METHOD FOR OPERATING A WIND TURBINE

Abstract

A method for operating a wind turbine is disclosed. The rotational speed and power of the wind turbine are reduced when the prevailing wind speed exceeds a predetermined first limit value. The rotational speed and power are reduced further with an increasing wind speed until the rotational speed reaches a predetermined minimum rotational speed and/or the power reaches a predetermined minimum power. The wind energy turbine maintains the minimum rotational speed, or the minimum power, if the wind speed increases even further.

Claims

1. A method for operating a wind turbine comprising: reducing a rotational speed of a rotor of the wind turbine and a power output of the wind turbine when a prevailing wind speed exceeds a predetermined first limit value; further reducing the rotational speed and the power output with an increasing wind speed beyond the predetermined first limit value until at least one of: the rotational speed reaches a predetermined minimum rotational speed and/or the power output reaches a predetermined minimum power; and if the wind speed further increases, maintaining at least one of the minimum rotational speed and the minimum power.

2. The method according to claim 1, further comprising: if the wind speed further increases, maintaining at least one of: the minimum rotational speed and the minimum power for higher wind speeds and refraining from switching off the wind turbine.

3. The method according to claim 1, comprising: when the predetermined minimum rotational speed is reached or when the predetermined minimum power is reached, adjusting blade angles of the rotor blades of the wind turbine so that the power output remains constant.

4. The method according to claim 1, wherein the predetermined first limit value depends on at least one of a gustiness and a gust frequency of the prevailing wind.

5. The method according to claim 1, wherein the minimum power is at least sufficient to supply operating devices of the wind turbine for operating the wind turbine.

6. A method for operating at least two wind turbines grouped in a wind park, wherein each wind turbine is operated according to the method of claim 1.

7. The method according to claim 6, wherein in the event of wind speeds above the first limit value, at least one wind turbine of the wind turbines generates enough power to supply operating devices of the at least one wind turbine that the at least one wind turbine sends excess to another wind turbine of the wind turbines for operating operating devices of the other wind turbine.

8. A wind turbine configured to be operated by the method according to claim 1.

9. The wind turbine according to claim 8, wherein the wind turbine is gearless and has a synchronous generator.

10. A wind park having at least two wind turbines according to claim 8.

11. The wind park according to claim 10, wherein in the event of a gale, each wind turbine generates enough power to respectively operate a further wind turbine.

12. The method according to claim 4, further comprising: before the prevailing wind speed reaches the predetermined first limit value, reducing at least one of the rotational speed and the power in the event of at least one of: a high speed gust or the gust frequency of the wind.

13. The method according to claim 5, wherein the minimum power is at least twice as much as necessary to supply the operating devices of the wind turbine, so that the wind turbine and at least one other wind turbine is operated.

14. The method according to claim 13, wherein the at least one other wind turbine is of a same size as the wind turbine.

15. The wind turbine according to claim 9, wherein the wind turbine has external excitation and is configured as a horizontal-axis wind turbine, and has rotor blades whose attitude angle is adjustable.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0047] The invention will be explained below by way of example with the aid of figures.

[0048] FIG. 1 schematically shows a wind turbine.

[0049] FIG. 2 schematically shows a wind park.

[0050] FIG. 3 schematically shows a diagram of the dependency of the power and the rotational speed on the wind speed.

[0051] FIG. 4 illustrates in a schematic diagram a wind speed profile for gusts, represented as a function of time.

DETAILED DESCRIPTION

[0052] FIG. 1 shows a wind turbine 100 having a tower 102 and a nacelle 104. A rotor 106 having three rotor blades 108 and a spinner 110 is arranged on the nacelle 104. During operation, the rotor 106 is set in a rotational movement by the wind and thereby drives a generator in the nacelle 104.

[0053] FIG. 2 shows a wind park 112 with, by way of example, three wind turbines 100 which may be identical or different. The three wind turbines 100 are therefore representative for, in principle, any number of wind turbines of a wind park 112. The wind turbines 100 provide their power, i.e., in particular the generated current, via a park electrical network 114. In this case, the respectively generated current or powers of the individual wind turbines 100 are added together, and a transformer 116 is usually provided which steps up the voltage in the park and then feeds it into the supply network 120 at an input point 118, which is generally also referred to as PCC. FIG. 2 is only a simplified representation of a wind park 112, which for example does not show a controller, even though there is naturally a controller. Also, for example, the park network 114 may be configured differently, for example by there also being a transformer at the output of each wind turbine 100, to mention only one other exemplary embodiment.

[0054] FIG. 3 shows the profile of the rotational speed n and of the power P as a function of the wind speed V.sub.w. Accordingly, starting from the initial wind speed V.sub.w0 the rotational speed n increases. It increases as far as the rated wind speed V.sub.wN. The power P begins at the starting wind speed V.sub.Pzu with a small value. At this wind speed V.sub.Pzu, for example, the excitation of the rotor of the generator is switched on, so that power is now generated for a first time, i.e., at this low speed. The power P then increases further up to the rated wind speed V.sub.wN. This range from V.sub.Pzu to V.sub.wN is also referred to as the partial load range. The profiles of the power P and of the rotational speed n are represented linearly here for simplicity and are usually strictly monotonically increasing, but not linearly, but rather with a curved profile.

[0055] At the rated wind speed V.sub.wN, both values now reach their rated values, i.e., the rotational speed n reaches its rated rotational speed n.sub.N and the power P reaches its rated power P.sub.N. Both are moreover usually system properties of the wind turbine, for which the latter is configured, and in particular the generator is configured. This applies in particular for a gearless wind turbine, in which the rotational speed n of the aerodynamic rotor is the same as the rotational speed n of the electrodynamic rotor of the generator.

[0056] When the wind speed increases further, the power P and the rotational speed n remain at their rated values. To this end, in particular, the attitude angle of the rotor blades with respect to the wind is modified. Specifically, with an increasing wind, the rotor blades are turned away from the wind, i.e., in the direction of a feathering position. This is carried out as far as the first limit wind speed V.sub.WG1. This first limit value V.sub.WG1 lies, in particular, at or at the end of wind force 9 according to the Beaufort scale and therefore at the transition from gale to severe gale.

[0057] At this first limit wind speed, the rotational speed n as well as the power P are then reduced as far as the second limit wind speed V.sub.WG2. There, they then reach their minimum values, namely the minimum power P.sub.min and minimum rotational speed n.sub.min.

[0058] The reduction of the power P and of the rotational speed n from the first limit wind speed V.sub.WG1 to the second limit wind speed V.sub.WG2 is represented approximately linearly in FIG. 3. A linear reduction is a preferred embodiment, although the reduction may be carried out in another way for the power P and/or the rotational speed n, for example a parabola or composite parabola or a sine function, for example a section of a sine function from 90 to 270 displaced into the positive range, to mention only one advantageous example.

[0059] The first limit wind speed V.sub.WG1 and the second limit wind speed V.sub.WG2 are used here synonymously for the first limit value of the prevailing wind speed and the second limit value of the prevailing wind speed. At the second limit wind speed, the power P and the rotational speed n then reach their minimum values P.sub.min and n.sub.min, respectively, at which they are then kept even if the wind speed V.sub.w continues to increase. The symbol is symbolically indicated there, in order to illustrate that these two minimum values are maintained even in the event of, in principle, arbitrarily higher wind speeds. Naturally, the wind speed does not reach the value co and this is used only for illustration.

[0060] In the schematic representation of FIG. 3, both the power P and the rotational speed n are respectively normalized to their rated values. According to the representation, the minimum rotational speed n.sub.min is about 25% of the rated rotational speed n.sub.N, and the minimum power P.sub.min is about 0% of the rated power P.sub.N, i.e., it is reduced to zero or almost zero in this embodiment, although a higher value may be envisaged. These are only illustrative values, and 25% of the rated rotational speed is a very high value, which should preferably be lower. It is, however, realistic for the minimum power P.sub.min to be reduced more greatly relative to its rated power P.sub.n than the minimum rotational speed n.sub.min to be reduced relative to its rated rotational speed n.sub.N, because otherwise there would still be a maximum torque in this range of very high wind speeds, i.e., above the second limit wind speed. Preferably, however, the torque is also reduced so that the power is correspondingly reduced even further with a reduction of the rotational speed. The reason resides in the relationship between the power P, the rotational speed n and the torque m according to the formula P=nm.

[0061] FIG. 4 illustrates very schematically the profile of the wind speed V.sub.w as a function of time t. The more strongly varying curve is intended to represent the actual wind speed, in particular the instantaneous wind speed V.sub.wi, whereas the very uniform curve represents the 1 minute average value V.sub.w1. The schematic example depicts about 13 minutes and the instantaneous wind speed V.sub.wi has three gusts B.sub.1 to B.sub.3 in the period of time represented. There is therefore a gust frequency of three gusts in 13 minutes, i.e., approximately one gust every four minutes. This would be a relatively low frequency of the gusts.

[0062] The outlined level of the gusts reaches about 7 m/s to 15 m/s above the 1 minute average value V.sub.wi. The gust height would in this case be on average about 10 m/s above the 1 minute average value and therefore twice as high as the minimum height of a gust, namely 5 m/s above the 1 minute average value. Here, the gustiness could thus be indicated by the value 2. This gustiness of 2 would be an average value and furthermore corresponds approximately to a conventional weather situation in which a gust is about two wind forces above the minimum wind speed.

[0063] According to one embodiment, the first limit wind speed V.sub.wG1 is reduced as a function of the gust frequency and/or the gustiness or strength of the gusts. This is represented in FIG. 3 by the dashed branch of the rotational speed n.sub.B and for the power P by the dashed branch P.sub.B. A gust-dependent shift is thus outlined there. The limit wind speeds V.sub.wG1 as well as V.sub.wG2 have not been modified in the representation, but as a result the wind speed values at which the power P or P.sub.B on the one hand and the rotational speed n or n.sub.B on the other hand are reduced have been shifted to lower wind speed values. These modified values are denoted as V*.sub.wG1 and V*.sub.wG2 on the abscissa.

[0064] If the two FIGS. 3 and 4 are considered together, the gust frequency, which is very low, would not, or would only to a small extent, lead to the indicated shift of the gust-dependent power P.sub.B and the gust-dependent rotational speed n.sub.B. The gustiness or strength of the gusts according to FIG. 4 has a moderate value and would therefore lead according to the corresponding embodiment to a shift of the power P.sub.B or rotational speed n.sub.B.