Electrical resonance change in a wind turbine

Field of invention

The present invention relates to the field of wind turbines.

A conventional wind turbine comprises a plurality of elements including a tower, a nacelle mounted to the tower and a plu rality of blades rotatable mounted to the nacelle.

Art Background

Such a conventional wind turbine has an electric resonance frequency which is normally fixed due to the size of the en closure. Radiated radio frequencies depend on how good the wind turbine emits the RF energy generated mainly by the en ergy conversion components. Limits are set in international standards such as in the requirements for radiated emissions for wind turbines. Here, some newly defined limit require ments are defined down to a frequency of about 150 kHz. How ever, wind turbines have a size corresponding to a few hun dred kHz and are included in a range set by the recent stand ards .

Usually, emissions are reduced by implementing filters and by carefully using EMC workmanship like shielding. However, due to the requirements for radiated emissions, it becomes very difficult and expensive to satisfy these limits as even a low current of some mA running in the surface of the wind turbine would exceed the limits. Normally, the dimension of the ele ment, i.e. the dimension of the enclosure of the element de fines the antenna characteristics of the element, but this dimension cannot readily be changed.

There may be a need for a wind turbine which satisfies the EMC standards under low costs without burdensome redesigns of the wind turbine. Summary of the Invention

This need may be met by the subject matters according to the independent claims. The present invention is further devel oped as set forth in the dependent claims.

According to a first aspect of the invention, a wind turbine comprises a plurality of elements including a tower, a na celle mounted to the tower and a plurality of blades rotata ble mounted to the nacelle, wherein at least one element of the tower, the nacelle and the blades comprises a coaxial im pedance member coaxially arranged about an axis of the ele ment. The axis of the element can be the longitudinal axis thereof, which is the axis of the longest extension. The ele ment is configured to exhibit electromagnetic oscillation along the axis with a resonance frequency of an order n, wherein the coaxial impedance member is preferably tuned to the resonance frequency of the order n.

Advantageously, the coaxial impedance member can function as a choke such that an electric current oscillating with the resonance frequency within the element is attenuated and the energy thereof is converted to heat in the coaxial impedance member. In addition or alternatively, the coaxial impedance member can function as a detuning member such that the reso nance frequency is split in two resulting resonance frequen cies being higher than the resonance frequency. With it, the EMC standards can readily be satisfied under low costs.

Preferably, the coaxial impedance member is configured to be placed at different positions along the axis of the element so that the choking and detuning effects can be optimized.

Preferably, the coaxial impedance member has a length sub stantially equal to approximately one-quarter wavelength of the resonance frequency. In particular, the coaxial impedance member can thus be tuned to the first order resonance fre quency which is thereby well absorbed.

Preferably, the coaxial impedance member is formed of a tape or a sleeve comprising electrically conductive material. More preferred, the electrically conductive material comprises a nanocrystalline or ferrite material. The coaxial impedance member allows detuning the resonance frequency in a simple and cost effective manner. Due to the very low common mode current in the tower surface, only a very small amount of tape material is needed. The arrangement of such a coaxial impedance member can readily be integrated in the manufactur ing process of the tower or the other elements of the wind turbine, and it is even possible to retrofit existing wind turbines by adding the coaxial impedance member.

In a second aspect of the present invention, a tower for a wind turbine comprises a coaxial impedance member coaxially arranged about a longitudinal axis of the tower.

In a third aspect of the present invention, a blade for a wind turbine comprises a coaxial impedance member coaxially arranged about a longitudinal axis of the blade.

In a fourth aspect of the present invention, a method of sup pressing electromagnetic radiation having a resonance fre quency of an order n in a wind turbine comprising a plurality of elements including a tower, a nacelle mounted to the tower and a plurality of blades rotatable mounted to the nacelle, the method comprises a step of arranging a coaxial impedance member coaxially about an axis of at least one element of the tower, the nacelle and the blades.

Preferably, the step of arranging the coaxial impedance mem ber comprises a step of placing the coaxial impedance member at a position along the axis of the at least one element, where a current distribution or current density in the ele ment exhibits a maximum. Preferably, arranging the coaxial impedance member comprises a step of wrapping a tape of an electrically conductive mate rial about the at least one element.

It has to be noted that embodiments of the invention have been described with reference to different subject matters.

In particular, some embodiments have been described with ref erence to apparatus type claims whereas other embodiments have been described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless other noti fied, in addition to any combination of features belonging to one type of subject matter also any combination between fea tures relating to different subject matters, in particular between features of the apparatus type claims and features of the method type claims is considered as to be disclosed with this application.

Brief Description of the Drawings

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi ment but to which the invention is not limited.

Fig. 1 shows a wind turbine and the different elements there of;

Fig. 2 shows a distribution of an electric RF-current in a wind turbine;

Fig. 3 shows electromagnetic oscillation with a first order resonance frequency fi along the axis of the tower; Fig. 4 shows electromagnetic oscillation with resulting reso nance frequencies along the axis of the tower equipped with a coaxial impedance member;

Fig. 5 shows an embodiment of a tower comprising a plurality of coaxial impedance members; and

Fig. 6 shows an embodiment of a tower comprising a coaxial impedance member made of nanocrystalline material.

Detailed Description

The illustrations in the drawings are schematically. It is noted that in different figures, similar or identical ele ments are provided with the same reference signs.

Fig. 1 shows a wind turbine 1. The wind turbine 1 comprises a nacelle 3 and a tower 2. The nacelle 3 is mounted at the top of the tower 2. The nacelle 3 is mounted rotatable with re gard to the tower 2 by means of a yaw bearing. The axis of rotation of the nacelle 3 with regard to the tower 2 is re ferred to as the yaw axis.

The wind turbine 1 also comprises a hub 4 with three rotor blades 6 (of which two rotor blades 6 are depicted in Fig.

1) . The hub 4 is mounted rotatable with regard to the nacelle 3 by means of a main bearing 7. The hub 4 is mounted rotata ble about a rotor axis of rotation 8.

The wind turbine 1 furthermore comprises a generator 5. The generator 5 in turn comprises a rotor 10 connecting the gen erator 5 with the hub 4. The hub 4 is connected directly to the generator 5, thus the wind turbine 1 is referred to as a gearless, direct-driven wind turbine. Such a generator 5 is referred as direct drive generator 5. As an alternative, the hub 4 may also be connected to the generator 5 via a gear box. This type of wind turbine 1 is referred to as a geared wind turbine. The present invention is suitable for both types of wind turbines 1.

The generator 5 is accommodated within the nacelle 3. The generator 5 is arranged and prepared for converting the rota tional energy from the hub 4 into electrical energy in the shape of an AC power. Thereby, the generator 5 generates noise in the shape of RF radiation.

Each element of the wind turbine 1 has a certain dimension. The tower 2 has a height A. The blades 6 form together a di ameter C, and each blade 6 has a length E. The nacelle 3 has a height B and a length D.

The wind turbine 1, especially the tower 2 thereof, is a large conductive structure for RF-energy. The wind turbine 1 can be considered as an antenna (particularly a monopole an tenna) . The physical dimension of length determines the elec tromagnetic resonance frequency. For each dimension A, B, C,

D and E, a specific resonance frequency f _n of an order n is present. Also each combination of any elements exhibits a specific resonance frequency f _n of an order n. Fig. 2 shows an example of a distribution of an electric RF-current in the wind turbine 1.

For the first order harmonics (n = 1), the quarter wavelength 1/4 li is the most effective length for an antenna, where resonance can be used for transmitting or receiving radio signals. Fig. 3 shows a first order harmonics within the tow er 2 along the longitudinal axis of the tower 2. The length 1 of the element corresponds to 1/4 li of the resonance fre quency fi of the first order harmonics (n = 1) .

According to the present invention, a coaxial impedance mem ber 10 is coaxially arranged about a longitudinal axis of the tower 2 and axially aligned with the tower 2. The term "coax ially arranged about a longitudinal axis" includes a case where the coaxial impedance member 10 is arranged coaxially to and radially outside of the tower 2, and a case where the coaxial impedance member 10 is arranged coaxially to and em bedded in the tower 2. The term "longitudinal axis" normally refers to the axis of the longest extension of the tower 2, i.e. in the upright direction of the tower 2. The coaxial im pedance member 10 can be arranged onto a circumferential sur face of the tower 2.

The same applies to embodiments where the coaxial impedance member 10 is arranged at another element such as the nacelle 3 and the blades 6. With regard to the nacelle 3 as shown in Fig. 1, one coaxial impedance member 10 can be coaxially ar ranged about the vertical axis, and another coaxial impedance member 10 can be arranged about the horizontal axis, i.e. the rotational axis 8.

Preferably, the coaxial impedance member 10 has an induct ance. The impedance member is tuned to the resonance frequen cy fi of the order 1. The coaxial impedance member 10 may have a length which is substantially equal to approximately one-quarter wavelength li (1/4 li) of the resonance frequency fi of the first order harmonics (n = 1) . By arranging the co axial impedance member 10, two advantageous effects are ob tained :

First, the coaxial impedance member 10 acts like choke so that the resonance frequency fi is choked. That means, the RF-current in the tower 2 is attenuated and converted to heat in the coaxial impedance member 10. The energy of the reso nance frequency fi of the order n = 1 is absorbed by the im pedance member 10. Preferably, the coaxial impedance member 10 is placed to a position, where a current distribution in the tower 2 exhibits a maximum so that a large amount of en ergy is absorbed.

Second, the resonance frequency fi of the order 1 is detuned. Fig. 4 shows that the physical length 1 (corresponding to the length A in Fig. 1) of the tower 2 is split in two parts by the coaxial impedance member 10. The resulting resonance fre quencies are higher than the original resonance frequency.

For example, if the coaxial impedance member 10 is arranged in the middle of the length A, the original resonance fre quency fi having a wavelength 1/4 li = A is split in two res onance frequencies fi _a and fi _b , each having a resulting wave length 1/4 Xi _a = 1/4 Xi _b = A/2. In other words, the coaxial impedance member 10 moves up (increases) the resonance fre quency. The coaxial impedance member 10 functions as a detun ing member. By detuning the resonance frequency, the amount of energy is potentially further reduced.

If the coaxial impedance member 10 is placed anywhere to the tower 2 except for the axial center, a first one of the two resulting resonance frequencies is higher than the original resonance frequency, and a second one of the two resulting resonance frequencies is not only much higher than the origi nal resonance frequency but also higher than the first one of the two resulting resonance frequencies.

Fig. 5 shows an embodiment, where the tower 2 is provided with a plurality (three) of coaxial impedance members 10 so that the original resonance frequency is split a plurality of times (three times) . In the embodiment described so far, the coaxial impedance members 10 are arranged on the tower 2. However, it is clear that a coaxial impedance member 10 can be arranged in addition or alternatively at any other element of the wind turbine 1 such as the nacelle 3 and/or one or more blades 6, for example on a circumferential surface thereof .

Fig. 6 shows that the coaxial impedance member 10 is formed of a tape of electrically conductive material and is arranged about and axially aligned with the tower 2. Due to the very low common mode current in the tower surface, only a very small amount of tape material is needed. The tape comprises an electrically conductive material which can include a nano crystalline or ferrite material. The nanocrystalline material can be a nanocrystalline alloy. The electrically conductive material can be applied onto a substrate member such as a flexible sheet. The sheet can be made of a synthetic resin such as polyester or metal such as aluminum.

Next, a method of suppressing an electromagnetic radiation in a wind turbine 1 is described. The coaxial impedance member 10 is configured to be placed at different axial positions of the tower 2. The coaxial impedance member 10 is preferably placed to a position, where a current distribution in the tower 2 exhibits a maximum. The maximum current distribution in the tower 2 can be found by measurements or can be deter mined based on empiric data. Generally, the axial position of the element means a position along a longitudinal axis of the element. The longitudinal axis of the element is normally the axis of the longest extension of the element.

If the coaxial impedance member 10 is formed of a tape, the same is wrapped or wound about the tower 2 at the determined position .

According to the present invention, the wind turbine 1 can satisfy the requirements for radiated emissions which define limits in the frequency range down to 150 kHz. The coaxial impedance member 10 allows detuning the resonance frequency in a simple and cost effective manner. The arrangement of the coaxial impedance member 10 can readily be integrated in the manufacturing process of the tower 2 or the other elements of the wind turbine 1. It is even possible to retrofit existing wind turbines 1 by adding the coaxial impedance member 10 ex post .

Furthermore, a choking effect can be achieved, where RF- energy is converted to heat, particularly when the coaxial impedance member 10 is arranged at a location of a high or the highest current density. For radiation emissions, a rela tive small amount of material is needed for the coaxial im pedance member 10 due to very low currents. It should be noted that the term "comprising" does not ex clude other elements or steps and "a" or "an" does not ex clude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be con strued as limiting the scope of the claims.