Design, Analysis, and Control of the Modular Multilevel DC/DC Converter for Medium- and High-Voltage DC Grids
Creators: Razani, Ramin
Publisher: name: University of Alberta Library
Year: 2022

<p>Nowadays, renewable energy sources have gained escalating importance due to environmental and economic reasons. However, these energy sources are primarily located in remote areas and distant from load centers. High-voltage dc (HVDC) and medium-voltage dc (MVDC) systems have been proposed in the last decades for efficient and reliable integration of renewable energy resources. To date, a noticeable number of these dc systems are established around the world. Recently, researchers have proposed the concept of “DC grids,” which can be realized by connecting the existing point-to-point dc systems. This structure can improve the efficiency and stability of the power system. However, one of the most concerning challenges related to this concept is the interconnection of already built dc systems. Because existing dc systems are built through time, they possibly have different voltage levels and grounding systems. To address this challenge, the dc/dc modular multilevel converter (MMC) is proposed in the literature as one of the most promising solutions. This converter offers the advantages of modularity, scalability, and high efficiency. <br>
Few studies have been conducted on the modeling and control of the dc/dc MMC. The literature falls short in several aspects, such as improved design, analysis of operation limits, fault-tolerant operation, converter analysis under uncertainty, and development of advanced controllers and efficient fault-blocking capability. This research aims to 1) develop an augmented design approach that considers both control and hardware aspects of the converter, 2) investigate the operation limit of the hybrid dc/dc MMC caused by the capacitors voltages unbalance, 3) develop a tailored fault-tolerant operation strategy without additional submodules (SMs), 4) analyze the unsymmetrical operation of the dc/dc MMC caused by parametric uncertainty, 5) develop an advanced controller based on the model predictive control for the dc/dc MMC, and 6) realize an efficient fault-blocking capability by proper selection of SMs. <br>
The first study in this thesis facilitates the dc/dc MMC design with a smaller number of SMs and higher efficiency. Unlike the previous literature, the analytical results of the second study show that the capacitors voltages balance in the hybrid dc/dc MMC limits the operation range of the converter. In the third study, first, the unique features of the dc/dc MMC are investigated. These features make the fault-tolerant operation possible without the need for additional SMs. Then, utilizing these features, a tailored fault-tolerant operation strategy is developed to cope with several SMs failures. When the parametric uncertainty comes into action, it can force the converter to work in unsymmetrical conditions. The fourth study develops steady-state models representing the behavior of the converter in unsymmetrical conditions, and then the maximum tolerable variation of parameters is found in different practical cases. An advanced controller based on the model predictive control is developed in the fifth study to improve the steady-state and transient performances of the dc/dc MMC. Finally, an efficient fault-blocking capability is realized by adequately selecting the number and type of SMs. Detailed time-domain simulations under the MATLAB/Simulink environment validate the analytical results. This research contributed to the fundamental understanding of the dc/dc MMC operation and significantly improved the converter efficiency, reliability, and steady-state and dynamic performances.</p>