Low voltage ride through system for a variable speed wind turbine having an exciter machine and a power converter not connected to the grid

1. A method for operating a variable speed wind turbine comprising:

converting wind energy into mechanical power using a rotor to rotate a drive train;

converting the mechanical power into electrical power utilising a doubly fed induction generator (DFIG) coupled to the drive train;

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 DFIG or to provide power required by the rotor of the DFIG;

transferring electrical energy between the rotor of the DFIG and the drive train, through the power conversion system and the exciter machine in response to a low voltage event in the grid.

2. The method defined in claim 1 , further comprising adjusting rotor currents in order to generate desired stator currents to meet grid connection requirements.

3. The method defined in claim 2 , wherein the rotor currents are adjusted in order to supply reactive current to the grid during the low voltage event.

4. The method defined in claim 2 , wherein the rotor currents are adjusted in order to supply real current to the grid during the low voltage event.

5. The method defined in claim 2 , wherein the rotor currents are adjusted in order to supply a mix of real and reactive current to the grid during the low voltage event.

6. The method defined in claim 2 wherein the rotor currents are adjusted in order to supply reactive power to the grid during the low voltage event.

7. The method defined in claim 2 wherein the rotor currents are adjusted in order to supply real power to the grid during the low voltage event.

8. The method defined in claim 2 wherein the rotor currents are adjusted in order to supply a mix of real and reactive power to the grid during the low voltage event.

9. A method for operating a power conversion system defined in claim 1 , wherein switching frequencies can be dynamically adjusted and modified.

10. The method defined in claim 6 , wherein a switching frequency of an exciter side converter of the power conversion system can be dynamically adjusted and modified.

11. The method defined in claim 6 , wherein a switching frequency of a rotor side converter of the power conversion system can be dynamically adjusted and modified.

12. A variable speed wind turbine comprising:

a rotor shaft including at least one blade;

a drive train coupled to the rotor shaft, the drive train including a doubly fed induction generator (DFIG), said DFIG having at least a stator connectable to a power grid;

an exciter machine coupled to the drive train; and

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;

wherein the exciter machine is used as a power supply for components of the wind turbine when the generator speed is at or above a predefined value.

13. The variable speed wind turbine system of claim 12 further comprising:

means for generating an alternating current (AC) and a direct current (DC) power in response to the generator speed being at or above a predefined value.

14. The variable speed wind turbine system of claim 12 further comprising:

means for generating an alternating current (AC) auxiliary power supply; and

means for generating a direct current (DC) auxiliary power supply.

15. The variable speed wind turbine system of claim 12 , further comprising:

an alternating current/direct current (AC/DC) converter coupled to the exciter machine which generates a direct current (DC) power supply; and

a direct current/alternating current (DC/AC) converter connected to the DC power supply which generates an alternating (AC) auxiliary power supply.