This paper proposes an application of rotary frequency converter for a wind farm system. 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. Here, a large rotary frequency converter is proposed 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. For the evaluation of the proposed scheme, two simulation tests are carried out. One is load reference setting test for discussion of the power exchange response to a power grid. Other is load frequency control test in order to discuss required capacity for the rotary frequency converter to improve the frequency quality.