The Research Of Modular Multilevel Converter Device

In recent years, the modular multilevel converter is widely used in power systems and industrial production for its advantages of modularization, easy extension, high efficiency and good harmonic characteristics, and so on. Besides, it shows huge technical advantages in the field of high voltage and high power application. The current domestic research mainly focuses on the MMC circuit topology, mathematical modeling, the voltage balance, circulation suppression and modulation strategy, and the MMC modulation strategy is a key technology. Stable and efficient modulation strategy is the basis of the regular work of the MMC. For MMC system, appropriate modulation strategy is critical to its working status and control performance in a variety of applications. So, the focus of this paper is on the modulation strategy of MMC.First of all, The circuit topology and working principle of traditional double star half-bridge MMC is analyzed in this paper, and the mathematical model of MMC is established. The principle of the MMC internal circulation is deduced, and its physical meaning is illustrated in detail, which provides a theoretical basis for the system design.And then, the nearest level modulation strategy and carrier phase-shifting modulation strategy are often used in the engineering application, which are studied in this paper, and the advantages and disadvantages of them are compared. Combined with the advantages of them, a hybrid modulation strategy suitable for medium voltage application is proposed. The working principle and mode of generation of the traditional modulation strategy and hybrid modulation strategy are studied in detail. And Matlab/Simulink simulation platform is built for the simulation test and verify. The simulation results suggest that the hybrid modulation strategy has lowered switching frequency under the premise of better to keep the output voltage waveform, thus reducing the loss of the system.Finally, MMC hardware system and software system are designed by using DSP + FPGA controlled architecture, and the MMC experiment platform is developed as well, on the basis of which, the proposed hybrid modulation strategy is verified. The experiment results prove that the proposed hybrid modulation strategy is effective.