A Robust Controller for a Grid-Tied Inverter Connected Through an LCL Filter

This article presents a control strategy for three-phase inverters connected to grids through an LCL output filter. For this topology, it is known that when output current and capacitor current are fed back through certain optimal gains, the system is unfailingly unstable for a grid inductance value called “critical.” For the critical inductance value, the system's resonance frequency is equal to one-sixth of the sampling frequency. In this article, it is first checked that capacitor voltage positive feedback can stabilize this system. In addition, it is found that there exists a proportional relationship between the common coupling point voltage and the capacitor voltage. Based on this relationship, it is proposed to add grid voltage feedforward instead of capacitor voltage feedback for obtaining a stable system for every grid inductance value, including the critical value. In order to validate our assertion, this control law is tested via simulations and also, experiments on a prototype are included. Simulation and experimental results show the good performance of the proposed scheme.