Background of the invention 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 ex- tent, 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 imbal- ance forces even during normal operation causing fatigue in the com-

ponents of the turbine. This must be compensated by increased dimen- sions 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.

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. US Patent No 4,565,929 discloses an example of a turbine, which is able to teeter +7O until making contact with the teeter stops. The function is satisfactory during normal conditions, which means that the fatigue be- haviour 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 wide- spread use.

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 US Patent 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 dis- turbance forces in the system under certain conditions.

Basic idea of the invention 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.

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 ri- gidity, in theory may be looked upon as a mass-spring system according to classical mechanics.

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 revo- lution of the turbine in a, with the turbine, co-rotating system of co- ordinates. Also the less significant tower shadow (the air stream that is disturbed by the tower), creates the same variation. On further conside- ration it should be realised that also the turbulence will create com- ponents 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 irregul- arities. 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 (Odisturbance which is equal to the rotational angular frequency (J) rotation, i. e.

(1) (ddisturbance= Orotation In the following this frequency is denominated the disturbance frequency.

It should be noted that this condition is valid in a, with the tur- bine, 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.

The majority of today's wind turbines operate at a rotational speed (angular frequency), which normally varies with a few per cent, depend-

ing on the slip of the inductor generator generally used. This value may increase up to about ten per cent with a special generator design. In- stead of fixed speed, the wind turbine operates within a rotational speed range. There are also generators with dual windings which operate with- in two different rotational speed ranges. It is possible to control the ro- tational speed to any value, usually a low at low wind speeds and a high at high wind speeds, by applying specific electric equipment. The rota- tional angular frequency of the turbine 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 dimension- ing of the turbine.

The mass inertia factor of the turbine Jturbine relatively to the teeter axis may be calculated. The contribution from the hub, however, is in- significant. Thus, the mass inertia factor of the turbine may be approxi- mated as the mass inertia factor of the blades. The hinge member is as- sumed 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 resonance of the turbine in relation to the hinge may be calculated as COresonance-k l Jturbin From now on this is called the eigenfrequency of the teeter hinge. It should be noted that, for clarity, the stabilising impact on teeter move- ments of the centrifugal force, i. e. increase of rigidity due to the centri- fugal force, has not been analysed here.

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 dis- turbance, 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.

The study reveals that a low disturbance frequency COdisturbance in relation to the eigenfrequency of the teeter hinge wresonance, i. e. the operation is subcritical according to classical mechanics, gives a system response that is slightly larger than the disturbance, i. e. the amplifi- cation is just exceeding 1, corresponding to an ideal hub with a relativ- ely high degree of rigidity. It is further revealed that the amplification is large when the disturbing frequency and the eigenfrequency of the sys- tem 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 eigen- frequency, i. e. the operation is supercritical, the amplification is signi- ficantly lower.

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 tur- bulence and wind shear, which happen a few times during the opera- tional life of a wind turbine, can be handled with reasonable loads and teeter angles.

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.

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 cor- rectly taking mass distribution, stationary and instationary aerodyna- mics, 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.

As mentioned above, the degree of criticality depends on the rela- tions 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 fre- quency is equal to the rotational speed. The mass inertia factor of the turbine is mainly determined by the mass distribution and by the geo- metry of the blades. For a specific blade geometry, the mass inertia fac- tor 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.

To summerize, in accordance with the invention, the hub is con- structed 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.

Short description of the drawings The invention will be further described in detail below with reference to the appended drawings, in which Figure 1 illustrates how a system consisting of a mass, a spring and a damper in general reacts on disturbances of different frequencies, Figure 2 shows the principal structure of a wind turbine generator with a horizontal axis wind turbine, Figure 3A shows a side elevation, partly as a sectional view, of a teeter hub according to the invention and Figure 3B shows the teeter hub as shown in Figure 3A in a front elevation view.

Detailed description of embodiments of the invention Figure 1 illustrates how a system consisting of a mass, a spring and a damper in general reacts on disturbances of different frequencies.

The amplification (Y-direction in Fig. 1), i. e. the ratio of the amplitude of the system to the amplitude of the disturbance, is shown as a function of the ratio of the disturbing frequency to the eigenfrequency of the sys- tem (X-direction in Fig. 1). Point A indicates a state in which the distur- bing frequency COdisturbance is low relatively to the eigenfrequency of the teeter hinge coresonance, i. e. the operation is subcritical according to classical mechanics, corresponding to an ideal hub with a relatively high degree of rigidity. The response is slightly larger than the distur- bance, i. e. the amplification is slightly larger than 1. In point B, the disturbing frequency and the eigenfrequency are equal, i. e. the opera- tion is critical. The amplification of the disturbance is large. Point C indicates a state in which the disturbing frequency is higher than the eigenfrequency, i. e. the operation is supercritical. The response is lower than in point A and significantly lower than in point B.

Figure 1 illustrates that operation in the range of point B, in which there is a significant amplification of the disturbance, should be avoid- ed.

Figure 2 shows the general structure of a wind turbine generator with a horizontal axis wind turbine. Two aerodynamically shaped tur- bine blades (1) are connected to the hub (2) with a fixed or pivotal (along the longitudinal axis) connection. The hub (2) is connected to the tur- bine shaft (3), which is supported by the bearings (4). The turbine shaft (3) is connected to the gearbox (5), which transforms the low rotation speed of the turbine to a rotation speed conformable to the generator (6). The components of the machinery are supported by the machinery bed (7), which is connected to the yaw bearing (8). The yaw bearing (8) is rotatable on the tower (10) by means of the yaw mechanism (9). The tower is connected to solid ground by a foundation (not shown). The various functions may be more or less integrated with each other, which however does not affect the following description.

In Figure 2 is indicated that the hub (2) is a teetered hub, which implies that the two turbine blades (1) are rigidly connected to the hub (2). The hub (2) is hinged to the turbine shaft (3) and may teeter an angle A, as shown, in each direction.

The number of blades is normally two, but in one preferred em- bodiment the structure principle is applied to a turbine with one blade, and with the missing blade compensated by a counter weight.

Figure 3 shows a teeter hub according to the invention. As above, the blades (1) are connected to the hub (2), which normally is a cast structure and is connected to the turbine shaft (3) by means of a hinge member. The hinge member includes a bearing (12), which normally is composed of two or four symmetrically disposed bearing elements. The spring elements (13) counteract the teeter movement and may be com- bined with dampers, either by selecting a spring material with some damping properties, or by providing dampers of some other type (not shown). The active part of both the bearing (12) and the spring elements (13) are preferably made of elastomeric material.

The bearing (12) and the spring elements (13) together form a hinge member (12,13) having a specific rigidity in relation to the axis of the hinge member and hence the bearing. In a preferred embodiment, the bearing (12) and the spring elements (13) have been integrated into one unit, e. g. a so-called flex-beam. In this case, as well as when neighbour- ing components (primarily the turbine blades) have some inherent soft- ness, the spring constant of the spring elements (13) may include the impact of these elements.

In preferred embodiments additional advantages may be achieved by making the spring (13) progressive (i. e. the spring constant increases with the dimensional change) or pre-stressed. A special type of pro- gressive spring is achieved when there is a play between the spring element and the co-acting element, which results in a spring constant that is zero during the initial part of the teeter movement.

As described above, the structural parameters should by selected such that operation is avoided in the range in which the disturbing frequency is close to the critical frequency, i. e. the eigenfrequency of the teeter hinge. In preferred embodiments, the parameters are selected such that the disturbing frequency either is lower than 0.9 times the eigenfrequency or higher than 1.1 times the eigenfrequency. In addition, according to preferred embodiments, the disturbing frequency is norm- ally higher than 0.1 times the eigenfrequency and lower than 10 times the eigenfrequency. Thus, the range between 0.1 and 0.9 times the eigenfrequency generates especially interesting preferred embodiments, in view of the requirements to avoid large extreme loads as described above.

As described above, the invention and the preferred embodiments of the invention as described imply essential technical and economical advantages when applied on one-and two-bladed wind turbine gene- rators in particular.

Preferred embodiments as described above illustrate how the in- vention may be applied on wind turbines with one or two blades. How- ever, the man skilled in the art may easily apply the invention on wind turbines with several blades and on neighbouring application areas, such as propellers for airplanes and ships, fans, turbines for other gaseous or liquideous working media, etc.