Cross-Forming Control and Fault Current Limiting for Grid-Forming Inverters

This article proposes a “cross-forming” control concept for grid-forming inverters operating against grid faults. Cross-forming refers tovoltage angle formingandcurrent magnitude forming. It differs from classical grid-forming and grid-following paradigms that feature voltage magnitude-and-angle forming and voltage magnitude-and-angle following (or current magnitude-and-angle forming), respectively. The cross-forming concept addresses the need for inverters to remain grid-forming (particularlyvoltage angle forming, as required by grid codes) while managingfault current limitation. Simple and feasible cross-forming control implementations are proposed, enabling inverters to quickly limit fault currents to a prescribed level while preserving voltage angle forming for grid-forming synchronization and providing dynamic ancillary services, during symmetrical or asymmetrical fault ride-through. Moreover, the cross-forming control yields an equivalent system featuring aconstantvirtual impedance and a “normal form” representation, allowing for the extension of previously established transient stability results to include scenarios involving current saturation. Simulations and experiments validate the efficacy of the proposed cross-forming control implementations.