| ABSTRACT:Voltage source converter (VSC) with high-voltage and high-power capability is one of the core components for the VSC-HVDC transmission. As a new type of multilevel VSC, modular multilevel converter (MMC) is an excellent candidate for VSC-HVDC transmission application for its unique structural advantage.The topology, working principle and mathematical model of MMC are analyzed first, and then the modulation technique suitable for MMC is studied in detail. The technical characteristics and feasibility of carrier phase-shifted SPWM (CPS-SPWM) strategy and staircase waveform modulation strategy are verified by both the single-phase three-level and five-level MMC model in MATLAB environment.Based on analysis of the working principle and mathematical model of MMC-HVDC, a double-loop PWM control strategy is proposed for achieving active and reactive power control independently. Also, a novel staircase waveform modulation strategy is proposed to realize the dc capacitor voltage balancing control. Both the single-phase three-level and five-level MMC model in MATLAB are constructed to verify the validity of this novel staircase waveform modulation strategy.Finally, a three phase three-level MMC prototype with a set of DSP-FPGA based master-slave control system is developed and the relative experiment is carried out. The experimental results verify the feasibility and effectiveness of the control strategy proposed in this thesis. |
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