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To promote the introduction of wind power conversion systems, which have an unstable power output, the adoption of energy storage system is being considered in Japan as a measure for mitigating variations in wind farm (WF) output. In a research project which started in 2003, a battery energy storage system (BESS) was coupled to an existing WF with the aim of stabilizing short-term fluctuations (with periods of seconds to tens of minutes) of WF output, and demonstration tests on smoothing of the fluctuations were carried out. This paper presents the results of analyzing the data obtained by the demonstration tests. In the demonstration facility, the vanadium redox-flow battery of 6 MW and 6 MWh capacity was installed in the 30.6 MW WF in Hokkaido and smoothed out the fluctuations using a first-order lag filter. A control for maintaining the charging level within the proper range and a control for reducing excessive power output of BESS were adopted so that a BESS with a small capacity compared with the WF capacity was able to smooth out the fluctuations. Furthermore, to reduce system losses, a control to pause the operation of some of the battery banks in the case of low WF output was also adopted. The demonstration tests were conducted from January 2005 to February 2008. It not only verified the effect of each adopted control but also optimized the control parameters until May 2007. After that, a continuous-operation test in which all the controls were installed was carried out from June 2007 to February 2008. The aim of the continuous-operation test was to demonstrate the ability of the BESS to smooth out fluctuations over a long period of time under a set of constant parameters. As a result of analyzing the data obtained through the continuous-operation test, the effectiveness and applicability of the BESS for smoothing out short-term fluctuations of the WF output were verified. The characteristics of the charging level and output of the BESS were also clarified. - With regard to the smoothing performance, the output fluctuation for a period of up to 30 minutes was reduced by over a factor of 3.