PLL-less Nonlinear Current-limiting Controller for Single-phase Grid-tied Inverters: Design, Stability Analysis and Operation Under Grid Faults

A nonlinear controller for single-phase grid-tied inverters, that can operate under both a normal and a faulty grid with guaranteed closed-loop stability, is proposed. The proposed controller acts independently from the system parameters, does not require a phase-locked loop (PLL) and can achieve the desired real power regulation and unity power factor operation. Based on nonlinear input-to-state stability theory, it is analytically proven that the inverter current always remains below a given value, even during transients, independently from grid variations or faults (short circuit or voltage sag). The desired performance and stability of the closed-loop system are rigorously proven since the controller has a structure that does not require any switches, additional limiters or monitoring devices for its implementation. Therefore, nonlinear stability of a grid-tied inverter with a given current-limiting property is proven for both normal and faulty grid conditions. The effectiveness of the proposed approach is experimentally verified under different operating conditions of the grid.