Research On The Voltage Balance Strategy Of Modular Multilevel Converter

Without clamping devices and separate DC sources, Modular MultilevelConverter (MMC) has a modular structure, so it’s easy to expand to any level. Withcommon DC-link, it’s convenient to work on four-quadrant operation. According tothe mentioned advantage, it attracts widespread attention.Because the voltage step which synthesize AC output voltage is provided bythe Sub-Module(SM) capacitors divide a DC source, the balance of the SMcapacitors voltage is crucial for the stable operation of the MMC. In the paper, thefactors which affect the balance of the SM capacitors voltage are analyzed. Thenfocusing on the nominal-frequency voltage average value and the low-frequencyvoltage ripple, the nominal-frequency voltage average control strategy and thelow-frequency voltage ripple suppression strategy are study.The topology and the basic operating principles and pre-charge strategy areintroduced. Then a simplified modulation strategy combining a capacitor voltagebalancing scheme which remains all SM capacitor voltage at the same level ispresented. Based on the modulation strategy, the arm as a whole are studied. Theessential relationship of the SM capacitor voltage and the circulating current isrevealed. In order to facilitate the research and simulation, a continuous model ofthe MMC is established.At nominal-frequency, by the design parameters of the capacitor, the SMcapacitor voltage ripple can be controlled within reasonable limits. The voltagebalance control objective is to maintain the average value constant. Based onrelationship of the SM capacitor voltage and the circulating current, the totalvoltage controller and the differential voltage controller are designed. Asingle-phase five-level model and a continuous model are built, the simulationresults show the design of the voltage control strategy has good performance andverify the continuous model is reasonable.At low-frequency, the SM capacitor voltage ripple increases rapidly. Based onthe analysis of the ripple, the low-frequency ripple of the arm power change tohigh-frequency ripple by the arm voltage and current reconstruction, so that theamplitude of the SM capacitor voltage ripple is greatly reduced. A three-phasefive-level model is build and presented low-frequency voltage ripple restrain strategy is simulated, simulation results verify the correctness of the strategy.Finally, a single-phase five-level MMC experimental prototype is designedand built. The paper presents the schematic diagram of the main unit of the SM.The controller consists of DSP and FPGA, DSP is main controller, FPGA isauxiliary controller. The flow chart of the program features in the FPGA and DSPsoftware and FPGA program simulation results are present. Finally, theexperimental waveforms of the prototype is given, Experimental resultsdemonstrate the correctness and validity of the proposed average of voltage controlstrategy.