In power transmission networks alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or undersea cables. This conversion removes the need to compensate for the AC capacitive load effects imposed by the transmission line or cable, and thereby reduces the cost per kilometer of the lines and/or cables. Conversion from AC to DC thus becomes cost-effective when power needs to be transmitted over a long distance.
The conversion of AC to DC power is also utilized in power transmission networks where it is necessary to interconnect the AC networks operating at different frequencies.
In any such power transmission network, converters are required at each interface between AC and DC power to effect the required conversion, and two such forms of converter are the line commutated converter (LCC) and the voltage source converter (VSC).
One form of known converter is based on the arrangement of large thyristors in twelve-pulse line commutated converter (LCC) structures to achieve the conversion between AC and DC power. These converters are capable of continuous operation at 3000 to 4000 Amperes and are suitable for plant installations capable of processing several gigawatts of electrical power.
Power plants based on these conventional converters absorb significant quantities of reactive power from the AC network to which they are connected. In addition, the twelve-pulse nature of the LCC structures leads to high levels of harmonic distortion in converter current. Consequently both of these factors mean that the conventional power plants require the use of large passive inductors and capacitors to provide the required reactive power and filter the harmonic currents. This leads to an increase in size, weight and costs of converter hardware.
In addition, inherent regulation effects arising from an impedance of the associated transformer and AC network leads to a reduction in DC side voltage with an increase in current flow. This is seen as an inherent negative slope in DC voltage against DC current characteristic as the power flow increases.