Direct Internal Voltage Control-Based Fault Current-Limiting Control Strategy for Grid-Forming Converters with LCL Filter

Grid-forming (GFM) converters enhance power system stability by emulating synchronous generators, but their limited overcurrent capability under grid faults poses a critical challenge to transient stability. Existing current-limiting methods often force a trade-off between fault current suppression and voltage support. To address this, a direct internal voltage control (DIVC)-based fault current-limiting strategy is proposed. The DIVC framework eliminates inner control loops and directly regulates the internal voltage amplitude and phase by leveraging measurements at the point of common coupling (PCC) and the converter output, enabling fast, accurate current control within a virtual synchronous generator (VSG) architecture. Under mild faults, the strategy prioritizes maintaining the terminal voltage to preserve voltage source behavior; under severe faults, it smoothly transitions to a current-limiting mode that preserves the terminal voltage phase angle to support transient synchronization. The scheme incorporates compensation-enabling criteria, dual-mode amplitude/phase compensation, and power reference modification. Experimental results under an 80% voltage sag demonstrate that the proposed method limits the transient current peak to 1.1 p.u. and ensures oscillation-free recovery within 0.1 s, significantly outperforming conventional current saturation and virtual impedance techniques. The proposed approach also exhibits strong current-limiting capability under unbalanced faults.