Project Report

From June to August 2011, under the National Science Foundation East Asia Pacific Summer Institute Award, I worked with the Digital Power Systems research team at Tsinghua University, Beijing, to model and simulate a microgrid and analyze the effects of its operation on the electric power distribution system. Defined as an aggregation of loads and microsources that rely on power electronics and control systems to operate as a single, self controlled system, microgrids are small power systems designed supply reliable, high-quality power to consumers, even in the case of an outage in the bulk power system. In addition to providing increased reliability, microgrids are a means of integrating distributed renewable generation and variable demand-side resources, which are key objectives of Title XIII of the Energy Independence and Security Act of 2007. While in Beijing, my research team and I collaborated with a local Chinese engineering service company to model a microgrid they had recently commissioned at the Caizhuan School in Zhengzhou city, Henan province. One of the first commercial installations in China, this microgrid is composed of 350 kilowatts of solar photovoltaic arrays, two 100 kilowatt-hour Lithium Iron Phosphate battery storage banks, about 500 kilowatts peak load from the student residence halls and cafeteria and a Battery Energy Storage Controller. Capable of operating completely islanded from the campus grid, the microgrid is monitored and controlled by a local Supervisory Control and Data Acquisition (SCADA) system that exchanges data with and provides remote control capability to the local municipal utility control center. Under the EAPSI award, I developed and implemented a Dynamic Programming algorithm to optimize the daily charge/discharge patterns of the battery storage system to achieve the lowest power consumption costs for the Caizhuan School. The algorithm takes into account the school’s unique Time-of-Use (TOU) rate structure, day-ahead load and solar photovoltaic power output forecasts, as well as the battery storage system’s operating constraints. Using historical SCADA data, I simulated battery operation over a seven day period. As expected, savings could be achieved by shifting energy consumption from high cost to low cost times, i.e. energy arbitration, and there is a real economic incentive for implementing energy storage systems in this manner. We found that for small implementations, the accuracy of the load and microsource generation forecasts is insignificant and savings are largely dependent primarily on the rate structure and the amount of energy arbitrated. By varying energy storage capacity in the simulations, we also found that battery operating constraints impose a natural limit on the benefits that can be realized from increased capacity. That is, battery energy storage systems need to be appropriately sized for the application in terms of both capacity and power rating. While in Beijing, I also researched the effects of the microgrid and battery energy storage system operation on the distribution network. Using distribution system simulation software developed by the Electric Power Research Institute (EPRI), I integrated the Caizhuan system with a standard power system model to investigate any potential abnormal operating conditions. Preliminary findings show that increased penetration of distributed generation affects distribution line voltages by reducing downstream load. This is beneficial in some regards due to reduced line losses and less stress on the grid. However, it can also cause un-anticipated operation of line-drop compensation regulators and adverse effects on feeder voltages with these regulators. In addition to academic research, while in Beijing China, I also engaged in a cultural events and outreach activities, including technical exchange visits to a provincial-level Smart Grid Research Center, the China State Key Lab at Tsinghua University and a local engineering firm that implements microgrid technology. And since I am fluent in oral and written Chinese and have experience living in China, I also acted as a cultural ambassador for other EAPSI fellows and program administrators. For instance, I was chosen by the U.S. Embassy and the Chinese Ministry of Science and Technology to host a break-out session for the first annual U.S. - China Young Scientists Forum. The aforementioned research is ongoing and the University of Minnesota and Tsinghua University are actively collaborating in this area.

National Science Foundation (NSF)
Office of International and Integrative Activities (IIA)
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Carter Kimsey
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Gantz Jesse
United States
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