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Title: Modeling, Control, and Fault-tolerant Operation of the Isolated Modular Multilevel DC-DC Converter for MVDC Applications
Committee:
Dr. Saeedifard, Advisor
Dr. Divan, Chair
Dr. Molzahn
Abstract: The objective of the proposed research is to achieve accurate modeling, optimized design and control of the isolated modular multilevel DC-DC (IM2DC) converter, filling the gaps existing in the technical literature. The current research on IM2DC converter only covers its basics of operation. The present mathematical modeling process for IM2DC converter is exactly the same as the dual-active bridge (DAB) converter, neglecting the internal dynamics of the modular multilevel converter (MMC). Moreover, the most widely used modulation technique for the IM2DC converter is the quasi-square-wave (QSW) modulation, which is always treated equally as the square-wave modulation and ignoring its staircases transitions regardless of the converter specifications. When the number of sub-modules (SM) is large or the switching frequency is high, the transition time becomes significant. Neglecting such transitions may jeopardize the converter performance analysis, e.g., the calculations of power transfer capability, current stress, and zero-voltage switching (ZVS) region. Besides, working as a conventional two-level modulation strategy, the QSW modulation lacks flexibility to support the design of optimal control. Last but not least, for the sake of reliable operation of the system, the fault study and a fault-tolerant operation method need to be investigated for the IM2DC converter. In this proposal, the harmonic state-space (HSS) equations are utilized to model the IM2DC converter, with the consideration of internal harmonic couplings to overcome the aforementioned modeling challenges. Such an accurate mathematical model subsequently benefits the operation analysis, passive component sizing and controller design of the converter. Next, a unified trapezoidal-wave (UTW) modulation scheme is proposed, armed with higher degrees of control freedom. Then, the optimal control method of the IM2DC converter based on UTW modulation will be studied. Furthermore, DC fault and power semiconductor failure fault modes of the IM2DC converter will be analyzed, and correspondingly, the necessary fault-tolerant operation methodologies will be proposed.
