Abstract-The modular multilevel converter (MMC) is distinguished by its modularity, that is the use of standardized submodules (SMs). To enhance reliability and avoid unscheduled maintenance, it is desired that MMC can remain operational without having to shut down despite some of its SMs are failed. Particularly, in this paper, a complete fault diagnosis and tolerant control solution, including the fault detection, fault tolerance, fault localization, and fault reconfiguration, has been proposed to ride through the IGBT open-circuit failures. The fault detection method detects the fault by means of state observers and the knowledge of fault behaviors of MMC, without using any additional sensors. Then MMC is controlled in a newly proposed tolerant mode until the specific faulty SM is located by the fault localization method, thus no overcurrent problems will happen during this time interval. After that, the located faulty SM will be bypassed while the remaining SMs are reconfigured to provide continuous operation. Throughout the fault periods, it allows the MMC to operate smoothly without obvious waveform distortion and power interruption. Finally, experimental results using a single-phase scaled-down MMC prototype with six SMs per arm show the validity and feasibility of the proposed methods. Index Terms-Fault diagnosis, fault tolerance, insulated gate bipolar transistor (IGBT), modular multilevel converter (MMC), open-circuit failure, redundancy, state observer, submodule (SM).

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The modular multilevel converter (MMC) has become more and more attractive in the field of high-voltage high-power dc/ac power conversions. In the very recent years, the dc/dc modular multilevel converter is proposed intended for high-voltage dc/dc applications, such as the solid-state transformers and future dc-grid networks, which is extended from the traditional MMC. However, it remains unknown how to optimize the operation of this dc/dc modular multilevel converter and how to dimension the parameters of the elements in it. In this paper, a detailed analysis is performed to indentify the operating principle of the dc/dc modular multilevel converter. Using these analyses, the selection rules and parameter design method of the components are given to maximize the utilization ratio, improve the power density, and increase efficiency. Finally, the validity and effectiveness of this paper are verified by simulation results.Index Terms-DC/DC conversion, modular multilevel converter (MMC), parameter dimensioning.

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