T1: Modeling and Control of Modular Multi-Level Converter

8:30AM - 10:20AM
Monday, June 3rd, 2019

Fred Lee
Virginia Polytechnic Institute and State University, USA



While a large amount of future renewable energy sources will be networked with high-voltage DC grids, integration between these high-voltage DC grids and the existing AC grids is challenging. Among the limited choices, the modular multi-level converter (MMC) is deemed the most prominent interface converter between the DC and AC grids. This subject has been widely pursued in recent years. One of the important design challenges is to reduce the capacitor size associated with each module. Currently, a rather large capacitor bank is required to store certain amount of line-frequency related circulating energy. A number of control strategies have been introduced to reduce the capacitor voltage ripples by injecting certain harmonic current. Most of these strategies were developed, to my understanding, with good engineering ingenuity together with trial and error. There is a lack of a simple analytical model to provide effective control means to address this issue.

To gain a better understanding of the intricate operation of the MMC, the authors propose a state-trajectory analysis technique in conjunction with the power flow analysis. The proposed model can easily delineate the desired power transfer from the unwanted circulating energy. The model can illustrate visually the convoluted current and power flow between source and load as well as the circulating energy swapping between capacitors in the upper and low arms. Based on the state plane analysis, a decoupled αβ model of MMC was proposed. This model can be further simplified into an equivalent circuit, which decouples the power flow between source and load and the circulation energy between upper arm and low arm of the MMC converter.

Based on the equivalent circuit, two control law can be identified, one is to achieve the maximize the power throughput and the other is to minimum circulating energy. Two examples of control are illustrated with significant capacitor size reduction beyond the current practice.  The authors believe that this model will pave the way for researchers to gain better understanding of this complex system and further development of more advanced control strategies.



Fred Lee received the B.S. degree in electrical engineering from National Cheng Kung University, Tainan, Taiwan, in 1968, and the M.S. and Ph.D. degrees in electrical engineering from Duke University, Durham, NC, USA, in 1972 and 1974, respectively. He is currently a University Distinguished Professor at Virginia Tech, Blacksburg, USA, and the Director of the Center for Power Electronics Systems (CPES), a National Science Foundation Engineering Research Center (NSF ERC) established in 1998, with four university partners-University of Wisconsin-Madison, Rensselaer Polytechnic Institute, North Carolina A&T State University, University of Puerto Rico-Mayaguez, and more than 80 industry members. The Center's vision is "to provide leadership through global collaboration to create electric power processing systems of the highest value to society." More than the Ten-Year NSF ERC Program, CPES has been cited as a model ERC for its industrial collaboration and technology transfer, as well as education and outreach programs. His research interests include high-frequency power conversion, renewable energy, high-density electronics packaging and integration, and modeling and control. He holds 69 U.S. patents and has published 238 journal articles and more than 596 refereed technical papers. During his tenure at Virginia Tech, he has supervised to completion 71 Ph.D. and 80 Master's students. Dr. Lee received the William E. Newell Power Electronics Award in 1989, the Arthur E. Fury Award for Leadership and Innovation in Advancing Power Electronic Systems Technology in 1998, and the Ernst-Blickle Award for achievement in the field of power electronics in 2005. He has served as the President of the IEEE Power Electronics Society (1993-1994). He was named to the National Academy of Engineering in 2011.