A Reduced Footprint Extended Arm Thyristor Leg Converter: A VSC with Enhanced Power Handling Capacity for HVDC Applications

LCCs (line commutated converter) & VSCs (voltage source converter) have their own advantages and limitations. The use of thyristors & IGBTs together in a converter station to blend in the properties of LCC & VSC would develop a hybrid converter with higher efficiency, superior control flexibility, higher power handling capability, fault tolerance capability etc. This paper relates to an interface arrangement, extended arm thyristor leg converter (EATLC), between an AC power system & an HVDC power system. EATLC has hybrid series-parallel chain-links which introduces current sharing among them. The on-state drop of thyristors is less than IGBTs and therefore major portion of current is made to flow through thyristors. The significant reduction of current in chain links reduces rating of devices and size of capacitors to be used. The advantages of both these converters, LCC & VCC, obtained in EATLC are carefully examined this paper. The results showcase the capability of EATLC to provide independent active and reactive power control despite presence of thyristors. The operation of EATLC for various circumstantial conditions such as standalone and grid connected are verified using PSCAD/EMTDC.