Energy storage assisted conventional units participating in power grid frequency modulation can improve the drawbacks of conventional unit frequency modulation, enhance the quality of frequency modulation, and help to improve the stability of the system. In this paper, the frequency modulation command signal is initially assigned by Variational Mode Decomposition (VMD). After considering the limitation of the generator ramp rate, and the power and State Of Charge (SOC) of the energy storage, the power constraints of each frequency regulation source in each minute are determined. Then, the redistribution method when the command signal exceeds the limit is proposed. This achieves the allocation of frequency regulation commands between the two resources in an optimal ratio over a period of minutes. Taking the cost-benefit model constructed as the goal, the PSO algorithm is used to optimize the energy storage capacity configuration. Finally, based on the measured Area Control Error (ACE) signal, the effects of different allocation schemes on energy storage capacity allocation, efficiency and technicality are analyzed, which verify the feasibility and effectiveness of the proposed method. This work is supported by National Natural Science Foundation of China (No. U1766204) and the Special Fund for Industrial Innovation of Jilin Province Development and Reform Commission (No. 2017C017-2).