The use of wind-power generation (WPG) increases supply-side variability, and hence reduces the reliability of power generation. Even if WPG is forecasted to deal with such a problem, the uncertainty of WPG cannot be fully resolved because of its randomness and intermittent nature. This paper proposes a two-step energy-storage system (ESS) sizing and operation strategy for enhancing the WPG reliability. From the analysis of WPG data, it is determined that the uncertainty that leads to deterioration in reliability affects the instantaneous changes in WPG significantly. In this regard, ESS sizing and operation strategies are designed based on the frequency-domain analysis using the discrete Fourier transform (DFT). The ESS sizing is firstly determined to filter out high-frequency components, and the ESS operation is determined considering the ESS sizing constraints for reducing the difference between the actual WPG and its forecasting. A case study is presented using the data from a large-scale wind farm with 4782 MW total capacity of 42 plants located in Columbia River Gorge, United States. The numerical results obtained demonstrate that the proposed ESS method can reduce the root-mean-squared error by up to 26% and further reduce by about 3% compared to the conventional method with the ESS. (C) 2017 Elsevier Ltd. All rights reserved.