| In recent years, with a surge of electricity consumption and the rapid development of power electronic technology, the study on high-voltage high-power electronic devices attracts widespread attention. Modular multilevel converter is a highly attractive multilevel topology for high voltage and power converting for its high modularity, transformer-less feature and possibility to be extended to higher levels. For modular multilevel inverter detailed studies on modulation strategy, capacitor voltage balancing method,capacitor voltage-fluctuation suppression method in low-frequency region, control system design and other aspects have carried on.Two modulation strategies suitable for MMC are discussed in this article,which are carrier disposition SPWM and carrier phase shifting SPWM.After that, the capacitor voltage balancing method based on the voltage rankings is improved according to the characteristics of CD-SPWM.In addition,some comparion between two modulation strategies are compared from different aspects,such as the THD of the output voltage,the capacitor voltage balancing effect, the difficulty of implementation,capacitor voltage adjustment speed.when the current frequency is low, the fluctuation of capacitor voltage become serious.To solve this problems the floating capacitor voltage fluctuation is studied firstly.Then,the voltage fluctuation expression derived. To reduce the voltage ripple amplitude of the 3-phase MMC and an algorithm which is by injecting high frequency zero-sequence voltage and phase to phase circulating current is proposed. Simulation results show that the method can significantly reduce the voltage fluctuation.Choosing a 3-phase MMC as control object, this article proposes a modular hierarchical controller. Based on the analysis and design, a 5-level MMC model is constructed in MATLAB/Simulink and the simulation results demonstrate the feasibility of the control system. |
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