This paper details the modeling and development of an improved controller design for a dc flywheel energy storage system (FESS) driving circuit. The driving system is based on a bidirectional buck-boost converter. The modeling of this converter including the parasitic resistances for all the components was carried out. In this model, the equivalent circuit of the machine was integrated in the converter state-space model for improved accuracy and controllability. The system has two operating modes; when the FESS is charging, the converter operates in the buck mode. A controller was designed to regulate the charging rate through controlling the machine's terminal voltage. In the discharging mode, the converter operates in the boost mode. A current controller takes over to control the injected current from the machine to the dc bus. The detailed design of both control loops was identified. Simulation results show the accuracy of the derived model and the enhanced performance of the FESS. Further, it is shown that the reversal of power-flow direction was performed seamlessly. A hardware prototype of the converter was implemented and the effectiveness of the developed system was experimentally verified. The experimental results are in excellent agreement with the simulation. |
