Maximizing Current Contribution of Grid-Forming MMCs under Asymmetrical Grid Faults

This paper explores the limitations of the fault current contribution of grid-forming modular multilevel converters (GFM-MMCs) under asymmetrical grid faults. Differing from the voltage source converters with a centralized dc-link capacitor, the arm current of MMCs comprises a circulating current for internal energy balancing, in addition to the dc-link current and ac output current. Under asymmetrical grid faults, the interaction between the positive- and negative-sequence components results in a power imbalance among the phases (or legs) of the MMC. This requires the presence of the dc circulating currents in each phase of MMC to eliminate this imbalance. The presence of this dc circulating current not only complicates the design of current limiting strategies but also constrains the fault current contribution of GFM-MMC. To address this issue, we propose a current limiting strategy ensuring that GFM-MMC can provide its allowable maximum fault current during asymmetrical grid faults.