Control of a High-Inertia Flywheel As Part of a High Capacity Energy Storage System

An alternative to high-energy capacity storage systems is described that uses a rotating high-inertia flywheel. When coupled with a synchronous machine, this system can be used to exchange electrical energy, store or retrieve, with the rotational energy of the flywheel; typically up to several kilowatts over tens of seconds can be achieved. This energy storage ability allows a flywheel to be used in a myriad of applications, including hybrid vehicle systems and wind generation. A system that improves the power conversion efficiencies using power electronics has a difficult task when considering that the electrical interface with the high speed generator has an operating frequency close to 1 kHz with a time varying voltage and frequency. This paper presents the results of a preliminary investigation into the development of a suitable control scheme for the power electronic converter that interfaces with a high speed generator mechanically coupled to a rotating flywheel. The investigation explored the possibility of operating the generator at unity power factor over a wide speed range and under constant power conditions. Simulation results show that unity-power factor operation improves the energy transfer efficiencies of the system since it minimizes the current flowing through the machine and hence minimizes the electrical losses. This is demonstrated by showing that 0.8 power factor operation results in a machine speed variation that is 300 rpm less than that obtained with unity power factor operation during an identical power removal cycle.