The HVDC technology (High Voltage Direct Current) is used to transmit electricity over long distances by overhead transmission lines or submarine cables. It is also used to interconnect separate power systems, where traditional alternating current (AC) connections cannot be used. Hitachi Energy pioneered the HVDC technology and is the undisputed world leader in the HVDC field. HVDC transmission offers, for instance, controllability and low total investment cost compared to AC solutions for long-range transmission.
In a high voltage direct current (HVDC) system, electric power is taken from one point in a three-phase
AC network, converted to DC in a converter station, transmitted to the receiving point by an overhead line or cable and then converted back to AC in another converter station and injected into the receiving AC network. Typically, an HVDC transmission has a rated power of more than 100 MW and many are in the 1,000 - 3,000 MW range. Hitachi Energy has successfully developed the world's first 1,100 kV converter transformer with a power rating of 12,000 MW, breaking the record for the highest DC voltage levels ever.
HVDC transmissions are used for transmission of power over long or very long distances, because it then becomes economically attractive over conventional AC lines.