An Optimized Capacitor Voltage Balancing Control for a Five-Level Nested Neutral Point Clamped Converter

This article presents a simple yet effective capacitor voltage balancing method for a five-level nested neutral point clamped (NNPC) converter. The NNPC has been recently developed for medium-voltage high-power applications and has interesting features. This hybrid topology has three flying capacitors in each leg. Like other flying capacitor-based topologies, it is necessary to regulate and balance the voltage of the flying capacitors in an NNPC converter to certain values. In this article, a new approach is proposed to accomplish this task. The proposed regulation technique employs line current direction and voltage deviation of flying capacitors to achieve regulation. The charging/discharging status of flying capacitors with respect to different switching states is used to define a priority index. This index is eventually used to select the proper switching state. A detailed investigation is presented to determine the best switching state for each level of the output voltage. The proposed method does not need any cost function and is very intuitive and simple to implement. Using simulations, the amplitude of the flying capacitor voltage ripple is measured, which shows reduction under all operating conditions. The feasibility and performance of the proposed method are confirmed by using a laboratory-type experimental setup.