An application of rotary frequency converter for a wind farm system is proposed in this paper. Wind power is getting attention for prospective renewal energy sources, and introduction of wind farm generation will solve environmental matters and energy exhaust problem. However, the quality of power system frequency will be degraded as the capacity of wind power generator increases. Attenuation of mismatch between the served power and load demand is most important problem in order to increase number of wind generators. A wind firm is connected to the proposed rotary frequency converter as a buffer to connect the wind farm and the power system. This provides energy storage function of the output power associated with irregular wind power. The converter has a synchronous generator and a doubly-fed machine. The synchronous motor is connected to the wind farm to supply AC voltage for induction generators of wind power generation units. The shaft of the doubly-fed machine is mechanically jointed to the synchronous motor, and rotates together. Tuning the current of the secondary winding supplied by cycloconverter or self-commutated inverter controls the output power. At present, doubly-fed machine is commonly used in pumped storage hydraulic power plants recently constructed. The proposed scheme is an expansion from the present application. In order to evaluate the proposed scheme, firstly, load reference simulation test is carried out for discussion of the power exchange response to a power grid. Secondly, experimental study is discussed in order to identify the required capacity for the rotary frequency converter to improve the frequency quality. These studies show preferable characteristic for the viewpoint of power system quality improvement.