DC-link capacitors sizing for three-level neutral-point-clamped inverters in four-wire distributed generation systems

When a three-level inverter is used in three-phase four-wire systems, the problem of mid-point voltage variation becomes significant because of the existence of balanced and unbalanced loads. Therefore sizing of the dc-link capacitors with respect to mid-point voltage variation should be studied. This paper presents an analytical study of dc-link capacitors sizing when using a three-level NPC inverter in four-wire systems, where positive, negative and zero sequence load currents are separately analyzed. The calculation of dc-link mid-point currents, mid-point voltage ripples and capacitor sizes are all mathematically derived. Result of this study on one hand establishes the calculation of DC-link capacitor sizes of a three-level NPC inverter when the load characteristics are known. On the other hand, it can be used to analyze the trade-off between using larger dc-link capacitors, and using an additional balancing circuit.