Grid-supporting three-phase inverters with inherent RMS current limitation under balanced grid voltage sags

In this paper, a novel nonlinear droop control method is proposed for three-phase grid-supporting inverters that rigorously guarantees limited RMS value of the inverter current and closed-loop system stability under both normal grid conditions and balanced voltage sags. Contrary to the traditional dq frame approaches which align the inverter output voltage with the d axis, the proposed controller aligns the inverter current with the d axis resulting in the desired current limitation and detailed closed-loop system stability conditions. Inspired by the recently presented state-limiting PI controller and using nonlinear invariant set theory, it is rigorously proven that the RMS value of the inverter current is always limited below a given value, even during transients or faults, without requiring additional adaptive saturation units, as commonly applied in conventional approaches. Furthermore, analytic conditions for the controller parameter selection are provided to ensure asymptotic stability for the entire closed-loop grid supporting inverter for the first time without depending on particular values of the filter and line parameters. To verify the effectiveness of the proposed controller compared to existing current-limiting control methods, extensive simulation and experimental results of a three-phase inverter are provided under a normal grid and under different balanced voltage sag scenarios.