The Working Mechanism And Control For Modular Multilevel Converter(MMC)

With the development of industrial, contradictions in energy area stand out. Energy conservation and emissions reduction has become a basic national policy of our country. Now, the research of multilevel converter turns into the focus of high voltage and high power area gradually. The traditional converter can not achieve the goal of four-quadrant operation properly whether the topology is diode-clamped, flying-capacitor or H-bridge cascaded. So it can not be directly used for mine hoists or other speed regulating system which needs quick start, braking and frequent reversing. At the same time, it can not realize the energy saving while the braking energy can not be directly feed back to the grid. The net side current waveform distortion is serious and the content of low order harmonics is high. In this paper we take the research of modular multilevel converter and its applying in inverter area as the target. We study the topology, mathematical model, working mechanism and control algorithm. The converter works properly and its advantage is approved.The main content for this paper:(1)First we discuss the development history of multilevel converter and the meaning of research for MMC. We build the mathematic model based on switching function through the analysis for MMC topology and working mechanism. Then the further study of modulation is done in order to lay the foundation for well working system.(2)For the balancing and stabilization control of SM capacitor voltage we take out the sequencing and modeling method. Though the detection of SM voltage, the sequencing process is done. For the balancing control, we keep the capacitor stay in charge or discharge according to the current direction in the bridge arm. For the stabilization control, we build the closed-loop controller which based on mathematical modeling.(3)We take the experiment on Matlab-Simulink and RT-Lab simulation platform. The result shows that the converter works properly. The SM capacitor voltage keeps in balancing and stabilization. The output voltage is in high quality and it has fewer harmonics. These results show that the control algorithm is practicable. Last the design for MMC control system, main circuit and key component selection is taken out.