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Keywords

DC distribution grid; modular multilevel converter; modulation strategy; harmonic analysis; voltage sharing control; circulation control

Abstract

Due to the low voltage level in the medium and low voltage distribution network, the number of modular multilevel converter (MMC) sub‑modules is also small. Therefore, the modulation strategy of MMC with few sub‑modules MMC has an important impact on the performance of the MMC system. In order to improve the total harmonic distortion (THD) of output waveform under nearest‑level modulation and reduce large switching loss under carrier phase‑shift modulation of the few‑sub‑module MMC system, a quantization error setting method is proposed for the minority module MMC system under nearest level approximation modulation. The quantization error is a new MMC hybrid modulation strategy that mixes the nearest level approximation and the carrier phase shift method. When the quantization error is less than the quantization error given by the system, the nearest level approximation modulation method is used, otherwise, the carrier phase shift modulation method is used. At the same time, combined with circulating current control and sub‑module voltage equalization control, to further ensure the normal operation of the minority sub‑module MMC under the mixed modulation. Finally, the simulation model of the MMC hybrid modulation with 4 sub‑modules is established, and the simulation study of the MMC hybrid modulation strategy of the few sub‑modules is carried out. The modulation error is controlled within the quantization error range given by the system.Key words: DC distribution grid; modular multilevel converter; modulation strategy; harmonic analysis; voltage sharing control; circulation control

DOI

10.19781/j.issn.1673-9140.2023.03.011

First Page

105

Last Page

113

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