Robust Model Predictive Controller Applied to Three-Phase Grid-Connected LCL Filters

Grid-connected inverters play an important role in renewable energy system nowadays, serving as an interface between the renewable energy source and the grid. However, the PWM modulation used to control the converters produces harmonic content that must be filtered properly. In this paper, an active damping strategy is used with a three-phase power converter with an LCL filter to achieve harmonic rejection. The control strategy which will be used is a continuous control set model predictive control (CCS-MPC) based on a state-space model of the system. This controller must ensure that the injected grid currents track sinusoidal references and reject harmonic disturbances from the grid voltage. This is achieved by using an augmented model of the system that contains resonant controllers. Following the unconstrained CCS-MPC methodology, a fixed gain controller that can be implemented similarly to a classical state-space feedback controller is obtained. An analysis of the impact of the CCS-MPC tuning parameters on the closed-loop response is made. Also, an a posteriori linear matrix inequality approach is used to show that the resulting closed-loop system is robust in regard to grid inductance uncertainties and variations. Simulation and experimental test results show that the proposed controller yields good results, complying with the IEEE 1547 Standard grid currents harmonic limits.