Simulation study of a modular multilevel high‐voltage direct current based on voltage source converter system for an offshore wind farm

SUMMARY Some new technologies of modular multilevel converter (MMC) for high-voltage direct current based on voltage source converter transmission applications were presented in this article. It focused on the triggering method of MMC and the strategy of capacitor voltage balancing. First, MMC topology and its characteristics were introduced. The method of comparing multiple triangular carriers with the reference wave for generating the trigger pulse was proposed. Then, the submodules in each arm were sorted according to their capacitor voltages, and the trigger pulses were assigned to them according to ranking result. This method ensured that the instantaneous values of capacitor voltages within the same arm were equal to each other. On the basis of the equivalent circuit of the converter, it was proved that the circulating current can make capacitor voltages unbalance between arms. Some component must be added to control the signals of the converter for reducing the capacitor voltages fluctuation and for improving the quality of output waveforms. Finally, the simulation study of the proposed trigger method and capacitor voltages balancing strategy for MMC-based high-voltage direct current based on voltage source converter system was implemented using “MATLAB/Simulink” software. The results show that the designed method is correct and effective. Copyright © 2011 John Wiley & Sons, Ltd.