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Advanced Modular Multilevel Converter for Medium-Voltage Motor Drive Application 

Jinjun Liu and Sixing Du

Brief Introduction:

    Medium-voltage motor drives are well recognized with the merits of energy saving and performance improvement for various industries. The drive systems usually impose critical requirements on medium-voltage invertors/converters due to the manifold control objectives and special motor requirements.  Among the limited choices, the modular multilevel converter (MMC) is regarded as a promising medium-voltage topology for motor drives because of its general DC bus, scalable voltage, and potential transformerless design. However, one of the big challenges of MMC-based motor drives is the exceeding fluctuations of submodule-capacitor voltage under zero/low-speed high-torque operation conditions. A number of control strategies have been introduced to cope with this problem. Most of them are developed by injecting high-frequency common-mode voltage, which further brings serious side-effects to medium-voltage motors.

    To gain better performances for zero/low-speed high-torque operations, the authors are inspired by classical flying-capacitor converter and propose a few advanced MMC topologies for motor drive applications. The proposed MMCs can easily overcome the issue of submodule-capacitor voltage fluctuation at zero/low-speed high-torque operation without injecting common-mode voltage. Based on the proposed converters, novel modulation schemes are proposed, and they could completely eliminate the common-mode voltage including the part from pulse-width modulation. The proposed converters are further developed into back-to-back transformerless medium-voltage high-power motor drive systems, where and startup issues and potential optimized designs are also discussed. The targeting audience of this tutorial would be engineers, graduate students, and academia faculties who are interested in the topic.  The level of the intended audience will be intermediate.

image.pngJinjun Liu (M’97–SM’10–Fellow’19) received the B.S. and Ph.D. degrees in electrical engineering from Xi’an Jiaotong University (XJTU), Xi’an, China, in 1992 and 1997, respectively.

He then joined the XJTU Electrical Engineering School as a faculty. From late 1999 to early 2002, he was with the Center for Power Electronics Systems, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA, as a Visiting Scholar. In late 2002, he was promoted to a Full Professor and then the Head of the Power Electronics and Renewable Energy Center at XJTU, which now comprises 21 faculty members and over 150 graduate students and carries one of the leading power electronics programs in China. From 2005 to early 2010, he served as an Associate Dean of Electrical Engineering School at XJTU, and from 2009 to early 2015, the Dean for Undergraduate Education of XJTU. He is currently a XJTU Distinguished Professor of Power Electronics. He coauthored 3 books (including one textbook), published over 400 technical papers in peer-reviewed journals and conference proceedings, holds nearly 50 invention patents (China/US), and delivered for many times plenary keynote speeches and tutorials at IEEE conferences or China national conferences in power electronics area. His research interests include design and modeling methods for high power converters, power quality control and utility applications of power electronics, and micro-grids for sustainable energy and distributed generation.

Dr. Liu received for eight times governmental awards at national level or provincial/ministerial level for scientific research/teaching achievements. He also received the 2006 Delta Scholar Award, the 2014 Chang Jiang Scholar Award, the 2014 Outstanding Sci-Tech Worker of the Nation Award, and the IEEE Transactions on Power Electronics 2016 Prize Paper Award. He served as the IEEE Power Electronics Society Region 10 Liaison and then China Liaison for 10 years, an Associate Editor for the IEEE Transactions on Power Electronics for 12 years, and from starting 2015, the Vice President for membership of IEEE PELS. He is on the Board of China Electrotechnical Society and was elected the Vice President of the CES Power Electronics Society in 2013. Since 2013, he has been the Vice President for International Affairs, China Power Supply Society (CPSS) and since 2016, the inaugural Editor-in-Chief of CPSS Transactions on Power Electronics and Applications. Since 2013, he has been serving as the Vice Chair of the Chinese National Steering Committee for College Electric Power Engineering Programs.

image.pngSixing Du received the Master and Ph.D. degrees in electrical engineering from Xi’an Jiaotong University (XJTU), Xi’an, China in 2011 and 2014, respectively. From 2015 to 2019, he has been a Postdoctoral Research Fellow in the Department of Electrical and Computer Engineering at Ryerson University, and University of Toronto, Respectively. He is currently a Distinguished Research Fellow (Ph.D supervisor) in the School of Electrical Engineering, Xi’an Jiaotong University. His research includes high-power convers and their applications to medium-voltage motor drives and power systems. He has invented several novel MMC topologies for various industrial applications, which not only overcome the major technical issues of conventional MMCs at zero/low-speed operation in medium-voltage (MV) motor drives, but also achieve single-stage DC-DC conversion for HVDC and MVDC applications. Dr. Du has published 20 journal papers as the first author, most of which are in peer-reviewed IEEE Transactions with a total of over 500 citations in the past 6 years. He also published a Wiley-IEEE Press book “Modular Multilevel Converter: Analysis, Control and Applications” in January 2018 as a leading author. He holds 7 issued Chinese patents in power electronics, power quality, and High-voltage direct current transmission (HVDC) as co-inventor. Dr. Du received Excellent Doctoral Dissertation Award from Shaanxi Provincial Department of Education in 2017, and other prestigious awards in past few years.