A model predictive control approach to reducing low order harmonics in grid inverters with LCL filters

Control of grid inverters aims to meet two challenging objectives: zero steady-state tracking offset in the grid current to achieve a high power factor, and attenuation of the inverter and grid harmonics to respect the limits imposed on the current harmonics injected into the grid. This paper proposes an observer-based model predictive control approach to meet these control objectives for single and three-phase grid inverters with LCL filters. A Kalman-filter based observer design is used to estimate the system states and harmonic disturbances based on the grid current measurement. A model predictive control scheme is employed to achieve offset-free reference tracking for the grid current, and rejection of low frequency harmonics. The scheme features not only rejection of grid voltage harmonics, but also rejection of low frequency inverter distortion caused, for example, by nonlinear filter inductances. The effectiveness of the scheme is demonstrated in simulation.