Methods of operating a wind turbine

Abstract

Methods of operating a variable speed wind turbine as a function of a wind speed, the wind turbine having a rotor with a plurality of blades, and one or more pitch mechanisms for rotating the blades. The method comprising a sub-nominal zone of operation for wind speeds below the nominal wind speed and a supra-nominal zone of operation for wind speeds above the nominal wind speed. In the supra-nominal zone, the blades are pitched so as to maintain the rotor speed substantially constant, and a tip speed ratio of the wind turbine is substantially continuously being determined and wherein an instantaneous minimum pitch angle is substantially continuously being determined based on the instantaneous tip speed ratio, and the blades are never pitched below the instantaneous minimum pitch angle. The disclosure further relates to a wind turbine suitable for carrying out such methods.

Claims

1. A wind turbine, comprising: a generator, a rotor with a plurality of blades, one or more pitch mechanisms for rotating the blades, a system for determining a rotor speed, a wind speed sensor, a control system comprising a pitch control system and a generator control system, the generator control system being adapted to control the generator, and the pitch control system being adapted to control the one or more pitch mechanisms, wherein the generator control system is adapted to determine generator torque commands in accordance with a steady-state control loop and the pitch control system is adapted to determine pitch angle commands in accordance with a transient control loop and the steady-state control loop, wherein the steady-state control loop is adapted to calculate generator torque set commands and pitch angle set commands based at least partially on an instantaneous rotor speed determined by the system for determining the rotor speed, the transient control loop is adapted to calculate an instantaneous tip speed ratio based on a wind speed measured by the wind speed sensor and the instantaneous rotor speed determined by the system for determining the rotor speed, and is further adapted to determine instantaneous minimum pitch angle commands based on the instantaneous tip speed ratio, and wherein in a supra-nominal zone of operation for wind speeds above a nominal wind speed, the pitch control system is adapted to determine whether the pitch angle command is above the instantaneous minimum pitch angle command, and in case of a positive result, the pitch control system follows the received pitch angle command; and in case of a negative result, the pitch control system follows the instantaneous minimum pitch angle command; and wherein the minimum pitch angle is defined as the pitch angle corresponding to an angle of attack that is a predefined amount or percentage below a critical angle of attack of a representative wind turbine blade section.

2. The wind turbine according to claim 1, wherein the wind speed sensor is an anemometer mounted on a nacelle of the wind turbine.

3. The wind turbine according to claim 1, wherein the system for determining the rotor speed comprises a generator speed sensor.

4. The wind turbine according to claim 2, wherein the instantaneous tip speed ratio is based on an average wind speed measured by the anemometer.

5. The wind turbine according to claim 2, wherein the wind speed measurement of the nacelle mounted anemometer is an average wind speed measured over a period of time of 1 -5 seconds.

6. The wind turbine according to claim 1, wherein the wind speed sensor is a LIDAR.

7. The wind turbine according to claim 1, wherein the representative wind turbine blade section is a section of the blade at 25% of a length of the blade.

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-2f illustrate aerodynamics of wind turbine blades and aerodynamic profiles in general;

(4) FIG. 3 illustrates a wind turbine;

(5) FIGS. 4a and 4b illustrate aerodynamics related to examples of methods according to the present invention; and

(6) FIG. 5 illustrates a wind turbine and a control system according to an example of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(7) FIG. 3 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.

(8) The wind turbine may have one or more pitch systems to rotate the blades 41, 42, 43 collectively or individually. FIG. 1 represents a typical power curve for a variable speed wind turbine with pitch capability. As mentioned before, above the nominal wind speed, the generator torque and rotor speed may be maintained constant even though the wind speed increases. This can be achieved by pitching the wind turbine blades, i.e. by rotating the blades along their longitudinal axes and with respect to the hub; by increasing the pitch of the blades, their angle of attack decreases and their contribution to the torque also decreases.

(9) The ideal power curve as depicted in FIG. 1 may most reliably be followed during steady-state conditions if the pitch system(s) is/are directly controlled by a sensor indicating the rotor speed. The rotational speed of the rotor may be measured directly or may be determined by measuring the rotational speed of the generator rotor. The pitch system is thus actuated to keep the speed constant, and if generator torque is maintained constant as well, this means that aerodynamic torque is maintained substantially constant as well.

(10) A problem arises in case of transients wind conditions. A particularly challenging situation may be a Mexican hat wind gust in the supra-nominal zone of operation. However, examples of the present invention may equally well be suitable in different transient conditions, such as sudden increases or decreases of wind speeds, for example in case of increased turbulence.

(11) In the case of a Mexican hat wind gust, the wind speed first decreases. Although the rotor may be slow to react, the rotor speed may diminish a little bit. The reaction of the pitch system may thus be to increase the angle of attack of the blades (i.e. decrease the pitch) so that the blades capture the wind better, i.e. increase their lift.

(12) After the decrease in wind speed, the wind speed suddenly increases. Once again, the rotor will be slow to react. The rotor speed may increase, and in reaction the pitch of the blades may be increased, but this may not be sufficient. Due to the previous decrease in pitch and corresponding increase in angle of attack, when the wind speed starts to rise, the angle of attack of the blade may surpass the critical angle of attack, and the blades may stall. With reference to FIG. 4a, the point of operation moves from point A, at tip speed ratio equal to .sub.1, to point B, which results from a similar pitch (the inertia of the rotor is not capable of tracking wind variations, so the input to the pitch drive system remains almost unaltered during the wind gust) and a significantly reduced tip speed ratio, .sub.2.

(13) A consequence of the stall may be relatively high loads. Another consequence may be a decrease in rotor speed. As such, the reaction of the pitch system may be to further decrease the pitch (and increase the angle of attack), thus aggravating the stall.

(14) In accordance with some examples of the invention, the tip speed ratio may be determined in a substantially continuous manner by measuring both wind speed and (generator) rotor speed. Given the real-time tip speed ratio, a minimum pitch angle may be determined which corresponds to a maximum angle of attack of the blade which should not be surpassed. This maximum angle of attack of the blade may correspond substantially to a critical angle of attack. Alternatively, it may correspond to an angle of attack with a predefined distance to the critical angle of attack.

(15) In the aforementioned example, when the wind speed starts to increase, the anemometer will be able to measure the increase in wind speed before the rotor speed increases. Whereas the input for the pitch system normally is the rotor speed, in this case, a boundary condition is set for the pitch angle not to surpass a minimum pitch angle as determined based on the anemometer (or LIDAR or other wind measuring device). With reference to FIG. 4a, the point of operation moves from point A to point C (instead of to point B), which is located to the right of .sub.crit,2, that is, to the right of the pitch angle that would lead to stalling of the blade. The blades will thus not stall and operation may continue in a more efficient manner, with lower blade loads. Furthermore, tower loads are also reduced as situations with high wind and low speed are avoided. These situations result in high thrust on the wind turbine rotor which would lead to abnormally high loads on the tower as well.

(16) The same effect can be also explained with reference to FIG. 4b. The curve indicated with normal illustrates the blade pitch angle as a function of the tip speed ratio under steady-state conditions as may be determined in accordance with a power curve such as the one illustrated in FIG. 1.

(17) The curve indicated with the label critical illustrates that for each tip speed ratio, a critical pitch angle, .sub.crit, exists. This curve defines the blade pitch lower boundary. Thus, given a certain tip speed ratio, pitch values below the second curve may result in stall. These pitch values are therefore to be avoided in order to keep loads under control.

(18) As already mentioned with reference to FIG. 4a, an initial steady state situation might correspond to the wind turbine operating in the supra-nominal zone with a tip speed ratio .sub.1. Under these circumstances, a blade pitch angle on the steady-state curve, .sub.1, would normally be found (operating point shown as A).

(19) In case of a wind gust, a significant shift may result in the tip speed ratio, which may be reduced from .sub.1 to .sub.2. Nevertheless, as the rotor inertia is too large to track said wind variations, the pitch set point as defined by prior-art methods, remains almost unaffected around .sub.1 (operating point B). Consequently, the new operating point, which is characterized by a too low pitch angle for the prevailing tip speed ratio, lies in the unstable region. Stall may occur, and loads may increase.

(20) This problem may be avoided, according to the present invention, by using a second blade pitch setpoint that defines a minimum boundary. This value is not dependent on the rotor speed but on the tip speed ratio. In one example, the curve labelled critical, defining .sub.crit values for each tip speed ratio may be used as this minimum boundary. The resulting operational point would thus be point C.sub.2.

(21) In another example, a curve defining minimum pitch angle values which lie slightly above the critical pitch angles may be used as such a minimum boundary. In FIG. 4b, such a curve is labelled with minimum. The resulting operating point may thus be point C.sub.1.

(22) The minimum pitch values may in some embodiments be e.g. the pitch angle corresponding to an angle of attack that is a predefined amount or percentage below a critical angle of attack of a representative wind turbine blade section for the supra-nominal zone of operation.

(23) Even though in FIG. 4b the curves giving the relationship between tip speed ratio and pitch angle may be straight lines, it is to be understood that this is not necessarily the case.

(24) FIG. 5 illustrates a wind turbine and a control system according to an example of the present invention. Wind turbine 40 comprises a generator and one or more pitch actuators 65. The pitch actuators or pitch mechanisms may be controlled by a pitch control system 60. To this end, the pitch control system 60 may send pitch commands 61 to the pitch actuators 65. A result is the setting of pitch angle 66 in the wind turbine 40.

(25) A generator control system 50 may send torque commands to a generator 55 and a converter 53 related to the generator. A result is the setting of the generator torque 56 in the wind turbine 40.

(26) Results of both settings in the wind turbine include a generator speed, .sub.gen, and electrical power P generated.

(27) A control system of the wind turbine may comprise a steady state control loop 70 and a transient control loop 80. The steady state control loop 70 may be adapted generally to control the wind turbine in such a way that a predefined power curve, e.g. such as the one in FIG. 1 is followed. The transient control loop may be adapted to ensure that the instantaneous pitch angles of the blades do not sink below a minimum pitch angle. This minimum pitch angle may be e.g. a critical pitch angle for a representative portion of the blade, e.g. at 25% of the blade length.

(28) The steady state control loop 70, a generator speed sensor 36 may measure the generator speed, .sub.meas. A comparison of the measured generator speed, .sub.meas, with an expected generator speed .sub.steady gives an error result . Based on the error, the torque controller 50 and the pitch controller 60 can determine pitch commands 61 and generator torque commands 51. Depending on the instantaneous operational range, the generator torque or the pitch angle of the blades, or both may be adapted to generally follow the predefined power curve.

(29) The transient control loop 80, comprises a tip speed ratio calculator TSR. The calculation of an instantaneous tip speed ratio may be based on a wind speed V.sub.wind as measured by a nacelle mounted anemometer 46. The measurement from the anemometer upon which the determination of the minimum pitch angle is based, may be an average wind speed as measured by the anemometer over a period of e.g. 1-5 seconds, e.g. 3 seconds. Alternatively, any other system for determining a representative wind speed can be used, such as e.g. a LIDAR.

(30) The calculation of the instantaneous tip speed ratio may be based on the measured generator speed, .sub.meas. The rotor speed used in the calculation may be the measured generator speed in the case of direct drive wind turbines, or may have a constant ratio with the generator speed in the case of wind turbines with a gearbox.

(31) The calculated instantaneous tip speed ratio, , may be sent to a minimum pitch commander 69. Based e.g. on curves such as the ones shown in FIG. 4b, the minimum pitch commander may send instantaneous minimum pitch commands .sub.min to the pitch controller 60. The pitch command 61 sent to the pitch mechanism(s) may be the pitch set command determined in the steady state control loop if it is above the minimum pitch command. If the pitch set command of the steady state control loop is below the instantaneous minimum pitch command the instantaneous minimum pitch command (as determined in the steady state control loop) is followed and sent to the pitch mechanism(s).

(32) In a supra-nominal zone of operation, corresponding to wind speeds above nominal wind speeds, the steady state control loop may send constant generator torque commands to the converter 53 and generator 55 and varying pitch commands 61 to the pitch actuator(s) 65. The pitch commands are varied so as to maintain the generator speed constant. At the same time, the transient control loop 80 ensures that the pitch angle does not fall below a minimum pitch angle, so that the blade does not stall.

(33) 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.