Method of starting a wind turbine

Abstract

Methods of starting a wind turbine from a standstill substantially until generator connection, the wind turbine having a rotor with one or more blades, a pitch system for rotating the blades along their longitudinal axes and a generator operationally connected with the rotor. In standstill, the blades are substantially in a feathered position and the generator is not generating electrical power. The methods may comprise measuring the wind speed representative for the wind turbine and measuring the rotor speed of the wind turbine, and when the rotor speed is not equal to zero, determining the tip speed ratio for the wind turbine, and determining the pitch angle of the blades as a function of the tip speed ratio to optimize the torque produced by the blades of the wind turbine rotor.

Claims

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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Particular embodiments of the present invention will be described in the following by way of non-limiting examples, with reference to the appended drawings, in which:

(2) FIG. 1 illustrates a typical power curve of a wind turbine;

(3) FIGS. 2a-2e illustrate aerodynamics of wind turbine blades and aerodynamic profiles in general;

(4) FIGS. 3a and 3b illustrate the process of pitching the wind turbine blades from a feather position towards an operational position; and

(5) FIG. 4 illustrates a wind turbine.

DETAILED DESCRIPTION

(6) FIG. 4 illustrates a wind turbine 40 having a rotor with three blades 41, 42 and 43. A nacelle 45 is mounted on wind turbine tower 44. An anemometer 46 is mounted on the nacelle 45. The anemometer 46 may be used to measure wind speed, however because of its location on the nacelle, behind the rotor, the wind speed measured by the anemometer may vary a lot and in general may not be very reliable.

(7) During start-up, in a first phase, the blades are rotated from a feather position towards an operational position. Depending on the wind conditions, such operational position may correspond to a zero degrees pitch angle (such as illustrated in FIG. 3a) or to a non-zero degrees pitch angle (e.g. as illustrated in FIG. 3b). In an example, this first phase may involve pitching the blades with a constant pitch rate.

(8) This pitch rate may be chosen such as to be suitable for any wind condition. In another example, two different pitch rates may be defined, one for high wind speeds, and another for low wind speeds. If the wind speed is higher than a threshold, the wind speed may be qualified as high. If the wind speed is lower than this threshold, the wind speed may be qualified as low.

(9) Depending on whether the wind speed is high or low, the pitch rate may be adapted. For low wind speed, a pitch rate of e.g. 3-5/second may be acceptable, with the risk of entering into stall being very low. The higher pitch rate will make sure that start-up is quick and efficient. For high wind speeds, a pitch rate of e.g. 0.5/second may be more suitable. If the blades are rotated at a higher rate, they may enter into stall, and the start-up may thus be unsuccessful.

(10) In further examples, three or more different pitch rates may be defined, each of them suitable for a certain range of wind speeds. Alternatively, the pitch rate might be varied continuously as a function of instantaneous wind speed, and/or the instantaneous pitch angle. For doing so, either an analytical expression or interpolated from values in a table might be employed.

(11) It may be suitable to use a relatively high first pitch rate. As the pitch angle is changed, and the wind turbine is getting closer to the rotor starting to move, it may be appropriate to use a relatively low pitch rate. The pitch rate may thus depend on the instantaneous pitch angle. To further optimize, also the instantaneous wind speed may be taken into account.

(12) Once the rotor starts rotating, the tip speed ratio may be used to determine the most suitable pitch angle at any given moment. As mentioned before, tip speed ratio may generally be defined as

(13) $=\frac{.Math.R}{V_w},$
wherein is the rotational speed of the rotor, R is the radius of the rotor swept area and V.sub.w is the wind speed.

(14) The wind speed may be measured e.g. using a LIDAR or using a nacelle mounted anemometer. If an anemometer mounted on the nacelle is used, an average wind speed over a short period of time e.g. 1-5 seconds may be used to have a more reliable indicator of the wind speed. In an example, an average wind speed measured during three seconds may be used. In further alternative examples, a wind speed representative for the (swept area) of a wind turbine may be measured at a different site, close to the wind turbine, instead of at the wind turbine. This may be done e.g. in a neighbouring wind turbine or using a measurement post.

(15) Thus, at any given moment, the actual wind speed may be known. Also, the rotor speed at any given moment may be measured e.g. by measuring the generator speed. As such, the tip speed ratio is known at any given moment. Given the tip speed ratio, and using the information from a graph such as the one illustrated in FIG. 2e, at any given moment the pitch angle that gives the most torque for the rotor may be determined.

(16) With reference to FIG. 2e, if the instantaneous tip speed ratio is .sub.1, the pitch angle for the maximum torque is given by the pitch angle corresponding to .sub.crit. Optionally, the pitch angle may be chosen slightly higher (and the angle of attack thus slightly smaller) than the critical angle for reasons of security, e.g. .sub.alt. For example, a 5% security margin may be taken into account. The torque generated is given by C.sub.Qalt, which still may be very close to the maximum possible torque for the instantaneous tip speed ratio.

(17) At any given moment, the instantaneous tip speed ratio may be determined in a reliable manner. Knowing the aerodynamics of the wind turbine blades, for a given tip speed ratio the ideal pitch angle can be determined. This may be implemented using e.g. a look-up table stored in the wind turbine control system, or alternatively in the SCADA of a wind park.

(18) This way, regardless of the wind conditions, the start-up of the wind turbine may be efficient and thereby shortened.

(19) Although only a number of particular embodiments and examples of the invention have been disclosed herein, it will be understood by those skilled in the art that other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof are possible. Furthermore, the present invention covers all possible combinations of the particular embodiments described. Thus, the scope of the present invention should not be limited by particular embodiments, but should be determined only by a fair reading of the claims that follow.