Research On The Control Strategy Of Modular Multilevel Converter

In recent years, power electronics technology is increasingly widespread in thefield of high-voltage power. Under the premise that there is no essential breakthroughin the technology of high voltage power electronics devices, multilevel inverter basedon medium voltage power electronic devices is the best choice to solve the high-voltagepower conversion. Meanwhile, with the development of new energy technologies andHVDC system, a new multilevel voltage source inverter topology: Modular MultilevelConverter (MMC), has been paid greatly attention to, because of its high degree ofmodularity, easy to expand, great output voltage harmonic characteristics and highequivalent switching frequency. At present, the academic research of MMC is relativelyin a small number, which needs in-depth study urgently.The topology and working principle of MMC is thoroughly analyzed, and itsmathematical model is established in this thesis, which lays the foundation for analysisof the inverter control strategy. Current provided by DC bus can only be available tothe load by means of sub-module suspended capacitors. Thus, the capacitor voltage willhave some fluctuations. The voltage balance control of a large number of suspendedcapacitors is to ensure that the operation of the system is stable and reliable. Fromperspective of arm power pulsation and energy pulsation, the factors affecting capacitorvoltage fluctuation are analyzed in details.The concept of Virtual Loop Mapping is applied to MMC in order to solve theproblem of inconsistent switching frequency among sub-modules based on traditionalCarrier Disposition PWM technology. And the strategy of capacitor voltage balancecontrol named Selective Virtual Loop Mapping (SVLM) based on CD-PWMtechnology is proposed in this thesis. This method does not require frequent sorting,which makes it suitable for large number of sub-modules. Besides, no additional controlcomponents are superimposed on the modulation wave. Simulation results shows thatthe proposed strategy can well balance the energy distribution among capacitors of theMMC sub-modules and can effectively suppress circulating current. Comprehensiveanalysis shows that the entire system has good dynamic and static characteristics.The proposed voltage balance control strategy is verified in the five-levelexperimental platform of MMC, which provides an experimental basis for application.