Study of MMC Capacitor Voltage Equalization Strategy with Improved Bubble Sorting
The traditional submodule capacitor voltage equalization strategy of modular multilevel converter(MMC)suffers from high switching frequency and high loss.For this reason,the MMC capacitor voltage equalization strategy with improved bubble sorting is designed.The gradient value is set to increase a certain margin between capacitor and voltage.During the sorting process,if the difference between two neighboring capacitor voltages is less than the gradient value,the indexing order of the submodules is not changed,thus reducing the change times that the switching state of the submodules.The equalization of capacitor voltages is controlled by adjusting the gradient value,and two voltage equalization strategies,constant gradient value and time-varying gradient value,are designed respectively.Matlab/Simulink is utilized to build the MMC model to validate the proposed strategies.The simulation results show that the proposed strategy can reduce the submodule switching frequency while ensuring the equalization of capacitor voltages compared with the traditional strategy;meanwhile,there is a good similarity between the capacitor voltages of each submodule.The strategy is of great significance in the research of engineering applications to reduce the switching losses of MMC.
Modular multilevel converter(MMC)SubmoduleNearest level modulation(NLM)Capacitor voltage equalizationSwitch frequencyBubble sortingGradient value
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