The conversion from AC to DC and vice versa is carried out in a converter station, with high power, high voltage electronic semiconductor valves. A semiconductor valve is a device that can carry current in one direction only and prevents current from flowing in the opposite direction. This feature is needed for the conversion from AC to DC and vice versa. The valves are controlled by a control system with computers. This allows the transmitted power to be precisely controlled, a feature unique to HVDC systems! (In an AC transmission the power cannot be directly controlled, but depends on external factors.)
A 600 MW classical converter station | 
A 330 MW HVDC Light converter station |
(Click for larger images)
A HVDC Light converter station has a much smaller size than a classical converter station, see figure. Practically all HVDC Light equipment is contained in the building. In a classical converter station, the semiconductor valves are in the large building and the small building on the side is for the control and auxiliary systems.
The AC/DC conversion technique is different between classical HVDC and HVDC Light since different semiconductor types are used in the valves. This gives HVDC Light some additional control features beyond that of classical HVDC.
If you have Google Earth on your computer you can see some of ABB's reference HVDC and HVDC Light converter on satellite pictures. We have provided Google Earth placemarks to the stations on this page.