CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Application No. 19176665.8, having a filing date of May 27, 2019, the entire contents of which are hereby incorporated by reference.

FIELD OF TECHNOLOGY

The following relates to a rotor for a wind turbine and the wind turbine.

BACKGROUND

Lightning protection is one of the biggest challenges in wind turbine design. Wind turbines are usually the tallest structures in a local environment, especially in offshore wind parks. Therefore, the probability of being hit by lightning is very high. Lightning strikes predominantly the tallest point of the wind turbine which is usually the blade.

Lightning currents need to be transferred between various rotating parts of the wind turbine. For example, the lightning current that flows through the blade needs to be transmitted to the ground via the hub, the nacelle and the tower. Usually, the blade is rotatably connected to the hub, the hub is rotatably connected to the nacelle and the nacelle is rotatably connected to the tower. Due to the parts rotating relative to each other, cables for conducting a lightning current are in most cases unsuitable. The pitch bearing at the connection of the blade to the hub is one of the critical parts where an improved lightning protection system is required.

Conventionally, for example, carbon brushes are used to conduct lightning current bypassing the bearing. However, the brushes are sliding parts which get worn out quickly. Thus, they need to be replaced frequently which is time and cost consuming. Alternatively, spark gaps are applied. However, they have the disadvantage that the blades tend to accumulate static electricity. This causes the risk of flashovers if sufficient grounding is not provided. Alternatively, the lightning current is passed from the blade down conductor through a high-voltage cable through an opening at the center of the inner ring of the pitch bearing. However, lightning current passing through the high-voltage cable generates huge magnetic fields inside the hub. This can damage the electronic components inside the hub. A further conventional approach is to directly pass a lightning current through the bearing. However, this causes a high risk of damaging the bearing. Replacing a bearing after starting operation of the wind turbine is a very expensive and time-consuming process.

SUMMARY

An aspect relates to an improved rotor for a wind turbine and an improved wind turbine.

Accordingly, a rotor for a wind turbine is provided. The rotor comprises a blade and a hub. The rotor further comprises a pitch bearing configured to support the blade rotatably about a longitudinal axis of the blade and relative to the hub. Furthermore, the rotor comprises one or more metal arcs for conducting a lightning current from the blade to the hub. Each of the one or more metal arcs comprises a first arc portion electrically and mechanically connected to the blade. Each of the one or more metal arcs comprises a second arc portion electrically and mechanically connected to the hub. Further, each of the one or more metal arcs comprises a bent portion connecting the first and second arc portions. Each of the one or more metal arcs is configured such that, when the blade rotates relative to the hub, the first arc portion is shifted relative to the second arc portion and a length of the first arc portion is reduced or increased at the expense of an increase or decrease of a length of the second arc portion.

This allows to conduct a lightning current from the blade to the hub bypassing the pitch bearing and still allows rotation of the blade relative to the hub. In particular, the metal arcs provide a low impedance path for the lightning current. Thus, damaging of the pitch bearing by a lightning current is avoided.

Furthermore, having the one or more metal arcs avoids sliding parts as in carbon brushes. Thus, the lifetime of the metal arcs is much larger than that of carbon brushes.

In addition, high magnetic fields inside of the hub due to a lightning current are prevented.

The wind turbine is an apparatus to convert the wind's kinetic energy into electrical energy. The wind turbine comprises, for example, the rotor having one or more of the blades and one or more of the pitch bearings. Each of the blades is connected rotatably to the hub by means of one of the pitch bearings. The hub is connected to a nacelle including a generator. The wind turbine further comprises a tower holding, at its top end, the nacelle. The tower of the wind turbine may be connected via a transition piece to a foundation of the wind turbine, such as a monopile in the seabed.

The one or more metal arcs are, in particular, made from a flexible metal. The one or more metal arcs are, for example, made from spring steel.

The rotor comprises, for example, one metal arc covering 360 degrees, two semicircular metal arcs covering each 180 degrees, three metal arcs covering each 120 degrees or four metal arcs covering each 90 degrees.

The metal arcs are, in particular, arranged along a circumference of the blade. The circumference of the blade is, for example, an outer circumference of the blade.

The second arc portion is, for example, arranged parallel to the first arc portion. Then, when the blade rotates relative to the hub, the first arc portion of each of the one or more metal arcs is shifted relative to the second arc portion such that the first and second arc portions remain parallel to each other.

According to an embodiment, each of the one or more metal arcs is configured such that in an initial rotation state of the blade relative to the hub, the length of the first arc portion is equal to the length of the second arc portion, and in a rotation state of the blade relative to the hub offset from the initial rotation state, the length of the first arc portion is different from the length of the second arc portion.

Having the first and second arc portions of the one or more metal arcs of equal length in the initial rotation state of the blade provides a symmetric and robust configuration of the metal arcs and their electrically and mechanically connection with the blade and the hub.

The initial rotation state of the blade relative to the hub corresponds, in particular, to a blade pitch angle of zero.

According to a further embodiment, the first arc portion of each of the one or more metal arcs comprises a main portion connected to the bent portion, the bent portion and the main portion being freely movable with respect to the blade. Further, the first arc portion of each of the one or more metal arcs comprises an end portion connected to the main portion, the end portion being fixedly connected to the blade. Further, both the main and end portions are electrically connected to the blade. Furthermore, the second arc portion of each of the one or more metal arcs comprises a main portion connected to the bent portion, the bent portion and the main portion being freely movable with respect to the hub. Further, the second arc portion of each of the one or more metal arcs comprises an end portion connected to the main portion, the end portion being fixedly connected to the hub, and both the main and end portions being electrically connected to the hub.

Having the main portions being freely removable and the end portions being fixedly connected allows to securely fix the one or more metal arcs to the blade and the hub and at the same time allow rotation of the blade relative to the hub over a large angular range.

In embodiments, for each of the first and second arc portions of each of the one or more metal arcs, the main portion is larger than the end portion in the initial rotation state of the blade relative to the hub. In particular, for each of the first and second arc portions of each of the one or more metal arcs, the main portion covers a larger arc segment of the metal arc compared to the end portion. For example, the length of the main portion along the arc comprises more than 70%, more than 80%, more than 90%, more than 95% of the first/second arc portion in the initial rotation state.

According to a further embodiment, the blade comprises a blade root section and the first arc portion of each of the one or more metal arcs is electrically and mechanically connected to the blade root section.

According to a further embodiment, the one or more metal arcs are arranged along an outer circumference of the blade.

Having the one or more metal arcs arranged along an outer circumference of the blade allows to better prevent high magnetic fields inside the hub due to lightning current passing through the metal arcs.

In particular, the one or more metal arcs are arranged along an outer circumference of the cylindrical blade root section.

According to a further embodiment, the blade comprises in its hollow interior an interior conductive ring for connection with one or more down conductors of the blade. The interior conductive ring is electrically connected with the first arc portion of each of the one or more metal arcs.

Having the interior conductive ring allows to distribute a lightning current from the blade, e.g., from the blade down conductor, evenly to the one or more first arc portions of the one or more metal arcs.

The interior conductive ring is, in particular, connected to an interior wall of a blade shell forming an inner cavity of the blade. The interior conductive ring is, in particular, connected to the interior wall along its entire circumference. The interior conductive ring is, in particular, made from metal.

The one or more down conductors may, for example, be split and connected to the interior conductive ring at various positions thereof.

According to a further embodiment, the interior conductive ring of the blade is electrically connected with the first arc portion of each of the one or more metal arcs by means of conductive elements extending from the blade interior to the blade exterior.

Having the conductive elements of the blade, allows to electrically connect the interior conductive ring with the first arc portion of the one or more metal arcs in the case that the one or more metal arcs are arranged along the outer circumference of the blade.

The conductive elements are, in particular, extending through the shell of the blade. The conductive elements are, in particular, made from metal.

According to a further embodiment, the pitch bearing comprises an inner ring mounted to the blade and an outer ring mounted to the hub. Further, the conductive elements of the blade are conductive bushings configured for mounting the blade to the inner ring of the pitch bearing by means of fasteners.

By using the conductive bushings configured for mounting the blade to the pitch bearing which are available anyway, no additional conductive elements are required for connecting the interior conductive ring of the blade with the one or more metal arcs.

The conductive bushings are, for example, threaded bushings. The fasteners are, for example, bolts.

In embodiments, the inner ring of the pitch bearing may be mounted to the hub and the outer ring of the pitch bearing may be mounted to the blade.

According to a further embodiment, the pitch bearing comprises an inner ring mounted to the blade and an outer ring mounted to the hub. Further, the conductive elements of the blade are arranged spaced apart and electrically isolated from conductive bushings of the blade being configured for mounting the blade to the inner ring of the pitch bearing by means of fasteners.

By using conductive elements of the blade which are arranged spaced apart and electrically isolated from the conductive bushings of the blade being configured for mounting the blade to the pitch bearing, it can be better avoided that lightning current is transmitted via the conductive bushings and the fasteners to the pitch bearing. Thus, it is better ensured that the lightning current is transferred completely by the one or more metal arcs and no current flows through the pitch bearing.

According to a further embodiment, the second arc portion of each of the one or more metal arcs is electrically and mechanically connected to an outer ring of the pitch bearing. In addition, the outer ring of the pitch bearing is electrically and mechanically connected to the hub.

According to a further embodiment, the hub comprises an outer circumferential protruding portion protruding in the direction of the blade. Furthermore, the second arc portion of each of the one or more metal arcs is electrically and mechanically connected to the protruding portion of the hub.

The protruding portion may protrude solely in the longitudinal direction of the blade. Alternatively, the protruding portion may comprise in addition to the portion protruding in the longitudinal direction of the blade also a portion protruding in a radial direction of the blade. Thus, the protruding portion has an angled configuration. The radial protruding portion may be configured for being connected to the second arc portion in the case that the first and second arc portions of each of the one or more metal arcs are arranged parallel to each other in a longitudinal direction of the blade.

According to a further embodiment, the one or more metal arcs are arranged outboard with respect to the pitch bearing.

Having the one or more metal arcs arranged outboard with respect to the pitch bearing allows to even better transmit lightning current from the blade to the hub bypassing the pitch bearing.

In embodiments, the one or more metal arcs may also be arranged inboard with respect to the pitch bearing. This may be in particular the case when the first and second arc portions of each of the one or more metal arcs are arranged parallel to each other in a radial direction of the blade.

According to a further embodiment, the first and second arc portions of each of the one or more metal arcs are arranged parallel to each other in a radial direction of the blade.

Having this parallel configuration of the first and second arc portions in the radial direction of the blade allows a direct transmission of the rotational force of the blade to the one or more metal arcs.

In particular, the first and second arc portions are arranged parallel to each other in a radial direction of the blade root section.

According to a further embodiment, the first and second arc portions of each of the one or more metal arcs are arranged parallel to each other in a longitudinal direction of the blade.

Having this parallel configuration of the first and second arc portions in the longitudinal direction of the blade provides a further design option for the rotor of the wind turbine blade.

According to a further aspect, a wind turbine is provided. The wind turbine comprises a rotor as described above.

Further possible implementations or alternative solutions of embodiments of the invention also encompass combinations—that are not explicitly mentioned herein—of features described above or below with regard to the embodiments. The person skilled in the art may also add individual or isolated aspects and features to the most basic form of embodiments of the invention.

