Two-Stage Variable Proportion Coefficient Based Frequency Support of Grid-Connected DFIG-WTs

This paper proposes a variable proportion coefficient based control scheme for the doubly-fed induction generator based wind turbines (DFIG-WTs) to implement frequency support by regulating rotor speed. To reduce the adverse impact caused by regulating DFIG-WT on the dynamic characteristic of the grid frequency, a two-stage switching control scheme is employed for the proposed method. In the first stage, the variable proportion coefficient is designed for emulating the virtual inertia of the DFIG-WT to provide inertia response. In the second stage, a fuzzy control scheme is employed to design the variable proportion coefficient for both quickly restoring the maximum power point tracking (MPPT) operation of DFIG-WTs and avoiding the secondary frequency drop to system. Case studies are undertaken based on WSCC 9-bus and IEEE 39-bus power systems, respectively. Simulation results show that the proposed control scheme can not only make grid-connected DFIG-WTs provide the friendly frequency support, but also help them to fully use the frequency regulation ability of synchronous generators for quickly restoring the MPPT operation.