Stabilizing dc-link voltage and neutral point current fluctuation in grid-connected three-level NPC converter under generalized grid fault conditions

This study introduces a stabilizing scheme that decreases the fluctuations in dc-link voltage and neutral point current in a grid-connected three-level NPC converter under generalized grid fault conditions. The fundamental component of the neutral point current is generated under grid-unbalanced conditions. Regardless of the type of unbalance in the grid, the behavior and pattern of the fundamental components of the neutral current are determined by the imbalance factor. The proposed control scheme, which involves regulating the negative sequence component of the ac input current, effectively reduces the size of the dc-link voltage ripple and the fundamental component of neutral point current under diverse types of grid unbalanced conditions. This is verified by simulations and experiments under various conditions. The proposed control method will improve the stability and reliability of the grid-connected converter under various types of imbalance conditions.