LAUSR

As compared to two-wire, unipolar DC microgrids (DCμGs), bipolar DCμGs apply a positive, neutral and negative conductor to increase the power transfer capability while retaining two voltage levels for supplying low- and high-power devices at an appropriate voltage. However, connecting devices between the positive or negative pole and the neutral conductor will result in unbalanced currents causing voltage to unbalance. Therefore, power electronic converters with voltage balancing capability are essential. Instead of installing dedicated voltage balancing converters, this study proposes to apply three-level DC–DC converters. These converters can interface battery storage with bipolar DCμGs and balance the pole-to-neutral voltages at the same time. However, high levels of unbalanced currents drive three-level DC–DC converters towards their theoretical unbalanced operating limits. To derive these, a method is presented which decomposes the governing equations in balanced and unbalanced components. Although the method is generally applicable to the three-level converters, the full-bridge three-level converter serves as a comprehensive example. The method enables to derive the unbalanced operating area of three-level DC–DC converters and to appropriately size the filter inductor. Furthermore, a novel modulation strategy is introduced in order to lower the inductor current ripple in unbalanced conditions, supported by experimental results.