Virtual EMF-based Current Limiting Control for Grid-Forming Inverters under Symmetrical and Asymmetrical Faults

This paper proposes a novel current-limiting strategy for grid-forming inverters under symmetrical and asymmetrical faults. The method enables fault ride-through while providing grid support in compliance with grid code requirements. During faults, the power references prescribed by the grid code are imposed, and the virtual EMF is modified accordingly through amplitude and phase feedforward terms in the power loops. These terms are derived from the space-vector diagram of the synchronous machine emulated in the synchronous power control, enabling effective current limitation while preserving grid-forming behavior and avoiding windup in both inner and outer control loops. Such a methodology is first developed for symmetrical grid faults and then extended to asymmetrical faults, where positive- and negative-sequence grid-current injection is prescribed. In this regard, the former is regulated by modifying the positive-sequence virtual EMF, while the latter is limited by a variable virtual reactance. The inner current loop relies on finite-control-set model predictive control to ensure fast dynamic and transient overcurrent limitation. The effectiveness of the proposed method is assessed on a controller hardware-in-the-loop platform and verified on an experimental test bench, both in strong and weak grid scenarios.