Abstract

In DC distribution grids, the nearest level modulation (NLM) applied to modular multilevel converters (MMCs) with small quantities of submodules fails to satisfy the harmonic requirement of AC side voltage. To double the number of levels on AC side, this paper proposes a novel half-level MMC (HLMMC) topology, where three-level submodules are superimposed into bridge arms. Meanwhile, a half-level modulation (HLM) strategy suitable for HLMMC is proposed to balance the capacitor voltage of two-level and three-level submodules respectively. To find an optimal proportion of the two types of submodules, operating characteristics under different submodule proportion are compared. Simulation analyses of HLMMC-HVDC are conducted by MATLAB/Simulink, and simulation results show that the proposed HLM strategy designed for the HLMMC topology possesses advantages of excellent voltage quality, low operating loss and extensible capacity.

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