classic and below 1 percent for HVDC Light of the transmitted power in each station.
Hence, in a side-by-side comparison, total HVDC transmission losses are still lower than the AC losses in practically all cases. HVDC cables also have lower losses than AC cables. The diagram below shows a comparison of the losses in 1,200 MW overhead line transmissions using AC and HVDC.
Losses overhead line AC versus DC
Lower investment cost
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.