High-voltage direct current (HVDC) transmission enables electric energy to be transmitted with lower losses compared to high-voltage alternating current (HVAC) transmission. At the same time, HVDC system requires fewer conductors to achieve the same transmission capacity as HVAC system. Each HVDC line requires two converter stations (rectifier and inverter), filters to smooth the voltage and the current, and a DC circuit, which is conducted via an overhead transmission line or cable connection (underground or undersea). The initial investment is relatively high due to the converter stations. HVDC is used primarily for longer connections when it becomes more cost-effective than HVAC due to reduced losses and the number of conductors.
Thyristor switches are used to transmit high power in converter circuits due to their capacity of carrying the highest power levels. In systems that do not require the transmission of such power levels, IGBT switches are used due to their superior dynamic properties, which consequently reduces harmonic components.
Currently, the most widely used type of HVDC link is the bipolar HVDC link. The connection uses two conductors, one with positive and one with negative polarity. The multi-terminal HVDC system is still quite rarely implemented, but its concept is increasingly used, as it allows the connection of several converter stations to a single DC grid, thus acting as a conventional HVAC transmission system. The break-even distance, where the investment in an HVDC system is more economical than in an HVAC system, is currently at distances above 500 km, or up to 50 km for the implementation with cable transmission.
Future challenges of HVDC transmission include the use of multi-terminal connections, the reduction of losses in the DC system and the development of new semiconductor switches for the new generations of converter stations.
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