High-voltage direct current


  High-voltage direct current.doc (Size: 421 KB / Downloads: 6)

A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. For underwater power cables, HVDC avoids the heavy currents required by the cable capacitance. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may still be warranted, due to other benefits of direct current links. HVDC allows power transmission between unsynchronized AC distribution systems, and can increase system stability by preventing cascading failures from propagating from one part of a wider power transmission grid to another.
The modern form of HVDC transmission uses technology developed extensively in the 1930s in Sweden at ASEA. Early commercial installations included one in the Soviet Union in 1951 between Moscow and Kashira, and a 10–20 MW system between Gotland and mainland Sweden in 1954.[1] The longest HVDC page link in the world is currently the Xiangjiaba-Shanghai 2,071 km (1,287 mi) 6400 MW page link connecting the Xiangjiaba Dam to Shanghai, in the People's Republic of China.[2] In 2012, the longest HVDC page link will be the Rio Madeira page link connecting the Amazonas to the São Paulo area where the length of the DC line is over 2,500 km (1,600 mi).[3].

High voltage transmission

High voltage (in either AC or DC electrical power transmission applications) is used for electric power transmission to reduce the energy lost in the resistance of the wires. For a given quantity of power transmitted and size of conductor, doubling the voltage will deliver the same power at only half the current. Since the power lost as heat in the wires is proportional to the square of the current, but does not depend in any major way on the voltage delivered by the power line, doubling the voltage in a power system reduces the line-loss loss per unit of electrical power delivered by a factor of 4. Power loss in transmission lines can also be reduced by reducing resistance, for example by increasing the diameter of the conductor; but larger conductors are heavier and more expensive.

Mercury arc valves

The grid controlled mercury arc valve became available for power transmission during the period 1920 to 1940. Starting in 1932, General Electric tested mercury-vapor valves and a 12 kV DC transmission line, which also served to convert 40 Hz generation to serve 60 Hz loads, at Mechanicville, New York. In 1941, a 60 MW, +/-200 kV, 115 km buried cable page link was designed for the city of Berlin using mercury arc valves (Elbe-Project), but owing to the collapse of the German government in 1945 the project was never completed.[10] The nominal justification for the project was that, during wartime, a buried cable would be less conspicuous as a bombing target. The equipment was moved to the Soviet Union and was put into service there.[11] Introduction of the fully static mercury arc valve to commercial service in 1954 marked the beginning of the modern era of HVDC transmission. A HVDC-connection was constructed by ASEA between the mainland of Sweden and the island Gotland.

Capacitor-Commutated Converters (CCC)

Line-Commutated Converters have some limitations in their use for HVDC systems, resulting from the inability of the thyristor to turn off current and its need for a period of reverse voltage after turn-off (turn-off time). An attempt to address these limitations is the “Capacitor-Commutated Converter (CCC)” which has been used in a small number of HVDC systems. The CCC is a conventional, Line-Commutated Converter HVDC system using thyristor valves, in which series capacitors are inserted into the AC line connections, either on the primary or secondary side of the converter transformer. The series capacitors partially offset the “commutating inductance” of the converter and help to reduce fault currents.