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A method of controlling a wind turbine generator is provided, the wind turbine generator converting mechanical energy to electrical. The method comprises: determining an electromagnetic power reference representing the electromagnetic power generated by the wind turbine generator, wherein the electr

A method of controlling a wind turbine generator is provided, the wind turbine generator converting mechanical energy to electrical. The method comprises: determining an electromagnetic power reference representing the electromagnetic power generated by the wind turbine generator, wherein the electromagnetic power reference is determined based on a desired output of the wind turbine generator; controlling the electrical power generated by the wind turbine generator using a control signal, wherein the control signal is derived from the electromagnetic power reference and is modified in dependence on an inverse power function of the wind turbine generator by incorporating minimal copper loss constraint and stator voltage limiting constraint such that non-linearity of the wind turbine generator is compensated in the control loop and it operates at its maximum efficiency. One effect of the method is that classical linear control loop design can be employed in spite of the plant being a non-linear identity.

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1. A method of controlling a wind turbine generator, the wind turbine generator converting mechanical energy to electrical energy, the method comprising: determining an electromagnetic power reference representing the electromagnetic power generated by the wind turbine generator, wherein the electro

1. A method of controlling a wind turbine generator, the wind turbine generator converting mechanical energy to electrical energy, the method comprising: determining an electromagnetic power reference representing the electromagnetic power generated by the wind turbine generator, wherein the electromagnetic power reference is determined based on a desired output of the wind turbine generator;determining a first reference from one of a first field power stator flux reference and a stator current reference based on a first criterion and the electromagnetic power reference, wherein the one of the first field power stator flux and stator current reference is linearized to compensate for non-linearity of the wind turbine generator;determining a second reference from one of a second field power stator flux reference and a stator current reference based on a second criterion and the electromagnetic power reference, wherein the one of the second field power stator flux and stator current reference is linearized to compensate for non-linearity of the wind turbine generator;selecting one of the first reference and the second reference; andcontrolling the electrical power generated by the wind turbine using a control signal, wherein the control signal is based on the selected one of the first reference and the second reference. 2. The method of claim 1, wherein determining the second reference from the one of the second field power stator flux reference and the stator current reference comprises:determining a first preliminary field power stator flux or stator current reference based on the electromagnetic power reference obtained at a partial field weakening speed (speed at which field weakening operation starts) of the wind turbine generator;determining a second preliminary field power stator flux or stator current reference based on the electromagnetic power reference obtained at a maximum speed of the wind turbine generator; anddetermining a weighted average of the first preliminary field power stator flux or stator current reference and the second preliminary field power stator flux or stator current reference in order to obtain the second field power stator flux or stator current reference. 3. The method of claim 1, wherein selecting one of the first reference and the second reference comprises: comparing an absolute value of the first reference and an absolute value of the second reference; andselecting the one of the first reference and the second reference which has the smaller absolute value. 4. The method of claim 1, further comprising: determining a third reference from one of a first reluctance power stator flux reference and stator current reference based on a third criterion and the selected one of the first reference and the second reference;determining a fourth reference from one of a second reluctance power stator flux reference and stator current reference based on a fourth criterion and on the selected one of the first reference and the second reference;selecting one of the third reference and the fourth reference; andcontrolling the stator flux or stator current of the wind turbine generator based on the selected one of the first and second reference and the selected one of the third reference and the fourth reference. 5. The method of claim 4, wherein selecting one of the third reference and the fourth reference comprises:comparing an absolute value of the third reference and an absolute value of the fourth reference; andselecting the one of the third reference and the fourth reference which has the smaller absolute value. 6. The method of claim 4, wherein the computation of the third reference and the fourth reference is carried out analytically. 7. The method of claim 4, wherein the third criterion comprises a generator minimal copper loss constraint. 8. The method of claim 4, wherein the fourth criterion comprises a generator voltage limiting constraint. 9. The method of claim 1, wherein the first reference and the second reference are determined using look-up tables or polynomial functions. 10. The method of claim 1, wherein the first criterion comprises a minimal copper loss constraint. 11. The method of claim 1, wherein the second criterion comprises a voltage limiting constraint. 12. The method of claim 1, wherein the wind turbine generator is an interior permanent magnet generator. 13. An apparatus for controlling electrical power generated by a wind turbine generator, the apparatus comprising: a determining unit adapted to determine an electromagnetic power reference, wherein the electromagnetic power reference represents the electromagnetic power generated by the wind turbine generator and is determined based on a desired output of the wind turbine generator, anda controlling unit adapted to control the electrical power generated by the wind turbine generator using a control signal;wherein the controlling unit comprises: a first determining sub-unit adapted to determine a first reference from one of a first field power stator flux reference and a stator current reference based on a first criterion in dependence on the electromagnetic power reference, wherein the one of the first field power stator flux reference and stator current reference is linearized to compensate for non-linearity of the wind turbine generator;a second determining sub-unit adapted to determine a second reference from one of a second field power stator flux reference and a stator current reference based on a second criterion in dependence on the electromagnetic power reference, wherein the one of the second field power stator flux reference and stator current reference is linearized to compensate for non-linearity of the wind turbine generator;a selection sub-unit adapted to select one of the first reference and the second reference; anda controlling sub-unit adapted to control the electrical power generated by the wind turbine using the control signal based on the selected one of the first reference and the second reference. 14. The apparatus of claim 13, wherein the first determining sub-unit and the second determining sub-unit are adapted such that the first reference and the second reference are calculated using look-up tables or polynomial functions. 15. The apparatus of claim 13, wherein the wind turbine generator is an interior permanent magnet generator. 16. A wind turbine generator, comprising an apparatus of claim 13. 17. The apparatus of claim 13, wherein the controlling unit further comprises: a third determining sub-unit adapted to determine a third reference from one of a first reluctance power stator flux reference and a stator current reference based on a third criterion and the selected one of the first reference and the second reference;a fourth determining sub-unit adapted to determine a fourth reference from one of a second reluctance power stator flux reference and a stator current reference based on a fourth criterion and on the selected one of the first reference and the second reference;a second selection sub-unit adapted to select one of the third reference and the fourth reference; anda second controlling sub-unit adapted to control the stator flux or stator current of the wind turbine generator based on the selected one of the first reference and the second reference and the selected one of the third reference and the fourth reference. 18. The apparatus of claim 17, wherein the second selection sub-unit selects one of the third reference and the fourth reference by: comparing an absolute value of the third reference and an absolute value of the fourth reference; andselecting the one of the third reference and the fourth reference which has the smaller absolute value. 19. The apparatus of claim 17, wherein the third criterion comprises a generator minimal copper loss constraint. 20. The apparatus of claim 17, wherein the fourth criterion comprises a generator voltage limiting constraint.