Research On Modular Multilevel Converter

In recent years, multilevel converter has been attracted much attention in the research field of high-voltage and high-power, and it constantly replaces the traditional two-level converter. Modular multilevel converter(MMC) as a new multilevel topology,it has been widely used in high-voltage direct-current transmission application. The modular structure of MMC is not only easy to expand its submodules, but also improve the system capacity and the voltage level. It also has the characteristics as same as the multilevel converters: low switching frequency, small output voltage harmonic, small voltage variation rate, low switching losses and so on.Firstly, this paper analyzes the topology characteristics of MMC and respectively elaborates the operation principle from the submodules and the overall structure.Compared with the advantages and disadvantages of a variety of modulation algorithm of MMC to determine to adopt the carrier phase shift SPWM(CPS-SPWM). Because it applies to the research of this subject. And the modulation process and principle are analyzed in detail.According to the existence of interphase circulating current and the unbalance of capacitor voltage, a balancing control algorithm is adopted based on the CPS-SPWM.The essence is to adjust the conduction time of submodules to adjust dynamically the capacitor voltage from the conservation of energy. Defining the relatively stability of circulating current restraining factor and capacitor voltage-sharing factor near the working point of the system, according to the capacitor voltage of submodules and the polarity of bridge arm current to calculate the longitudinal offset of the submodules modulation wave, then superimposed to the initial modulation wave. Compared the final modulation wave with the triangular carrier to produce the corresponding PWM driving signal.A simulation model of MMC based on Matlab/Simulink is built to verify the control algorithm of open loop and closed loop. The experiment platform of this system is built based on simulation. Then the related experiments are done and the output waveforms are obtained.