University of Tennessee, Knoxville TRACE: Tennessee Research and Creative Exchange Doctoral Dissertations Graduate School 12-2015 Analysis and Control of VSC Based HVDC System under Single- Line-to-Ground Fault Conditions Xiaojie Shi University of Tennessee - Knoxville, [email protected] Follow this and additional works at: https://trace.tennessee.edu/utk_graddiss Part of the Electrical and Electronics Commons Recommended Citation Shi, Xiaojie, "Analysis and Control of VSC Based HVDC System under Single-Line-to-Ground Fault Conditions. " PhD diss., University of Tennessee, 2015. https://trace.tennessee.edu/utk_graddiss/3608 This Dissertation is brought to you for free and open access by the Graduate School at TRACE: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Doctoral Dissertations by an authorized administrator of TRACE: Tennessee Research and Creative Exchange. For more information, please contact [email protected]. To the Graduate Council: I am submitting herewith a dissertation written by Xiaojie Shi entitled "Analysis and Control of VSC Based HVDC System under Single-Line-to-Ground Fault Conditions." I have examined the final electronic copy of this dissertation for form and content and recommend that it be accepted in partial fulfillment of the equirr ements for the degree of Doctor of Philosophy, with a major in Electrical Engineering. Leon M. Tolbert, Major Professor We have read this dissertation and recommend its acceptance: Fred Wang, Fangxing Li, James Ostrowski Accepted for the Council: Carolyn R. Hodges Vice Provost and Dean of the Graduate School (Original signatures are on file with official studentecor r ds.) Analysis and Control of VSC Based HVDC System under Single-Line-to-Ground Fault Conditions A Dissertation Presented for the Doctor of Philosophy Degree The University of Tennessee, Knoxville Xiaojie Shi December 2015 Acknowledgement First of all, I would like to take this opportunity to express my sincere gratitude to my advisor, Dr. Leon M. Tolbert, for his invaluable supervision, encouragement, and support throughout my Ph.D. studies. He taught me techniques for solving problems, and led me to a new world full of challenges and opportunities. His great characters of affability and humor have made my graduate research an enjoyable journey to me. I would also like to thank Dr. Fred Wang for his patience, guidance and belief during my study. He always shows me a bright way when I am wandering in puzzlement, and leads me to think deeper and more comprehensively. His gentle personality, profound knowledge and rigorous attitude toward research will benefit both my career and my whole personal life. Special thanks to Dr. Fangxing Li and Dr. James Ostrowski for serving as my committee members and giving me suggestions on my dissertation. I would like to thank Mr. Yalong Li, Mr. Bo Liu, and Dr. Zhiqiang Wang for their mentorship, help and friendship during the projects. Without them, I cannot move forward smoothly in my research and finish this dissertation. I also owe gratitude to my colleagues in the power electronics lab in the University of Tennessee, Knoxville, including but not limited to, Dr. Shengnan Li, Dr. Dong Jiang, Dr. Lijun Hang, Dr. Jingxin Wang, Dr. Wenjie Chen, Dr. Xiaoling Yu, Dr. Wanjun Lei, Dr. Faete Filho, Dr. Mithat Can Kisacikoglu, Dr. Lakshmi Reddy Gopi Reddy, Dr. Zhuxian Xu, Dr. Fan Xu, Dr. Ben Guo, Dr. Zheyu Zhang, Dr. Jing Xue, Mr. Weimin Zhang, Mr. Bradford Trento, Dr. Bailu Xiao, Dr. Jing Wang, Ms. Yutian Cui, Mr. Wenchao Cao, Mr. Yiwei Ma, Dr. Liu Yang, Ms. Yang Xue, Mr. Edward Jones, Dr. Xiaonan Lu, Dr. Haifeng Lu, Dr. Sheng Zheng, Mr. Fei Yang, Mr. Tong Wu, Mr. Shuoting Zhang, Mr. Chongwen Zhao, Ms. Ling Jiang, Ms. Lu Wang and Mr. Ren Ren. It was a great pleasure to work with all of them, and I really cherish their help and friendship. ii Thank you to all the staff members of the power electronics group at the University of Tennessee, Mr. Robert B. Martin, Mr. William Rhodes, Ms. Judy Evans, Ms. Dana Bryson, Mr. Erin Wills, Ms. Chien-fei Chen, and Mr. Chris Anderson, for making this lab a wonderful and safe place for research. I also have the deepest gratitude to my dearest parents and my husband Zhiqiang Wang for their constant belief and love. Without your support and encouragement, I am not able to finish my degree so smoothly. Special thanks to my husband Zhiqiang, who worked closely with me as both friend and mentor for many years. I feel so fortunate to have you by my side and I cannot imagine a life without you. This dissertation was supported primarily by the Engineering Research Center Program of the National Science Foundation and Department of Energy under NSF Award Number EEC-1041877 and the CURENT Industry Partnership Program. iii Abstract To take full advantage of multilevel modular converter (MMC) and extend its application in high voltage direct current (HVDC) transmission systems, the ability to deal with severe unbalanced conditions, especially under single-line-to-ground (SLG) fault, is a key requirement. This dissertation deals with the development of a fault handling strategy, which helps HVDC systems achieve continuous energy supply and desired operation performance under temporary SLG fault conditions while ensuring a smooth and fast black start for MMC-HVDC particularly if the system has to shut down when a permanent SLG fault occurs. Several related research topics will be discussed in this dissertation. First, a steady-state model of MMC for second order phase voltage ripple prediction under unbalanced conditions is proposed. From the steady-state model, a circular relationship is found to evaluate the magnitudes and initial phase angles of different circulating current components. Moreover, the derivation of the equivalent dc impedance of a MMC is discussed as well. Second, the analysis and control of a MMC based HVDC transmission system under three possible SLG fault conditions are discussed. Based on the derived steady state model, a novel double line frequency dc voltage ripple suppression control is proposed. This controller, together with the phase and circulating current control, could enhance the overall fault-tolerant capability of the HVDC system without additional costs. Third, the small signal model of the capacitor charging loop in a MMC inverter is first derived according to the internal dynamics of the MMC inverter. Based on this model, a novel startup strategy incorporating an averaging capacitor voltage loop and a feedforward control is proposed for enhanced dynamic response and system stability. Finally, a cascaded droop control scheme is proposed for multi-terminal HVDC systems to deal with dc overvoltage under onshore converter side SLG fault. Using ac voltage magnitude as an intermediate variable, the dc voltage information of offshore converters can be transferred to windfarms (WFs) and iv adjust their active power generation without any communication. With such control scheme, performance of the studied four-terminal HVDC system under various fault conditions are analyzed and compared. v Table of Contents 1 Introduction ........................................................................................................................... 1 1.1 Background and Motivation .......................................................................................................... 1 1.2 Research Objectives and Approaches ......................................................................................... 16 1.3 Dissertation Organization ........................................................................................................... 17 2 Literature Review ................................................................................................................ 19 2.1 Steady State Modelling of MMC under SLG Fault Condition ................................................... 20 2.2 Control Schemes of Point to Point MMC-HVDC under AC Unbalanced Conditions ............... 21 2.3 Startup Scheme ........................................................................................................................... 24 2.4 Control Strategies of MTDC under SLG Fault Conditions......................................................... 27 2.5 Summary ..................................................................................................................................... 32 3 Steady State Modelling of MMC under SLG Fault Conditions ...................................... 33 3.1 System Configuration under SLG Fault Conditions ................................................................... 33 3.2 Steady-state Model for Second Order Phase Voltage Ripple Prediction .................................... 35 3.3 Steady-state Model for Circulating Current Prediction............................................................... 42 3.3.1 Resistive Load ................................................................................................................. 43 3.3.2 MMC Load ...................................................................................................................... 45 3.4 Experimental Verification ........................................................................................................... 50 3.4.1 MMC Rectifier with Resistive Load under SLG Fault ................................................... 51 3.4.2 MMC Inverter under SLG Fault ...................................................................................... 53 3.4.3