本論文主要基於電能平衡與電流追蹤法則，探討多階層模組化轉換器之擴充性設計、故障排除以及模組回插等功能。使用的控制法為直接數位控制，透過抵消考量 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。
藉由直接數位控制，模組可獨立控制自身電容電壓，在追蹤電流的同時將電感衰減納入考慮。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，將電感值隨電流變化和控制法則通式存放於控制器中。此控制法的優點在於設計可疊加性單一模組時，更能體現其價值。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，以利於模組的疊加。
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，且系統整體不離線的情況下，判斷故障的模組並且排除故障部分，亦或是將新的模組回插，也是一項隨著系統擴大而漸趨重要的部分。
本研究將直接數位控制法應用於多階層模組化轉換器，並且介紹轉換器架構和控制法達成的各種功能。主要貢獻在探討模組故障排除與模組回插情況下的行為，並提高其可擴充性，進行單一模組的設計。

This thesis presents fault resolution control principle, cell plug-in and module design 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 can compensate the variation effects of dc voltage, switching period and inductance when calculating the duty ratios of switches in a controller.
Through the direct current digital control, each module is able to control its own capacitor voltage and consider the inductance variation with its current. The control law can be formulated readily and saved into a controller, making it possible for inductance 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 the component of signal transmission between local controller and the central controller, 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. System configuration and control methods of current tracking, voltage regulation and fault resolution have been presented. The major contributions of this research are investigating the behavior of MMC under fault resolution and redesigning the module structure based on system scaling purpose.

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