A semiconductor valve is a device that can carry current in one direction only, and which prevents current from flowing in the opposite direction. This feature is essential for conversions of this type. Semiconductor valves are operated by a computerized control system, which enables transmitted power to be precisely controlled, a feature unique to HVDC systems. In AC transmissions, the power cannot be directly controlled, but depends on external factors.
An HVDC Light converter station is much smaller than a converter station for HVDC Classic. The HVDC Light semiconductor valves and converter reactors are placed inside, and DC and AC yards can be located in buildings on request. In a classical converter station, the semiconductor valves are housed in a large building; the small building is for the control and auxiliary systems.
The AC/DC conversion technique for classical HVDC and HVDC Light is different, since different semiconductor types are used in the valves. This gives HVDC Light some additional control features that are not found in classical HVDC systems.