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Reactors for High Voltage DC Transmission (HVDC)

Electrical power is usually generated, transmitted and distributed as alternating current (AC). In a number of circumstances however it is advantageous to introduce direct current (DC) links in the electricity network.

High-voltage direct current (HVDC-) technology is employed, if electrical bulk power has to be transmitted over long distances by overhead lines or submarine cables. It is also used to interconnect independent AC power sytems by so-called back-to-back interconnectors, when traditional alternating AC connections can't be used. This might be the case, if the AC power systems are operating asynchronously or when the traditional AC connection of the power systems would result in a too high short-circuit power level.

In an HVDC system air-core dry-type reactors are major components and are used for various purposes. As an example the one-line diagram below shows a typical HVDC system, in particular the converter stations at the ends of the HVDC transmission line with an emphasis on the major air-core reactor applications.

HVDC reactors

HVDC smoothing reactors (a) are connected in series with the HVDC transmission line or in the intermediate DC circuit of a back-to-back interconnector. They are installed for the purpose of reducing the harmonic currents in the DC system, reducing the rate of current increase during fault conditions and of improving the dynamic stability of the HVDC system.

HVDC filter reactors are installed on the AC side (c) as well as on the DC side (b) of the converter station. AC filters serve two purposes at the same time, providing reactive power and reducing harmonic currents. DC filters are installed on the line side of the HVDC smoothing reactor in shunt with the DC line.

In HVDC systems three filter types are mainly used, employing different configurations of reactors, capacitors and resistors, namely

  • Single tuned filters
  • Double tuned filters
  • High pass filters
Typical HVDC filter configurations (AC-Filter)

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