To mitigate the deterioration of the system frequency response (SFR) caused by the increasing penetration of wind power, wind turbine generators are required to release their kinetic energy during frequency events to provide dynamic frequency support. Existing additional frequency control (AFC) primarily focuses on simulating output power with insufficient attention given to actual energy variations, thus posing challenges in optimizing energy management while maintaining control continuity. Moreover, abrupt control switching to avoid over-deceleration has a negative impact on the SFR. To address these issues, this paper proposes a novel AFC based on the kinetic energy trajectory (KET). An explicit smooth KET is first designed to serve as the energy reference. Specifically, the KET generates a positive power deviation when frequency drops and stores as much energy as possible when frequency rises. Then, the KET recovers energy to its initial state as frequency stabilizes. An adaptive sliding mode control is also employed to ensure the convergence of energy towards the KET, while enhancing robustness against various errors. Finally, the effectiveness and superiority of the proposed control are verified through an extended SFR model and a modified IEEE 9-bus system.