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High Voltage Direct Current (HVDC)
Power electronics enable conversion between AC and DC, or between AC at different frequencies. This makes it possible to transmit power in DC form and also enables exploitation of variable frequency systems and close control of power injections into the network.
High Voltage Direct Current (HVDC) transmission can be used to connect asynchronous AC systems and for the transfer of bulk power over long distances with lower losses than the traditional AC solution. HVDC is the preferred option for connecting large offshore wind farms that are a significant distance from the shore. When planned, HVDC links between parts of the main interconnected transmission system have the ability to provide additional capacity and relieve congestion. There are three principal technology options in HVDC.
A line commutated converter (LCC) uses thyristors, and commutation is carried out by the AC system voltage. A Capacitor Commutated Converter (CCC) can be used as an alternative to the traditional LCC in HVDC installations. The difference is that a CCC has a capacitor between the valve bridge and the converter transformer. A Voltage Source Converter (VSC) uses bidirectional rectifiers/inverters to convert between AC and DC for HVDC transmission. VSC is an alternative to the traditional LCC and CCC current source HVDC and offers more flexibility through the use of Insulated Gate Bipolar Transistors (IGBTs), which are more controllable than the thyristors used in LCC and CCC.