Enhanced Frequency Control Method for Microgrid-Connected Flywheel Energy Storage System

Flywheel energy storage system (FESS) can be used for frequency regulation in microgrids. In this article, an enhanced frequency control system is presented for FESS to reduce the frequency variations of microgrid. A three-layer control system is proposed for machine-side converter of the FESS including dc-link voltage controller, speed controller, and field-oriented control system. Unlike the conventional control methods, in the proposed method, the frequency controller is implemented in the grid-side converter to increase the ramp rate of the FESS and reduce the frequency deviation against load changes. Also, the proposed three-layer control system of the machine-side converter reduces the dc-link voltage variations. Furthermore, a mode coordination system is proposed to reduce the dc-link voltage variations when the operation mode of FESS changes from flywheel speed control mode to the microgrid frequency control mode. To demonstrate the advantageous of the proposed method in comparison to the conventional method, at first, the linearized model of the FESS in microgrid is extracted, and then the small-signal analysis is carried out for optimal design of proportional integral (PI) controllers. Two control strategies are theoretically presented and experimentally compared in a prototype microgrid including a diesel generator emulator and a load emulator. The frequency control system is studied in different loading conditions. Experimental results verify the theoretical concepts proposed in this article, and show the superiority of the proposed method over the conventional method.