A variable-speed variable-frequency（VSVF） operational mode for the induction generator wind turbine system is proposed based on the fractional frequency power generation & transmission technique. A control strategy aimed at optimizing wind power output using the vector control technology is introduced without measuring the wind velocity. A VSVF wind-power system model is developed with PSCAD/EMTDC， considering the effect of wake effects on the wind turbine coefficient. Based on the model， two types of control strategy， i.e. optimal wind power output control strategy and optimal wind power capturing control strategy are analyzed and compared. Simulation results demonstrate that the optimal wind power output control strategy is more efficient than the optimal wind power capturing control strategy in improving the output power quality.