Model-Based Design of Coordinated Grid-Forming Control for FESS-DFIG Systems Using Load-Current Feedforward and Sigmoid-Based Rotor-Energy Regulation

Grid-forming doubly fed induction generator (DFIG) systems integrated with flywheel energy storage systems (FESSs), hereafter referred to as FESS-DFIG systems, must be designed to provide rapid frequency support while maintaining DC-link voltage stiffness and respecting rotor-speed limits under finite kinetic-energy reserves. This study presents a model-based engineering design and verification framework that allocates these coupled requirements between the grid-side converter (GSC) and the rotor-side converter (RSC). For the GSC, stator–rotor coupling terms derived from the dq model are implemented as load-current feedforward signals to reduce the transient power imbalance across the DC link. For the RSC, virtual synchronous control is combined with a sigmoid-based rotor-energy constraint whose minimum-speed limit, transition width, and shape coefficient serve as physically interpretable design parameters for balancing frequency support against mechanical protection. The resulting architecture replaces abrupt support withdrawal with a continuous transition from inertial response to rotor-speed recovery. MATLAB/Simulink verification under load disturbances shows that compared with the conventional feedback-only dual-loop PI controller under the same 0.2 p.u. step-load disturbance, the proposed load-current-feedforward design reduces the maximum DC-link voltage deviation by approximately 33%, suppresses the secondary frequency dip and power oscillations caused by hard-switching logic, and maintains the rotor speed at the prescribed safety boundary of 0.8 p.u. Tests under low and high kinetic-energy conditions further demonstrate stable operation without changing the overall control architecture. The proposed framework therefore provides a systematic design basis for integrating DC-link regulation, grid-forming response, and rotor-energy management in converter-interfaced wind-energy systems.