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“HVDC”

Definition

HVDC is the acronym of High Voltage Direct Current.

HVDC refers to a technology to transmit high electric power across long distances, typically several hundreds kilometers

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The modern form of HVDC transmission uses technology developed extensively in the 1930s in Sweden by ABB.

Early commercial installations included one in Russia in 1951 between Moscow and Kashira, and a 10–20 MW system between Gotland and the mainland Sweden in 1954

The longest HVDC link in the world is currently the Xiangjiaba-Shanghai 2,071 km, 6400 MW link connecting the Xiangiiaba Dam to Shanghai in China.

In 2012, the longest HVDC link will be the Rio Madeira link connecting the Amazonas to San Paulo area where the length of the DC line is over 2,500 km.

The HVDC technology is used to transmit electricity over long distances by overhead transmission lines or submarine cables.

It is also used to interconnect separate power systems, where traditional alternating current (AC) connections can not be used. 

In a high voltage direct current (HVDC) system, electric power is taken from one point in a three-phase AC network, converted to DC in a converter station, transmitted to the receiving point by an overhead line or cable and then converted back to AC in another converter station and injected into the receiving AC network.

Typically, an HVDC transmission has a rated power of more than 100 MW and many are in the 1,000 – 3,000 MW range.

HVDC transmissions are used for transmission of power over long or very long distances, because it then becomes economically attractive over conventional AC lines, since DC transmission can be done at no losses.

The increased interest in recent years for transporting clean and renewable energy from remote hydro generation plants has also increased the interest in higher DC transmission voltage than presently used (i.e. 600 kV DC).

This has led to development of Ultra High Voltage Direct Current (UHVDC) at 800 kV DC.

In the lower range  HVDC may also be used to transmit just a few tens of Megawatts (MW) power underground and under water, also over long distances.

It offers numerous environmental benefits, including “invisible” power lines, neutral electromagnetic fields, oil-free cables and compact converter stations.

HVDC increases the reliability of power grids, and the technology extends the economical power range of HVDC

HVDC is quick to install and provides an alternative to conventional AC transmission systems and local generation.

Possible applications include:

 – Connecting wind farms to power grids

 – Underground power links

 – Providing shore power supplies to islands and offshore oil & gas platform

 – Connecting asynchronous grids

 – City center infeed

Overall the HVDC takes advantage on Conventional AC transmission as soon as either the power to transmit increases, either the distance increases, and of course when both increase because of the very low electrical losses left in the cable during the transmission.

Even if some losses must be taken in the AC/DC and DC/AC power conversion stations, these losses with HVDC technology are negligible compared with the losses of the same power to be transmitted through conventional AC lines.

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