A Novel Analysis of the Influence of Zero-Axis Control on Neutral-Point Potential Self-Balancing of Three-Level Converters

The neutral-point potential balance issues in three-level converters have obtained great attention. The popular view thinks that the neutral-point voltage deviation can be suppressed by regulating the injected zero-sequence component, whether via carrier modulation or space vector modulation techniques. However, this paper presents a novel finding: the efficacy of different frame controllers on the self-balancing of neutral-point potential in three-level converters differs when a comprehensive analysis of zero-sequence dynamics, including neutral-point current and PWM modulation, is conducted. That is, the proportional-resonant (PR) controller in the abc frame effectively introduces a zero-axis PR control of the zero-sequence component, which subsequently degrades the stability of neutral-point potential self-balancing. In contrast, the PI control in the dq frame does not incorporate any additional control of the zero-sequence component, thereby enhancing the self-balancing capability of the neutral-point potential. To substantiate this novel finding, a series of simulations and experimental validations were performed.