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Wind power fluctuation raises the security concern of grid frequency deviation, especially for an isolated power system. Thus, better control methodology needs to be developed to smooth the fluctuation without excessive spillage. Based on an actual industrial power system, this paper proposes a smoothing controller to suppress the power fluctuation from doubly-fed induction generator (DFIG)-based wind farm. This controller consists of three main functionality components: risk assessment model, wind turbine rotor speed optimizer, and rotor speed upper limiter. In order to avoid unnecessary energy loss, this paper designs a risk assessment model of grid frequency deviation, which is capable of locally estimating the maximum grid frequency deviation risk of the next dispatch cycle. A wind turbine speed optimizer then uses the estimated frequency deviation risk to search for the optimal power curve with reduced output so that a trade-off between fluctuation smoothing and energy loss is achieved. Subsequently, the controller limits the maximum rotor speed to shift down the power curve of wind power plant based on the optimal wind turbine rotor speed. Therefore, the power fluctuation is smoothed along with the down-regulated power curve. A numerical case study demonstrates the effectiveness and economy of this smoothing controller for the studied isolated system.