12 Facts About HVDC converter

Some HVDC converter systems take full advantage of this bi-directional property.

Early HVDC systems, built until the 1930s, were effectively rotary converters and used electromechanical conversion with motor-generator sets connected in series on the DC side and in parallel on the AC side.

However, all HVDC systems built since the 1940s have used electronic converters.

The market for voltage-source converter HVDC is growing fast, driven partly by the surge in investment in offshore wind power, with one particular type of converter, the Modular Multi-Level Converter emerging as a front-runner.

The term line-commutated indicates that the conversion process relies on the line voltage of the AC system to which the HVDC converter is connected in order to effect the commutation from one switching device to its neighbour.

Basic LCC configuration for HVDC converter uses a three-phase Graetz bridge rectifier or six-pulse bridge, containing six electronic switches, each connecting one of the three phases to one of the two DC terminals.

DC output voltage of the HVDC converter steadily becomes less positive as the firing angle is increased: firing angles of up to 90° correspond to rectification and result in positive DC voltages, while firing angles above 90° correspond to inversion and result in negative DC voltages.

HVDC systems based on voltage-source converters normally use the six-pulse connection because the converter produces much less harmonic distortion than a comparable LCC and the twelve-pulse connection is unnecessary.

Simplest waveform that can be produced by a two-level HVDC converter is a square wave; however this would produce unacceptable levels of harmonic distortion, so some form of pulse-width modulation is always used to improve the harmonic distortion of the HVDC converter.

However, where each valve of the two-level HVDC converter is effectively a high-voltage controlled switch consisting of a large number of IGBTs connected in series, each valve of a MMC is a separate controllable voltage source in its own right.

Typical MMC for an HVDC application contains around 300 submodules connected in series in each valve and is therefore equivalent to a 301 level converter.

Various other types of HVDC converter have been proposed, combining features of the two-level and Modular Multi-Level Converters.