Date of Defense
12-12-2024 5:00 PM
Location
F1-1164
Document Type
Dissertation Defense
Degree Name
Doctor of Philosophy in Electrical Engineering
College
College of Engineering
Department
Electrical and Communication Engineering
First Advisor
Prof. Hussain Shareef
Keywords
Renewable Energy-based Distributed Generation systems, Voltage Source Inverter, grid-forming, Feedback Linearization, Disturbance Observer, Double Fundamental Frequency Oscillations.
Abstract
This thesis addresses two critical issues in Renewable Energy-based Distributed Generation (REDG) systems that operate autonomously to supply unbalanced and nonlinear loads. The first issue concerns voltage regulation in grid-forming three-phase Pulse Width Modulation Voltage Source Inverters (PWM VSIs), which convert DC power generated by Distributed Generators (DGs) into three-phase AC power. These inverters often face challenges when feeding unbalanced and nonlinear loads, leading to poor voltage quality. To address this, a voltage controller was developed for grid-forming three-phase PWM VSIs to deliver symmetrical and high-quality voltage to such loads. The proposed controller integrates a Disturbance Observer (DO) into a state feedback controller designed using the Feedback Linearization (FL) framework to address a range of disturbances, including non-oscillatory disturbances and oscillatory disturbances that represent the harmonic oscillations induced by unbalanced and nonlinear loads. The controller’s steady-state and transient responses were verified through simulation and experimentation, with a comparative analysis conducted against a conventional cascaded PI control scheme. The findings demonstrate the controller’s capability to accurately track the reference voltage while complying with IEEE voltage quality standards under the tested conditions. The second issue addressed in this thesis is the occurrence of Double Fundamental Frequency Oscillations (DFFO) in the voltage across the DC link, particularly when unbalanced AC loads are connected to the system. These oscillations, which circulate in the DC link capacitor as a result of feeding unbalanced AC loads, can lead to the degradation of the capacitor at the DC link and subsequently a reduction in its lifespan. To mitigate these oscillations, an energy storage (ES)-based compensation strategy was developed. This strategy is executed by incorporating a Disturbance Observer (DO)-based control scheme to operate the interfacing DC-DC converter of the ES. This control scheme enables the ES to effectively compensate for DFFO in the DC link voltage while maintaining its power balancing function. The effectiveness of this strategy was evaluated through a comparative study with a conventional control scheme that cannot compensate for the DFFO, under various operating modes and transient conditions. The results demonstrated that the proposed strategy effectively mitigates DFFO, leading to a stable and smooth DC link voltage under the tested conditions. Additionally, the strategy reduced the need for large DC link capacitors, which are typically used to dampen the DFFO. The research findings highlight the importance of advanced control schemes for power electronic converters (PECs) in ensuring the reliable performance of REDG systems when supplying loads that present unbalance and nonlinearity. Moreover, the integration of disturbance observer techniques into control strategies significantly enhances controller performance under loads that draw unbalanced and distorted currents and improves response to load transients. Furthermore, these control strategies can also help eliminate the need for larger circuit components that are typically required to mitigate the oscillations within the system. The proposed solutions contribute to the optimization of REDG systems, ensuring stable and efficient operation under diverse load conditions. A broader consequence of these developments is the enhanced reliability and efficiency of REDG systems, facilitating their seamless integration into power networks and significantly advancing the development and deployment of renewable energy technologies.
Included in
DISTURBANCE OBSERVER-BASED CONTROL SCHEMES FOR OFF-GRID RENEWABLE ENERGY-BASED DISTRIBUTED GENERATION SYSTEMS FEEDING UNBALANCED AND NONLINEAR LOADS
F1-1164
This thesis addresses two critical issues in Renewable Energy-based Distributed Generation (REDG) systems that operate autonomously to supply unbalanced and nonlinear loads. The first issue concerns voltage regulation in grid-forming three-phase Pulse Width Modulation Voltage Source Inverters (PWM VSIs), which convert DC power generated by Distributed Generators (DGs) into three-phase AC power. These inverters often face challenges when feeding unbalanced and nonlinear loads, leading to poor voltage quality. To address this, a voltage controller was developed for grid-forming three-phase PWM VSIs to deliver symmetrical and high-quality voltage to such loads. The proposed controller integrates a Disturbance Observer (DO) into a state feedback controller designed using the Feedback Linearization (FL) framework to address a range of disturbances, including non-oscillatory disturbances and oscillatory disturbances that represent the harmonic oscillations induced by unbalanced and nonlinear loads. The controller’s steady-state and transient responses were verified through simulation and experimentation, with a comparative analysis conducted against a conventional cascaded PI control scheme. The findings demonstrate the controller’s capability to accurately track the reference voltage while complying with IEEE voltage quality standards under the tested conditions. The second issue addressed in this thesis is the occurrence of Double Fundamental Frequency Oscillations (DFFO) in the voltage across the DC link, particularly when unbalanced AC loads are connected to the system. These oscillations, which circulate in the DC link capacitor as a result of feeding unbalanced AC loads, can lead to the degradation of the capacitor at the DC link and subsequently a reduction in its lifespan. To mitigate these oscillations, an energy storage (ES)-based compensation strategy was developed. This strategy is executed by incorporating a Disturbance Observer (DO)-based control scheme to operate the interfacing DC-DC converter of the ES. This control scheme enables the ES to effectively compensate for DFFO in the DC link voltage while maintaining its power balancing function. The effectiveness of this strategy was evaluated through a comparative study with a conventional control scheme that cannot compensate for the DFFO, under various operating modes and transient conditions. The results demonstrated that the proposed strategy effectively mitigates DFFO, leading to a stable and smooth DC link voltage under the tested conditions. Additionally, the strategy reduced the need for large DC link capacitors, which are typically used to dampen the DFFO. The research findings highlight the importance of advanced control schemes for power electronic converters (PECs) in ensuring the reliable performance of REDG systems when supplying loads that present unbalance and nonlinearity. Moreover, the integration of disturbance observer techniques into control strategies significantly enhances controller performance under loads that draw unbalanced and distorted currents and improves response to load transients. Furthermore, these control strategies can also help eliminate the need for larger circuit components that are typically required to mitigate the oscillations within the system. The proposed solutions contribute to the optimization of REDG systems, ensuring stable and efficient operation under diverse load conditions. A broader consequence of these developments is the enhanced reliability and efficiency of REDG systems, facilitating their seamless integration into power networks and significantly advancing the development and deployment of renewable energy technologies.