Unbalanced Fault Ride-Through in Grid-Forming Modular Multilevel Converters

Unbalanced grid conditions generate negative-sequence (NS) components in power systems and lead to oscillating output power from converters, posing technical challenges to the fault ride-through (FRT) capabilities of grid-forming (GFM) converters. The current limiter in the GFM converter avoids overcurrent issues during contingencies but it compromises system synchronization stability under grid voltage sags if the power synchronization loop (PSL) is used. This paper proposes a GFM control scheme based on the current synchronization loop (CSL) as a generalized solution for enhancing system synchronization stability under both balanced and unbalanced grid voltage sags. Theoretical analysis informs the selection of power control coefficients to coordinate unbalanced FRT objectives, including oscillating active power elimination and negative-sequence voltage suppression. An adaptive unbalanced limiter for GFM modular multilevel converters (MMCs) is further proposed based on the characteristics of the internal arm current, making full use of the power semiconductor current ratings and improving fault current contribution. The combination of the adaptive current limiter with the CSL-based GFM control allows GFM MMCs to provide additional reactive power to support positive-sequence voltage and suppress NS voltage at the PCC, enhancing their unbalanced FRT capabilities. All these grid supporting functionalities are quantified and validated with hardware experimental results from a laboratory-scale, grid-connected GFM MMC system.