Research On Modular Multilevel Converter(MMC) And Development Of An Experimental Prototype

In recent years, modular multilevel converter (MMC) has been widely used in the field of high-power energy conversion systems, especially in HVDC Flexible, due to its low power device switching frequency, superior harmonic performance, high modularity, etc.Thus, significant research has been done to address the technical challenges associated with MMC. In order to design a MMC experimental system, the following study has been conducted in this dissertation:(1)The structure of MMC and its operational principle are detailedly analyzed. Then analyze the mechanism of capacitor voltage fluctuation and its interaction with the circulating current from the perspective of energy conservation. Noted that, the capacitor voltage fluctuation mainly includes the fundamental and second harmonic component, and the main affecting factors are analyzed. And also, summarize the impact of the circulating current on capacitor voltage fluctuation, AC output voltage and arm current.(2) Two modulation strategies have been introduced:Carrier Phase-Shifted SPWM (CPS-SPWM) and Nearest Level Modulation (NLM), and two kinds of voltage balancing control strategies are adopted. To reduce the switching times of power devices in traditional voltage balancing control strategy, an optimized method has been proposed, which can effectively reduce the switching frequency to reduce switching losses. The simulation and experiment have been done to validate the optimized methods.(3) An experimental prototype has been designed and the calculation of key parameters has been studied. Firstly, the control architecture used in the prototype has been introduced. Then, the values of capacitor in submodules and arm inductor have been analyzed and designed under different conditions.(4) In order to precharge the experimental prototype, two precharge circuits have been designed, respectively on the AC side and DC side. The charging dynamic process is analyzed at full length so as to search a suitable precharging control strategy. To ensure the effective charging current control for MMC, an optimized precharging control strategy is presented.(5) A submodule design scheme including a controller has been proposed, in which a CPLD is controller to achieve local control and protection. Both power circuit and control system are introduced in submodule, designed to simplify the communication and has a complete protection system. In addition, the test method of submodule is briefly described.(6) In the end, some experiments have been done on the MMC experimental prototype to verify the theoretical analysis and parameters designed. Particularly, different control strategies have been designed for MMC under different working conditions, which are proved to have excellent steady-state and dynamic-state performance by experiments.