| Modular multilevel converter (MMC) is a novel multilevel topology of voltage source converter (VSC). Compared with the traditional VSC, it’s distinct structure make it have many merits, such as the high level, the high-quality output voltage, the low switching frequency, the low switching losses, the use of standard power electronic devices as well as the capacity of four quadrant operation and so on. Therefore, the application of MMC in the high-voltage high-power field such as HVDC transmission system, power quality control and etc. has attracted many engineers’and scientists’ attention in the world. The modulation technique one of the core technology of MMC is a significant guarantee of MMC to obtain a good operation performance. In view of the application of MMC in the low and middle voltage occasions, the main modulation strategies are studied in this thesis.Firstly, this thesis introduces the topology structures of MMC and the power unit, analyzes the working principle of half-bridge MMC in detail. The continuous mathematical model is established to analyze its physical interpretation and to provide the theoretical basis for modulation instructions and withstand voltage of the power unit.Based on the comparison of different modulation schemes such as the space vector modulation, the staircase modulation, the carrier phase shift modulation and the nearest level modulation for MMC, the staircase modulation strategy and the carrier phase shift sine pulses width modulation strategy are deeply studied for the low and middle voltage MMC. The Fourier series representation of the output voltage waveform is derived. This paper proposes a modified particle swarm optimization (MPSO) algorithm to solve the switch angle for the staircase modulation strategy. Considering the voltage balancing requirements, the square wave pulses rotation method and staircase modulation strategy are combined together to reduce the voltage ripple of floating capacitor. Both the simulation and experimental results of five-level MMC prototype verified the feasibility and effectiveness of the proposed strategy.Double modulation wave CPS-SPWM strategy is studied for MMC in this paper. Two digital implementation schemes about double modulation wave CPS-SPWM are designed based on the main control system with DSP-FPGA. And their influence for control performance is researched through the simulation and experiment to provide references for engineering design. Considering the voltage balancing requirements in the double modulation wave CPS-SPWM strategy, a modified double modulation wave CPS-SPWM strategy that the traditional CPS-SPWM strategy and capacitive voltage sorting method are combined together is presented to reduce the voltage ripple of floating capacitor in MMC. At the same time, a combination of the modified double modulation wave CPS-SPWM strategy and the traditional double modulation wave CPS-SPWM strategy improve greatly the startup performance of MMC.This thesis also studies the application of modified double modulation wave CPS-SPWM strategy in the MMC with an open end transformer (OETMMC). The basic structural characteristics and a simple mathematical model were analyzed to demonstrate the merit of OETMMC and difference of the modulation instruction between the OETMMC and the traditional MMC. Then, the modulation performance in OETMMC is studied using the simulation software. |
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