Control and performance of a flywheel energy storage system based on a doubly-fed induction generator-motor for power conditioning

A flywheel energy storage system based on a doubly-fed induction generator-motor basically consists of a wound-rotor induction machine and a cycloconverter or a voltage-source PWM rectifier-inverter which is used as an AC exciter. Adjusting the rotor speed makes the generator-motor either release the kinetic energy to the power system or absorb it from the utility grid. Thus, the generator-motor has the capability of achieving not only reactive power control, but also active power control based on the flywheel effect of the rotating parts. This paper proposes a new control strategy for a doubly-fed induction generator-motor, which is characterized by the combination of vector control and decoupling control. The control strategy enables the induction generator-motor to perform active power control independent of reactive power control even in transient states. The validity of the theory developed in this paper, along with the effectiveness and viability of the control strategy, is confirmed by computer simulation. In addition, this paper discusses a transient behavior of a magnetizing current in the induction machine.