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A protection system and a protection method are provided for protecting a double fed induction generator and a gearbox during a grid fault. The protection system includes a plurality of controlled impedance devices. Each of the controlled impedance devices is coupled between a respective phase of

A protection system and a protection method are provided for protecting a double fed induction generator and a gearbox during a grid fault. The protection system includes a plurality of controlled impedance devices. Each of the controlled impedance devices is coupled between a respective phase of a stator winding of the double fed induction generator and a respective phase of a grid side converter. The protection system also includes a controller configured for coupling and decoupling impedance in one or more of the plurality of controlled impedance devices in response to changes in at least one of a utility grid voltage and a utility grid current.

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The invention claimed is: 1. A protection system, comprising: a plurality of controlled impedance devices, each coupled between a respective phase of a stator winding of a double fed induction generator and a respective phase of a grid side converter; and a controller configured for coupling and de

The invention claimed is: 1. A protection system, comprising: a plurality of controlled impedance devices, each coupled between a respective phase of a stator winding of a double fed induction generator and a respective phase of a grid side converter; and a controller configured for coupling and decoupling impedance in one or more of the plurality of controlled impedance devices in response to changes in at least one of a utility grid voltage and a utility grid current. 2. The protection system of claim 1, wherein each controlled impedance device comprises an impedance device in parallel with a semiconductor device, the semiconductor device being configured for coupling and decoupling the impedance device based on control signals received from the controller. 3. The protection system of claim 2, wherein the semiconductor device comprises a bidirectional semiconductor device. 4. The protection system of claim 2, wherein the semiconductor device comprises a triac, an assembly of thyristors, or anti parallel thyristors. 5. The protection system of claim 2, wherein the impedance device comprises impedance of a predetermined magnitude. 6. The protection system of claim 1, wherein each controlled impedance device comprises a current dependent variable impedance device. 7. The protection system of claim 1, wherein the controller is configured for coupling the impedance in one or more of the plurality of controlled impedance devices when the utility grid voltage drops below or the utility grid current rises above a first threshold value. 8. The protection system of claim 7, wherein the impedance increases the stator voltage when coupled. 9. The protection system of claim 7, wherein the controller is configured to control the stator voltage in phase with the utility grid voltage to provide unity power factor or to control the stator voltage to provide reactive power to a utility grid. 10. The protection system of claim 1, wherein the controller is configured for decoupling the impedance in one or more of the plurality of controlled impedance devices when the utility grid voltage rises above or the utility grid current drops below a second threshold value. 11. The protection system of claim 1, wherein the controlled impedance device further comprises an impedance device in parallel with a semiconductor device, the semiconductor device being configured for coupling the impedance device when the utility grid voltage drops below or the utility grid current rises above a first threshold value and decoupling the impedance device when the utility grid voltage rises above or the utility grid current drops below a second threshold value based on control signals received from the controller. 12. The protection system of claim 1, wherein each controlled impedance device comprises a current dependent variable impedance device, the current dependent variable impedance device configured for offering high impedance when stator current rises above a first threshold value and a very low impedance when stator current drops below a second threshold value. 13. A wind turbine generator, comprising: a wind turbine coupled to a double fed induction generator; a plurality of controlled impedance devices, each coupled between a respective phase of a stator winding of the double fed induction generator and a respective phase of a grid side converter; and a controller configured for coupling and decoupling impedance in one or more of the plurality of controlled impedance devices in response to changes in at least one of a utility grid voltage and a utility grid current. 14. The wind turbine generator of claim 13, wherein the wind turbine is coupled to the double fed induction generator via a gearbox. 15. The wind turbine generator of claim 13, wherein each controlled impedance device comprises an impedance device in parallel with a semiconductor device, the semiconductor device being configured for coupling and decoupling the impedance device based on control signals received from the controller. 16. The wind turbine generator of claim 13, wherein each controlled impedance device comprises a current dependent variable impedance device. 17. The wind turbine generator of claim 13, wherein the controller is configured for coupling the impedance in one or more of the plurality of controlled impedance devices when the utility grid voltage drops below or the utility grid current rises above a first threshold value. 18. The wind turbine generator of claim 13, wherein the controller is configured for decoupling the impedance in one or more of the plurality of controlled impedance devices when the utility grid voltage rises above or the utility grid current drops below a second threshold value. 19. A protection method, comprising: coupling and decoupling impedance in one or more of a plurality of controlled impedance devices in response to changes in at least one of a utility grid voltage and a utility grid current, wherein each of the plurality of controlled impedance devices is coupled between a respective phase of a stator winding of a double fed induction generator and a respective phase of a grid side converter. 20. The method of claim 19, wherein coupling and decoupling the impedance comprises coupling and decoupling an impedance device via a semiconductor device based on control signals received from a controller, the semiconductor device being connected in parallel with the impedance device in each of the plurality of controlled impedance devices. 21. The method of claim 19, wherein coupling and decoupling the impedance comprises coupling and decoupling the impedance in a current dependent variable impedance device based on control signals received from a controller. 22. The method of claim 19, wherein coupling the impedance comprises coupling the impedance when the utility grid voltage drops below or the utility grid current rises above a first threshold value. 23. The method of claim 19, wherein decoupling the impedance comprises decoupling the impedance when the utility grid voltage rises above or the utility grid current drops below a second threshold value.