A new topology of multilevel VSC converter for hybrid HVDC transmission system

In this paper, the existing Modular Multilevel Converter (MMC) topologies for Line Commutated Converter (LCC)-Voltage Source Converter(VSC) connected as a hybrid High Voltage DC (HVDC) transmission system are reviewed and a new topology of multilevel converter for a hybrid HVDC is introduced. Among the existing MMC topologies for the hybrid HVDC, an MMC structure consisted of Half-Bridge SubModule(HBSM)s and Full-Bridge SubModule(FBSM)s has characteristics such as 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, where each arm of MMC is consisted of mixed HBSM and FBSM, to balance the submodule capacitor voltages under sliding of DC bus voltage. For solving the defect of the conventional structure of MMC, an asymmetric MMC, where in a leg an arm is consisted of HBSM and other arm of FBSM, is devised. The proposed asymmetric MMC can regulate the DC bus voltage freely without uncontrollable submodule capacitor voltages. The problems of the conventional structure of MMC and the validity of asymmetric MMC are verified by both computer simulation and experiment results.