His research interests include multilevel converter topologies, control and power loss analysis.

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A four-level π-type converter is a Neutral Point Clamped (NPC) multilevel converter topology for low/medium voltage applications. As the inherent issue with this topology, the DC-link capacitor voltage balancing is challenging, especially when it operates as a single-end inverter/rectifier with unity power factor. This paper proposes a closed-loop DC-link voltage balancing algorithm of a π-type converter that is effective and simple to implement. In principle, this approach is based on Redundant Level Modulation (RLM). The RLM utilizes additional voltage levels in each switching window to gain extra controllability of the DC-link capacitor voltages without affecting the average output voltage. An algorithm based on mathematical and logical operations is developed to utilize RLM to achieve the closed-loop voltage balancing. The proposed control method is effective over the full modulation index (M = 0 ~ 1.15) and full power factor range (cos φ = 0 ~ 1). The algorithm is implemented in a test rig, and the experiment confirms its effectiveness.

In this paper, an analytical model has been developed to analyze the device power loss and the efficiency of a new four-level π-type converter. The efficiency of the π-type converter has been evaluated against a conventional two-level converter, three-level T-type converter as well as the three-level NPC converter. It has been found out that the four-level π-type converter has a higher efficiency when switching frequency is above 5 kHz. It can achieve 97% efficiency at 50 kHz switching frequency under the rated current. An experimental four-level π-type inverter topology has been built and the efficiency has been validated.

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