Microgrid stability studies have primarily focused on complete knowledge systems where all state equations are known. These include both linearized small signal methods as well as nonlinear large signal methods. However, many modern applications require microgrids to be constructed as a culmination of individually designed subcomponents. Each subcomponent may have incomplete knowledge of its adjoining subcomponents and therefore presents a unique engineering challenge. Interface design guidance and stability assessment practices for incomplete knowledge systems are critical for ensuring safe operation. The state-of-the-art has only established these practices in the small signal realm, but they fall short of addressing some complexities found in modern systems. Practical large signal methods of ensuring stability at an interface with incomplete knowledge are nonexistent in the literature. The research presented in this thesis addresses not only the shortfalls of small signal methods but also provides design and assessment guidance for large signal stability of DC power electronics-based microgrids.
Small signal stability is a necessary qualifier for ensuring large signal stability. This thesis begins by addressing the shortfalls of the state-of-the-art small signal methods by proposing two stability criterions: The Generalized Nyquist Criterion for DC Networks and The Relative Stability Criterion. The small signal methods found in the research literature fail to address complex interfaces that arise due to system redundancy requirements or common mode parasitic feedback paths. The methods found in the research literature also fail to quantifiably enforce a desired transient performance of an integrated system. Poorly damped resonant modes can be just as hazardous as classically defined unstable operation. These two methods provide the foundation to begin large signal stability analysis by establishing small signal stability of a system’s operating point.
A large signal stability criterion for interconnected systems, analogous to small signal impedance-based black-box methods, has been long sought after in the research community. Traditional large signal studies have primarily focused on complete knowledge systems due to the limitations of nonlinear mathematics. There has been some work on small gain methods but these are not practical for higher order generalized DC microgrid power systems. The work included in this thesis expands on the small signal concept of passivity and passivity indices to newly define a Domain of Passivity and establish the first practical large signal stability criterion: The Generalized Passivity-based Stability Criterion. The stability assessment and design tool presented in this thesis utilizes the stabilizing concept of passivity that has been traditionally used in linear analysis and extends it to general nonlinear large signal practices.
This thesis concludes by exploring the use of nonlinear control for enforcing global system passivity and therefore meeting the proposed passivity-based stability criteria. For systems with large transient loads, a cascaded converter topology can be appealing for DC microgrids due to its natural transient filtration properties and protection of critical upstream energy sources. The Interconnected and Damping Assignment Passivity Based Control (IDA-PBC) design framework is applied to a cascaded converter system. It is shown that in addition to large signal stability, the addition of energy-based control parameters can provide an intuitive methodology for tuning system dynamics.
|Commitee:||Kabalan, Mahmoud, Nataraj, Chandrasekhar, Niebur, Dagmar|
|Department:||Department of Electrical and Computer Engineering|
|School Location:||United States -- Pennsylvania|
|Source:||DAI-B 82/7(E), Dissertation Abstracts International|
|Subjects:||Electrical engineering, Engineering|
|Keywords:||Asymptotic stability, Interconnected systems, Lyapunov methods, Nonlinear dynamical systems, Power system stability, Stability criteria|
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