Dual 3L-NPC Converter System With Improved Power Quality for Bipolar DC Distribution

Three-level neutral point clamped (3L-NPC) converters are a favorable candidate for the grid interfacing converter stage in bipolar dc distribution systems. This article proposes the use of dual 3L-NPCs where, by virtue of connecting their ac ports in a differential fashion across a center-tapped transformer winding, two main benefits can be realized: 1) full bipolar voltage balancing capability that can accommodate any degree of imbalance between two dc poles, and 2) improved dc-side power quality in terms of reduced ripples in the pole voltages and currents. The proposed converter system also avoids the reliance on more complex zigzag transformer arrangements used in prior art. Detailed theoretical analyses of the pole voltage balancing and the ripple reduction mechanisms are conducted, with the results guiding the development of a suitable control strategy. The advantages of the proposed scheme and the correctness of the theoretical analyses are validated through both simulation and experimental results.