Design of Adaptive Current Limiters in Modular Multilevel Converters

Current limiters in power converters restrict the output current and protect power semiconductors during contingencies. However, such limitation compromises their fault current contribution to power systems. This letter proposes an arm current-based adaptive limiter for modular multilevel converters (MMCs), considering the impact of dc circulating current. The magnitude limit for MMC output current adaptively changes with the voltage at the point of common coupling, to make full use of the current ratings of power semiconductors under both balanced and unbalanced grid conditions. The proposed limiter allows for additional reactive current output during voltage sags and enhances the short-circuit capacity of the MMC. All these capabilities are quantified and validated with experimental results of the low-voltage ride-through performance of a grid connected grid-forming MMC system.