Empowering the electric grid: Can SMES coupled to wind turbines improve grid stability?
Introduction
The variability in wind speed coupled with conditional changes in the level of energy consumption with respect to time has made the need for energy storage indispensable in increased penetration of wind energy. Energy sources with predominantly stable output power levels tend to more easily integrate into the electric grid than those with unstable levels [1]. Fink found that for 5 different energy sources representing a range of timescales, integration of natural gas, thermal and nuclear energy were easier than renewables [1]. Increased wind energy integration into the electric grid is a long-standing challenge in electrical engineering. A traditional method for such integration utilizes the principle of micro-siting, wherein assessment tools are used to determine the position of a wind turbine on a parcel of land so as to maximize wind power production that can be injected into the grid [2]. The success of this method ultimately depended on the accuracy of the wind data at the potential site and the information on site constraints [2]. Much of the current focus in improving wind energy integration has been on accurate wind forecasting and improvements in power electronics. Efforts to improve wind forecasting have included probabilistic models [3], [4], site specific constraints [5] and aggregating larger wind data [6]. In addition, the recent progress and explosion in power electronics especially flexible alternating current transmission systems (FACT devices) has aided in improving the transmission capacity of wind energy but has not enabled an increase in the maximum output of the transmission line. Thus once a transmission line's capacity is reached, wind energy will have to be curtailed [7], [8]. Energy storage on the other hand provides a wide range of power system security related benefits such as spinning reserve, frequency control, peak shaving and power quality [9]. Although, several publications on energy storage technologies exist in literature [10], [11], [12], [14], [16], a comparison between multiple storage technologies and their efficacy specific to excess wind curtailment has not been established previously. Our paper goes beyond recently published literature by focusing on four major areas (1) to show that storage systems with greater efficiency and faster charge and discharge times exhibit a strong linearity to a reduction in excess wind energy wastage, (2) to analyze current magnet grade superconductors and design a novel state-of-the-art SMES using MgB2 that mitigates momentary interruptions in wind turbines, (3) to establish the electrical performance and stability of the SMES wire using a BJR model and von-Mises stress analysis and (4) to demonstrate using EMTDC™/PSCAD™ simulations that the SMES coupled with wind turbines is effective at alleviating momentary interruptions for ∼50 s in 3 MW wind turbines.
Section snippets
Methods
A literature search was performed for articles containing the keywords “wind energy”, “grid integration” and “storage.” Articles were considered for inclusion if they contained any data on time profiles, load levelling and voltage instability. The year 1985 was established as a cutoff since this period largely coincides with a shift from the traditional centralized utility system consisting of thermal power plants to a decentralized system with an integrated mix of traditional and
Results and discussions
Countries like Denmark have high penetration of wind energy into their electric grid without much energy storage chiefly due to the greater interconnectivity in the Danish/European electric grid when compared with the U.S. Thus the lack of large scale interconnectivity in the U.S. electric grid warrants the need for a storage system with increased penetration of wind/renewable energy.
Conclusion
The excess wind energy plot was used to demonstrate that a strong negative correlation existed between the cycles to failure of a storage device and the excess wind energy on the system. This implied that a SMES with its high efficiency of ∼95% and instantaneous response time is perfectly suited to mitigate wind energy integration problems when compared to traditional forms of energy storage. To combat momentary interruptions for ∼50 s in 3 MW wind turbine a1MJ SMES storage was designed. The
Acknowledgments
The authors would like to thank Dr. Yukikazu Iwasa (Head, Magnet Technology Division, MIT Francis Bitter Magnet Laboratory) for his valuable guidance on magnet design.
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