Application of Improved Phase-Shifted Pulse Width Modulation with Third harmonic injection to Hybrid Modular Multi-level Converter

Hybrid modular multi-level converters (Hybrid-MMCs) which are modular multi-level converters including full and half bridge sub-modules, are recently used in high-voltage direct current (HVDC) transmission systems. Compared to its non-hybrid counterpart, these converters have several advantages such as the ability to nullify the DC side fault current and controlling AC side reactive power during the faults. This paper proposes a novel modified phase shifted PWM method (PS-PWM) which uses a combination of an improved PS-PWM with cancelled mismatch pulses and a third harmonic injection method. The proposed method not only reduces output voltage harmonic content and uneven loss distribution between sub-modules but also extends the linear operating range of the inverter, which improves the DC bus utilization. The mathematical analysis is derived for the proposed method and in order to study the efficiency of the system using proposed method, the loss calculation has been done and compared with traditional PS-PWM method. Simulation results in Matlab/Simulink depict the suitable performance of the presented scheme.