Modeling and Control of Flywheel-Integrated Generators in Split-Shaft Wind Turbines

This paper introduces the modeling and control of split-shaft drivetrains where the system's inertia is adjusted to store the energy. Accordingly, a flywheel is mechanically coupled with the rotor of a doubly-fed induction generator. The generator is driven by a split-shaft drivetrain that decouples the turbine's shaft from the shaft of the generator to provide independent control of their angular velocities. Hence, the turbine controller can track the point of maximum power (MPPT) while the generator controller can adjust the generator speed. Accordingly, The flywheel, which is directly connected to the shaft of the generator, is charged and discharged by controlling the generator speed. In this process, the flywheel can modify the electric power generation of the generator on-demand. Since the drivetrain is a split-shaft, the turbine speed is not affected by this energy storing process. This improves the quality of injected power to the grid. The structure of the flywheel energy storage can be simplified by removing its dedicated motor/generator and the power electronics driver. This significant modification can only occur in the split-shaft drivetrain. Two separate supervisory controllers are developed in the form of fuzzy logic regulators to generate a real-time output power reference. Furthermore, small-signal models are developed to analyze and improve the MPPT controller. Extensive simulation results demonstrate the feasibility of such a system and its improved quality of power generation.