FRETTING CORROSION AND METHODS OF OPERATING A WIND TURBINE

Abstract

Methods of operating a wind turbine including one or more geared pitch systems. The method comprises a sub-nominal zone of operation for wind speeds below nominal and a supra-nominal zone of operation for wind speeds equal to or above the nominal. The sub-nominal zone of operation comprises a first operational range extending from a cut-in wind speed to a first wind speed and a second operational range extending from the first wind speed to a second wind speed, and the first operational range comprises two or more sub-ranges, wherein a different constant pitch position is defined for each of the sub-ranges. The method further comprises in the first operational range, using the geared pitch systems for setting the blades in the pitch position defined for each sub-range as a function of the current wind speed.

Claims

1. A method of operating a wind turbine in steady state conditions as a function of wind speed, wherein the wind turbine includes a rotor with a plurality of blades, a generator, one or more geared pitch systems for rotating the blades along their longitudinal axis and a system for varying a torque of the generator, and the method comprising: operating in a sub-nominal zone of operation for wind speeds below a nominal wind speed and in a supra-nominal zone of operation for wind speeds equal to or above the nominal wind speed, wherein the sub-nominal zone of operation comprises a first operational range which extends from a cut-in wind speed to a first wind speed and a second operational range extending from the first wind speed to a second wind speed, wherein the first operational range comprises two or more sub-ranges, wherein a different constant pitch position is defined for each of the sub-ranges; in the first operational range, using the geared pitch systems for setting the blades in the pitch position defined for each sub-range as a function of the current wind speed, and varying the torque of the generator to maintain a rotor speed substantially constant; and in the second operational range, varying the torque of the generator as a function of wind speed so as to maintain a constant tip speed ratio.

2. The method of claim 1, wherein the sub-ranges of the first operational range overlap in an intersection area and the pitch position defined for each sub-range is maintained along the intersection area.

3. The method of claim 1, wherein the sub-ranges are consecutives such that an upper limit of a lower sub-range coincides with a lower limit of an upper sub-range.

4. The method of claim 1, wherein the different constant pitch positions in the first operational range vary from each other approximately 1-5.

5. The method of claim 1, further comprising dividing the second operational range in two or more sub-ranges, wherein a different constant pitch position is defined for each of the sub-ranges and using the geared pitch systems for setting the blades in the pitch position defined for each sub-range as a function of the current wind speed.

6. The method of claim 5, wherein the different constant pitch positions in the second operational range vary from each other approximately 0.5-to approximately 1.

7. The method of claim 1, wherein the sub-nominal zone of operation further comprises a third operational range extending from the second wind speed to the nominal wind speed and the method further comprises in the third operational range, varying the torque of the generator to maintain the speed of the rotor substantially constant and equal to a nominal speed defined for the rotor.

8. The method of claim 7, further comprising dividing the third operational range in two or more sub-ranges, wherein a different constant pitch position is defined for each of the sub-ranges and using the geared pitch systems for setting the blades in the pitch position defined for each sub-range as a function of the current wind speed.

9. The method of claim 7, wherein a single pitch position is defined for the entire third operational range.

10. A wind turbine comprising: a rotor with a plurality of blades, a generator, one or more geared pitch systems for rotating the blades around along their longitudinal axis, a system for varying the generator torque and a control unit configured to: operate in a sub-nominal zone of operation for wind speeds below a nominal wind speed and in a supra-nominal zone of operation for wind speeds equal to or above the nominal wind speed, wherein the sub-nominal zone of operation comprises a first operational range which extends from a cut-in wind speed to a first wind speed and a second operational range extending from the first wind speed to a second wind speed, wherein the first operational range comprises two or more sub-ranges, wherein a different constant pitch position is defined for each of the sub-ranges; in the first operational range, use the geared pitch systems for setting the blades in the pitch position defined for each sub-range as a function of the current wind speed, and varying the torque of the generator to maintain a rotor speed substantially constant; and in the second operational range, vary the torque of the generator as a function of wind speed so as to maintain a constant tip speed ratio.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Non-limiting examples of the present disclosure will be described in the following, with reference to the appended drawings, in which:

[0030] FIG. 1 shows a typical power curve of a wind turbine;

[0031] FIGS. 2A and 2B show different examples of output obtained using the present methods,

[0032] FIGS. 3 and 4 show further examples of output that may be obtained using further examples of the present methods.

DETAILED DESCRIPTION OF EXAMPLES

[0033] The power curve of FIG. 1 has been discussed before. In FIG. 1, the continuous line shows the pitch angle versus incoming wind speed V. In this theoretical steady state situation, in the sub-nominal zone of operation, i.e. in the first, second and third operational ranges I, II and III, (from e.g. approximately 3 m/s to approximately 11 m/s) the pitch angle is not varied from a minimum pitch angle of e.g. 0. In alternative cases, the minimum or default pitch angle may be any other amount close to 0 defined as default pitch angle depending on circumstances.

[0034] Throughout the following figures the same reference numbers will be used for matching parts.

[0035] FIGS. 2A2A and 2B2B show examples of an output pitch angle versus incoming wind speed V obtained using a method substantially as hereinbefore described. In these examples the first operational range I may be divided in two sub-ranges and a different constant pitch angle may be defined for each sub-range. In alternative examples, other number of sub-ranges may be provided e.g. three or more, each having a different constant pitch angle. In these examples, the minimum or default pitch angle may be e.g. 2. Alternatively, any other amount close to 0 may be defined as default pitch angle depending on circumstances.

[0036] In the example of FIG. 2A, curve 10 shows that a first sub-range a may extend, for example from e.g. 3 m/s to 4.5 m/s and the defined pitch angle .sub.a may be approximately 7 and the second sub-range b may extend from e.g. 4.5 m/s to 6 m/s and the defined pitch angle .sub.b may be approximately 4.5. In this example, the sub-ranges a and b may be consecutives and may only overlap in one of their limits C corresponding to an upper limit of sub-range a and a lower limit of sub-range b. In this example limit C may thus be established at 4.5 m/s. In this example, two different defined pitch positions are thus defined in which other teeth of the actuating gear and annular gear may be contacting each other depending on the current wind speed. This avoids or at least reduces fretting corrosion between contacting teeth. In alternative examples, more sub-ranges may be defined each having a defined pitch angle. In some examples the different pitch angles/positions provided for each sub-range may vary from each other around 2-3, for example 2.5. In further examples, this variation may generally be between 1 to 5. In some examples, the step between one pitch position and another may be such that it involves changing a single tooth of the crown. The amount of degrees thus depends on the number of crown teeth. In alternative examples, the step between sub-ranges may involve changing more than one tooth.

[0037] In the example of FIG. 2B2B, curve 10 shows that a first sub-range a may extend from e.g. 3 m/s to 5 m/s and the defined pitch angle .sub.a may be approximately 7 as in the example of FIG. 2A2A but the second sub-range b may extend from e.g. 4 m/s to 6 m/s and the defined pitch angle .sub.b may be approximately 4.5. In this example, an intersection area IA may be defined between e.g. 4 m/s to 5 m/s. In this example, the method may further comprise maintaining the pitch angle defined for a sub-section along the intersection area IA. This means that the value of the pitch angle within the intersection area IA may depend on the previous pitch angle value. In this example, if a value of the pitch angle before reaching the intersection area IA belongs to the sub-range a, the defined pitch angle .sub.a will be maintained up to 5 m/s (upper limit of sub-range a). On the contrary if a value of the pitch angle before reaching the intersection area IA belongs to the sub-range b, the defined pitch angle .sub.b will be maintained up to 4 m/s (lower limit of sub-range b). By doing this, actuations of the pitch system are reduced thus minimizing hysteresis problems. Power consumption of the pitch systems is thus also reduced. In alternative examples, more than two sub-ranges may also be foreseen.

[0038] In these examples, the defined pitch position for each sub-range may also vary from each other approximately 2-3. In further examples, this variation may be slightly smaller or larger. Also, depending on circumstances other teeth of the actuating gear (pinion) and the annular gear will be contacting each other thus extending lifetime of the gear and/or the pinion.

[0039] In some examples, the method may further comprise dividing the second operational range II in two or more sub-ranges, each having a different defined pitch position and using the geared pitch system for setting the blades in the pitch position defined for each sub-range as a function of the current wind speed substantially as explained for the first operational range in connection with FIGS. 2A2A and 2B2B. FIG. 3 shows an example of these methods.

[0040] The example of FIG. 3 differs from that of FIG. 2B in that the second operational range II may also be divided in sub-ranges. In particular curve 10 shows two sub-ranges c and d. In alternative examples dividing the second operational range in two or more sub-ranges may also be combined with a first operational range as shown in the example of FIG. 2A. Furthermore, more sub-ranges may also be foreseen.

[0041] In curve 10 sub-range c may extend, for example, from 6 m/s to 7.5 m/s and the defined pitch angle .sub.c may be the minimum pitch angle, e.g. approximately 2, and the second sub-range d may extend from e.g. 7.5 m/s to 8.5 m/s and the defined pitch angle .sub.d may be e.g. 1.5. In this example, the sub-ranges c and d may be consecutives in a similar manner as explained for the first operational range in accordance with FIG. 2A. In alternative examples, the sub-ranges may overlap as explained for the first operational range in connection with FIG. 2B.

[0042] In general in the second operational range, as the blades are normally designed for maximum performance in this operational range, the defined pitch position for each sub-range may vary from approximately 0.5 to approximately 1. This way at least other regions of the same teeth of the actuating gear (pinion) and the annular gear will be contacting each other. This reduces at least in part premature wear of contacting teeth without substantially interfering in the optimum performance of this operational range. These very slightly different pitch positions may also take into account the phenomenon of torsion of the blades at higher wind speeds within the second range. As the blades are subjected to the torsion, their aerodynamic performance may not correspond exactly to their simulated or calculated theoretical performance. A slightly negative pitch may compensate for this effect.

[0043] As was shown for the first operational range, in this example, stepwise changes of pitch positions are employed. This thus means that the pitch system does not need to act continuously and wear and energy use of the pitch system can thus be less.

[0044] In the examples shown so far, the pitch position is maintained constant along the third operational range. The problem of fretting corrosion could thus theoretically persist if the wind speed were to stay within the third operational range for longer periods of time. However, it is relatively unlikely that the wind speed stays within the third operational range for a long time without incidentally increasing beyond the nominal wind speed. In the supra-nominal zone, the blades will be pitched so as to maintain aerodynamic torque substantially constant. The pitch position will thus at least incidentally be changed, so that different teeth touch other and new grease or lubricant can be provided at the location most vulnerable to the problem of fretting corrosion. This grease or lubricant is brought there by the different teeth that enter into contact.

[0045] In still further examples, the method may further comprise dividing the third operational range in two or more ranges and defining a different constant pitch position for each sub-range as a function of wind speed substantially as explained for the first and second operational ranges in connection with FIGS. 2A, 2B and 3. FIG. 4 shows an example of these methods. It is submitted that all possible combinations of this example with the examples of FIGS. 2A, 2B and 3 may also be foreseen.

[0046] The example of FIG. 4 differs from that of FIG. 3 in that the third operational range III may also be divided in sub-ranges, in particular curve 10 shows two sub-ranges e and f. In alternative examples, three or more sub-ranges may also be foreseen.

[0047] In curve 10 sub-range e may extend, for example, from 8.5 m/s to 10 m/s and the defined pitch angle .sub.e may be approximately 2.5 and the second sub-range f may extend from e.g. 9.5 m/s to 11 m/s and the defined pitch angle .sub.f may be e.g. 3. In this example, the sub-ranges e and f may overlap in an intersection IA as explained for the first operational range in accordance with FIG. 2B. In alternative examples, the sub-ranges may be consecutive as explained for the first operational range in connection with FIG. 2A.

[0048] Again in this example, by employing stepwise changes as opposed to continuous changes, the wear and energy consumption of the pitch system can be relatively low.

[0049] In this example, the blades may be rotated to a positive pitch position thus reducing drag and reducing lift, i.e. reducing the loads on the blade.

[0050] It will be clear to the person skilled in the art that the precise numeric values of wind speeds defining sub-ranges a, b, b, c, d, e and f and/or pitch angles .sub.a, .sub.b, .sub.c, .sub.d, .sub.e, and .sub.f may vary in further examples. The precise numeric values may depend e.g. on site, tower height, blade design, and pitch system design (e.g. number of teeth of pinion and of annular gear).

[0051] Although only a number of examples have been disclosed herein, other alternatives, modifications, uses and/or equivalents thereof are possible. Furthermore, all possible combinations of the described examples are also covered. Thus, the scope of the present disclosure should not be limited by particular examples, but should be determined only by a fair reading of the claims that follow.

[0052] This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.