Power electronics for FACTS equipments

At the heart of FACTS installations are the power electronic convertors which form the smart part of the equipment. They are based on high power semiconductor devices configured to control megawatts and megavars. The author describes how, today, virtually all of the commercially available FACTS equipments use power electronic convertors employing conventional, high power, line-commutating thyristors. To achieve the high voltage and high power ratings required, the power convertors employ series connected `back-to-back´ thyristor assemblies, called valves. The individual thyristors are gated by local electronic units which are controlled by light beam signals transmitted along optical glass fibre cables that are immune from electrical interference and provide isolation from high voltages. The typical thyristor has a silicon diameter of 100 mm and a rated blocking voltage of up to 6 kV. Current ratings depend, naturally, upon the capability of the cooling system employed: the use of high efficiency water-cooled heat sinks maximises the utilisation of the thyristors enabling `back-to-back´ pairs to be operated at ratings in excess of 4 kA RMS