Technical Field

The invention relates to a method for operating and/or maintaining a wind turbine, wherein the wind turbine comprises a hub element at which a number of blades is mounted, wherein a slewing bearing is arranged between the hub element and a blade for rotationally supporting the blade relatively to the hub element, wherein first connection means are provided for fixing the blade to a first ring of the slewing bearing, wherein second connection means are provided for fixing the hub element to a second ring of the slewing bearing, and wherein a pitch drive is provided for engaging with the first ring of the slewing bearing and actively adjusting a pitch angle of the blade.

Background

In a wind turbine of the generic kind a number of blades is arranged at a hub element; the hub element rotates during normal operation of the wind turbine around an axis. Each blade is supported by means of a slewing bearing (pitch bearing) in such a manner that the blade can be rotated around the longitudinal axial of the blade relative to the hub element to adjust the pitch of the blade. Thus, the blade can be positioned in such a manner at the hub element that the wind streaming can be exploited in an optimal manner. Accordingly, the blade is rotated around its longitudinal axial through a relatively small swivel angle, e.g. between 0° and 35°, which is the working range of the blade during power production. For adjusting the blade, a pitch drive is employed. Typically, the bearing ring to which the blade is attached comprises a geared ring that engages with a motor-driven pinion gear.

Over time, fatigue effects and wear occurs in the slewing bearing. These effects are concentrated in the working range of the slewing bearing, due to the limited angular adjustment range and also due to the fact that the bending loads from the blade mostly act in one specific region of the bearing, known as the loaded zone.

If replacement of the slewing bearing becomes necessary, this is very expensive, not only in terms of the cost of a new bearing, but also in terms of the replacement procedure. In a general, a crane is used to support the blade while it is detached from the hub and the bearing is dismounted.

In US 2012/0141280 Al a method is described for bringing the geared ring of a slewing bearing into an unused working section, thereby increasing the service life of the slewing bearing. In the disclosed method, a hoisting device is employed to support the blade while the blade is detached from the geared ring and the geared ring is rotated to a relatively fresh section. While a crane is not necessary to carry out the disclosed process, a drawback of this pre- known solution is that use of the required hoisting is also rather expensive.

Thus, it is an o b j e c t of the present invention to propose a wind turbine arrangement of the kind mentioned above which allows an easier and more cost effective rotation of a slewing bearing ring, relative to the hub element and blade. Summary of the invention

A s o l u t i o n according to the invention is characterized in that the method for operating and/or maintaining a wind turbine comprises the steps of: a) Mounting third connection means for establishing a firm connection between the hub element and a blade; b) Dismounting the first connection means between the blade and the first ring of the slewing bearing, so that the first ring can rotate relatively to the blade; c) Turning the first ring of the slewing bearing around a predetermined angle by means of the pitch drive relatively to the blade and/or relatively to the hub element; d) Mounting the first connection means between the blade and the first ring of the slewing bearing; e) Dismounting the third connection means between the hub element and the blade.

Thus, as a general concept, the first bearing ring of the slewing bearing to which the blade is attached is rotated through a predetermined angle to bring the bearing ring to a relatively fresh or unused section of its circumference. For doing so, neither a crane nor a complex device is required, but the pitch drive is used. The first bearing ring may be the inner ring or the outer ring of the bearing. According to a preferred embodiment of the invention, both the first and second rings of the slewing bearing are rotated through the predetermined angle. Thus, a raceway section of the second bearing ring which has been in the loaded zone is rotated to a location where smaller loads are experienced and is replaced with a relatively fresh raceway section. Suitably, the step of dismounting b) then further comprises dismounting the second connection means between the hub element and the second bearing ring, so that the second ring can rotate relatively to the hub element. Furthermore, the step of turning c) comprises turning both the first and second bearing rings and the step of mounting d) further comprises mounting the second connection means between the hub element and the second bearing ring.

When the first bearing ring is rotated using the pitch drive, friction between the rolling elements of the slewing bearing and the raceways will in most cases be sufficient to cause the second bearing ring to rotate also. Preferably, to ensure that the second bearing ring rotates through the predetermined angle, the method comprises, between steps b) and c), a following step of: bl) Mounting fourth connection means for establishing a firm connection between the first ring and the second ring of the slewing bearing.

Preferably, before, after or simultaneously with mentioned step e) the fourth connection means are dismounted again.

Preferably, the blade is brought into a vertical orientation before above mentioned step a) is carried out. The force of gravity on the blade will then help prevent that the first and second bearing rings become locked between the hub element and the blade, so that they can be rotated. The predetermined angle can be according to a preferred solution between 60° and 180°. Thus, it may be possible to repeat the method of the invention several times before the slewing bearing has reached the end of its lifetime. This depends on the design of the slewing bearing. For example, in some designs, the bearing is executed with a filler plug through which rolling elements are inserted into the bearing. At the location of the plug, the raceway is unhard- ened and is thus arranged in a zone where minimal loading occurs. A rotation through 180 ⁰ is then advantageous, to ensure that the unhardened part is not located in the loaded zone.

The third connection means for establishing a firm connection between the hub element and a blade may comprise two flange-like rings which are firmly connected with the hub element and with the blade respectively. The two flange-like rings can extend in a radially outer or radially direction of the hub element and the blade respectively. The two flange-like rings can be connected to the hub element and to the blade respectively by welding. The third connection means can further comprise connection rods which connect the two flange-like rings.

An alternative embodiment suggests that the third connection means for establishing a firm connection between the hub element and a blade comprise a plurality of brackets which can be mounted at a radially inner region between the hub element and the blade. Alternatively, the third connection means for establishing a firm connection between the hub element and a blade comprise a plurality of brackets which can be mounted at a radially outer region between the hub element and the blade. Preferably, a number of brackets are distributed equidistantly around the circumference of the hub element and blade respectively. The first and second connection means are mostly bolted connections. Also, the third and fourth connection means may comprise bolted connections.

The hub element may comprise two or more blade receptions, wherein the method is carried out sequentially for all blades.

As mentioned, the proposed method may be repeated after a predetermined operation time of the wind turbine.

Thus, the idea of the present invention is based on the concept that the blade is temporarily firmly attached to the hub element. The bearing rings of the slewing bearing are released from their connection to the hub element and blade respectively. The pitch drive is then used to turn preferably both bearing rings to a "fresh" or "unused" position in which the slewing bearing (i.e. a part of the same) can operate optimally for an extended period. Thus, the life of the slewing bearing will be increased significantly.

The method of the invention provides for "in situ" shifting of the slewing bearing in a straightforward manner. In effect, the slewing bearing is refurbished on site according to a highly cost effective procedure.

The mentioned re-adjustment of the rings of the slewing bearing can beneficially be done without removing the blade from the hub element. The readjustment of the inner ring and the outer ring of the slewing bearing can be done without removing or rotating the hub or the blade. Additional devices for the mentioned shifting can be avoided by the fact that the regular pitch drive is used for shifting. Advantageously, no extra actuator is needed. The shifting is done only with the regular pitch drive.

As mentioned above it is possible to use a temporary connection between the hub element and the blade on the inside of the construction as well at on the outside of the same. By the temporary connection between the hub element and the blade, a by-passing of the slewing bearing is established. Obviously, there are many possibilities to design such third connection means for connecting the hub and the blade, so that the slewing bearing no longer transfers loads and thus can be manipulated, serviced, or shifted.

Beneficially, there is the potential to significantly increase the service life of a regular slewing bearing used in the pitch control of wind turbine blades.

Brief description of the drawings

The drawings show embodiments of the invention.

Fig. 1 shows a partial perspective view of a hub element of a wind turbine on which three blades are mounted,

Fig. 2 shows a sectional perspective view of the transition between the hub element and a blade during normal operation of the wind turbine,

Fig. 3 shows a sectional perspective view of the transition between the hub element and a blade, wherein third connection means are mounted for firmly connecting the hub element with the blade, Fig. 4 shows a sectional perspective view according to Fig. 3, wherein the connection between the hub element and the blade and the respective rings of a slewing bearing are dismounted,

Fig. 5 shows an alternative embodiment of the invention in the depiction according to Fig. 3,

Fig. 6 shows a further alternative embodiment of the invention in the depiction according to Fig. 3, where brackets are used which are mounted in a radially inner region and

Fig. 7 shows a further alternative embodiment of the invention in the depiction according to Fig. 3, where brackets are used which are mounted in a radially outer region.

Detailed description of the invention

In Fig. 1 a hub element 1 of a wind turbine is shown. It is designed to take three blades 2. Each blade 2 is rotatable about its longitudinal axis for adjusting a pitch angle of the blade. Thus, a respective pitch drive 4 is arranged at the transition zone between the hub element 1 and the blade 2. From the pitch drive 4 only a geared ring is shown. The geared ring is mounted to or forms an integral part of a first bearing ring, to which the blade is mounted. The pitch drive further comprises means (not shown) for driving the geared ring, such as a pinion gear on a motor. Alternatively, the first bearing ring may comprise a stud that is connected to the output piston of a linear actuator. The pitch drive is designed according to the state of the art so that additional explanations are redundant.

In Fig. 2 details become apparent concerning the design of the transition zone between the hub element 1 and the blade 2. To allow the adjustment of the pitch angle, i.e. the rotation of the blade 2 relative to the hub element 1 around the longitudinal axis of the blade 2, a slewing bearing 3 is employed. The slewing bearing 3 has a first ring 6 (inner ring) and a second ring 8 (outer ring), between which rolling elements (balls) 14 are arranged. A radially inner circumference of the first ring 6 comprises the geared ring of the pitch drive 4.

The first ring 6 of the slewing bearing 3 is firmly connected to the blade 2 by first connection means 5, which are bolts. Similarly, the second ring 8 of the slewing bearing 3 is connected to the hub element 1 by second connection means 7, which are also bolts.

Between the first ring 6 and the blade 2 a first flange-like ring 12 is arranged, which is connected to the blade 2 by means of welding in this example. Furthermore, between the second ring 8 and the hub element 1 a second flange- like ring 11 is arranged, which is connected to the hub element 1 also by welding.

The situation as shown in Fig. 2 is the regular operation situation when the wind turbine is working. Thus, the two rings 6, 8 of the slewing bearing 3 are firmly connected with the hub element 1 and the blade 2 respectively. The bearing 3 allows a proper adjustment of the blade 2 relatively to the hub element 1. After a certain operation time, wear takes place in the slewing bearing 3 but - due to the relatively small swivel angle of the slewing bearing and the dominant loads in one direction - this wear extends along a relatively small part of the circumference of the bearing rings 6, 8 only. Thus, a worn part of the bearing ring circumference adjoins to parts of the circumference which are relatively un-worn or fresh. Now it is aimed to re-adjust the slewing bearing 3 to bring parts of it into operation which have not yet been subject to wear and which have not been situated in a loaded zone of the bearing. Thus, the cost- intensive substitution of the slewing bearing 3 should be avoided.

To do so, the following process is carried out:

In a first step third connection means 9 are mounted for establishing a firm connection between the hub element 1 and the blade 2 as depicted in Fig. 3. Here, it can be seen that the third connection means 9 comprise the flange-like rings 11 and 12 between which a plurality of connection rods 13 are mounted around the circumference of the slewing bearing 3.

The sleeve-like connection rods 13 are fixed with screws 15 and 16 with the respective flange-like rings 11 , 12. Thus, the slewing bearing 3 is by-passed and no longer needs to transmit any force to hold the blade 2 on the hub element 1.

In the next step - as depicted in Fig. 4 - the first connection means 5 (i.e. the bolts) between the blade 2 and the first ring 6 of the slewing bearing 3 are dismounted, so that the first ring 6 can now rotate relative to the blade 2. Now, the second connection means 7 (i.e. the bolts) between the hub element 1 and the second ring 8 of the slewing bearing 3 are dismounted, so that the second ring 8 can rotate relatively to the hub element 1.

Now, fourth connection means 10 are mounted for establishing a firm connection between the first ring 6 and the second ring 8 of the slewing bearing 3. The fourth connection means 10 are shown in Fig. 4 only very schematically.

Now, using again the pitch drive 4, the first and second rings 6 and 8 of the slewing bearing 3 are turned around a predetermined second angle, e.g. through 90° in a clockwise direction, relative to the hub element 1. Thus, the whole slewing bearing 3 is rotated accordingly.

When the rings 6, 8 have reached their desired position, the fourth connection means 10 between the first ring 6 and the second ring 8 of the slewing bearing 3 are dismounted again.

Now, the first connection means 5 between the blade 2 and the first ring 6 of the slewing bearing 3 as well as the second connection means 7 between the hub element 1 and the second ring 8 of the slewing bearing 3 are mounted again.

Finally, the third connection means 9 between the hub element 1 and the blade 2 are dismounted again to bring the slewing bearing in normal operation conditions.

Thus, an un-used part of the slewing bearing is brought into the working range of the bearing. The substitution of the slewing bearing 3 has been avoided. This process can be repeated after a certain operation time, to bring further fresh parts of the bearing into the working range.

In Fig. 5 an alternative to the above discussed solution is shown. Basically, the same principle is used. The only difference is that here the flange-like rings 11 and 12 extend substantially radially inward of the hub element 1 and blade 2 respectively. Thus, the third connection means 9 are not arranged in the radially outer side of the hub element 1 as depicted in Fig. 3 but in the radially inner region.

So, the concept as depicted in Fig. 5 is basically identical to the embodiment according to Fig. 2 - Fig. 4, but now the spacers are located on the inside of the construction, making it possible to establish the connection between the hub element 1 and the blade 2 from within the hub. A service engineer does not need to climb outside to establish this connection.

A certain drawback in the described embodiments is that it can be difficult to get the regular bolts of the first or second connection means past the radial plates (flange-like rings). In the external concept (Fig. 2 to Fig. 4) the bolts connecting the bearing to the hub would have to go through the radial plate at the blade side. In the internal concept (Fig. 5) the bolts connecting the blade to the bearing would have to go through the radial plate on the hub side.

In both embodiments the radial plates (flange-like rings 11, 12) have many holes which allow bolt removal. These holes reduce the strength and stiffness of the radial plate. A possible solution how to avoid these problems is shown in Fig. 6. As can be seen here the third connection means 9 are established by brackets 17 to connect the hub element 1 with the blade 2. Thus, some of the connection bolts of the first and/or second connection means 5, 7 are removed locally, then the brackets 17 are mounted on those positions; then all the other bolts of the first and/or second connection means 7 are removed.

A certain number of brackets 17 is mounted here equidistantly around the inner circumference of the hub element 1 and blade 2.

Preferably, the brackets 17 are designed as lightweight elements (e.g. with a mass up to 15 kg per bracket 17) that can be carried manually into a tower by a service engineer. The service engineer can mount the brackets 17 on the inside of the construction, he does not have to climb outside to mount them.

But also the latter case can be taken into account as shown in the embodiment according Fig. 7.

In some applications of wind turbines, a hydraulic drive and torque/stiffening plates are used. These can possibly interfere with the brackets 17 in the radially inner region as shown in Fig. 6. In this case the embodiment according to Fig. 7 proposes that brackets 18 are mounted on the outside.

To simplify the construction, the connection between the hub element 1 and the blade 2 is preferably established by mounting the brackets 18 directly to the hub and the blade. It is possible to use the same bracket 18 at any position around the circumference of the hub element 1 and blade 2. Reference Numerals:

Hub element

Blades

Slewing bearing

Pitch drive

First connection means

First ring of the slewing bearing

Second connection means

Second ring of the slewing bearing

Third connection means

Fourth connection means

Flange-like ring

Flange-like ring

Connection rod

Roller element (ball)

Screw

Screw

Bracket

Bracket