Analysis and Control of Neutral Point Current Deviation in Grid Tied 3-level NPC Converter under Various Grid Unbalanced Conditions

This paper introduces analysis and control method regarding the variation of neutral point current in a grid tied 3-level NPC converter under various grid unbalanced operating conditions. Various faults cases having both unbalanced amplitude and phase are systematically categorized and described using the unified metric called imbalance factor. The fundamental component of neutral point current is generated under grid imbalance cases. The pattern and behavior of this fundamental component of neutral point current highly depends on the imbalance factor irrespective of the particular type of grid fault cases. The control scheme of regulating the negative sequential component of ac input current effectively reduces the size of fundamental component of neutral point current under the wide range of grid imbalance. The control scheme would make the operation of a grid tied 3-level NPC converter more reliable and stable under various types of grid faults.