The frequency stability of the power system is challenged by the high penetration of power electronic interfaced renewable energy sources (RES). This paper investigates the improvements of frequency responses of fully decoupled wind power generators (FDWG) by proposing a novel implementing of ultracapacitors (UC) within a hybrid scheme in real-time simulations of wind power plants. UCs are selected as ideal power sources in fast active power-frequency control due to their high power density and fast-reacting speed. Batteries and UCs combined hybrid energy storage systems (HESS) are formed to complement their characteristics. Droop-based and derivative-based control and virtual synchronous power (VSP) are the selected strategies to control power system frequency stability. The best frequency performance trading off with HESS cost is found by solving an optimization problem. The proposed optimization algorithm is used to define the HESS size and controller parameters. The optimization results are analysed to illustrate the improvements of frequency stability control comparing the results of droop and derivative-based control with the VSP control strategy.