Control method for a wind turbine

Abstract

The invention relates to a method of controlling a wind turbine, the wind turbine comprising wind turbine blades attached to a rotor hub and a control system for pitching the blades relative to the rotor hub. The method comprises determining a wind speed and providing a normal pitch mode of operation to control the output power of the wind turbine, where the pitch mode of operation comprises pitch reference values in dependence of the wind speed. The output power of the turbine is controlled according to the normal pitch mode of operation as a function of the wind speed if the wind speed is lower than a first upper level wind speed threshold, and according to a modified mode of operation if the wind speed exceeds the first upper level threshold wind speed, wherein the modified mode of operation comprises decreasing the output power according to a de-rating function which is a function of time. The invention further relates to a control system for controlling a wind turbine according to the above mentioned control method.

Claims

1. A method of controlling a wind turbine, the wind turbine comprising blades attached to a rotor hub and a control system for pitching the blades relative to the rotor hub, the method comprising: determining a wind speed; controlling, when the wind speed is less than a first upper level wind speed threshold, an output power of the wind turbine according to a normal pitch mode of operation, wherein in the normal pitch mode the control system generates pitch reference values dependent on the wind speed; and controlling the output power of the wind turbine according to a modified mode of operation, wherein in the modified mode of operation: when the wind speed exceeds the first upper level wind speed threshold, the output power is decreased according to a predefined, time-based de-rating function that is independent of the wind speed; and when the wind speed exceeds a second upper level wind speed threshold that is greater than the first upper level wind speed threshold, the output power is further decreased according to the predefined, time-based de-rating function.

2. The method of controlling a wind turbine according to claim 1, wherein in the modified mode of operation, the output power is increased according to a time-based up-rating function when the wind speed drops below a first lower level wind speed threshold that is less than the first upper level wind speed threshold.

3. The method of controlling a wind turbine according to claim 2, wherein the normal pitch mode of operation is resumed when the wind speed is lower than the first upper level wind speed threshold and the output power is increased to reach the output power as provided by the normal pitch mode of operation.

4. The method of controlling a wind turbine according to claim 1, wherein, in the modified mode of operation, the wind turbine is stopped when at least one of: the wind speed exceeds a predetermined stop wind speed, and the output power reaches a predetermined minimum power threshold.

5. The method of controlling a wind turbine according to claim 2 wherein the method further comprises increasing the output power according to the up-rating function as a result of the wind speed dropping below a second lower level wind speed threshold that is less than a corresponding upper level wind speed threshold.

6. The method of controlling a wind turbine according to claim 2, wherein the de-rating function comprises a decreasing part and a constant part and wherein the up-rating function comprises an increasing part and a constant part.

7. The method of controlling a wind turbine according to claim 2, wherein the modified mode of operation further comprises controlling a rotational speed of the wind turbine in addition to controlling the output power.

8. The method of controlling a wind turbine according to claim 7, wherein, in the modified mode of operation, the rotational speed of the wind turbine is increased or decreased in accordance with the lower and upper level wind speed thresholds.

9. The method of controlling a wind turbine according to claim 7, wherein at least one of: the output power is controlled to decrease at a greater rate than a rate of decreasing the rotational speed, and the rotational speed is controlled to increase at a greater rate than a rate of increasing the output power.

10. The method of controlling a wind turbine according to claim 2, wherein at least one of: the de-rating function and the up-rating function, comprises controlling the output power at a constant rate.

11. The method of controlling a wind turbine according to claim 10, wherein the constant rate output power is within the range of 0.5 kW/s to 1.5 kW/s.

12. A control system for a wind turbine configured to perform: receiving a determined wind speed; controlling, when the wind speed is less than a first upper level wind speed threshold, an output power of the wind turbine according to a normal pitch mode of operation, wherein in the normal pitch mode the control system generates pitch reference values dependent on the wind speed; and controlling, when the wind speed exceeds the first upper level wind speed threshold, the output power and rotational speed of the wind turbine according to a modified mode of operation wherein in the modified mode of operation: the output power is decreased according to a pre-defined, time-based de-rating function that is independent of the wind speed, and wherein at least one of: the output power is controlled to decrease at a greater rate than a rate of decreasing the rotational speed, and the rotational speed is controlled to increase at a greater rate than a rate of increasing the output power.

13. The control system according to claim 12, wherein the greater rate of decreasing the output power is 10% or less than the rate of decreasing the rotational speed.

14. The control system according to claim 12, wherein in the modified mode of operation, the output power is increased according to a time-based up-rating function when the wind speed is below a first lower level wind speed threshold that is less than the first upper level wind speed threshold.

15. A method of controlling a wind turbine comprising: determining a wind speed; controlling, when the wind speed is less than a first upper level wind speed threshold, an output power of the wind turbine according to a normal pitch mode of operation, wherein in the normal pitch mode a control system of the wind turbine generates pitch reference values dependent on the wind speed; and controlling, when the wind speed exceeds the first upper level wind speed threshold, the output power of the wind turbine according to a modified mode of operation, wherein the output power is decreased according to a predefined, time-based, de-rating function that is independent of the wind speed, and the de-rating function comprises a decreasing part and a constant part.

16. The method according to claim 15, wherein, the modified mode of operation, the output power is increased according to a time-based up-rating function when the wind speed drops below a first lower level wind speed threshold that is less than the first upper level wind speed threshold.

17. The method according to claim 16, wherein the up-rating function comprises an increasing part and a constant part.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following different embodiments of the invention will be described with reference to the drawings, wherein:

(2) FIG. 1 shows a simulated example of the wind speed as a function of time for a number of wind turbines in a power plant,

(3) FIG. 2 shows the power produced as function of time for the wind turbines of FIG. 1 when controlled according to an embodiment of the invention,

(4) FIG. 3 is a flow chart illustrating a control method according to an embodiment of the invention,

(5) FIG. 4 illustrates the resulting power as a function of the wind speed resulting from Monte-Carlo simulations of 200 wind turbines controlled according to an embodiment of the invention a time span of 1800 s and at wind speeds around a first upper level threshold wind speed of 25 m/s,

(6) FIG. 5 shows an example of a conventional power curve during normal pitch mode of operation and with the power controlled as a function of the wind speed, and

(7) FIG. 6 shows an embodiment of the invention of the de-rating of the output power and rotational speed of the wind turbine as a function of time.

DESCRIPTION OF EMBODIMENTS

(8) FIG. 1 shows a simulated example of the wind speed V, 102 as a function of time t, 101 for eight different wind turbines in a wind power plant. All turbines start from a wind speed V of 25 m/s at t=0. The simulations are performed as a Monte-Carlo simulation using a statistical tool which for every 200 second assumes that the change in wind speed is a normal distribution with a mean value of 0 m/s and a standard deviation of 1 m/s. Hereby is obtained a realistic wind speed scenario for a wind power plant with wind speeds around the stop or cut-out wind which is 25 m/s for many modern turbines. The control method according to an embodiment of the invention is then applied to all the wind turbines with the first upper level wind speed threshold V.sub.up,1 equal to the stop wind of 25 m/s.

(9) When the wind speed is lower than the first upper level wind speed threshold (here set to 25 m/s), the wind turbines are controlled according to a conventional pitch mode of operation. FIG. 5h shows an example of a conventional or normal power curve with the power P, 201 set directly as a function of the wind speed V, 102. As can be seen from the FIG. 5 the power increases with increasing wind speeds until a nominal power is reached. The nominal power in this example is 3000 kW which is reached at a nominal wind speed of 15 m/s. At wind speeds above the nominal wind speed, the turbine is controlled with a view to maintain the nominal power while reducing or controlling the loads on the turbine. During such normal mode of operation, the output power of the turbine is therefore controlled directly as a function of the wind speed by the corresponding setting of pitch reference values as a function of the wind speed.

(10) FIG. 2 shows the power produced P, 201 by the eight turbines exposed to the wind as illustrated in FIG. 1 and when applying the control method and the parameters in the simulation as:

(11) Start ramp down at V.sub.up,1=25 m/s

(12) Start ramp up at V.sub.low,1=22 m/s

(13) Max Power: P.sub.max=3000 kW

(14) Power ramp or de-rating rate: 1 kW/s.

(15) As can be seen from the FIGS. 1 and 2, the wind turbine a (marked with +) at no time experiences a wind speed above the 25 m/s as the first upper level wind speed threshold V.sub.up,1 and therefore continues unaffected with the normal pitch mode of operation which here is maximal power of P.sub.max=3000 kW. As long as the wind speed is above V.sub.up,1=25 m/s the turbines de-rates with the power ramp in this example of 1 kW/s. This can be seen to happen to four of the turbines already at the first time step, encircled 103, whereas the turbines b and c (marked with crosses and filled circles, respectively) only start de-rating later and the turbine d (marked with triangles) even later at the time t=1000. As soon as the wind speed drops below V.sub.low,1=22 m/s the turbines start up-rating again until the normal power for the current wind speed according to the normal pitch mode of operation is reached. Such up-rating only happens for two of the turbines e and f near the end of the simulation.

(16) If a conventional stopping control had been applied to the eight turbines exposed to the winds of FIG. 1, four of the turbines would have shut down completely after the first time step as the wind speed then is above 25 m/s.

(17) In an embodiment the control method may further specify a minimum power, P.sub.min, for example equal to 1200 kW below which the turbine is stopped irrespective of the wind speed. Hereby is obtained a realistic gearbox protection level to avoid torque reversals. This minimum power was not reached in the simulations shown in FIGS. 1 and 2.

(18) FIG. 4 shows the power P,201 versus wind speed V,102 resulting from similar Monte-Carlo simulations as described above except that 200 turbines have been simulated and for time spans of hour at wind speeds around 25 m/s. Each dot in the figure then marks the current power and wind speed for a wind turbine at the time T of 1800 s in FIG. 4.

(19) It appears that the produced power generally decays with the wind speed with some scatter. The situations with high wind and high power which cause high turbine loading can be seen to be avoided with the proposed control method. For safety reasons it can be considered to shut down the turbines at e.g. 35 m/s. As can be seen from FIG. 4, all turbines have a full or reduced power production illustrating that annual energy production can be reasonably expected to be considerably improved as less power is lost when the turbines are de-rated rather than stopped at high wind loads.

(20) FIG. 3 is a flow chart illustrating different steps in a control method according to an embodiment of the invention. If a measured wind speed V, 301, is below a first upper level wind speed threshold V.sub.up,1 (step 302) the normal pitch mode of operation is followed as illustrated in 303 with the sketched standard power curve. If the first upper level wind speed threshold V.sub.up,1 is exceeded, a modified mode of operation of de-rating the power as a function of the time is followed as illustrated in 304. Applying the modified mode of operation of de-rating, the power is set to decrease with time independently of the current wind speed.

(21) This is continued until the wind speed drops below a first lower level wind speed threshold, V<V.sub.low,1 (step 305), in which case the power is again up-rated, 306. The power is increased until normal power for that wind speed is reached (step 307) and of course as long at the wind speed continues to be lower than the first upper level threshold value. When the normal power is reached again, the normal pitch mode of operation is resumed.

(22) In an embodiment, the modified mode of operation further includes maintaining the power constant when the wind speed drops below the first upper level wind speed threshold and until the first lower level wind speed threshold is reached (in which case the power is again up-rated) or until the wind speed exceeds the upper level first wind speed threshold (in which case the de-rating as a function of time is resumed).

(23) The flow chart of FIG. 3, only illustrates the process for the first upper and lower level wind speed thresholds. However, the process may be expanded to include also further levels of upper and lower level wind speed thresholds.

(24) In FIG. 6 the curves show a de-rating of the output power P, 201 and of the rotational speed , 601 as a function of the time t, 101 according to one embodiment of the invention. Initially both the output power and the rotational speed are at their nominal values. At the time t.sub.1 the wind speed exceeds the pre-set first upper level wind speed threshold, V>V.sub.up,1 and a modified mode of operation is initiated. In this embodiment both the output power P, 201 and the rotational speed , 601 are decreased as a function of the time t, 101. The de-rating of the power P, 201 is performed by decreasing the output power at a constant rate for a first period of time or time interval t.sub.1 followed by keeping the output power constant for a second time interval t.sub.e. Thus the de-rating function comprises a decreasing part (in time span t.sub.1) and a constant part (in time span t.sub.2). This procedure is then repeated at t.sub.2 where the wind speed exceeds the second upper level wind speed threshold, V>V.sub.up,2. At time t.sub.3 the wind speed drop below a lower level wind speed threshold, V<V.sub.low,1. Then the critical wind effect is considered to be over and the output power is up-rated again. The up-rating may likewise be performed as sketched in FIG. 6 by successive time intervals of increasing the power and of maintaining a constant or approximately constant output power.

(25) The rotational speed , 601 is in this embodiment de-rated correspondingly to the output power as outlined in the lowermost curve in FIG. 6. However, the time intervals t.sub.t and t.sub.2 during which the rotational speed is decreased linearly at a constant rate or maintained at a constant level need not be the same as the time intervals t.sub.1 and t.sub.2 governing the corresponding de-rating function of the output power.

(26) The output power may advantageously be reduced faster than the rotational speed to avoid high torque situations. This effect is obtained when the output power is reduced less than 10% faster than the rotational speed, such as for example in the range of 1-3% faster. Similarly the rotational speed may be increased faster than the output power to avoid high torque situations during up-rating, such as 1-10% faster than the power or 1-3% faster.

(27) While preferred embodiments of the invention have been described, it should be understood that the invention is not so limited and modifications may be made without departing from the invention. The scope of the invention is defined by the appended claims, and all devices that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein.