Design and implementation of the low computational burden phase-shifted modulation for DC–DC modular multilevel converter

摘要

DC-DC modular multilevel converter (DC-DC MMC) with the medium-frequency or high-frequency interlinked transformer is an attractive candidate for high-voltage-direct-current system and DC grid. This study proposes the phase-shifted modulation for DC-DC MMC, aiming to reduce the switching losses, minimise the computational burdens and obtain the satisfactory performance. According to the features of DC-DC MMC, the least but necessary control parameters are extracted, and their control cycles are also properly designed to adapt to the digital implementation. Then, the modulation algorithm is designed and the parameter mapping is realised by the simplest algorithm. Finally, the voltage fluctuations of the submodules (SMs) under such a modulation are predicted off-line, and the adapted sort-and-selection strategy is designed for voltage balance of the SMs. With the proposed modulation, the switching frequencies of all SMs are strictly equivalent to the fundamental frequency; the sampling, parameter mapping and sort-and-selection frequencies are also the fundamental frequency. These features alleviate not only the switching loss in the power stage, but also the computational burden in the modulation and control systems. The analysis and design of the proposed modulation will be given in this study, and verified by the experiments.