Voltage Source Converter h.v.d.c.
The converters considered so far use naturally commutated thyristors and are connected to a d.c. circuit that has a large inductance thus maintaining the flow of current. Current always flows in the same direction (from anode to cathode of the thyristors) and power is reversed by changing the voltage polarity of the d.c. circuit. This conventional h.v.d.c. technology has been used since the mid-1950s with mercury arc valves and from around 1980 with thyristors.
Since about 2000, an alternative technology using voltage source converters (VSC), has become available although at lower power levels than conventional h.v. d.c. The valves of VSC h.v.d.c. use semiconductor devices that can be turned on and off, that is, they are force commutated. This ability to turn the valves off as well as on, allows the converters to synthesise a voltage wave of any frequency, phase and magnitude, within the rating of the equipment. Hence both the rectifier and inverter of a VSC h.v.d.c. link can operate at any power factor, exporting as well as importing reactive power from the a.c. systems. VSC h.v.d.c. does not need synchronous generators to be present in the a.c. networks and in fact can supply a passive (dead) load. In contrast to conventional h.v.d.c. the power flow through the link is reversed by changing the direction of the d.c. current while the d.c. voltage remains of the same polarity. The voltage of the d.c. link is maintained by capacitors. Using a VSC, a much closer approximation to a sine wave is created and so the large filters of conventional h.v.d.c. schemes are not needed.