Multi-Input–Multi-Output Control of a Utility-Scale, Shaftless Energy Storage Flywheel With a Five-Degrees-of-Freedom Combination Magnetic Bearing

The modeling and control of a recently developed utility-scale, shaftless, hubless, high strength steel energy storage flywheel system (SHFES) are presented. The novel flywheel is designed with an energy/power capability of 100 kWh/100 kW and has the potential of a doubled energy density when compared to conventional technologies. In addition, it includes a unique combination magnetic bearing (CAMB) capable of providing five-degrees-of-freedom (5DOF) magnetic levitation. Initial test results show that the CAMB, which weighs 544 kg, can provide a stable lift-up and levitation control for the 5543 kg flywheel. The object of this paper is to formulate and synthesize a detailed model as well as to design and simulate a closed-loop control system for the proposed flywheel system. To this end, the CAMB supporting structures are considered flexible and modeled by finite element modeling. The magnetic bearing is characterized experimentally by static and frequency-dependent coefficients, the latter of which are caused by eddy current effects and presents a challenge to the levitation control. Sensor-runout disturbances are also measured and included. System nonlinearities in power amplifiers and the controller are considered as well. Even though the flywheel has a large ratio of the primary-to-transversal moment of inertias, multi-input–multi-output (MIMO) feedback control demonstrates its effectiveness in canceling gyroscopic toques at the designed operational spinning speed. Various stages of proportional and derivative (PD) controllers, lead/lag compensators, and notch filters are implemented to suppress the high-frequency sensor disturbances, structural vibrations, and rotor imbalance effects.