The balancing properties of DC link compensation for 3-phase Neutral Point Clamped (NPC) Converter

One of the major benefits of a three level NPC multilevel converter is that it requires only one DC source. However, the intermediate Neutral Point (NP) voltage of the converter must be maintained midway between the two outer dc-bus voltages, to avoid semiconductor device overvoltage stress and output modulation distortion. Various approaches have been proposed to regulate the NP voltage, using active or (more recently) passive balancing strategies. Strategies have also been proposed for a multilevel converter to use feedforward techniques to adjust the modulation processes to compensate for variations in the intermediate DC link bus voltages. This paper now combines these concepts to explore how a DC link compensation strategy can enhance the natural balancing processes of a NPC multilevel converter, particularly when combined with a passive balance booster filter. The paper uses a harmonic model of the converter modulation processes to predict the natural balancing process under various operating conditions. The analysis identifies that DC link compensation provides a significant improvement in natural balancing, particularly at low modulation depths. The conclusions are validated by both simulation and experimental investigations.