High voltage direct current (HVDC) transmits electricity long distances much more efficiently than conventional alternating current (AC), emitting less radiation, with less line losses, using less wires. HVDC transmission towers require less space, less materials to construct, and are less expensive.
HVDC converter stations, one at each end of an HVDC line, cost more than terminals for HVAC (high voltage AC). Thus, for short distances, AC may be cheaper. However, HVDC lines use less wires and less transmission towers, costing less per kilometer (km) than HVAC.
In addition, HVDC only needs a converter at each end, while AC needs intermediate stations if the lines are long, adding yet more cost for AC. And AC has an upper limit of how far it can transmit, while HVDC does not.
And besides construction costs being lower for longer distances, HVDC lines also transmit more of the electricity (have less line losses). HVAC requires more electricity input to produce the same amount of electricity downstream, resulting in permanently higher recurring costs (to pay for the additional electricity that is wasted over and over again).
The graph of Figure 2 illustrates cost comparison of AC vs. DC transmission systems. The vertical axis corresponds to overall cost of each system, and the horizontal axis specifies how long the transmission lines are from end to end of each system.
For short transmission distances, AC costs less because the terminating AC stations cost less than the DC converter stations. For longer transmission distances, DC costs less. The break even point is the transmission distance for which a DC system becomes cheaper than AC.
To the right of this graph (not shown), the AC cost curve swoops up sharply, as the cost of extra wires and stations piles up. The cost of DC becomes less steep to the right of the graph, with lower cost per kilometer as the line gets longer.
Historically, the break even distance (for HVDC to be less expensive than AC) was 500 to 800 kilometers (km). In recent years, the cost of LCC HVDC has dropped. A break even distance of about 200 km (124 miles) is possible depending on the project.
HVDC has many other uses, besides overland transmission. For example, HVDC can be used instead of AC for submarine cables, with a break even distance of about 20 to 50 km depending on the project.
Also, HVDC can be used as back-to-back stations (with little or no DC transmission lines) to link (interconnect) different AC grids together, even if the AC grids are different frequencies (which is not possible to do with AC interconnection).
Other uses of HVDC include transmission of collected non-dispatchable energy (variable energy resources VERs).
Standard (classic) HVDC converts AC to DC, transmits the DC electricity, then converts it back to AC. Conversion of DC, to and from AC, is done at a converter station at each end of the DC line. Most converter stations are Line Commutation Converters (LCC), which is covered in this article.
1. Armando L. Figueroa-Acevedo, Michael S. Czahor, David E. Jahn, A comparison of the technological, economic, public policy, and environmental factors of HVDC and HVAC interregional transmission, AIMS Energy, 2015, 3(1): 144-161. doi/pdf