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Emerging Neutral-Point Clamped and Hybrid Multilevel Converters: Topology, 

Modulation and Control 

Yongdong Li and Kui Wang

Brief Introduction:

    Multilevel converters have received numerous attractions in past decades because of the demands for high-power and high-voltage power conversions in industrial applications. Although mature and effective, researchers, from both academia and industry, are always seeking for new solutions to topology, modulation, and control for developing multilevel converters with high reliability, high efficiency, high power density, and low cost.

    Among all the multilevel converters, the three-level neutral-point clamped (NPC) converter is one of the most practical topologies, and has been widely used in industrial applications, because of simple structure and high power density. The major challenge for this topology is the neutral-point (NP) voltage balancing problem. In this tutorial, the existing NP voltage balancing schemes will be reviewed and summarized. Thereafter, a universal zero-sequence voltage injection approach will be presented and demonstrated.

    When the voltage level of the NPC topology is extended to four or more, the NP voltages become more difficult to be balanced. It has been proven that, the NP voltages of four-level NPC (4L-NPC) converters cannot be balanced over the full range of power factors and modulation indexes with the well-known phase-disposition PWM (PDPWM) or nearest three-vector (NTV) PWM, limiting the application of the 4L-NPC converters in the industrial world. To break the theoretical limitation and improve the voltage balancing performance, a novel carrier-overlapped PWM (COPWM) scheme, which satisfies the volt-second balance principle and enables the natural balance of three dc-link capacitor voltages, is proposed for four-level NPC converters. On the basis of the COPWM, a closed-loop NP voltage balance scheme is further proposed.

    For concurrently increasing voltage levels and saving clamping devices, hybrid multilevel topologies, which combine the feature of different types of multilevel topologies, have been widely investigated in recent years. In this tutorial, three practical hybrid multilevel topologies, namely, five-level active NPC (ANPC) converter, hybrid-clamped four-level converter, and symmetrical hybrid nine-level converter, will be introduced for medium-voltage drive applications. In these topologies, only quite a few flying capacitors are employed, effectively reducing the total device counts. To demonstrate the unique features, this part starts with the operation principle and carrier-based modulation scheme for these three hybrid multilevel topologies. Thereafter, a simple zero-sequence-voltage injection approach will be presented to show the way of NP voltages balancing in these hybrid multilevel topologies.

image.pngYongdong Li was born in Hebei, China, in 1962. He received B.S. degree from Department of Electrical Engineering, Harbin Institute of Technology, Harbin, China, in 1982, and M.S. and Ph.D degrees from Department of Electrical Engineering and Automation, Institut National Polytechnique de Toulouse, France, in 1984 and 1987, respectively. He was a post-doctor from 1988 to 1990, and became a full professor since 1996 in Department of Electrical Engineering, Tsinghua University, Beijing, China.

Prof. Li has successfully accomplished several NSF projects and industry-sponsored projects, including the first industry-standard high-power multilevel inverter in China, 1.5 MW DFIG for wind generation, and etc. He has authored 3 books, and more than 300 papers in national and international conferences and journals. His research interests include high voltage power electronic converters and their applications in variable speed drives, digital control of ac motors and their applications in NC machine-tools, traction drives for high speed train and propulsion drives for ships, and renewable energy generation systems.

Prof. Li is currently the Chairman of IEEE IAS Beijing Chapter, IEEE Senior Member, IET Fellow, Deputy Director of Power Electronic Engineering Research Center of Tsinghua University, Vice-Director of Institute of New Concept EV, Senior Member of China Electro-technique Society, Vice-Chairman of China Power Electronics Society, Vice-Chairman of Electrical Automation Committee of China Automation Association, Member of Board of Directors of PCIM Asia.

image.pngKui Wang was born in Ezhou, Hubei, China, in 1984. He received the B.S. and Ph.D. degrees in Electrical Engineering from Tsinghua University, Beijing, China, in 2006 and 2011, respectively. He is currently an Assistant Researcher with Department of Electrical Engineering, Tsinghua University, Beijing China.

Dr. Wang is the first author or corresponding author for 21 IEEE transaction papers, and also the author or co-author for over 80 papers in other journals or conference proceedings. He is the holder of 12 Chinese patents and 1 U.S. patent. His research interests include topology and control of multilevel converters, energy storage systems, and wide band-gap semiconductors applications.

Dr. Wang is a member of IEEE Power Electronics Society, IEEE Industrial Electronics Society, and IEEE Industry Applications Society. He has become an IEEE Senior Member since 2019.