Power fluctuations caused by wind speed variations affect power quality, especially in weak or isolated grids, and degrade the reliability of wind power converters. This paper develops a wind power smoothing scheme by making use of the large inertial energy of a wind turbine system (WTS). The proposed method, which explores the energy storage capability of the WTS shaft, is easy to implement by adding two additional terms into the existing maximum power point tracking (MPPT) control reference. Based on the law of conservation of energy, the new controller includes two parts: one part is to duplicate the original power trajectory under the MPPT control and the other is to compensate the fluctuations of it. In this way, two control objectives, namely, optimized wind power capture and its smoothing can be achieved simultaneously since the rotor rotates around the optimum speed points. Meanwhile, the mechanical stress of a WTS is alleviated with less oscillating torque reference, and the stability of a WTS is maintained by disabling the smoothing function when rotor speed hits the speed limits. RTDS simulations of doubly-fed induction generator (DFIG)-and direct-driven permanent magnet synchronous generator (PMSG)-based WTSs are used to demonstrate the effectiveness of the proposed power smoothing control algorithm. The proposed method is validated on both a single WTS and a wind farm. Quantitative analysis is then carried out to evaluate the relationship between the smoothing performance and the efficiency of wind energy capture.