UBC Theses and Dissertations
Hierarchical control and stability analysis of AC microgrids with virtual-oscillator controlled sources Mahdavyfakhr, Mohammad
Microgrids are envisioned as building blocks of evolving power systems, where they have a potential for improved resiliency, higher energy efficiency, and integration of renewable resources on a smaller scale. At the same time, the increased complexity of integrating new and distributed energy resources (DERs) presents several control-related challenges for the safe and optimal operation of AC microgrids. The new challenges also include the increasing number of power-electronically interfaced loads and sources that behave as constant power loads (CPLs) or constant power sources (CPS), and are known to have destabilizing effect on the system. All of this necessitates research for better analysis tools, control schemes, and technological solutions that ensure proper coordination of all components within a microgrid. To improve the operation of envisioned AC microgrids, this thesis proposes an economical hierarchical control scheme that incorporates control levels with different dynamics. This scheme not only ensures proper voltage and frequency regulation and power sharing among different resources, but it also minimizes the operational costs considering the price of available energy resources. At the next level, this thesis investigates dynamic interactions between the DERs and CPLs within a microgrid. A recently proposed virtual oscillator controlled source (VOS) is investigated and compared against the traditional/generic control used with DERs. It is demonstrated that impedance-based analysis can accurately predict the instability and sideband oscillations resulted from the interaction with the CPL, as well as used to tune the controllers. It is also shown that while the VOS has a faster dynamic response, its stability boundaries may actually be smaller. It is envisioned that the presented research will contribute to the development of AC microgrids with high penetration of renewables, energy storage systems, as well as various types of electronic loads.
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