HVDC Classic HVDC Classic is one of two HVDC technologies used today 6. Based on thyristor 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 semiconductors are line commutated, meaning that the turn-off signal is controlled by the AC line voltage reversal