Modular multilevel converter (MMC) was proposed in 2003 to extend power electronic converters to high voltage applications. Each MMC contains several identical submodules in series. MMC allows redundant submodules since its operation would not be disturbed by redundant submodules. This is a unique feature compared to other types of multilevel converters. In addition, the installation and uninstallation of submodules is easy. This modular feature makes MMC stand out for medium/high-voltage high-power applications. However, as the number of modules increases, the control complexity of voltage balance of each submodule sharply increases. Conventionally, the MMC submodule voltage cannot be balanced by open-loop modulation methods without voltage monitoring and control. This dissertation proposes a Γ-matrix modulation (ΓMM) that completely eliminates the voltage monitoring and control. In another word, the submodule voltage is self balanced.The MMC submodule voltage balancing nature with respect to each switching pattern is comprehensively analyzed in Chapter 2. The mathematical analysis reveals that the MMC is self balanced by nature if considering all possible switching patterns. Based on the enlightenment of Chapter 2, the ΓMM is proposed in Chapter 3 to bridge the gap between mathematical analysis and MMC switching operations. With this novel modulation, the MMC achieves self voltage balancing. The two-, three-, four-, and eleven-level MMCs are studied to verify the effectiveness of ΓMM. Also, compared to the conventional MMC, the ΓMM based MMC has smaller submodule capacitance and smaller arm inductance. This small capacitance and inductance feature extremely reduces the volume and weight of MMC.To understand the mechanisms of the self balance phenomena of MMC, a state-space model of MMC is proposed in Chapter 4. The existing MMC modeling are developed on different degrees of assumptions and simplifications. This makes them unsuitable for understanding the nature of this circuit from its physical basement. Compared to existing MMC modeling, the proposed state-space model well captured the MMC dynamics. With this state-space model, the MMC capacitor voltage convergence and divergence can be well observed. Four-level MMC with both full-rank Γ and non-full-rank Γ are studied to demonstrate that this model could explain both convergence and divergence of the capacitor voltage. In addition, a generalized MMC model is derived. The generalized model can be applied to higher level MMC. An eleven-level MMC case study is provided to verify the proposed model when extended to higher level. The arm inductor voltage assumption is discussed in Chapter 5.
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