Modeling, Control, and Fault-tolerant Operation of the Isolated Modular Multilevel DC-DC Converter for MVDC Applications
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Author(s)
Yin, Shiyuan
Advisor(s)
Saeedifard, Maryam
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Abstract
The global transition to green energy generation is accelerating, marked by the increasing integration of renewable energy sources into power grids. Simultaneously, as the electrification of energy demands progresses, there is also growth in energy storage systems and the adoption of novel DC loads, such as electric vehicles and data centers. Such transformations necessitate the modernization of the legacy power grid to adapt to evolving energy requirements. Given this circumstance, DC grid technologies are gaining prominence to accommodate the increasing penetration of renewable energy sources, energy storage systems, and DC loads. DC transformers, as essential components in DC grids, are responsible for interconnecting DC systems with different voltage levels and facilitating flexible power-flow control.
This research is dedicated to the investigation of the isolated modular multilevel DC-DC (IM2DC) converter for medium-voltage DC applications. The objective of this research is to achieve accurate modeling, versatile modulation, performance optimization, and fault protection design of the IM2DC converter, addressing existing gaps in the technical literature. The intricate circuit structure of the IM2DC converter poses challenges in the design of modulation methods and the accurate modeling of the system. To address these challenges, a unified trapezoidal wave (UTW) modulation scheme is proposed for the IM2DC converter, unifying various existing modulation strategies into a cohesive framework. The UTW modulation enables independent control over the rising/falling transition times and amplitudes of AC voltages on both sides, providing the potential for future performance optimization of the IM2DC converter. Furthermore, the introduced harmonic state-space (HSS) modeling method effectively tackles two primary challenges. Firstly, the frequency-domain modeling approach of the HSS equations seamlessly integrates the implementation of the UTW modulation. Secondly, the HSS model adeptly handles the complex harmonic interactions within the converter, ensuring the desired level of accuracy. Following that, the zero-voltage switching (ZVS) boundaries of the IM2DC converter are delineated, considering various system parameters using the HSS model. The analysis then delves into the effects of modulation variables on arm RMS currents under UTW modulation. Subsequently, methods for selecting modulation variables to minimize arm RMS current are proposed to effectively decrease conduction losses. Next, a sensorless method is introduced for balancing SM capacitor voltages in the IM2DC converter. A predetermined switching signal rotation pattern enables SMs to restore voltage within a rotation period. Then, a power boost technique is proposed that leverages the ripple in SM capacitor voltages. This approach enables the design of smaller SM capacitance values while increasing the power transfer capability, leading to an improvement in the power density of the IM2DC converter. Finally, a DC fault detection and protection mechanism is designed to promptly limit and clear fault currents while ensuring uninterrupted DC voltage on the healthy side. Additionally, fault detection and fault tolerant control strategies are developed for the presence of an open-circuit fault in a semiconductor device, effectively avoiding disruptions of the system.
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Date
2024-05-17
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Dissertation