Three-phase grid-connected inverters equipped with nonlinear current-limiting control

Voltage source inverters are essential devices to integrate renewable energy sources to the main grid and control the injection of real and reactive power. Due to their inherent nonlinear dynamics, the stability and particularly the current limitation of power controlled inverters represent challenging tasks under grid variations or unrealistic power demands. In this paper, using the synchronously rotating dq transformation, a nonlinear current limiting controller is proposed for threephase inverters connected to the grid through an LCL filter. The proposed controller introduces a cascaded control structure with inner current and voltage control loops and an outer power controller that includes a droop function to support the grid and rigorously guarantee a limit for the grid currents. Using nonlinear closed-loop system analysis and based on input-to-state stability theory, the limits for the d- and q-axis grid currents are proven independently from each other without adding any saturation units into the system that can lead to instability. Extensive simulation results of the proposed nonlinear currentlimiting controller are provided to demonstrate its effectiveness and current-limiting property.