An Improved Three-Level Neutral Point Clamped Converter System With Full-Voltage Balancing Capability for Bipolar Low-Voltage DC Grid

Bipolar low-voltage dc grids are an emerging and promising solution for power distribution. As a natural candidate for the distribution converter, the three-level neutral-point-clamped (3L-NPC) converter has inherent but limited capability in balancing the dc voltages during asymmetrical operation. This article begins by investigating the voltage balancing limits of the conventional 3L-NPC in detail. To overcome the identified limitations, an improved 3L-NPC converter system with a zigzag transformer and a neutral line is proposed. With the designed control strategy, the proposed converter system can provide full bipolar voltage balancing capability without extra devices or penalties on component ratings. The effectiveness and advantages of the proposed scheme are verified by simulation and experimental results.