Generalised model predictive control scheme for three-level converter under unbalanced grid voltage considering full power factor operation

With the increase of grid-connected renewable power generation units, the three-level converter (TLC) is widely utilized. However, when unbalanced grid voltage occurs, the converter should provide a certain of reactive power to satisfy the grid code, which decreases the power factor and degrades neutral-point (NP) voltage balance. The existing model predictive control methods for NP voltage balance in TLC mainly focus on high power factor operation, which are not applicable to low power factor operation. To solve this problem, this study proposes a generalized model predictive control (GMPC) scheme for the TLC. By analyzing the influence of P-type and N-type small vectors on NP voltage, it is indicated that the charging and discharging operation of the dc-link capacitors are determined by the grid currents directions with the defined small vectors. Since the directions of grid currents are changed with different power factors, the effect of one small vector on NP voltage is change. In the proposed GMPC scheme, the direction of grid currents and the capacitor voltage difference are simultaneously used to select the appropriate small vector to mitigate the NP voltage unbalance effectively in the full power factor range. The validity is verified by simulation and experimental results.