Wind turbine yaw system and method of controlling the same

1. A wind turbine yaw system, comprising:

a yaw gear,

at least two pinion gears;

at least two drive units, wherein each drive unit is associated to one of the at least two pinion gears for driving the respective pinion gear; and

a control system, comprising

a controller for generating a drive unit control signal for each drive unit for controlling the respective drive unit according to a drive unit reference signal, the drive unit reference signal comprising at least one desired operational parameter value for the respective drive unit so as to realize the at least one desired operational parameter value in the respective drive unit;

at least one feedback loop for each drive unit feeding at least one drive unit feedback signal comprising at least an actual value of the at least one desired operational parameter of the respective drive unit back to the controller,

each drive unit comprises a frequency converter between the controller and the motor for controlling the speed of the motor, and

each feedback signal represents at least an actual value of the at least one desired operational parameter of the respective frequency converter,

wherein the controller is adapted to generate the drive unit control signals based on the reference signal and the feedback signals,

wherein the drive unit control signals delivered to the frequency converters are chosen such that the difference in the at least an actual value of at least one desired operational parameter values contained in the feedback signals of the two drive units are reduced preferably to zero.

2. The wind turbine yaw system as claimed in claim 1 , wherein

each drive unit comprises at least one torque sensor sensing a torque the respective drive unit yields, and

each feedback signal represents at least the output of the respective torque sensor.

3. The wind turbine yaw system as claimed in claim 1 , wherein

each drive unit comprises an electric motor, and

each feedback signal represents at least one operational parameter of the respective electric motor.

4. The wind turbine yaw system as claimed in claim 3 , wherein each feedback signal represents at least the load experienced by the respective electric motor as an operational parameter of the electric motor.

5. The wind turbine yaw system as claimed in claim 3 , wherein each feedback signal represents at least the current consumed by the respective electric motor.

6. The wind turbine yaw system as claimed in claim 3 , wherein

each drive unit comprises a gear between the electric motor and the pinion gear, and

each feedback signal represents at least one operational parameter of the gear.

7. The wind turbine yaw system as claimed in claim 2 , wherein the torque sensor is located at the high speed side of the gear.

8. The wind turbine yaw system as claimed in claim 2 , wherein the torque sensor is located at the low speed side of the gear.

9. The wind turbine yaw system as claimed in claim 3 , wherein

each drive unit comprises a position encoder encoding the rotational position of a shaft connecting the motor and the pinion gear or, if a gear is present between the motor and the pinion gear, a shaft between the motor and the gear, a shaft between the gear and the pinion gear or of a gear wheel in the gear, and

the feedback signal represents at least the output of the respective position encoder.

10. A wind turbine, comprising:

a tower;

a nacelle located atop the tower which carries a rotor rotatable by wind about a rotor axis; and

a yaw system arranged between the nacelle and the tower for allowing the nacelle to be rotated about a tower axis so as to align the rotor axis with a direction of the wind, the yaw system comprising:

a yaw gear,

at least two pinion gears;

at least two drive units, wherein each drive unit is associated to one of the at least two pinion gears for driving the respective pinion gear; and

a control system, comprising

a controller for generating a drive unit control signal for each drive unit for controlling the respective drive unit according to a drive unit reference signal, the drive unit reference signal comprising at least one desired operational parameter value for the respective drive unit so as to realize the at least one desired operational parameter value in the respective drive unit;

at least one feedback loop for each drive unit feeding at least one drive unit feedback signal comprising at least an actual value of the at least one desired operational parameter of the respective drive unit back to the controller,

each drive unit comprises a frequency converter between the controller and the motor for controlling the speed of the motor, and

each feedback signal represents at least an actual value of the at least one desired operational parameter of the respective frequency converter,

wherein the controller is adapted to generate the drive unit control signals based on the reference signal and the feedback signals,

wherein the drive unit control signals delivered to the frequency converters are chosen such that the difference in the at least an actual value of at least one desired operational parameter values contained in the feedback signals of the two drive units are reduced preferably to zero.

11. A method of controlling a wind turbine yaw system with a yaw gear, at least two pinion gears, at least two drive units, wherein each drive unit is associated to one of the pinion gears for driving the respective pinion gear, the method comprising:

controlling each drive unit according to a drive unit reference signal comprising at least one desired operational parameter value for the respective drive unit so as to realize the at least one desired operational parameter value in the respective drive unit;

feeding at least an actual value of the at least one desired operational parameter of each drive unit to a controller by a feedback signal;

providing each drive unit a frequency converter between the controller and the motor for controlling the speed of the motor;

using the controller to generate drive unit control signals for the drive units based on the respective reference signal and the feedback signals; and

choosing drive unit control signals and delivering them to the frequency converters such that the difference in the at least an actual value of at least one desired operational parameter values contained in the feedback signals of the two drive units are reduced preferably to zero.

12. The method as claimed in claim 11 , wherein the drive unit control signals are generated based on the respective reference signal and differences between the operational parameter values of the respective feedback signals.

13. The method as claimed in claim 11 , wherein each drive unit control signal for a drive unit is generated based on the difference between the at least one desired operational parameter value of the reference signal and the at least one operational parameter value of the feedback signal obtained from the drive unit to be controlled by the respective drive control signal.

14. The method as claimed in claim 11 , wherein each drive unit comprises an electric motor and the feedback signal of a drive unit represents at least one operational parameter of the respective electric motor.

15. The method as claimed in claim 11 , further comprising providing a frequency converter for each drive unit for controlling the speed of the electric motor and/or a gear between the electric motor and the pinion gear, and wherein the feedback signal of a drive unit represents at least one operational parameter of the respective frequency converter and/or the respective gear.

16. The method as claimed in claim 15 , further comprising providing a frequency converter for each drive unit for controlling the speed of the electric motor and/or a gear between the electric motor and the pinion gear, and wherein the feedback signal of a drive unit also represents at least one operational parameter of the respective frequency converter and/or the respective gear.