Modulation Strategy Research For Modular Multilevel Converter

As solar, wind power and other new energy access grid, and emergence of urban distribution network expansion and the island load power supply, flexible HVDC technology has broad application prospects. German scholar Rainer Marquardt in 2002 put forward modular multilevel converter (MMC) topology for the first time, the topology due to the module of design, good redundancy, and low output harmonics become the core equipment of a new generation of the flexible HVDC technology. After ten years of research the development, MMC has made great progress and has been applied in practical engineering. But there are still MMC as VSC-HVDC topology, and put into operation. Although MMC research has made great progress, but there are still many issues worthy of further exploration.This paper mainly researches the converter’s control strategy of MMC. It is mainly about these aspects:(1) The structure of MMC and its operational principle are detailedly analyzed. Then the equivalent mathematical model of MMC is built. At the same time Hardware in the loop of MMC based on RT-LAB is established, the hardware system design and MMC system modeling are described in detail. This experiment platform is a foundation for future study.(2) The advantages and disadvantages of various types of MMC modulation method is analyzed, and focused on the carrier shift modulation and the nearest level modulation. The relationship between carrier shift angle and level number is analyzed in detail.n+1 level and 2n+1 level implementation methods are given and extended to carrier cascade and NLM mode. The modified nearest level modulation which the level number of ac output voltage increases to 2n+1 through shifting modulation wave. Finally, the effectiveness and feasibility of the above analysis are verified by simulation and experiment.(3) The principle of capacitor voltage fluctuation are detailedly analyzed, pointed out that the voltage fluctuation is proportional to the AC or DC side current and the modulation ratio, nversely proportional to the fundamental frequency and the capacitor capacitance. Two kinds of voltage balancing control strategies are adopted to accomplish the goal of voltage balancing control in the carrier-shift modulation, traditional sort algorithm is modified combine to CD-SPWM and NLM modulation, reduced unnecessary switching switching.(4) Firstly, establishing circulating current mathematical model in normal and fault condition, indicating that it is a negative sequenced, frequency-doubled current but it is not frequency-doubled in fault condition. A second order generalized integrator and lowpass filter is proposed to obtain Even-numbered harmonics,then adopting PR controller to suppress circulating current. Simultaneously, a low frequency voltage balance is introduced when MMC applying into high-voltage high-power drive systems.