FIELD OF THE INVENTION

[0001] The invention relates to a system for a turbine with a gaseous or liquideous working medium, in particular a wind turbine for a wind turbine generator. The turbine comprises a shaft, which is rotatable at a certain angular frequency, a hub, on which at least one turbine blade is attached, and a hinge member, disposed between the shaft and the hub. The hinge member comprises a bearing and spring elements, together forming a rigidity against movements in the hinge member. The turbine blade has a mass inertia factor relatively to the hinge member and is adapted to move through the gaseous or liquideous flow, which has a flow direction essentially perpendicular to the rotational plane of said turbine blade and has a varying velocity in said direction, such that the system is exposed to disturbance forces. The invention also relates to a wind turbine generator with such a system.

BACKGROUND OF THE INVENTION

[0002] Conventionally, wind turbine generators have rigid hubs, which means that the blades of the wind turbine have a rigid connection with the hub. The function in acceptable when the number of blades is at least three, since three symmetrically arranged blades, to a certain extent, are capable of levelling out the imbalance forces that are created due to irregularities in the wind field. A reduction of the number of blades to two is desirable, since this means a considerable reduction of the blade cost as well as other advantages, such as a less complicated assembly. The yearly energy yield for the two-bladed turbine, calculated for a certain turbine diameter, is only reduced with 2-3%. However, a two-bladed, rigid hub wind turbine is exposed to considerable imbalance forces even during normal operation causing fatigue in the components of the turbine. This must be compensated by increased dimensions of all the main components, such that this two-bladed solution, due to the excessive cost, is no longer justified. As a consequence, this type of wind turbine is no longer manufactured.

[0003] The teetered hub became the solution of the problems of the two-bladed, rigid hub wind turbine. It is characterised by the two blades being rigidly fixed to a hub, which is hinged to the turbine shaft. U.S. Pat. No. 4,565,929 discloses an example of a turbine, which is able to teeter ±7° until making contact with the teeter stops. The function is satisfactory during normal conditions, which means that the fatigue behaviour is advantageous. However, during extreme wind conditions with high turbulence and wind shear, such contacts with the teeter stops may occur that result in more severe moments than in a rigid hub wind turbine. Thus, it is the extreme load cases that are critical. None of the turbines with this simple type of teeter hub have reached any widespread use.

[0004] In order to solve the problems caused by the extreme loads, it has been proposed to control the teeter movement by combining the teeter stops with damping. One example is disclosed in U.S. Pat. No. 5,354,175, in which it is proposed to limit the teeter movement by a controllable hydraulic damping. None of these hub types have been used extensively, which is due to a lack of knowledge of how a hub should be designed in order to prevent a serious increase of the disturbance forces in the system under certain conditions.

BASIC IDEA OF THE INVENTION

[0005] The object of the present invention is to provide a system for a turbine, in particular a system for a wind turbine for a wind turbine generator, which minimises the effects of the imbalance forces caused by the irregularities in the wind field, and thus the risk of fatigue, and of the extreme loads in the structure.

[0006] The invention is based on the understanding that a wind turbine, e.g. a two bladed wind turbine, with a teeter hinge having a certain rigidity, in theory may be looked upon as a mass-spring system according to classical mechanics.

[0007] The wind field comprises both a systematic variation, wind shear, which means that the mean wind speed is higher during the upper part of the revolution of the turbine, and a stochastic variation, turbulence. It is obvious that the wind shear creates one load cycle for each revolution of the turbine in a, with the turbine, co-rotating system of coordinates. Also the less significant tower shadow (the air stream that is disturbed by the tower), creates the same variation. On further consideration it should be realised that also the turbulence will create components of the same frequency, since the turbine blades move swiftly (50-100 m/s) compared with the wind (about 5-25 m/s) and its irregularities. Each turbine blade will thus hit a specific irregularity of the wind several times, which means that the resulting disturbance also in this case will have a frequency ω_disturbance which is equal to the rotational angular frequency ω_rotation, i.e.

ω_disturbance=ω_rotation (1)

[0008] In the following this frequency is denominated the disturbance frequency.

[0009] It should be noted that this condition is valid in a, with the turbine, co-rotating system of co-ordinates, which is relevant for those forces that affect the turbine. In a system of co-ordinates that is fixed to the nacelle or tower the disturbance frequency is proportional to the result of the multiplication of the number of blades and the rotational frequency.

[0010] The majority of today's wind turbines operate at a rotational speed (angular frequency), which normally varies with a few per cent, depending on the slip of the inductor generator generally used. This value may increase up to about ten per cent with a special generator design. Instead of fixed speed, the wind turbine operates within a rotational speed range. There are also generators with dual windings which operate within two different rotational speed ranges. It is possible to control the rotational speed to any value, usually a low at low wind speeds and a high at high wind speeds, by applying specific electric equipment. The rotational angular frequency of the turbine ω_rotation shall be construed in the present invention as the highest rotational speed range which is used during normal, main circuit connected operation. This is possible since the high rotational speeds normally is used when the wind speeds are fairly high or high and the wind turbine has a high output power, which constitute the operation conditions that are decisive for the dimensioning of the turbine.

[0011] The mass inertia factor of the turbine J_turbine relatively to the teeter axis may be calculated. The contribution from the hub, however, is insignificant. Thus, the mass inertia factor of the turbine may be approximated as the mass inertia factor of the blades. The hinge member is assumed to be of the type, in which the movement is counteracted by springs, which makes it possible to calculate a spring constant k for the hinge member. The spring constant constitutes a value of the rigidity of the hinge member. According to classic mechanics, the eigenfrequency ω_resonance of the turbine in relation to the hinge may be calculated as

ω_resonance={overscore (k/Jturbin )} (2)

[0012] From now on this is called the eigenfrequency of the teeter hinge. It should be noted that, for clarity, the stabilising impact on teeter movements of the centrifugal force, i.e. increase of rigidity due to the centrifugal force, has not been analysed here.

[0013] In order to elucidate the general reaction of such a mass-spring-system on disturbances of varying frequencies, the amplification, i.e. the ratio of the amplitude of the system to the amplitude of the disturbance, has been studied. A moderate damping has been added to the system, in correspondence with an actual state in which the air will dampen the teetering movement of the blades and the hinge member may be furnished with damping elements.

[0014] The study reveals that a low disturbance frequency ω_disturbance in relation to the eigenfrequency of the teeter hinge ω_resonance, i.e. the operation is subcritical according to classical mechanics, gives a system response that is slightly larger than the disturbance, i.e. the amplification is just exceeding 1, corresponding to an ideal hub with a relatively high degree of rigidity. It is further revealed that the amplification is large when the disturbing frequency and the eigenfrequency of the system are equal, i.e. the operation is critical. It is likely that earlier attempts to use teetering hubs with counteracting springs have given this effect. When the disturbing frequency is higher than the eigenfrequency, i.e. the operation is supercritical, the amplification is significantly lower.

[0015] The cases mentioned above illustrate the conditions during normal operation. A wind turbine with a teeter hinge having a certain rigidity additionally has the advantage that the states during extreme turbulence and wind shear, which happen a few times during the operational life of a wind turbine, can be handled with reasonable loads and teeter angles.

[0016] The conditions during normal operation primarily determine the fatigue of the materials of the structure, while the extreme operation states are decisive for the extreme loads. A hub with a certain rigidity presents an improved balance between the fatigue load cases and the extreme load cases.

[0017] The study as described above illustrates that operation in the range of large amplification of the disturbance, i.e. when the disturbing frequency and the eigenfrequency are equal, should be avoided. These results have been confirmed by simulations in the time domain with a reasonably comprehensive computer turbine model, said model correctly taking mass distribution, stationary and instationary aerodynamics, hinges, rigidity, damping, wind distribution, increase of rigidity due to the centrifugal force, etc, into consideration for wind turbines at different wind speeds. The simulations has revealed that the moment in the hub becomes as much as ten times larger when the rigidity of the hub has the critical value as compared with a higher or lower value.

[0018] As mentioned above, the degree of criticality depends on the relations between the disturbing frequency, the mass inertia factor of the turbine and the rigidity of the teeter hinge. In the construction phase, these values may be selected without restrictions. The disturbing frequency is equal to the rotational speed. The mass inertia factor of the turbine is mainly determined by the mass distribution and by the geometry of the blades. For a specific blade geometry, the mass inertia factor may be influenced by the choice of construction materials and by adding ballast material. The rigidity of the teeter hinge is determined by the stiffness of the different hinge elements, which normally are made of rubber or some other elastomeric material. Thus, it is relatively easy to change the rigidity, also in an existing teeter hinge, by exchanging the rubber elements to new ones with a different Young's modulus and possibly with a modified geometry.

[0019] To summerize, in accordance with the invention, the hub is constructed such that the operation is either supercritical or subcritical. By putting the invention into practise, the loads decrease considerably and both technical and economical advantages are achieved.