Embodiments of the present disclosure relate to a method and system for determining a reference power for an energy storage system (ESS) in a wind power generation system. In an embodiment of the present disclosure, the method comprises filtering an actual wind power output to obtain a desired power output delivered to a grid; determining a reference output power for the energy storage system from the actual wind power output and the desired power output delivered to the grid; and adjusting the reference output power for the energy storage system based on a change in predicted wind power output at the next time point. With embodiments of the present disclosure, it can provide a direct control strategy for smoothing wind farm output fluctuations and limiting power ramping rate within required range, which is more practically feasible than the existing strategies and could result in better control performance.