== Advantages of HVDC over AC transmission ==
A long distance point to point HVDC transmission scheme generally has lower overall investment cost and lower losses than an equivalent AC transmission scheme. HVDC conversion equipment at the terminal stations is costly, but the total DC transmission line costs over long distances are lower than AC line of the same distance. HVDC requires less conductor per unit distance than an AC line, as there is no need to support three phases and there is no [[skin effect]].
Depending on voltage level and construction details, HVDC transmission losses are quoted as about 3.5% per 1,000 km, which are 30 – 40% less than with AC lines, at the same voltage levels.<ref>[http://www.energy.siemens.com/hq/en/power-transmission/hvdc/hvdc-ultra/#content=Benefits Siemens AG – Ultra HVDC Transmission System]</ref> This is because direct current transfers only active power and thus causes lower losses than alternating current, which transfers both active and reactive power.
HVDC transmission may also be selected for other technical benefits. HVDC can transfer power between separate AC networks. HVDC powerflow between separate AC systems can be automatically controlled to support either network during transient conditions, but without the risk that a major [[Power outage|power system collapse]] in one network will lead to a collapse in the second. HVDC improves on system controllability, with at least one HVDC link embedded in an AC grid—in the deregulated environment, the controllability feature is particularly useful where control of energy trading is needed.
The combined economic and technical benefits of HVDC transmission can make it a suitable choice for connecting electricity sources that are located far away from the main users.
Specific applications where HVDC transmission technology provides benefits include:
* [[Submarine power cable|Undersea cables]] transmission schemes (e.g., the 580 km [[NorNed]] cable between Norway and the [[Netherlands]],<ref>Skog, J.E., van Asten, H., Worzyk, T., Andersrød, T., Norned – World’s longest power cable, [[International Council on Large Electric Systems|CIGRÉ]] session, Paris, 2010, paper reference B1-106.</ref> Italy's 420 km [[SAPEI]] cable between [[Sardinia]] and the mainland,<ref>http://new.abb.com/systems/hvdc/references/sapei</ref> the 290 km [[Basslink]] between the Australian mainland and [[Tasmania]],<ref>[[Basslink]] website</ref> and the 250 km [[Baltic Cable]] between [[Sweden]] and [[Germany]]<ref>[http://www.abb.com/hvdc ABB HVDC] website</ref>).
* Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps', usually to connect a remote generating plant to the main grid, for example the [[Nelson River DC Transmission System]] in [[Canada]].
* Increasing the capacity of an existing [[power grid]] in situations where additional wires are difficult or expensive to install.
* Power transmission and stabilization between unsynchronised AC networks, with the extreme example being an ability to transfer power between countries that use AC at different frequencies. Since such transfer can occur in either direction, it increases the stability of both networks by allowing them to draw on each other in emergencies and failures.
* Stabilizing a predominantly AC power-grid, without increasing fault levels ([[prospective short circuit current]]).
* Integration of renewable resources such as wind into the main transmission grid. HVDC overhead lines for onshore wind integration projects and HVDC cables for offshore projects have been proposed in North America and Europe for both technical and economic reasons. DC grids with multiple voltage-source converters (VSCs) are one of the technical solutions for pooling offshore wind energy and transmitting it to load centers located far away onshore.<ref>[http://magazine.ieee-pes.org/files/2012/11/10mpe06-adapa-2213011-x.pdf] website</ref>
== Disadvantages ==