Research On Capacitor Voltage Balancing Control Of Modular Multilevel Converter

With the rapid development of power electronic technology, modular multilevel converter (MMC) with its unique structural advantages, overcome the deficiency of the traditional multilevel converter, is becoming one of the hot research topics in the field of high voltage high-power electric power transformation. In this paper, the operation principle of MMC, traditional floating capacitor voltage balanced control and its key problems such as optimization carried on the thorough research.Firstly, this thesis analyzes the topology structure and operation principle of MMC, establishes the mathematical model of MMC, and studies the carrier phase-shift sinusoidal pulse width modulation and nearest level control in the MMC. Because of the cascaded structure, the energy is distributed in the floating capacitor of power units. Floating capacitor voltage balancing control is the focus and the difficulty in MMC research. So the study on the power unbalancing mechanism of MMC, which has not yet been fully resolved, is conducted in-depth, and establishes an equivalent loss model of the power module.This thesis studies the current floating capacitor voltage balance control modes:by adding external balance control circuit and by improving their balance control algorithm. This dissertation proposes two improved topologies, hybrid modular multi-level converter (HMMC), and a new topological structure of the intermediate links. Also, it studies the close-loop control and open-loop control algorithm, including PI regulating method, indirect feedback method, capacitor voltage sorting method and NLC control methods’theoretical analysis and mathematical deduction, summarizes their advantages and disadvantages and applicable scope. On the basis of traditional voltage balancing strategy, this thesis focuses on the optimization and improvement, based on optimization of NLC modulation technology and PI control method based on the CPS-SPWM modulation technology optimization strategy, circulating current is considered and adding circulating current suppressing strategy.Based on the theoretical analysis and software simulation, a five-level single-phase half bridge MMC prototype was built and tested. The experimental results further verify the feasibility and effectiveness of the proposed optimized control strategy. With the optimized control strategies, better voltage balancing performance has been reached.