Electric power transmission through interconnectors using high-voltage direct-current (HVDC) involves usually two converter stations and a transmission line. Generally overhead lines are used, but an important class of HVDC projects use submarine power cables. A back-to-back station has no transmission line and joins two separate AC grids at a single point. Historical HVDC systems used the Thury system of motor-generators but these have all been made obsolete by later developments such as mercury-arc valves, thyristors, and IGBT power transistors.