Dissertation Announcement for Ali Zakerian – 12/08/2022 at 9:00 AM
November 29, 2022
Dissertation Title: Relaxing dc Capacitor Voltage of Power Electronic Converters to Enhance their Stability Margins.
Date and time: Thursday, December 08, 2022, 9:00 AM
Venue: Simrall-228 (Conference Room)
Candidate: Ali Zakerian
Degree: Doctor of Philosophy, Electrical and Computer Engineering
Committee:
Dr. Masoud Karimi-Ghartemani
Professor of Electrical and Computer Engineering
(Major Professor)
Dr. Yong Fu
Professor of Electrical and Computer Engineering
(Committee Member)
Dr. Seungdeog Choi
Associate Professor of Electrical and Computer Engineering
(Committee Member)
Dr. David A. Wallace
Assistant Clinical Professor of Electrical and Computer Engineering
(Committee Member)
Abstract:
Recently, due to the increasing adoption of distributed energy resource (DER) technologies including battery energy storage (BES) and electric vehicle (EV) systems, bidirectional power converters are becoming more popular. These converters are broadly utilized as interface devices and provide a bidirectional power flow in applications where the primary power supply can both supply and receive energy. A dc capacitor, called the dc link, is an important component of such bidirectional converters. For a wide range of applications, the converter is required to control the dc-link voltage.
Commonly, a proportional-integrating (PI) controller is used by the dc capacitor voltage controller to generate a set-point for the inner current controller. This approach tightly regulates the dc-link voltage to a given value. The research presented in this dissertation shows that such an approach compromises the stability margins of the converter for reverse power flow and weak grid conditions. It is shown that by allowing a small variation of dc capacitor voltage in proportion to the amount of power flowing through the converter, the stability and robustness margins are improved. This approach also simplifies the design process and can be applied to both dc/dc and dc/ac (single-phase and three-phase) converters. Moreover, it grants an inherent power sharing capability when multiple converters share the same dc link terminals; removing the need to a communication link between parallel converters. The proposed controller is equipped with a current limiting mechanism to protect the converter during low-voltage/over-current transients. Detailed analyses, simulations, comparisons, and experimental results are included to illustrate the effectiveness of the proposed control approach.
To mathematically establish the properties of the proposed method in a single-phase dc/ac application, this dissertation also derives a new and systematic modeling approach for a grid-connected bidirectional single-phase inverter controlled in stationary frame. Implementing the control system in the stationary frame has advantages over rotating frame. However, the combination of dc and ac state variables and nonlinearities make its stability analysis challenging. In the proposed model, an imaginary subsystem is properly generated and augmented to allow a full transformation to a synchronous rotating frame. The proposed modeling strategy is modular and has a closed form which facilitates further extensions. It is successfully used to demonstrate enhanced stability margins of the proposed controller.