0278-0046 (c) 2018 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. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIE.2018.2833019, IEEE Transactions on Industrial Electronics IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS Abstract—The thermal design of highly reliable modular multilevel converter (MMC) is significant for the voltage-source-converter based high-voltage direct current (VSC-HVDC) system, especially under the unbalanced AC grid fault. In this paper, the analytical thermal model of MMC considering the fault ride-through strategy is established to explore the MMC dynamic thermal behavior under the single-phase-to-ground fault. With the established thermal model, the proportional relationship between the MMC DC current and sub-module thermal imbalance factor is derived. By exploring the features of DC current, it is discovered that the asymmetrical AC voltage makes the DC current different among three phases, leading to the thermal imbalance among three phases. To solve this issue, the neutral-point-shift based active thermal control method, which can reshape the MMC AC voltage and balance the three-phase DC current, is proposed. With the proposed control method, the thermal distribution among three phases becomes balanced and the junction temperature of the most stressed power device is significantly reduced. Furthermore, the performance analysis shows that the proposed method brings little effects on the circulating current suppression. Finally, the major theoretical conclusions are verified by a laboratory MMC test bench. Index Terms—Modular multilevel converter, unbalanced AC grid fault, active thermal control. NOMENCLATURE ijp /ujp MMC upper arm current/voltage of phase j ioj /uoj MMC AC current/voltage of phase j Ioj/Uoj Amplitude of ioj/uoj θoj Phase displacement between ioj and uoj Uo1 + /Uo1 - Positive-/negative-sequence amplitude of uoj Uo1 Rated amplitude of uoj φoj + /φoj - Positive-/negative-sequence phase angles of uoj Manuscript received August 15, 2017; revised January 10, 2018 and March 11, 2018; accepted April 16, 2018. This work is sponsored by the National Nature Science Foundations of China (51490682, 51677166), National Basic Research Program of China (973 Program 2014CB247400) and Zhejiang Provincial Natural Science Foundation (LR16E070001). (Corresponding author: Wuhua Li). Y. Dong, H. Yang, W. Li and X. He are with the College of Electrical Engineering, Zhejiang University, Hangzhou 310027, China (e-mail: dongyufei@zju.edu.cn; yangheya@zju.edu.cn; woohualee@zju.edu.cn; hxn@zju.edu.cn). Dp Voltage dip severity idcj MMC DC current of phase j kidcj MMC DC current modulation ratio of phase j Tjx Junction temperature of switch x(x is T1~D2) Tjmax Maximum junction temperature in sub-module uNO Injected zero-sequence voltage α Phase angle of uNO (j=a,b,c) I. INTRODUCTION ompared with the line-commutated-converter based high-voltage direct-current (LCC-HVDC), the voltage sourced converter based high-voltage direct-current (VSC-HVDC) has some prominent advantages, such as independent active/reactive power regulation, black start-up capability and small footprints [1]-[4]. The modular multilevel converter (MMC) is considered as one of the promising candidates among numerous VSC-HVDC topologies due to its lower harmonic components, higher conversion efficiency and lower manufacturing difficulty [5]-[7]. Since the power rating of VSC-HVDC system is continuously increasing, the failure or shutdown of MMC results in more serious problems than ever before. Hence, it is becoming a requirement in many applications that the MMC should not only be able to withstand the severe grid disturbance but also provide sufficient support to the AC or DC grids [6], [8]-[13]. A typical example can be seen in the MMC rectifier of VSC-HVDC system that powers the industrial plants. When the AC grid fault occurs, the strong over-current capability of MMC is usually expected to ensure the constant DC-side voltage and system robustness [6], [8]-[9]. Clearly, the increased output current requirement pushes MMC into a severe operation condition. Thus, the design of highly reliable MMC with strong grid support capacity is significant. The power devices are considered as one of the critical components for the reliability of power converters. And the main reason for power device failure is the thermal stress [14]-[17]. Thus, the thermal performance of power devices becomes a critical criterion for evaluating the reliability of the power converter. The thermal behaviors of the wind and solar power converters under the AC grid fault are investigated in [18]-[22], where the factors like the grid codes, converter control strategies are taken into consideration. It is discovered that the unbalanced thermal distribution and the complicated Neutral-Point-Shift Based Active Thermal Control for Modular Multilevel Converter under Single-Phase-to-Ground Fault Yufei Dong, Heya Yang, Wuhua Li, IEEE Member, and Xiangning He, IEEE Fellow C