A New Topology of Multilevel VSC Converter for a Hybrid HVDC Transmission System

This paper introduces a new multilevel converter topology for a hybrid HVDC system comprising line-commutated converter (LCC) and voltage source converter (VSC). Among the existing modular multilevel converter(MMC) topologies for the hybrid HVDC, a mixed MMC structure with half-bridge submodules (HBSMs) and full-bridge submodules (FBSMs) has characteristics of reduced system cost, low operation loss, but still keeping capability to cope with dc short-circuit fault. However, it is very difficult for the conventional hybrid MMC structure to balance the submodule capacitor voltages under dc-bus voltage sliding since each MMC arm is a mixture of HBSMs and FBSMs. To solve the defect of the conventional hybrid MMC structure, an asymmetric mixed MMC, in which one arm is made of series-connected HBSMs and other arm is made of FBSMs, is devised. The proposed asymmetric MMC can regulate the dc-bus voltage freely without uncontrollable submodule capacitor voltages. The problems of the conventional MMC structure and the validity of asymmetric MMC are verified by both computer simulation and experimental results.