Control Strategy and Dynamic Performance of a Doubly-Fed Flywheel Generator-Motor

A doubly-fed flywheel generator-motor consists of a wound-rotor induction machine and a cycloconverter or a voltage-source PWM rectifier-inverter which is used as an ac excitor. Adjusting the rotor speed makes the generator-motor either release the kinetic energy to the power system or absorb it from the power system. Thus, the generator-motor has the capability of achieving, not only reactive power control, but also active power control based on a flywheel effect of the rotor. A 200-MJ flywheel generator-motor for the purpose of line-frequency regulation on a 132-kV bus has been installed and commissioned by the Okinawa Electric Power Company. However, little literature has been published on control strategy and dynamic performance analysis of doubly-fed flywheel generator-motors. This paper proposes a new control strategy for a doubly-fed flywheel generator-motor, which is characterized by the combination of vector control and decoupling control. The control strategy enables the flywheel 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 of the control strategy, is confirmed by computer simulations using EMTDC. In addition, this paper discusses the transient behavior of a magnetizing current of the flywheel generator-motor. The flywheel generator-motor based on leading edge power electronics and electric machine technologies shows promise as a versatile power conditioner, in particular, being capable of repetitively absorbing or releasing electric energy for a short period of time less than a minute.