IEEE JOURNAL OF EMERGING AND SELECTED TOPICS IN POWER ELECTRONICS, VOL. 7, NO. 3, SEPTEMBER 2019 1777 Thermal Optimization of Modular Multilevel Converters With Surplus Submodule Active-Bypass Plus Neutral-Point-Shift Scheme Under Unbalanced Grid Conditions Wuhua Li , Member, IEEE, Heya Yang , Jing Sheng, Chushan Li, Min Chen , Xiangning He , Fellow, IEEE , and Xiaowei Gu Abstract— The thermal design of highly reliable modular multilevel converters (MMCs) is significant for the voltage-source converter based high-voltage direct current (VSC-HVdc) systems, especially under the ac unbalanced fault. In this paper, the surplus submodule is employed as new control freedom to optimize the thermal behavior of the MMCs, because its number is linearly increased with the ac voltage dip. With the surplus submodule active bypassed (SSAB), the total submodules of the MMC in one arm can be taken turns to reduce the thermal stress of the hottest power device. In order to obtain equal ac voltages in three phases to improve the SSAB strategy, the neutral-point-shift (NPS) scheme is introduced under the unbalanced conditions, especially single-phase-to-ground fault. Through the balancing of the amplitude of the MMC ac voltages with the NPS scheme, the thermal stress asymmetry of theMMC under the single-phase- to-ground fault is mitigated, and the junction temperature of the most stressed power devices is significantly reduced. Finally, the simulation and experiment results show that the junction temperature rise of the most stressed power device is reduced by nearly 30% with the SSAB plus NPS scheme. Index Terms—Modular multilevel converters (MMCs), ther- mal stress optimization, unbalanced operation. NOMENCLATURE i jp /u jp MMC upper arm currents/voltage. i oj /u oj MMC ac current/voltage. i dc j / U dc MMC dc current/voltage. I o1 /U o j Amplitude of i oj /u oj . Manuscript received December 25, 2018; revised April 11, 2019; accepted May 18, 2019. Date of publication June 10, 2019; date of current version July 31, 2019. This work was supported in part by the National Key Research and Development Program of China under Grant 2018YFB0904600, in part by the National Nature Science Foundation of China under Grant U1834205 and Grant 51677166, and in part by the Zhejiang Provincial Key Research and Development Program under Grant 2018C01SA150059. Recommended for publication by Associate Editor Joseph O. Ojo. (Corresponding author: Min Chen.) W. Li, H. Yang, J. Sheng, M. Chen, and X. He are with the College of Electrical Engineering, Zhejiang University, Hangzhou 310027, China (e-mail: woohualee@zju.edu.cn; yangheya@zju.edu.cn; zjdxsj2013@zju. edu.cn; calim@zju.edu.cn; hxn@zju.edu.cn). C. Li is with the College of Electrical Engineering, Zhejiang University, Hangzhou 310027, China, and also with Zhejiang University–University of Illinois at Urbana–Champaign Institute, Zhejiang University, Haining 314400, China (e-mail: chushan@intl.zju.edu.cn). X. Gu is with the School of Information Science and Technology, Zhejiang Sci-Tech University, Hangzhou 310018, China (e-mail: gxw@zstu.edu.cn). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JESTPE.2019.2921795 u v j /u Lj AC voltages on Bus1/ac inductance. U + v 1 /U - v 1 Positive-/negative-sequence amplitude of u v j . ϕ + v j /ϕ - v j Positive-/negative-sequence phase angles of u v j . U v 1 Rated amplitude of u v j . D p Voltage dip severity. N rated / N min j Rated/minimum number of active submodules. R total / R new Total/newly increased number of sur- plus submodules. k bypass Ratio of bypass time to total opera- tion time of submodule. T j x Junction temperature of switch x (x is T 1 – D 2 ). T j max Maximum junction temperature in submodule. u NO Injected zero-sequence voltage. α Phase angle of u NO. ( j = a , b, c) I. I NTRODUCTION D RIVEN by the growing demands on offshore wind farm integration and asynchronous power grid interconnec- tion, the installation of voltage-source converter-based high- voltage direct-current (VSC-HVdc) transmission has been continuously increasing in recent years [1]–[3]. Compared with the line-commutated-converter based high-voltage direct- current (LCC-HVdc) transmission, VSC-HVdc has the merits of independent active/reactive power regulation, black start- up capability, and small footprints [4], [5]. The modular multilevel converters (MMCs), featuring small harmonic com- ponents, high conversion efficiency, and low manufacture demand, are considered as the breakthrough technology for VSC-HVdc [6]–[9]. As the power rating of VSC-HVdc system keeps increasing, the MMCs should be not only able to with- stand the severe grid disturbances but also provide sufficient power support during the grid fault [10]–[12]. Accordingly, the design of highly reliable MMCs is significant. Power semiconductor devices are the major fragile com- ponents in the power conversion systems, and the thermal 2168-6777 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.