Operation of microgrids with conventional and virtual energy storage systems
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2022
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Abstract
Distribution systems are now increasingly becoming more active due to the sustainable integration of Distributed Energy Resources (DER). While this has enabled a cleaner and more efficient generation, it has also resulted in new challenges for the operation of modern power systems. In this context, the operation of isolated microgrids is particularly challenging, as these systems are characterized by a low inertia and significant renewable integration, and must be capable of an autonomous operation without the support of other electrical grids. Thus, the present thesis focuses on the design of an Energy Management System (EMS) for the reliable and economic operation of modern isolated microgrids.
Isolated microgrid operation requires considering additional aspects typically omitted in the operation of robust bulk power systems. In particular, as demonstrated in this thesis, second-to-second renewable power fluctuations need to be considered in the microgrid EMS, since these fluctuations can have a large impact on the system’s frequency regulation due to its low inertia. Furthermore, to ensure an economic yet reliable operation, modern flexible technologies capable of counterbalancing these short-term fluctuations, such as Battery Energy Storage Systems (BESS) and Demand Response (DR), need to be integrated in the microgrid EMS. Hence, the present thesis focuses on designing a microgrid EMS model that integrates short-term renewable power fluctuations, their impact on frequency regulation, and the role that BESS and DR can play for their management. In the first part of the thesis, models are presented to characterize short-term renewable power fluctuations and their impact on microgrid operations, including the role that BESS can play to manage power fluctuations and the battery degradation resulting from providing this service. These models are then used to develop a practical EMS considering short-term renewable fluctuations and BESS flexibility, which is validated through exhaustive simulations on two realistic test microgrids, showing the operational benefits of the proposed EMS and highlighting the need to properly model short-term fluctuations and battery degradation in EMS for isolated microgrids. In the second part of the thesis, the above EMS model is extended to also incorporate the impact of short- term power fluctuations on the microgrid’s frequency regulation performance. For this purpose, accurate linear equations describing the frequency deviation and Rate-of-Change-of-Frequency (RoCoF) resulting from these fluctuations are developed, which are then used to build a frequency-constrained EMS model capable of guaranteeing an adequate frequency regulation performance in line with current DER operating standards. Exhaustive transient simulations on a realistic test microgrid considering detailed frequency dynamic and control models are presented, demonstrating the accuracy of the proposed frequency-constrained EMS and the operational benefits resulting from its implementation.
Finally, the integration of DR techniques for an enhanced microgrid operation is discussed. In particular, the smart control of Thermostatically Controlled Loads (TCL) is studied, as these type of loads comprise a significant share of the total residential demand, and have the capability of managing second-to-second power imbalances without significantly affecting customer comfort. Since computational limitations prevent the direct integration of TCLs within operational models, alternative computationally efficient aggregate models representing TCL flexibility and frequency dynamics are proposed, which are referred to as Virtual Energy Storage Systems (VESS) due to their close resemblance to Conventional Energy Storage Systems (CESS) such as batteries. The proposed aggregate VESS models are then used to design a practical EMS integrating TCL flexibility, and study the impact of TCL integration on microgrid operation and frequency control. Computational experiments using detailed frequency transient and thermal dynamic models are presented, demonstrating the accuracy of the proposed aggregate VESS models, as well as the economic and reliability benefits resulting from using these aggregate models to integrate TCLs in microgrid operation.
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Tesis (Doctor in Engineering Sciences and Doctor of Philosophy)--Pontificia Universidad Católica de Chile, 2022