Topologies and Modulation Methods for Five-Level Active-Neutral-Point-Clamped Inverters

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Kou, Lei
Five-Level , Inverter , Modulation Method
DC to AC power conversion is a key technology in the modern set-up of generation, transmission, distribution and utilization of electric power. Among the various schemes of prevalent inverters, the multi-level inverters have been innovated as necessary cost-benefit DC to AC devices with a wide range of applications. They have been in the focus for decades because of interesting features, such as high-quality output voltage, operation in high voltage/power, low stress on switches, etc. While the conventional multi-level inverter topologies have proven to be a viable alternative in a wide range of high-power medium-voltage/high-voltage applications, there has been an active interest in the evolution of newer topologies. Reduction in overall part count as compared to the conventional topologies has been an important objective. In this thesis, two novel five-level inverter topologies with reduced number of active switches are proposed, which are called the Six-Switch Five-Level Active-Neutral-Point-Clamped (6S-5L-ANPC) and the Seven-Switch Five-Level Active-Neutral-Point-Clamped (7S-5L-ANPC) inverters respectively. As compared with the conventional eight-switch (8S) 5L-ANPC inverters, the 6S-5L-ANPC reduces two active switches. The 7S-5L-ANPC inverter aims to solve the flying-capacitor voltage drop problem of the 6S-5L-ANPC inverter operating in reactive power condition. The operating principles of two novel topologies are presented. The loss comparison results between the 6S-5L-ANPC/7S-5L-ANPC and the conventional 8S-5L-ANPC inverters show the proposed topologies can achieve the same performance as the conventional 8S-5L-ANPC inverters in high power applications with the reduction of active switches. In addition, a new DC-link capacitor voltages balancing technique for 5L-ANPC inverters is proposed to achieve better dynamic response and no flying-capacitor voltage ripples in steady state over the conventional method. The proposed DC capacitor voltages balancing method can be applied to all 5L-ANPC inverters. Finally, three different hybrid modulation methods are proposed to reduce or eliminate the flying-capacitor voltage drop of the 6S-5L-ANPC inverter under reactive power operation. Simulation models and experimental prototypes are developed to verify the effectiveness of the proposed topologies, DC capacitor voltages balancing strategy and hybrid modulation methods.
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