HVDC > HVDC Classic > What is HVDC
The HVDC technology 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 can not be used.
HVDC stands for High Voltage Direct Current and is today a well-proven technology employed for power transmission all over the world. In total about 70,000 MW HVDC transmission capacity is installed in more than 90 projects. ABB has supplied the converter stations in more than 47 of these projects with a total transmission capacity of about 40,000 MW.
There are three different categories of HVDC transmissions:
1. Point to point transmissions
2. Back-to-back stations
3. Multi-terminal systems
The development of the HVDC technology started in the late 1920s, and only after some 25 years of extensive development and pioneering work the first commercially operating scheme was commissioned in 1954. This was a link between the Swedish mainland and the island of Gotland in the Baltic sea. The power rating was 20 MW and the transmission voltage 100 kV. At that time mercury arc valves were used for the conversion between AC and DC, and the control equipment was using vacuum tubes.
A significant improvement of the HVDC technology came around 1970 when thyristor valves were introduced in place of the mercury arc valves. This reduced the size and complexity of HVDC converter stations substantially. The use of microcomputers in the control equipment in today's transmissions has also contributed to making HVDC the powerful alternative in power transmission that it is today.
In 1995 ABB announced a newconcept for HVDC converter stations, HVDC with Capacitor Commutated Converters (CCC), that further improves the performance of HVDC transmissions. And in 1997 a completely new converter and DC cable technology called HVDC Light was introduced.