Modular multilevel converter design

Modular multilevel converter (MMC) has many submodules in each phase. Hence, it has many operating advantages such as modularity, flexibility, ability to generate high voltage and low harmonic content at the output. Selecting the arm inductance to obtain satisfactory MMC performance is one of the critical design constraints of the MMC. Since it affects the fault/circulating current magnitudes, efficiency as well as the harmonic content at the output. In this thesis, a novel procedure has been conducted to find the optimum value of the arm inductor when considering the factors of DC voltage utilization, harmonics and efficiency of the MMC. On the other hand, capacitor voltage balancing is an important issue in MMC operation. Unequal SM capacitor voltages cause improper operation of the MMC, resulting in high circulating currents, reduced efficiency and low harmonic quality at the output. Carrier swapping method can overcome the voltage balancing problem without any need for voltage and current sensors. In this thesis, a swapping method has also been proposed suitable for applying to different switching modulation schemes for MMC. More specifically, this swapping method has been used for the first time for Nearest Level Modulation (NLM) scheme in the literature. The optimum swapping frequency for each modulation scheme has been determined by performing case studies on the MMC. For all studies, the models are first developed in the simulation environment for verification and finally, the experimental studies are conducted to validate the results.