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Modeling, Analysis and Advanced Control of Voltage- and Current-Source Converters in Renewable Energy-Based Active Distribution Systems
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- Author / Creator
- Radwan, Amr A A
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This thesis addresses the integration of renewable energy resources into the grid-connected and isolated distribution systems using voltage- and current-source converters. Motivated by its promising potential and attractive features, a one-stage current-source converter (CSC) is selected as an effective interfacing device for the photovoltaic (PV) generators to the utility-grid. The operation of the grid-connected CSC-based PV system is investigated under different operating conditions, parameters variation, and control topologies. From the dc-side, it is found that the variation in the weather conditions, e.g., solar irradiance levels, might affect the dynamic performance of the vector-controlled grid-connected CSCs. Small-signal dc-side impedance models for the CSC and the PV generators are developed to investigate the system stability. Active compensation techniques are proposed so that the system performance is well maintained under different operating points. From the ac-side, a susceptible potential to instabilities is yielded under the very weak grid conditions. This potential significantly increases in the vector-controlled converters. The implementation of the phase-locked loop (PLL) is found to have a detrimental impact on the system stability as the grid impedance increases. Two solutions for the very weak grid integration have been proposed. Firstly, the power synchronization control (PSC) scheme is developed for CSCs in PV applications so that the PLL is no longer needed, and hence a seamless integration to the very weak grid is achieved. Secondly, supplementary compensation loops are proposed and added to the conventional PLL-based vector-controlled CSC so that the negative impacts of the PLL are completely alleviated. In both cases, small-signal state-space models are developed to investigate the system stability and provide a detailed design approach for the proposed controllers. A larger system level integration is considered in this thesis by interfacing multiple distributed generation (DG) units to the conventional distribution system. A generalized hybrid alternating-current (ac)/direct-current (dc) system that constitutes ac and dc subgrids is created to efficiently accommodate the dc-type renewable sources (such as PVs, batteries, fuel cells, etc.) to the ac-type conventional distribution system. Both subgrids are interconnected using voltage-source converters (VSCs) to facilitate a bidirectional power exchange via multiple inversion-rectification processes. To achieve an accurate and efficient operation, a supervisory power management algorithm is proposed. The algorithm operates successfully regardless of the control mode of the individual DG units. However, the integration of severe dynamic loads, such as direct online inductions motors (IMs) into the ac-side of the hybrid system reflects very lightly damped modes, which in turns induce instabilities, particularly in the isolated mode of operation. Therefore, active compensation techniques have been proposed to increase the system damping. Throughout this thesis, time domain simulations under Matlab/Simulink® environment for the systems under study are presented to validate the analytical results and show the effectiveness of the proposed techniques.
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- Subjects / Keywords
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- Graduation date
- Spring 2016
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.