Abstract
2N+1 level modulation is attractive because it can greatly improve the harmonic characteristics of the modular multilevel converter (MMC) AC output voltage. However, there are also some drawbacks to the 2N+1 level modulation. When 2N+1 level modulation is applied in the MMC, a high-frequency voltage will be imposed on the arm inductors. Meanwhile, it will also result in the asymmetric high-frequency circulating current in the arms and the high-frequency current ripple in the dc-link. Based on the above issues, this paper proposes to transfer the high-frequency voltage caused by the 2N+1 level modulation to the AC side of the MMC. Meanwhile, a full-bridge submodule is inserted between the arms and the AC side to generate the additional high-frequency voltage. The proposed method can allow the MMC to output 2N+1 level AC voltage without generating the high-frequency circulating current and the dc-link current ripple. It makes up for the deficiency of the traditional 2N+1 level modulation. Finally, the simulations are conducted based on Matlab/Simulink to verify the effectiveness of the proposed method.
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