Control method for doubly-fed electric generator

1. A method of controlling a doubly-fed electric generator with a frequency converter including a torque and flux controller, which frequency converter is connected to a rotor of the generator, wherein the method comprising:

determining a rotor flux;

determining a stator flux;

calculating a stator flux positive sequence component from the stator flux;

calculating a rotor flux amplitude reference by using the stator flux positive sequence component;

calculating a rotor flux error from the rotor flux amplitude reference and the rotor flux;

converting the stator flux positive sequence component from a stator reference frame to a rotor reference frame;

calculating an estimated torque by using the stator flux positive sequence component in the rotor reference frame;

calculating a torque error from the estimated torque and a torque reference; and

selecting a voltage vector by using the rotor flux, the torque error and the rotor flux error.

2. The method according to claim 1 , wherein the generator is part of a wind turbine.

3. The method according to claim 1 , wherein the rotor flux amplitude reference is calculated from the amplitude of the stator flux positive sequence component and a reactive power reference.

4. The method according to claim 1 , wherein the stator flux positive component is converted from the stator reference frame to the rotor reference frame by multiplying the stator flux positive component in the stator reference frame by e −jθm , where θ m is a rotor angle in respect of the stator.

5. The method according to claim 4 , wherein the rotor angle θ m is determined by using a rotational speed sensor.

6. The method according to claim 1 , wherein the estimated torque is calculated as a cross product of the rotor flux and the stator flux positive sequence component in the rotor reference frame.

7. A frequency converter comprising:

means for determining a rotor flux;

means for determining a stator flux;

means for calculating a stator flux positive sequence component from the stator flux;

means for calculating a rotor flux amplitude reference using the stator flux positive sequence component;

means for calculating a rotor flux error from the rotor flux amplitude reference and the rotor flux;

means for converting the stator flux positive sequence component from a stator reference frame to a rotor reference frame;

means for calculating an estimated torque by using the stator flux positive sequence component in the rotor reference frame;

means for calculating a torque error from the estimated torque and a torque reference; and

means for selecting a voltage vector by using the rotor flux, the torque error, and the rotor flux error.

8. A method of controlling a doubly-fed electric generator with a frequency converter connected to a rotor of the generator, wherein the method comprising:

calculating a stator flux positive sequence component from a stator flux of the generator;

calculating a rotor flux amplitude reference by using the stator flux positive sequence component;

calculating a rotor flux error from the rotor flux amplitude reference and a rotor flux of the generator;

converting the stator flux positive sequence component from a stator reference frame to a rotor reference frame;

calculating an estimated torque using the stator flux positive sequence component in the rotor reference frame;

calculating a torque error from the estimated torque and a torque reference; and

selecting a voltage vector using the rotor flux, the torque error and the rotor flux error.

9. The method according to claim 8 , wherein the generator is part of a wind turbine.

10. The method according to claim 8 , wherein the rotor flux amplitude reference is calculated from the amplitude of the stator flux positive sequence component and a reactive power reference.

11. The method according to claim 8 , wherein the stator flux positive component is converted from the stator reference frame to the rotor reference frame by multiplying the stator flux positive component in the stator reference frame by e −jθm , where θ m is a rotor angle in respect of the stator.

12. The method according to claim 11 , wherein the rotor angle θ m is determined by using a rotational speed sensor.

13. The method according to claim 8 , wherein the estimated torque is calculated as a cross product of the rotor flux and the stator flux positive sequence component in the rotor reference frame.