Thyristors, also called silicon-controlled rectifier (SCRs), are controllable semiconductors that conduct when their gates receive a current pulse. They can carry very high currents and can block very high voltages. They are connected is in series to form a thyristor valve, which allows electricity to flow during the positive half of the alternating current voltage cycle but not during the negative half. Since all three phases of the HVAC system are connected to the valves, the resultant voltage is unidirectional but with some residual oscillation. Smoothing reactors are provided to dampen this oscillation.
Recently, insulated gate bipolar transistors (IGBTs), using pulse-width modulation (PWM), have been used as valves. This technology was initially developed to be used with underwater HVDC cable installations when one of the terminals is connected to a weak electrical source such as an offshore wind farm.
An IGBT is basically a bipolar junction transistor (BJT) with a semiconductor gate structure in which the gate is controlled using voltage instead of current.
HVDC transmission lines can either be single pole or bipolar, although most are bipolar, that is, they use two conductors operating at different polarities such as ± 500 kV. HVDC submarine cables are either of the solid type with oil-impregnated paper insulation or of the self-contained oil-filled type. New applications also use cables with extruded insulation made of cross-linked polyethylene.
