Improved nearest level modulation strategy for modular multilevel converters
Zhou Yang1, HU Peng-fei1, JIANG Dao-zhuo1, ZHANG Ke-bo2
1. College of Electrical Engineering, Zhejiang University, Hangzhou 310027, China;2. State Grid Ningbo Yinzhou Power Supply Company, Ningbo 315100, China
In order to solve the fluctuations of DC voltages and circulating currents of modular multilevel converters by using traditional nearest level modulation (NLM), an improved NLM method for the modular multilevel converter was proposed. In the proposed method, full consideration to capacitance voltage of the sub-module was given and capacitance voltage balancing strategy of optimization was used. Instead of each module voltage, the module average voltage was used because the voltage of each module is approximately equal. Through the analysis of the circulating current’s generation mechanism, mathematical model was established. According to the mathematical model, in view of the impact of the sub-modules’ voltage fluctuations, the control strategy of NLM has been proposed, which is, feeding back actual capacitance voltage to curb circulating current. In this way, the improved NLM both eliminates the errors between actual arm voltage and reference voltage and inhibits the fluctuations of DC voltages and circulating currents. By the simulation in PSCAD/EMTDC, the results show that the improved NLM can suppress the fluctuation of DC voltage and circulating currents both in steady and dynamic state, which means that the system has a high reliability.
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