本論文主要基於電能平衡與電流追蹤法則，探討雙星型連接結構之多階層模組化轉換器之重視擴充性設計以及故障排除法則。使用的控制法則為分切合整數位控制，其透過抵消系統參數如直流鏈電壓、切換週期及電感變化對受控體的影響，來設計控制器並得到開關責任比率。
藉由直接數位電流控制，每台模組可獨立控制自身電壓，在追蹤電流的同時考慮電感隨電流增大而衰減的特性。控制法經推導後能以通式展現，並且在得知元件特性的情況下，能將電感值隨電流變化和控制法則通式存放於控制器中，易於實現軟體。此優點在於設計可疊加性單一模組時更能體現其價值──在每台模組中皆建立獨立的控制器，並且自行計算開關責任比率，可以有效的減少模組之間所共用的架構與訊號，從而加強系統的模組化，以利在更高電壓應用時模組的疊加。
系統模組化除提高可擴充性外，還具有加強系統適應性的優點。在發生故障時能夠判斷發生故障的模組，並且在系統不整體離線的情況下，控制並排除故障部分，也是一項隨著系統擴大而漸趨重要的部分。
本論文將控制法應用於雙星型連接、半橋模組的多階層模組化轉換器，並且介紹轉換器架構和控制法所達成的各種功能。主要貢獻在探討多階層模組化轉換器在故障排除情況下的行為，並根據提高其可擴充性的目的，進行單一模組的設計。

This thesis presents fault resolution control principle and module redesign of a modular multilevel converter (MMC) while emphasizing its system scalability as well as modularity. The proposed control strategy is a kind of direct current digital control. This control method eliminates the variation effects of dc voltage, switching period and inductance when calculating the duty ratio of switches in a controller.
Through the direct current digital control, each module is able to control its own capacitor voltage while considering the inductance variation regarding of current in the same time. This control law can be formulated readily and saved into a controller, making it possible for inductances tabulation during programming. This advantage can be fully illustrated in the design of cascading modules for the controller itself which can be merged into every single module and let them calculate their own duty ratios of switches. This feature effectively reduces a common structure and feedback signals between modules, enhancing modularity of a system and benefiting to module cascading in higher voltage applications.
Aside from increasing the scalability of a system, modularity also improves flexibility. Achieving module fault diagnosis and resolution while maintaining system online operation has become a critical part with continuous growth of system scale.
The proposed control method is applied to a double-star chopper-cell (DSCC) modular multilevel converter (MMC) in this study. System configuration and control method of current tracking, voltage regulation and fault resolution have been presented. The major contributions of this research are investigating the behavior of DSCC-MMC under fault resolution and redesigning the module structure based on system scaling purpose.

[1] N. M. Kangwa, C. Venugopal and I. E. Davidson, "A review of the performance of VSC-HVDC and MTDC systems," in Proc. IEEE PES, 2017, pp. 267-273.
[2] S. Kouro et al., "Recent Advances and Industrial Applications of Multilevel Converters," in IEEE Trans. Industrial Electron., vol. 57, no. 8, pp. 2553-2580, 2010.
[3] P. Ladoux, N. Serbia, L. Rubino, and P. Marino, “Comparative study of variant topologies for MMC,” in Proc. SPEEDAM, 2014, pp. 659-664.
[4] R. A. Krishna and L. P. Suresh, "A brief review on multilevel inverter topologies," in Proc. ICCPCT, 2016, pp. 1-6.
[5] M. Carpaneto, M. Marchesoni, and L. Vaccaro, “A new cascaded multilevel converter based on NPC cells,” in Proc. IEEE ISIE, 2007, pp. 1033-1038.
[6] J. Rodrigues, S. Bernet, J. O. Pontt and, S. Kouro, "Multilevel voltage sources converter topology for industrial medium voltage drives,” IEEE Trans. Industrial Electron., vol. 54, no. 6, pp. 2930-2945, 2007.
[7] J. I. Y. Ota, T. Sato and H. Akagi, "Enhancement of Performance, Availability, and Flexibility of a Battery Energy Storage System Based on a Modular Multilevel Cascaded Converter (MMCC-SSBC)," in IEEE Trans. Power Electron., vol. 31, no. 4, pp. 2791-2799, 2016.
[8] P. H. Wu; Y. t. Chen; P. T. Cheng, "The Delta-Connected Cascaded H-Bridge Converter Application in Distributed Energy Resources and Fault Ride Through Capability Analysis," in IEEE Trans. Industry Applications, vol. PP, no. 99, pp.1-1
[9] P. Sochor and H. Akagi, "Theoretical Comparison in Energy-Balancing Capability Between Star- and Delta-Configured Modular Multilevel Cascade Inverters for Utility-Scale Photovoltaic Systems," in IEEE Trans. Power Electron., vol. 31, no. 3, pp. 1980-1992, 2016.
[10] N. Thitichaiworakorn, M. Hagiwara and H. Akagi, "Experimental Verification of a Modular Multilevel Cascade Inverter Based on Double-Star Bridge Cells," in IEEE Trans. Industry Applications, vol. 50, no. 1, pp. 509-519, 2014.
[11] K. Oguma and H. Akagi, "Low-Voltage-Ride-Through (LVRT) Control of an HVDC Transmission System Using Two Modular Multilevel DSCC Converters," in IEEE Trans. Power Electron., vol. 32, no. 8, pp. 5931-5942, 2017.
[12] I. R. F. M. P. da Silva, C. B. Jacobina, A. C. Oliveira, G. A. de Almeida Carlos and M. B. de Rossiter Corrêa, "Hybrid Modular Multilevel DSCC Inverter for Open-End Winding Induction Motor Drives," in IEEE Trans. Industry Applications, vol. 53, no. 2, pp. 1232-1242, 2017.
[13] Y. Okazaki, H. Matsui, M. Hagiwara and H. Akagi, "Design considerations on the DC capacitor of each chopper cell in a modular multilevel cascade inverters (MMCI-DSCC) for medium-voltage motor drives," in Proc. IEEE ECCE, 2014, pp.3393-3400.
[14] H. Akagi, “Classification, terminology, and application of the modular multilevel cascade converter (MMCC),” in Proc. IPEC, 2010, pp. 508-515.
[15] M. Hagiwara, R. Maeda and H. Akagi, "Control and Analysis of the Modular Multilevel Cascade Converter Based on Double-Star Chopper-Cells (MMCC-DSCC)," in IEEE Trans., Power Electron., vol. 26, no.6, pp.1649-1658, 2011.
[16] S. Allebrod, R. Hamerski, R. Marquardt, "New transformerless scalable modular multilevel converters for HVDC-transmission,” in Proc. IEEE PESC, 2008, pp. 174-179.
[17] K. Shi, F. Shen, D. Lv, P. Lin, M. Chen, and D. Xu, “A novel start-up scheme for modular multilevel converter,” in Proc. IEEE ECCE, 2012, pp. 4180–4187.
[18] UC3843 datasheet, Fairchild, 2002.
[19] LOC110 datasheet, IXYS, 2016.
[20] OJE-SS-112HMF datasheet, TE, 2011.