Method of operating a wind turbine

1. A method of operating a variable speed wind turbine as a function of a wind speed, the variable speed wind turbine having a rotor with a plurality of blades and one or more pitch mechanisms for rotating at least one blade of the plurality of blades, the method comprising:

in steady-state conditions following a power curve describing the operation of the variable speed wind turbine,

the power curve comprising a sub-nominal zone of operation for wind speeds below a nominal wind speed and a supra-nominal zone of operation for wind speeds above the nominal wind speed, and wherein

in steady-state conditions in the sub-nominal zone of operation, each blade of the plurality of blades is not pitched, and wherein

the sub-nominal zone of operation comprises a first operational range, a second operational range and a third operational range, wherein

the first operational range extends from a cut-in wind speed to a first wind speed, wherein a rotor speed is kept substantially constant at a first value,

the second operational range extends from the first wind speed to a second wind speed, wherein both the rotor speed and a generator torque are varied as a function of wind speed while a pitch angle of each blade of the plurality of blades is kept constant, and

the third operational range extends from the second wind speed to the nominal wind speed, wherein the rotor speed is kept substantially constant at a second value, and

the method further comprising in transient conditions in which the variable speed wind turbine cannot follow the power curve in the second operational range, changing the pitch angle of each blade of the plurality of blades as a function of a change in a tip speed ratio caused by the transient conditions so as to increase a power coefficient of the plurality of blades during the transient conditions by:

measuring instantaneous wind speed;

computing an instantaneous tip speed ratio for the measured instantaneous wind speed; and

changing the pitch angle of each blade of the plurality of blades to approach a maximum power coefficient based on the instantaneous lip speed ratio.

2. The method according to claim 1 , wherein the pitch angle of each blade of the plurality of blades is selected so as to achieve the maximum power coefficient available at the instantaneous tip speed ratio during the transient conditions.

3. The method according to claim 1 , wherein the instantaneous tip speed ratio during the transient conditions is determined based on the instantaneous wind speed measured by a nacelle mounted anemometer.

4. The method according to claim 3 , wherein the instantaneous wind speed measured by the nacelle mounted anemometer is an average wind speed measured over a short period of time.

5. The method according to claim 4 , wherein the short period of time is 1-5 seconds.

6. The method according to claim 5 , wherein the short period of time is between 2-4 seconds.

7. The method according to claim 6 , wherein the short period of time is approximately 3 seconds.

8. The method according to claim 1 , wherein the instantaneous tip speed ratio is determined based on a generator speed.

9. The method according to claim 2 , wherein the instantaneous tip speed ratio is determined based on the instantaneous wind speed measured by a nacelle mounted anemometer.

10. The method according to claim 9 , wherein the instantaneous wind speed measured by the nacelle mounted anemometer is an average wind speed measured over a short period of time.

11. The method according to claim 10 , wherein the short period of time is 1-5 seconds.

12. The method according to claim 2 , wherein the instantaneous tip speed ratio is determined based on a generator speed.

13. The method according to claim 3 , wherein the instantaneous tip speed ratio is determined based on a generator speed.

14. The method according to claim 4 , wherein the instantaneous tip speed ratio is determined based on a generator speed.

15. The method according to claim 5 , wherein the instantaneous tip speed ratio is determined based on a generator speed.

16. The method according to claim 6 , wherein the instantaneous tip speed ratio is determined based on a generator speed.

17. The method according to claim 7 , wherein the instantaneous tip speed ratio is determined based on a generator speed.

18. The method according to claim 11 , wherein the instantaneous tip speed ratio is determined based on a generator speed.