A Time-Domain Analysis Model for Modular-Multilevel-Based Grid-Forming Converters in Isolated Power Supply

The modular-multilevel-based grid-forming converter (MMGC) is considered a viable solution in isolated island power supply. However, the existing analysis model cannot meet the requirements of its performance evaluation and circuit parameter design. This article proposes a high-accurate time-domain steady-state analysis model for MMGC, where the coupled relationships among the load, the MMGC, and the dc-side system can be fully reflected, and the amplitudes and phase angles of electrical quantities, including their harmonic components, can be directly calculated. Then, the influence of LC filters on the submodule capacitor is studied. The study found that the submodule capacitance is highly correlated with the filter capacitance and almost uncorrelated with the filter inductance. It is also found that the filter capacitance selection should consider the load characteristic; a larger filter capacitance is required in the inductive load supply scenario. Finally, the studies are verified by both simulation and experiment.