A doubly-fed induction generator (DFIG) system is described. The DFIG system includes an induction electric machine including a stator having a stator winding and a rotor having a rotor winding. The stator winding is electrically connected to at least one output terminal and the rotor winding is electrically connected to the at least one output terminal by means of a power converter. The power converter includes a first active rectifier/inverter with alternating current AC terminals electrically connected to the rotor winding, and direct current DC terminals, and a second active rectifier/inverter with DC terminals electrically connected to the DC terminals of the first active rectifier/inverter by a DC link, and AC terminals electrically connected to the at least one output terminal. A controller is adapted to control the first active rectifier/inverter so that the frequency of the AC current at its AC terminals is substantially constant during at least one of a “line charging mode” and an “islanded mode”.