Design of high power density sub module in modular multilevel converter for VSC-HVDC project

Modular multilevel converter (MMC) is a novel multilevel converter in the technical field of high voltage direct current based on voltage sourced converter (VSC-HVDC). This paper presents a high density sub module, which uses the series-connected IGBTs technique, in order to increase voltage level or power transmission capability for MMC. The topology of MMC and the working principle of sub module are firstly introduced. Then the system structure, software and hardware design, modulation method, loss calculation of sub module are illustrated. Afterwards, a practical back-to-back model has been established and simulated by using the software PSIM. Then an analysis of high power density sub module is mainly developed using series voltage balancing technique. Experimental results during operation and fault condition demonstrate that the designed sub module is reliable and the proposed RCD voltage balancing circuit for insulated gate bipolar transistor (IGBT) is feasible and validity for high density sub module.