Browsing by Author "Lee, Lung-An"

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    Critical values of cyber parameters in a dynamic microgrid system
    Lee, Lung-An; Liu, Chen-Ching; Wang, Jingyu; Appiah-Kubi, Jennifer; Schneider, Kevin P.; Tuffner, Francis K.; Ton, Dan T. (2022-01)
    An islanded microgrid is cyber-physical system, and the control relies on the communication system significantly. Improper parameters of the cyber system can result in instability of a microgrid system. To evaluate the impact of a networked control system on control performance, a cyber model is developed to represent data acquisition periods and communication delays. Simplification of the networked control system model is proposed to enhance the computational performance, making the analytical method applicable for large-scale systems. Based on the analysis, a two-dimensional stability region of a microgrid in the space of cyber parameters can be obtained. To validate the proposed method, a microgrid control scheme is proposed for power dispatch and regulation based on the droop and proportional-integral (PI) feedback control. The analytical method is compared to the time-domain simulation, and it is shown that the stability regions are nearly identical. The critical values of cyber parameters are determined based on the analytical results. The proposed control strategy with the given cyber parameters is validated for transient stability following dynamic events. Simulation results indicate that the design of a microgrid as a cyber-physical system needs to be guided by critical values for cyber parameters to prevent system instability.
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    Microgrid as a Cyber-Physical System: Dynamics and Control
    Lee, Lung-An (Virginia Tech, 2023-05-15)
    As a result of climate change, extreme events occur more frequently and at higher severity, causing catastrophic power outages with significant economic losses. Microgrids are deployed as a technology to enhance power system resilience. A microgrid may include one or more distributed energy resources (DERs), including synchronous generators, solar panels, wind turbines, and energy storage systems which are decentralized power sources primarily in a distribution system to enable system recovery from catastrophic events. Microgrids can be operated in a utility-connected mode or an islanded mode in separation with the hosting transmission or distribution system. As major disasters occur, intentional islanding of a microgrid is a strategy to serve critical loads, within or outside the microgrids, until the utility service is restored. To operate microgrids, dispatch and control capabilities are required that would significantly improve the dynamic performance of the microgrid. An islanded microgrid can be used to serve critical load as a resiliency source when a severe outage occurs. In an islanded mode, control of a microgrid relies on the communication system significantly. Hence, microgrids are cyber-physical systems and, therefore, the cyber system plays a crucial role in the performance of the cyber-power system. Improper parameters of the cyber system can result in instability of a microgrid system. Simplification of the networked control system model is needed to enhance the computational performance, making the analytical method practical for large-scale power systems. To reduce the emission of carbon dioxide and alleviate the impact of climate change, the electric power industry has been integrating renewable energy into the power grid. The high penetration of renewable energy at an unprecedented level also raises new issues for the power grid, e.g., low inertia, degraded power quality, and higher uncertainties. Power electronics technology is used for power conversion of renewable energy. As the level of penetration of renewable energy increases, the inverter-based resources (IBRs) are being installed at a fast pace on the power grid. Compared to conventional synchronous generators (SGs), a major technical challenge of IBRs is their low inertia which can lead to system instability. In this context, the work of this dissertation results in major contributions regarding control algorithms for microgrid resilience, stability, and cyber-physical systems. Specifically, three novel contributions are presented: 1) A coordinated control scheme is proposed to achieve the goals of power dispatch and system regulation for an islanded microgrid. The proposed control scheme improves system dynamics; 2) A method is developed for the determination of critical values for the data reporting period and communication delay. Based on the proposed method, a 2-dimensional stability region of a microgrid in the space of cyber parameters is derived and critical values of cyber parameters are identified based on the stability region; 3) A control scheme is proposed to improve system stability of a hybrid-DER microgrid. The analysis serves to illustrate the stability regions of the hybrid-DER microgrid. A control methodology based on two-time scale decomposition is developed to stabilize the system.