Abstract: A cooperative optimal control scheme combining active disturbance rejection voltage control and power feedforward compensation was proposed for a flywheel energy storage system (FESS) based on a neutral-point-clamped (NPC) converter. A grid-side second-order active disturbance rejection control (ADRC) strategy was developed, in which an extended state observer (ESO) was designed to observe disturbances such as machine-side power variations and compensate them into the control variable. The real-time deviation between the power setpoint and the grid-side output power was incorporated into the setpoint of the flywheel machine-side power. The effectiveness of the proposed strategy was validated through simulations. Results show that during full-power charge/discharge switching, the maximum deviation of the DC-link capacitor voltage is reduced by more than 91.5%, the average voltage recovery time is shortened by 94.79%, and the power output accuracy within the rated operating range is improved by 55.21%, in comparison with the conventional PI control scheme.