Four-Level Hybrid Neutral Point Clamped Converters

In terms of low-voltage (<1.5 kV) and high-power (>10 kW) industry applications, power converters without any flying capacitors are preferred to ensure high efficiency, high power density, and light weight. As for these requirements, this article proposes a four-level hybrid neutral point clamped (4L-HNPC) converter and a four-level hybrid active neutral point clamped (4L-HANPC) converter. Compared with the existing three-level and four-level converters, the proposed 4L-HNPC and 4L-HANPC converters show higher efficiency or more even loss distribution. Compared with five-level converters, the proposed converters show higher power density and lower weight due to the absence of flying capacitors. To solve the dc-link capacitor imbalance issue, this article proposes a variable-reference voltage-balancing control method that aims at controlling the middle capacitor voltage at 1/3 of the dc-link voltage via a proportional-integral (PI) regulator. Meanwhile, the other two capacitor voltages are automatically balanced. Compared with the existing voltage-balancing control methods, the proposed variable-reference voltage-balancing control method is more suitable for digital implementation as only one triangular carrier is used. Both simulation and experimental studies have been carried out to verify the proposed four-level converter topologies and variable-reference voltage-balancing control method.