Grid-Forming Control With Current Limitation for MMC Under Unbalanced Fault Ride-Through

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Abstract
This letter proposes a grid-forming control method for Modular Multilevel Converter (MMC) with AC-side current limitation capabilities under fault ride-through scenarios (balanced and unbalanced faults). The novelty of the proposed controller is that no physical capacitor bank at the AC-side connection is considered as it is usually done. This is achieved with two cascaded controllers (for voltage and current) and elliptic limitations in double synchronous rotating reference frame. In normal mode, the currents are imposed solely by the AC-side load and the current controller is in by-pass mode. In case of AC-side faults, the current controller is automatically enabled to effectively limit the currents if needed. This is achieved without switching logic between normal and limitation mode. Simulation results prove the effectiveness of the proposed strategy.

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