Abstract
Modular multilevel converter (MMC) is a promising converter topology to be applied in medium-voltage dc (MVDC) system due to its advantages of superior fault response, high quality output voltage and high efficiency. This paper is focused on the analysis of the features of MMC dc impedance in MVDC system and its influence on MVDC system stability. First, the closed-loop terminal impedance of a MMC at the dc output side is derived and verified by compared with that measured through a circuit-based model. It is also found that the capacitor size and dynamic will not affect the MMC impedance if the capacitor is designed to limit up to 20% voltage ripple, which will simplify the stability analysis. A shipboard MVDC system consisting of a MMC and other load converters has been established. All the impedance characteristics of load converters and their mutual interactions with the MMC are explored. With the developed MMC dc impedance and stability analysis method, the MVDC system stability characteristics can be predicted, which can be used for system design to maintain stability. The simulation results are consistent with the analysis.
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