Method for operating a wind turbine, method for determining the temperature of a permanent magnet and controller for a wind turbine

1. A method for operating a wind turbine comprising an electrical system, the electrical system comprises a permanent magnet generator comprising a rotor and a stator, and a power electronic device electrically connected to the permanent magnet generator, wherein the power electronic device and a circuit breaker are electrically disposed in series between the permanent magnet generator and a grid, wherein the permanent magnet generator comprises permanent magnets and a generator winding into which a voltage is induced by the permanent magnets when the rotor is rotating, the method comprising:

generating a signal for tripping the circuit breaker based on a field modification that is modifying the field of the permanent magnets of the permanent magnet generator.

2. The method according to claim 1 , wherein the generating a signal for tripping the circuit breaker is further based on a rotational dynamic of the rotor of the permanent magnet generator.

3. The method according to claim 1 , wherein the field modification is a field modification in direction of a d-axis, wherein the d-axis is an axis in direction of the magnetic field of the permanent magnets in a complex plane.

4. The method according to claim 1 , wherein the method further comprises generating the signal for tripping the circuit breaker if a magnet wheel voltage exceeds a predetermined threshold voltage.

5. The method according to claim 4 , wherein the method further comprises determining the magnet wheel voltage based on the field modification and a rotational dynamic of the rotor; and comparing the magnet wheel voltage to the predetermined threshold voltage.

6. The method according to claim 4 , wherein the predetermined threshold voltage corresponds approximately to a maximum withstand capability voltage of the power electronic device.

7. The method according to claim 1 , wherein the field modification is determined based on the detection of a value of I d , wherein I d is the part of a current in the generator winding in direction of a d-axis, wherein the d-axis is an axis in direction of the magnetic field of the permanent magnets in a complex plane.

8. The method according to claim 1 , wherein the permanent magnet generator further comprises at least one excitation winding arranged at at least one of the permanent magnets, wherein the at least one excitation winding is configured to modify the magnetic field of the respective permanent magnet of the permanent magnet generator, wherein the field modification is determined based on the detection of a current in the at least one excitation winding and the detection of a value of I d , wherein I d is the part of a current in the generator winding in direction of a d-axis, wherein the d-axis is an axis in direction of the magnetic field of the permanent magnets in a complex plane.

9. The method according to claim 1 , wherein a temperature of the permanent magnets of the generator is determined based on a rotational dynamic of the rotor and the field modification.

10. The method according to claim 1 , wherein the power electronic device is selected from the group including an inverter, a commutator, a transformer, and a circuit breaker.

11. A method for determining a temperature of at least one permanent magnet of a permanent magnet machine comprising a rotor and a stator, wherein the permanent magnet machine comprises at least one permanent magnet and a machine winding into which a voltage is induced by the at least one permanent magnet when the rotor is rotated, the method comprising:

determining the temperature of the at least one permanent magnet of the permanent magnet machine on the basis of a rotational dynamic of the rotor and a field modification that is modifying the field of the at least one permanent magnet of the permanent magnet machine.

12. The method according to claim 11 , wherein the field modification is determined based on the detection of a value of I d , wherein I d is the part of a current in the machine winding in direction of a d-axis, wherein the d-axis is an axis in direction of the magnetic field of the at least one permanent magnet in a complex plane.

13. The method according to claim 11 , wherein the permanent magnet machine further comprises at least one excitation winding arranged at at least one of the permanent magnets, wherein the at least one excitation winding is configured to modify the field of the magnetic field of the respective permanent magnet of the permanent magnet machine, wherein the field modification is determined based on the detection of a current in the at least one excitation winding and the detection of a value of I d , wherein I d is the part of a current in the machine winding in direction of a d-axis, wherein the d-axis is an axis in direction of the magnetic field of the at least one permanent magnet in a complex plane.

14. A controller for a wind turbine, the wind turbine comprising an electrical system comprising a permanent magnet generator comprising a rotor and a stator, and a power electronic device operatively connected to the permanent magnet generator, and a circuit breaker electrically disposed in series between the permanent magnet generator and a grid, wherein the permanent magnet generator comprises permanent magnets and a generator winding into which a voltage is induced by the permanent magnets when the rotor is rotating, wherein the controller is configured to be connected to a control circuit of the circuit breaker, wherein the controller is further configured to generate a signal for tripping the circuit breaker based on a field modification of the magnetic field of the permanent magnets of the permanent magnet generator.

15. The controller according to claim 14 , wherein the controller is configured to generate the signal for tripping the circuit breaker further based on a rotational dynamic of the rotor of the permanent magnet generator.

16. The controller for a wind turbine according to claim 14 , wherein the controller is configured to detect of a value of I d , wherein I d is the part of a current in the generator winding in direction of a d-axis, wherein the d-axis is an axis in direction of the magnetic field of the permanent magnets in a complex plane.

17. The controller for a wind turbine according to claim 14 , wherein the permanent magnet generator further comprises at least one excitation winding arranged at at least one of the permanent magnets, wherein the at least one excitation winding is configured to modify the magnetic field of the respective permanent magnet of the permanent magnet generator, wherein the field modification is determined based on the detection of a current in the at least one excitation winding and the detection of a value of I d , wherein I d is the part of a current in the generator winding in direction of a d-axis, wherein the d-axis is an axis in direction of the magnetic field of the permanent magnets in a complex plane.

18. The controller for a wind turbine according to claim 14 , wherein the controller is further configured to determine a temperature of the permanent magnets of the permanent magnet generator on the basis of a measured rotor speed and the field modification.

19. A wind turbine comprising a controller according to claim 14 .

20. The wind turbine according to claim 19 , wherein the power electronic device is selected from the group including an inverter, a commutator, a transformer, and a circuit breaker.