Abstract
Energy storage systems cannot be considered energy sources but having the ability of storing and discharging energy. They have a vital rule in the reliability and power quality of power systems. There are several energy storage systems (e. g. compressed air storage, battery, supercapacitors, hydrogen storage and flywheels) which can be selected according to the application requirements. Flywheels are used in electric power systems for frequency support, uninterruptible power supply, voltage sag mitigation, flexible AC transmission and power leveling. The flywheel energy storage system consists of a flywheel, an electric machine and a power conversion system. In this paper, energy storage systems used in power system applications are surveyed focusing on flywheel energy storage systems.