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HVDC

HVDC or high-voltage, direct current systems are used for electric power transmission as an alternative technology to conventional alternating-current systems. Among the earliest installations were in the USSR in 1951 and a 10-20 MW system in Gotland, Sweden in 1954. [1]

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Advantages of HVDC over AC transmission Some of the advantages are:

  • Converter electronics allows control of magnitude and direction of power flow.
  • HVDC converter electronics can be used to help manage the AC grid.
  • HVDC can carry more power per conductor, because for a given power rating the constant voltage in a DC line is lower than the peak voltage in an AC line. This voltage determines the insulation thickness and conductor spacing. Above a certain break-even distance (about 50 km for submarine cables, and 600-800 km for overhead cables [3]), the lower cost of the HVDC cable outweighs the cost of the electronics.
  • Allows interconnection of asynchronous networks.
  • Suitable for many solar or wind generation solutions.
  • Results in smaller footprints for transmission and conversion and hence lower land costs.
  • Absence of reactive power loss allows use of long high-capacitance undersea cables (e.g. 250 km Baltic Cable between Sweden and Germany [3]).

Technology Early systems used mercury-arc rectifiers, which were unreliable. The thyristor valve was first used in HVDC systems in the 1960s. The thyristor is a solid-state semiconductor device similar to the diode, but with an extra control terminal that is used to switch the device on at a particular instant during the AC cycle. The insulated-gate bipolar transistor (IGBT) is now also used.

Because the voltages in HVDC systems, around 500 kV in some cases, exceed the breakdown voltages of the semiconductor devices, HVDC converters are built using large numbers of semiconductors in series.

The low-voltage control circuits used to switch the thyristors on and off need to be isolated from the high voltages present on the transmission lines. This is usually done optically. In a hybrid control system, the low-voltage control electronics sends light pulses along optical fibres to the high-side control electronics. Another system, called direct light triggering, dispenses with the high-side electronics, instead using light pulses from the control electronics to switch light-triggered thyristors (LTTs).

Applications HVDC is used to transfer power between two AC grids of different frequencies, voltages or load capacities. Such links are often used to share power between neighbouring countries, sometimes via a sea cable. It is also used for power distribution in wind farms, where power from turbines rotating at different speeds has to be fed into a common grid.

A HVDC link in which the two AC-to-DC converters are housed in the same building is called a back-to-back HVDC link. This uses AC transmission lines to connect to the two external grids. The system is also called HVDC light.

Long-distance HVDC links can be monopolar or bipolar. A monopolar link uses either a single wire, with the earth used as the return path, or a pair of wires, one at high voltage and the other at ground potential. Such a system can carry typically 1500 MW. [2]

A bipolar link uses two wires, one at a high positive voltage and the other at a high negative voltage. This system has two advantages over a monopolar link. First, it can carry twice as much power as a monopolar link, typically 3000 MW (the current is the same, but the potential difference between the wires is doubled). [2] Second, it can continue to operate despite a fault in one of the wires or in one module of the converter equipment, by using the earth as a backup return path.

Multi-terminal HVDC links, connecting more than two points, are possible but rare. An example is the 2000 MW Hydro Québec system opened in 1992. [3]

References

[1] Narain G. Hingorani in IEEE Spectrum magazine, 1996.
[2] Siemens AG "HVDC Basics" page.
[3] ABB website http://www.abb.com/hvdc



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