Modeling and control of a novel three-phase series-connected modular multilevel converter in T configuration

A novel three-phase series-connected modular multilevel converter in T configuration (TSC-MMC) for high voltage direct current transmission system is presented. The paper studies the dynamic mathematical model and the control scheme based on the proposed topology. Compared to conventional MMC, the number of IGBTs in TSC-MMC is reduced when withstanding the same dc voltage, which effectively decreases the system cost. Due to the series connection, no circulating currents exist in TSC-MMC. When the auxiliary arms on the dc side consist of full-bridge sub-modules (FBSMs), fault currents can be eliminated by blocking the cascaded FBSMs when dc fault occurs, without using circuit breakers. Moreover, an optimization of reactive power distribution between the main arms and the auxiliary arms is presented to improve the system performance. The simulation results obtained in MATLAB/Simulink are provided to verify the new topology as well as the proposed control strategies.