Modelling and Control of Flywheels Integrated in Wind Turbine Generators

Wind turbines have large inertia. However, in hydraulic wind turbine drivetrains, the displacement ratio of the hydraulic machinery reduced the inertia experienced at the generator. Restoring this inertia requires a direct connection of a large flywheel to the rotor of the generator. This paper introduces the modeling and control of such a system. The rotating mass of the storage is mechanically coupled with the rotor of a doubly-fed induction generator. The drivetrain decouples the shaft of the turbine from the shaft of the generator to provide independent control of their angular velocities. Full control over the speed of the generator and its flywheel provides a means to charge and discharge the energy storage, on-demand. Consequently, the quality of power generated by wind turbines improves as the power fluctuations, imposed by wind speed intermittencies, reduces. The structure of flywheel energy storage (FESS) can be simplified and the supporting power electronics and its dedicated motor-generator can be eliminated. Only in hydraulic drivetrain systems, such integration and save of the components is possible. Two supervisory controllers are introduced and utilized using the fuzzy logic regulator to determine the output power reference. Furthermore, small-signal analysis is provided to investigate and improve the speed of tracking the maximum power point. Extensive simulation results demonstrate the feasibility of such a system and its improved quality of power generation.