High Voltage Direct Current
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, costing less per kilometer (km) than HVAC, so that for longer lines the cost savings of less wires outweighs the extra cost of converters, making HVDC cheaper than AC. 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 for how far it can transmit, while HVDC does not.
Figure 1.3 Cost comparison. [AIMS Energy]
The graph of Figure 1.3 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) may now be possible depending on the project.
HVDC has many other uses. 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).
Figure 1.4 Submarine HVDC interconnector connecting Scotland and Northern Ireland. [Siemens 2008]
Another use for HVDC is to aggregate non-dispatchable energy, which is also referred to as variable energy resources (VERs).
Aggregating the output of VERs over many individual units substantially increases bulk system reliability and decreases overall supply fluctuations as well. HVDC lines can also help to transfer power from generation-excess regions to generation-deficient regions to balance the system.
Assessing HVDC Transmission for Impacts of Non-Dispatchable Generation, Energy Information Administriation Report, June 2018
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.
References for this page:
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
2. High Voltage Direct Current Transmission Proven Technology for Power Exchange, Siemens, 2008. pdf
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