SYSTEMS COMPRISING A DOUBLE FED INDUCTION MACHINE AND A FLY WHEEL, AND METHODS OF OPERATING SUCH SYSTEMS

An embodiment of the invention relates to a system (10) comprising a double fed induction machine (20) comprising a stator (22) and a rotor (21); a fly wheel (30) coupled to the rotor (21); and a control device (40) for providing a rotor voltage and a rotor current to the rotor (21); wherein the control device (40) is connected to the rotor (21) and the stator (22) and capable of generating the rotor voltage and rotor current in response to an electrical signal (M) that is applied to the stator (22). The control device (40) comprises a multilevel converter (50) and a control unit (60) for controlling the multilevel converter (50).