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A variable speed wind turbine having a doubly fed induction generator (DFIG), includes an exciter machine mechanically coupled to the DFIG and a power converter placed between a rotor of the DFIG and the exciter machine. Thus, the power converter is not directly connected to the grid avoiding the in

A variable speed wind turbine having a doubly fed induction generator (DFIG), includes an exciter machine mechanically coupled to the DFIG and a power converter placed between a rotor of the DFIG and the exciter machine. Thus, the power converter is not directly connected to the grid avoiding the introduction of undesired harmonic distortion and achieving a better power quality fed into the utility grid. Moreover, the variable speed wind turbine includes a power control and a pitch regulation.

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What is claimed is: 1. A variable speed wind turbine system comprising: a first rotor shaft including at least one blade; a doubly fed induction generator coupled to the first rotor shaft and having a stator connectable to a power grid and having a shaft; an exciter machine coupled to the doubly fe

What is claimed is: 1. A variable speed wind turbine system comprising: a first rotor shaft including at least one blade; a doubly fed induction generator coupled to the first rotor shaft and having a stator connectable to a power grid and having a shaft; an exciter machine coupled to the doubly fed induction generator shaft; and a power conversion system isolated from the grid and electrically connected to a rotor of the doubly fed induction generator and to the exciter machine to control the doubly fed induction generator. 2. The variable speed wind turbine of claim 1, further comprising a power control system to control wind turbine power output delivered to the grid. 3. The variable speed wind turbine of claim 1, further comprising a pitch controller to limit demanded power to the exciter. 4. The variable speed wind turbine of claim 1, wherein the generator is a non-slip ring induction generator. 5. The variable speed wind turbine of claim 1, wherein the exciter is a reversible electrical machine. 6. The variable speed wind turbine of claim 1, wherein the exciter is a synchronous machine. 7. The variable speed wind turbine of claim 1, wherein the exciter is an asynchronous machine. 8. The variable speed wind turbine of claim 1, wherein the exciter is a DC machine. 9. The variable speed wind turbine of claim 1, wherein the power conversion system comprises two active electronic power converters joined by a DC link Bus with an AC side of one of the active power converter connected to the rotor circuit of the doubly fed induction generator and the AC side of the other active power converter connected to the exciter machine. 10. The variable speed wind turbine of claim 1, wherein the power conversion system is a bi-directional power converter isolated from the grid. 11. The variable speed wind turbine of claim 1, wherein the power conversion system is a cycloconverter isolated from the grid. 12. The variable speed wind turbine of claim 1, wherein the power conversion system is a matrix converter isolated from the grid. 13. The variable speed wind turbine of claim 1 wherein the control of the doubly fed induction generator is based on the grid flux orientation (GFO). 14. The variable speed wind turbine of claim 1 wherein the control of the exciter machine is based on the DC Bus voltage regulation. 15. A variable speed wind turbine system comprising: a rotor including at least one blade; a drive train coupled to the rotor, the drive train including at least one doubly fed induction generator (DFIG), said DFIG having a stator connectable to a power grid; exciter machine means for consuming power generated in a rotor of the DFIG in super-synchronous mode and for generating power for the rotor of the DFIG in sub-synchronous mode and coupled to the drive train; a power conversion means isolated from the grid for converting power transferred between the rotor of the DFIG and the exciter machine means. 16. The variable speed wind turbine of claim 15 further comprising: a power control means for controlling wind turbine power output delivered to grid. 17. The variable speed wind turbine of claim 15 further comprising: a pitch control means for performing pitch regulation. 18. A method for a variable speed wind turbine comprising: converting wind energy into mechanical power by driving a rotor; converting mechanical power into electrical power utilising a doubly fed induction generator (DFIG) in a drive train. supplying power to a power grid from a stator of the DFIG; using an exciter machine coupled to the drive train and a power conversion system isolated from the power grid to receive power generated by a rotor of the doubly fed induction generator or to provide power required by the rotor of the doubly fed induction generator. 19. The method of claim 18 further comprising controlling the stator power supply based on grid flux orientation (GFO). 20. The method of claim 18 further comprising controlling the power of the exciter machine based on DC Bus voltage regulation. 21. The method of claim 18 wherein the rotor receives power from the exciter machine when the doubly fed induction generator is operating in a sub-synchronous mode. 22. The method of claim 18 wherein the rotor provides power to the exciter machine when the doubly fed induction generator is operating in a super-synchronous mode. 23. A variable speed wind turbine comprising: a rotor including at least one blade; a drive train coupled to the rotor, the drive train including at least a doubly fed induction generator (DFIG), said DFIG having at least a stator connectable to a power grid; at least an exciter machine coupled to the drive train; and at least a power conversion device isolated from the grid and electrically coupled to a rotor of the doubly fed induction generator and to the exciter machine to transfer electrical power between the rotor and the exciter machine. 24. The variable speed wind turbine of claim 23, further comprising a power control system to control wind turbine power output delivered to the grid. 25. The variable speed wind turbine of claim 23, further comprising a pitch controller to limit demanded power to the exciter. 26. The variable speed wind turbine of claim 23, wherein the generator is a non-slip ring induction generator. 27. The variable speed wind turbine of claim 23, wherein the exciter is a reversible electrical machine. 28. The variable speed wind turbine of claim 23, wherein the exciter is a synchronous machine. 29. The variable speed wind turbine of claim 23, wherein the exciter is an asynchronous machine. 30. The variable speed wind turbine of claim 23, wherein the exciter is a DC machine. 31. The variable speed wind turbine of claim 23, wherein the power conversion system comprises two active electronic power converters joined by a DC link Bus with an AC side of one of the active power converter connected to the rotor circuit of the doubly fed induction generator and the AC side of the other active power converter connected to the exciter machine. 32. The variable speed wind turbine of claim 23, wherein the power conversion system is a bi-directional power converter isolated from the grid. 33. The variable speed wind turbine of claim 23, wherein the power conversion system is a cycloconverter isolated from the grid. 34. The variable speed wind turbine of claim 23, wherein the power conversion system is a matrix converter isolated from the grid. 35. The variable speed wind turbine of claim 23 wherein the control of the doubly fed induction generator is based on the grid flux orientation (GFO). 36. The variable speed wind turbine of claim 23 wherein control of the exciter machine is based on the DC Bus voltage regulation.