Editing Electric power transmission (section)

== High-voltage direct current ==
{{Main|High-voltage direct current}}

High-voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is transmitted over very long distances, the power lost in AC transmission becomes appreciable and it is less expensive to use direct current instead. For a long transmission line, these lower losses (and reduced construction cost of a DC line) can offset the cost of the required converter stations at each end.

HVDC is used for long [[Submarine power cable|submarine cables]] where AC cannot be used because of cable capacitance.<ref>Donald G. Fink, H. Wayne Beatty, ''Standard Handbook for Electrical Engineers 11th Edition'', McGraw Hill, 1978, {{ISBN|0-07-020974-X}}, pages 15–57 and 15–58</ref> In these cases special [[high-voltage cable]]s are used. Submarine HVDC systems are often used to interconnect the electricity grids of islands, for example, between [[Great Britain]] and [[continental Europe]], between Great Britain and Ireland, between [[Tasmania]] and the Australian mainland, between the North and South Islands of New Zealand, between [[New Jersey]] and [[New York City]], and between New Jersey and [[Long Island]]. Submarine connections up to {{convert|600|km}} in length have been deployed.<ref name="guarnieri 7-3">{{Cite journal|last=Guarnieri|first=M.|year=2013|title=The Alternating Evolution of DC Power Transmission|journal=IEEE Industrial Electronics Magazine|volume=7|issue=3|pages=60–63|doi=10.1109/MIE.2013.2272238|s2cid=23610440}}</ref>

HVDC links can be used to control grid problems. The power transmitted by an AC line increases as the [[Electric power#Alternating current|phase angle]] between source end voltage and destination ends increases, but too large a phase angle allows the systems at either end to fall out of step. Since the power flow in a DC link is controlled independently of the phases of the AC networks that it connects, this phase angle limit does not exist, and a DC link is always able to transfer its full rated power. A DC link therefore stabilizes the AC grid at either end, since power flow and phase angle can then be controlled independently.

As an example, to adjust the flow of AC power on a hypothetical line between [[Seattle]] and [[Boston]] would require adjustment of the relative phase of the two regional electrical grids. This is an everyday occurrence in AC systems, but one that can become disrupted when AC system components fail and place unexpected loads on the grid. With an HVDC line instead, such an interconnection would:
* Convert AC in Seattle into HVDC;
* Use HVDC for the {{convert|3000|mi|km}} of cross-country transmission; and
* Convert the HVDC to locally synchronized AC in Boston,
(and possibly in other cooperating cities along the transmission route). Such a system could be less prone to failure if parts of it were suddenly shut down. One example of a long DC transmission line is the [[Pacific DC Intertie]] located in the Western United States.