Model Predictive Control for Flexible Reduction of Active Power Oscillation in Grid-tied Multilevel Inverters under Unbalanced and Distorted Microgrid Conditions

The unbalanced and distorted grid phenomena is expected to be a critical factor for future microgrids due to extensive use of unbalanced and nonlinear loads. These microgrid conditions keep changing depending on the load profile. The DC-link voltage fluctuation, which happens due to polluted grid conditions, not only leads to instability of the system but also compromises the lifetime of the batteries and DC-link capacitors. Active power oscillation can only be reduced by compromising with the harmonic distortion of the grid currents. Therefore, to cope with various expected unbalanced and distorted microgrid conditions, a trade-off between active power oscillation and total harmonic distortion of grid current is required. This paper proposes a novel flexible reference current generation technique by using a tuning parameter to reduce active power oscillation flexibly. The generated reference current comprises not only the positive and negative sequence currents but also lower order harmonic components. The peak values of all the components present in the reference current are decided in a stationary reference frame. Since model predictive controller tracks multi-frequency reference currents without any additional requirement and with good transient performance, it has been used to control the neutral point clamped grid-tied three-level inverter. The concept is validated through simulation and experimental results.