Conversion Efficiency of the Buck Three-level DC-DC Converter in Unbalanced Bipolar DC Microgrids

This article evaluates the power conversion efficiency of the buck three-level dc-dc converter, when operating in unbalanced bipolar dc microgrids. Bipolar dc microgrids adopt a positive, neutral, and negative wire to double the power transfer capability, reduce conduction losses, and provide two voltage levels. Additional converters are however required to balance the pole-to-neutral voltages in the presence of unbalanced loading conditions. Previous work has shown that the buck three-level dc-dc converter features voltage balancing capability and can serve, at the same time, as an interface for battery storage and photovoltaic systems for example. Nevertheless, available conversion loss models are only valid for balanced loading conditions. Therefore, this article derives a conversion loss model for a buck three-level dc-dc converter, also valid in unbalanced conditions. The model is decomposed in balanced and unbalanced components in order to separate losses arising in balanced and unbalanced conditions. Furthermore, the model accounts for nonideal common-mode currents as experimental results will reveal that they have a profound impact on the conversion efficiency in unbalanced and balanced conditions.