SRF-based Current-Limiting Droop Controller for Three-Phase Grid-Tied Inverters

A nonlinear droop controller for three-phase grid-connected inverters that guarantees a rigorous current limitation and asymptotic stability for the closed-loop system is proposed in this paper. The proposed controller is designed using the synchronous reference frame (SRF)and can easily change its operation between the PQ-set mode, i.e. accurate regulation of real and reactive power to their reference values, and the droop control mode. Furthermore, nonlinear input-to-state stability theory is used to guarantee that the grid current remains limited below a given value under both normal and abnormal grid conditions (grid faults). Asymptotic stability for any equilibrium point of the closed-loop system is also analytically proven. The proposed control approach is verified through extended real-time simulation results of a three-phase inverter connected to both a normal and a faulty grid.