Analysis and Calculation of Zero-Sequence Voltage Considering Neutral-Point Potential Balancing in Three-Level NPC Converters

The neutral-point (NP) potential of the three-level neutral-point-clamped converters is needed to maintain balancing. Zero-sequence voltage is the only freedom degree when carrier-based pulsewidth modulation is utilized. Appropriate zero-sequence voltage should be identified to control the NP potential. The relationship between the neutral current and injected zero-sequence voltage is studied comprehensively, and two balancing algorithms of NP potential, respectively adopting searching-optimization and interpolation methods, are presented. The theoretical optimum zero-sequence voltage for controlling NP potential can be obtained by the latter proposed algorithm. Simulation and experimental results are shown to verify the validity and practicability of the proposed algorithms.