Energy storage sizing for low-inertia microgrids, and lessons learned from a potential microgrid



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The coordinated control of multiple distributed generators in a microgrid and the preservation of adequate system inertia in real-time operations are some of the principal technical challenges for stable microgrid operation. One issue in particular pertains to grid-tied inverters, which, as mandated by present standards, are only permitted to operate at unity power factor, thereby requiring the microgrid’s synchronous generators to operate at a low power factor. This behavior accordingly introduces ramifications by limiting the generator’s active power output, which would compromise frequency and voltage stability margins. Consideration is also given to the effect of line impedances, since interconnecting microgrid lines can be described by a variety of X/R ratios that affect the control and flow of active and reactive power. Moreover, the absence of a stiff grid presents control challenges for grid-tied inverters due to the inverters’ tendency to regulate the voltage at the point of common coupling. These same inverters also jeopardize microgrid stability due to their low equivalent inertia as traditional forms of generation (i.e., spinning sources) become displaced by inertia-less inverters. Because of this low microgrid inertia, fluctuations in the output power of renewable energy sources or changes in local load levels may lead to power quality or frequency/voltage stability concerns. Therefore, energy storage sizing is investigated in this dissertation, as it is closely related to the stability analysis of microgrids. Furthermore, an existing residential community (in Austin, TX) described by a moderate penetration of photovoltaic sources and electric vehicle charging is considered, and the implications of said community being retrofitted to a microgrid are examined.