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 are 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 active power oscillation, which happens due to polluted grid conditions, results in dc-link voltage fluctuation. Therefore, not only lifetime of the batteries and dc-link capacitors are compromised but also the stability of the system is affected. Active power oscillation can only be reduced by compromising with the harmonic distortion of the grid currents. Therefore, to cope with various expected abnormal microgrid conditions, a tradeoff between active power oscillation and total harmonic distortion of grid current is required. This article proposes a novel flexible reference current generation technique by using a tuning parameter to reduce the 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 multifrequency reference currents without any additional requirement and with good stability margin, it has been used to control the neutral point clamped grid-tied three-level inverter. The concept is validated through simulation and experimental results.