Wind Turbine Generator Systems (WTGSs) are considered to be one of effective approach to solve global warming and energy issues. However, the stability of the output power generated from WTGSs depends on the weather conditions and the natural environment, and any instability in the power causes frequency fluctuations in the power grid. Several methods have been examined to stabilize such fluctuations using batteries. In this paper, we describe the development of an inverter using sodium sulfur (NAS) batteries to stabilize the power from a wind farm and our field experiences indicating excellent operating performance. This system is the world's largest constant-output stabilization system using 34 MW NAS batteries for a 51 MW wind farm at Futamata in the Tohoku district of Japan.