Method of starting a wind turbine

1. A method of starting up a wind turbine from a standstill substantially until generator connection, the wind turbine having a rotor with a plurality of blades, a pitch system for rotating each of the blades along their respective longitudinal axes and a generator operationally connected with the rotor,

wherein in standstill a rotor speed of the wind turbine is zero, the blades are substantially in a feathered position and the generator is not generating electrical power, the method comprising:

measuring a wind speed representative for the wind turbine and measuring the rotor speed of the wind turbine,

with the rotor speed equal to zero when starting up the wind turbine, rotating the blades with the pitch system at a constant positive pitch rate until the rotor starts to rotate,

and when the rotor starts to rotate and rotor speed is not equal to zero,

determining a tip speed ratio for the wind turbine, and

determining a pitch angle of the blades as a function of the tip speed ratio and, with the pitch system, changing the pitch angle of the blades to the determined pitch angle.

2. The method according to claim 1 , wherein

if the wind speed when starting up the wind turbine is below a threshold, the constant pitch rate is equal to a first value, and

if the wind speed when starting up the wind turbine is above the threshold, the constant pitch rate is equal to a second value,

the second value being a lower pitch rate than the first value.

3. The method according to claim 2 , wherein the first value is between 3-5° per second.

4. The method according to claim 3 , wherein the second value is approximately 0.5 per second.

5. The method according to claim 1 , wherein the wind speed representative for the wind turbine is measured using a LIDAR.

6. The method according to claim 1 , wherein the wind speed representative for the wind turbine is measured using an anemometer arranged on the nacelle of the wind turbine.

7. The method according to claim 6 , wherein the wind speed representative for the wind turbine is an average wind speed measured by the anemometer over a period of time.

8. The method according to claim 7 , wherein the period of time is between one and five seconds.

9. The method according to claim 8 , wherein the period of time is between two and four seconds.

10. The method according to claim 1 , wherein the pitch angle of the blades as a function of the tip speed ratio is a critical pitch angle.

11. The method according to claim 1 , wherein the pitch angle of the blades as a function of the tip speed ratio is a predefined percentage above a critical pitch angle.

12. The method according to claim 11 , wherein the predefined percentage is 5%.

13. The method according to claim 1 , wherein setting the pitch angle of the blades as a function of the tip speed ratio comprises obtaining the pitch angle from a look-up table.

14. A wind turbine control system configured to control a wind turbine having a plurality of blades from standstill substantially until a moment of connecting a generator, comprising:

wherein the wind turbine control system is configured

to receive a rotor speed and receive a wind speed representative for a wind turbine;

with the rotor speed equal to zero when starting up the wind turbine, rotate the blades at a constant pitch rate, wherein the constant pitch rate is determined as a function of the received wind speed when starting the wind turbine until the rotor starts to rotate,

and

when the rotor starts to rotate and rotor speed is not equal to zero

to determine a tip speed ratio for the wind turbine, and

to determine a pitch angle of the blades as a function of a tip speed ratio; and

change the pitch angle of the blades to the determined pitch angle by sending the determined pitch angle to one or more pitch systems of the wind turbine.

15. A wind turbine comprising a wind turbine control system according to claim 14 .

16. The wind turbine according to claim 15 , further comprising an anemometer for measuring the wind speed.