Energy storage system can enhance the flexibility of power system containing wind power through buffering the fluctuation and uncertainty of the net load. In order to taking into account the intra hourly ramping needs, this paper proposes an optimal capacity allocation model of energy storage considering the two time scale scheduling cycles:hourly scale and intra-hour scale. In the hourly time scale, the energy storage capacity determined by the model can provide sufficient power to support the load demand. In the intra-hour time scale, the energy storage capacity and power of the model ensures that the system has sufficient ramping capability to meet the intra-hour load following and response to short-term net load fluctuation. This model can effectively improve the reliability of wind power connected to the system, and supply reference the connection between energy storage capacity configuration with the existing system scheduling operation mode. Finally, the actual wind farm data is used in simulation, and the results verify the method is effective.