A third AC source fed by frequency converters without DC-ESE is generated by the Flywheel energy storage system. Flywheels are energy storage based on kinetic energy in a spinning mass[3]. Conversion from kinetic to electrical energy is accomplished by electromechanical machines. In the case of flywheelmotor/generator fed by MC the converters match the decreasing speed of the flywheel during discharge and the increasing speed during charging with a fixed frequency AC electrical system