Power imbalances between generation and consumption will cause frequency deviations in the grid. In modern power systems, intermittent renewable energy sources (RES) have resulted in more frequent frequency violations, as traditional power plants cannot compensate for power gaps timely. Electric vehicles (EVs) can participate in load frequency control (LFC) through aggregators and are capable of reacting faster to control commands than conventional frequency control reserves (FCR) in generators. Thus EVs hold great promise in assisting with LFC. This paper proposes a composite control scheme that fully utilizes EVs for LFC in both normal scenarios and contingencies. The designed droop control can greatly reduce instantaneous frequency deviations (IFD) in emergencies, while the tube model predictive control (Tube MPC) can ensure smooth frequency trajectories during normal operations. Based on realistic models, simulation results illustrate the effectiveness of the proposed method.