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

TL;DR

This paper introduces a novel 'cross-forming' control approach for grid-forming inverters that maintains grid synchronization and limits fault currents effectively during grid faults, enhancing stability and compliance.

Contribution

It proposes a new control paradigm that combines voltage angle forming with current magnitude forming, enabling inverters to limit fault currents while maintaining grid-forming functions.

Findings

Effective fault current limiting demonstrated in simulations and experiments

Maintains voltage angle forming during faults for grid stability

Extends transient stability analysis to include current saturation scenarios

Abstract

This article proposes a "cross-forming" control concept for grid-forming inverters operating against grid faults. Cross-forming refers to voltage angle forming and current 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 (particularly voltage angle forming, as required by grid codes) while managing fault 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 a constant virtual 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.