Conventional radial AC distribution systems cannot effectively accommodate the rapidly increasing renewable energy sources (RESs) and new loads such as fast charging stations of electric vehicles. To address the pressing challenges, active distribution grids and DC systems have attracted significant interests with their many potential benefits. Considering that AC and DC systems will coexist in future distribution grids wherever suitable, hybrid AC/DC distribution is regarded as a promising and practical solution for future distribution systems. The focus of this work, multiport interlink modular multilevel converter-based solid-state transformers (iMMC-SSTs), is expected to play a key enabling role in hybrid distribution systems to integrate different grid entities, including both AC and DC networks at both medium and low voltage levels. The iMMC-SST features capabilities such as bidirectional power transfer, fault isolation and restoration, system reconfiguration, and voltage regulation. However, power electronics-based SSTs are more vulnerable under abnormal conditions, which hinders their adoption in practical systems. The high number of circuit elements are potential fault sources in the iMMC-SST. The possible faults of the SST and the connected feeders can destroy balance of the system and even result in second faults. A comprehensive protection scheme for the iMMC-SST is indispensable to ensure the device's safety and improve the system's reliability and robustness. Based on the fault location, abnormal conditions are in general divided into external and internal types. In this work, grounding scheme for the SST is designed and investigated to address typical external fault conditions such as the single line-to-ground (SLG) short-circuit fault and single pole-to-ground (SPG) short-circuit fault. For internal abnormal conditions, power switch faults are of major concerns of the iMMC-SST since a switch failure will lead to arm voltage imbalance, circulating current increase, and second faults. The submodule (SM) switch open-circuit (OC) fault analysis is presented considering different operation modes of the iMMC-SST in Chapter 4. Unlike traditional MMC applications, the iMMC-SST has different fault characteristics, and the previous fault diagnosis and fault-tolerant schemes developed for other applications are not applicable here. Based on detailed analysis of the fault behaviors, a fault-tolerant scheme based on the global redundant module and unbalanced control is proposed in Chapter 5. Similarly, the dual active bridge (DAB) switch OC fault is studied in detail, and a DC current injection fault-tolerant method is proposed to address the overcurrent and transformer saturation issues in Chapter 6. The proposed solutions and their analysis are verified with MATLAB simulations and experiments with scaled-down laboratory prototypes.
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Thesis advisor: Wang, Jiacheng (Jason)
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