A Switching Sequence Optimization Method (SSOM) to Eliminate the Dead-Time Unexpected Output Levels for Four-Level Nested Neutral Point Clamped Converter

The four-level nested neutral point clamped (4L-NNPC) converter is a new multilevel converter with a simple structure and no power device in series, which is suitable for medium-voltage high-power applications. However, it experiences severe dead-time unexpected output levels problem in the output voltage when the low-output-frequency modulation method is applied. This article proposes a switching sequence optimization method (SSOM) on the basis of carrier-based 3 level (CB3L) PWM that can eliminate the dead-time unexpected levels and ensure the voltage balance of the flying capacitors at the low output frequency. The reasons for the unexpected levels generated by the state transition are first analyzed, and the relationship between the dead time and unexpected levels is investigated. Then, the basic principle and implementation of the proposed SSOM are elaborated in detail. Simulation and experimental results are presented to verify the effectiveness of the proposed method.