HVDC Classic HVDC Classic is one of two HVDC tech nologies used today 6. Based on thy ristor power semiconductors, it is used primarily for bulk electrical transmission over long distances and for interconnect ing separate power grids where conventional AC methods are not suitable HVDC Classic transmission typically has power rating between 100 and 8,000 MW. t uses overhead lines (for distances over 600 km) subsea cables (for distances of 50 to 100km), or a combination of cables and lines. Thyristor power sem conductors are line commutated, mean ing that the turn-off signal is controlled by the AC line voltage reversal