Received February 12, 2021, accepted February 12, 2021, date of current version March 4, 2021. Digital Object Identifier 10.1109/ACCESS.2021.3059556 COMMENTS AND CORRECTIONS Corrections to ‘‘MMC Based MTDC Grids: A Detailed Review on Issues and Challenges for Operation, Control and Protection Schemes’’ JAMSHED AHMED ANSARI 1,2 , (Member, IEEE), CHONGRU LIU 1 , (Senior Member, IEEE), AND SHAHID AZIZ KHAN 1 1 School of Electrical and Electronic Engineering, North China Electric Power University, Beijing 102206, China 2 Department of Electrical Engineering, Sukkur IBA University, Sukkur 65200, Pakistan Corresponding author: Chongru Liu (chongru.liu@ncepu.edu.cn) In the above article [7], the following citations / references were missing. The caption of Fig. 1 should be read ‘‘FIGURE 1. Schematic diagram of MMC [1].’’ The seventh sentence of Section III-A should be read ‘‘The detailed switch modelling (DSM) of MMC is infeasible for large-scale MTDC systems due to huge time consumption in simulations [2].’’ The eighth sentence of Section III-A should be read ‘‘Average modelling of MMC is not capable to investigate DC side transients [3].’’ The fourth sentence of Section IV-A should be read ‘‘In a master-slave control, one converter station is selected as a master controller which is used for controlling voltage while others control power [4].’’ The sixth sentence of Section IV-A should be read ‘‘However, a voltage droop control is different from the abovementioned techniques, because it possesses decentralized control and master-slave and voltage margin possess centralized control [5].’’ The eighth and ninth sen- tences of Section IV-A should be read ‘‘A master-slave con- trol method is employed in a Nan’ao five-terminal MTDC system [4]. A master-slave control faces instability issues in case of failure of the master converter station [6].’’ Delete the sentences of Section III-A as ‘‘Detailed equiva- lent circuit models (ECM) are able to calculate capacitor voltage for HB MMC. However, they are not applicable for full-bridge (FB) submodules. ECM with fault block- ing capability are considered for MMCs with self-blocking capability.’’ REFERENCES [1] L. Zhang, Y. Zou, J. Yu, J. Qin, V. Vittal, G. G. Karady, D. Shi, and Z. Wang, ‘‘Modelling, control, and protection of modular multilevel converter-based multi-terminal HVDC systems: A review,’’ CSEE J. Power Energy Syst., vol. 3, no. 4, pp. 340–352, Dec. 2017. [2] J. Peralta, H. Saad, S. Dennetière, J. Mahseredjian, and S. Nguefeu, ‘‘Detailed and averaged models for a 401-level MMC–HVDC system,’’ IEEE Trans. Power Del., vol. 27, no. 3, pp. 1501–1508, Apr. 2012. [3] S. Khan and E. Tedeschi, ‘‘Modeling of MMC for fast and accurate simulation of electromagnetic transients: A review,’’ Energies, vol. 10, no. 8, p. 1161, Aug. 2017. [4] H. Rao, ‘‘Architecture of Nan’ao multi-terminal VSC-HVDC system and its multi-functional control,’’ CSEE J. Power Energy Syst., vol. 1, no. 1, pp. 9–18, Mar. 2015. [5] T. K. Vrana, J. Beerten, R. Belmans, and O. B. Fosso, ‘‘A classification of DC node voltage control methods for HVDC grids,’’ Electr. Power Syst. Res., vol. 103, pp. 137–144, Oct. 2013. [6] T. M. Haileselassie and K. Uhlen, ‘‘Impact of DC line voltage drops on power flow of MTDC using droop control,’’ IEEE Trans. Power Syst., vol. 27, no. 3, pp. 1441–1449, Aug. 2012. [7] J. A. Ansari, C. Liu, and S. A. Khan, ‘‘MMC based MTDC grids: A detailed review on issues and challenges for operation, control and protection schemes,’’ IEEE Access, vol. 8, pp. 168154–168165, 2020. VOLUME 9, 2021 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ 34681