NO342746B1 - Procedure for reducing axial power variations in a wind turbine. - Google Patents

Abstract

A method which continuously reduces the variations of the axial force of the rotor and thus reduces the output loads on the rotor blades and towers while the resulting effect on the generator is not significantly affected or kept within acceptable limits relative to the limitations on the drive, generator and electric grid. A method of using the rotor axial force to actively counteract the movements of a floating wind turbine. The method further describes how rotational forces about the vertical axis (12) of the tower (4) are controlled and counteracted by the cyclic variation of pitch angles and associated forces on the individual rotor blades. The method also describes how the aerodynamic force variation on each blade can be reduced due to different wind speeds at different altitudes (vertical wind shear) and in the horizontal direction parallel to the rotor plane (horizontal wind shear).

Description

This invention relates to a method for regulating the angle of the rotor blades about their own longitudinal axis on a wind power plant in such a way that the thrust of the rotor on the tower is controlled and kept within desired sizes without the average effect of the wind power plant being significantly affected. This has the advantage that the load variations on the rotor blades and tower are reduced so that the output of these exposed components is significantly reduced.

In this patent application, the following definitions are used:

1) Instantaneous wind speed is defined as the instantaneous wind speed measured at the moment. 2) Average or smoothed wind speed is defined as the average or approximate average of the instantaneous wind speed for a certain period. This period will typically be longer than 3 seconds and normally be in the order of 10 minutes to an hour, but it can also be longer. When the wind speed is used to control the wind turbine, scaling or fractions of such measured values will also come under this definition. 3) Pitch angle is defined in this patent application as the torsion of a rotor blade's rigid body around its own longitudinal axis in relation to a fixed starting position for this angle. By "pitching" the blades, the forces on the rotor for a given instantaneous wind speed can be varied. 4) Rotor axial force is defined as the thrust force which is transmitted from the rotor towards the mill housing and which is directed mainly along the axis of rotation of the rotor axis. This force is made up of the total thrust in the direction of the wind from the rotor blades and can be both positive and negative at different times during the wind power plant's operation. 5) Nominal wind speed is defined as the wind speed at which the wind power plant first achieves full effect. This can typically be in the range of 12-14m/s. 6) Conversion unit is the unit that generates or transfers energy from the wind/rotation of the rotor blades into electrical power or other mechanical power. This unit can typically be a generator, a mechanical pump, an exchange unit etc. In the following description, the term generator is mostly used, but it is clear that generator can be replaced by a suitable conversion unit of each type as mentioned here.

It is desirable to be able to place commercial large horizontal axis wind turbines on foundations in deep water. This is desirable in order to be able to increase potential areas for wind power utilization, gain access to areas with high average wind speeds and to be able to build wind farms near oil and gas installations in order to be able to electrify these using wind power.

In deep water, a floating construction will be advantageous to limit the size and costs of towers and foundations.

Such a floating construction will primarily be affected by two types of forces that will control the movement pattern and stresses on the floating construction. These are wave forces against the floating part of the construction and thrust force on the rotor from the wind, here called the rotor's axial force.

For a wind power plant on land or in shallow water that is fixed against the ground or seabed, the dominant forces acting on the structure will usually be thrust on the rotor from the wind in addition to gravitational forces.

For large wind power plants (with outputs of typically 1 MW or higher) there are currently two main types of regulation mechanisms that are used to check that the rotor produces a constant power, equal to the plant's nominal power, for wind speeds that are higher than what is necessary to achieve full power (nominal power).

One method is stall regulation of the rotor blades. This method turns the blades into the wind so that the relative angle of attack of the wind against the airfoil is increased and the rotor blades achieve stall. That is the wing gradually loses its lifting power as the flows over the rotor blade go from laminar to turbulent. This releases the excess energy.

The other regulation method is pitch regulation of the blades by turning the blades in the opposite direction compared to stall regulation so that the wind is released by reducing the relative angle of attack of the wind against the wing profile. This reduces the lifting force of the rotor blade and less energy is extracted from the wind. This invention relates to this last regulation method, which is here called pitch regulation.

For large rotor diameters, there is a risk that the wind speed may have large variations over the rotor area both vertically and horizontally. This can cause greater fatigue problems of the blades than for smaller plants if the blades' individual thrust in the wind direction is not controlled and regulated appropriately. It can also lead to large moments that try to turn the rotor out of the wind if the wind speed at a given time is significantly greater on one half of the rotor (about the vertical axis) than the opposite.

With known technology with pitch control as described above, the rotor speed for wind speeds above the nominal wind speed will be regulated so that the rotor's power, which is equal to the rotor's torque multiplied by the angular speed (revolution speed in radians), is kept as constant as possible equal to the nominal power of the wind power plant. To achieve this, a control unit controls the pitch angle of the rotor blades continuously. Power and rotor axial force are not linear quantities as a function of variation of the wind speed. When the wind speed changes and the effect from the rotor is kept constant by pitching the rotor blades, the rotor axial force will change at the same time. In this way, the rotor's axial force (thrust in the direction of the wind) can have large variations. These force variations cause large output loads on the blade and tower structure, which in many cases can become dimensioning for these structural elements.

To illustrate this, one can look at the effect of the pitch control using known technology: If the instantaneous wind speed is increased from the nominal wind speed (e.g. 13 m/s) to twice that (26 m/s) while keeping the power and the rotor's rotation speed constant, the blade's pitch change will be about 20 degrees to prevent the rotor's effect from increasing at the higher wind speed. The result of this pitch change of the blades is that the rotor's axial force (thrust in the wind direction) is simultaneously approximately halved, which causes a fatigue load on the blade and tower structure.

For a weather event with, for example, a 10-minute average wind speed of 19 m/s, such fluctuations in the wind speed will typically occur with large variations in the overall axial force of the blades and the rotor if the effect is to be kept constant. Similar fluctuations in the axial force will occur for all average wind speeds above the nominal wind speed to a greater or lesser extent if pitch control is used according to known techniques.

There will always be a certain delay between the aerodynamic instantaneous torque on the rotor and the generator torque. This is mainly due to inertial forces of the rotor, drive unit and generator. Since pitch regulation according to known techniques uses measured values of rotation speeds or generator power, the pitch regulation will be inaccurate and delayed in relation to the instantaneous forces acting on the rotor, including the rotor's instantaneous axial force. This means that for sudden reductions in the instantaneous wind speed, the rotor blades will have too large a blade pitch angle and the rotor's axial force can be drastically reduced or even negative. For the example above, a sudden reduction of the wind speed from 26 m/s to 13 m/s and if the pitch regulation was not changed quickly enough could lead to a reduction of the rotor's instantaneous aerodynamic axial force from 50% of the nominal axial force to 26 m/s s wind to - 30% of the nominal axial force at 13 m/s wind, i.e. in the opposite direction of the wind. Together, this amounts to a change of 80%) of the nominal rotor axial force. Such large fluctuations in the axial force due to of the delayed pitch control will especially be a problem for the higher average wind speeds.

This will also mean that rotor blades designed for a low annual average wind speed will not be able to be used for a location with a high annual average wind speed. Due to the increased output loads that occur due to the greater variations in the thrust of the blades in the direction of the wind, rotor blades for areas with high average wind speeds must be dimensioned more powerfully. This can mean more expensive and heavier blades. The fact that a location with a high average wind speed will have more operating hours will, however, also increase the requirement for the fatigue strength of the rotor blades.

For a floating wind power plant, the effect described above that increased wind speed reduces the rotor's axial force due to the pitch regulation (for wind speeds above the nominal wind speed) can also have negative effects on the wind power plant's movement pattern. When the tower and rotor move against the wind, the relative wind speed against the rotor will increase, which will lead to a reduced rotor thrust due to the pitch control trying to maintain a constant effect, which in turn will increase the tower's movement against the wind. Conversely, when the tower and rotor move back in the same direction as the wind, the relative wind speed towards the rotor will decrease and the blades will automatically pitch (twist) using known techniques to maintain nominal power into the generator. This in turn leads to increased rotor thrust which in turn will increase the tower's movement in the direction of the wind. The result of this is an additional excitation and amplification of the tower's movements if pitch regulation using the known technique is used. This has been shown to lead to large increases in the fatigue loads for floating wind turbines.

Patent US-4201514 describes a method for regulating the pitch angle of the individual rotor blades in relation to variations in the wind speed. The regulation describes how the torque of the individual blades about the rotor axis is automatically kept constant at changing wind speeds. This has the same effect as otherwise known technique as described above. This means that the forces acting in the direction of the blade's direction of rotation, i.e. perpendicular to the wind direction, and which cause the blade to rotate are kept constant. This has the side effect that the blade's thrust in the wind direction varies correspondingly as described above for other known techniques. Thus, this known technique will have the same effect on blade and tower output due to varying thrust forces on the rotor blades when the rotor's torque is attempted to be kept constant.

DE 19628073 describes a method for adjusting the angles of rotor blades in a windmill to reduce aerodynamic imbalance. The contribution to output power of each blade is monitored to minimize the imbalance by changing the blade angle.

US 6619918 describes a wind turbine having sensors on the blades to adjust the pitch angles to maintain a safety distance between the tip of the blades and the windmill.

The invention aims to remedy the disadvantages of known technology.

In the method according to the claims, disadvantages of known technology are improved or eliminated.

In one embodiment of the invention, a method is provided for controlling power for a wind power plant comprising a conversion unit, in that when the conversion unit's output power lies within a given interval, the pitch angle of the rotor blades is changed with regard to minimizing variations in the thrust of the rotor blades in the wind direction individually or collectively, and when If the conversion unit's output power is outside this interval, the pitch angle of the rotor blades is changed with regard to bringing the output power within the interval.

In another embodiment of the invention, the minimization of variations in the thrust of the rotor blades in the direction of the wind is done by regulating against a calculated target value for the thrust of the rotor blades in the direction of the wind, as the target value for the thrust in the direction of the wind is different for different average wind speeds.

In yet another embodiment of the invention, the target value for the thrust of the rotor blades in the wind direction is adjusted in relation to the average conversion unit power or rotor speed over a given period of time.

In a further embodiment of the invention, the target value for the thrust of the rotor blades in the direction of the wind is predefined and linked to given average wind speeds.

In one embodiment of the invention, the thrust of the rotor blades in the wind direction is additionally adjusted by changing the rotor speed by adjusting the generator's torque and/or rotor brakes.

In yet another embodiment, the momentary thrust of the rotor blades in the wind direction can be determined directly or indirectly by means of strain gauges, wind speed measurements, by measuring geometric deflection of the blades, measurement of the generator's torque and/or measurement of the generator's power together with simultaneous measurement of the blade(s) pitch angles, and/or by measuring or using the blades' pitch moment around the pitch bearing's axis of rotation by either mounting the blades leaning backwards in the pitch bearing or shaping the blades so that the wind's pressure center of gravity on the blade is behind the pitch bearing's axis of rotation in relation to the rotor's direction of rotation.

In one embodiment, the pitch angle of the rotor blades is also changed with regard to minimizing directional errors for the wind power plant.

In one embodiment, the direction error is corrected if it lies outside a given interval.

In a further embodiment of the invention, the pitch angle of the rotor blades is adjusted differently for different rotational positions.

In one embodiment, the pitch angle of the rotor blades is adjusted individually and/or independently of each other.

In one embodiment, the wind field is predicted in a plane that is essentially perpendicular to the wind direction by using directly or indirectly measured values of the wind forces acting on the rotor blade(s) that are in front of the rotor's direction of rotation.

In one embodiment, the thrust of the rotor blades in the direction of the wind is actively used to counteract movements of the wind turbine's tower by regulating the pitch angles of the rotor blades.

In one embodiment of the invention, one or more anemometers are placed at favorable location(s) on the wind power plant so that the spatial distribution of the wind speed can be recorded and interpolations between the various anemometers can be made to form a picture of the distribution of the wind over the rotor's sweep area. This can be done by placing anemometers at significantly different heights and significantly different horizontal positions. This spatial distribution of the instantaneous wind speed can then be used to individually regulate the pitch of the rotor blades, or all the blades can be pitch regulated together.

The wind field in a plane that is mainly perpendicular to the wind direction can be predicted by using directly or indirectly measured values of the wind forces acting on the rotor blade(s) that are in front of the rotor's direction of rotation.

The rotor can advantageously be placed downwind of the tower so that the anemometers record the wind's speed before it hits the rotor. In addition, directly or indirectly measured values of the thrust on the blade in front, in relation to the rotor's direction of rotation, of a given blade can be used to predict the wind field that the given blade will move into. In this way, the blade's optimal pitch angle can is pre-calculated so that there is little or no delay between aerodynamic forces and the pitch response of the rotor blades. Thus, sudden changes in the instantaneous wind speed can be predicted. Using the horizontal distance between the wind gauges mounted upwind and the rotor's vertical plane and the wind speed, the time delay can be calculated from the time the measurements are made until the current wind speed will occur in the rotor. The control unit that manages the pitch regulation is given access to all these measurements and can at any time use this information to optimize the pitch angles of the blades. In this way, especially the large rotor axial force reductions that occur with sudden instantaneous reductions in the instantaneous wind speed can be avoided because the pitch regulation for known technology does not take place quickly enough.

For instantaneous wind speeds above the wind power plant's nominal wind speed, the blades will initially twist so that the axial force on the rotor is reduced. This is counteracted by increasing the rotation speed of the rotor through reduced or no pitch response, while the generator's torque is possibly simultaneously reduced following input from the control unit, which will also help to increase the rotor's rotation speed. Both the rotor's axial force and the effect on the generator can then be kept approximately constant at an optimal pitch angle within a smaller increase in wind speed. With a wind increase of approx. 10%, the rotation speed must be increased according to this method by approx. 10%> in order to achieve an unchanged rotor axial force and effect on the generator. The pitch angle must be changed at the same time.

A similar method is used when reducing the instantaneous wind speed, but in this case the rotor's rotation speed is reduced while the generator's torque is possibly simultaneously increased following input from the control unit.

For larger changes in the instantaneous speed of the wind, the following is done:

When the instantaneous wind speed is reduced in relation to the average wind speed measured over a longer period than the update frequency of the pitch regulation, for example over a 10 minute period, the pitch angle of the blades changes less than with purely power-controlled pitch regulation. This has the result that the rotor's axial force remains unchanged, but that the effect on the generator is somewhat less than the nominal effect. 10%) reduction of wind speed gives approx. 10%> reduction of the effect while the axial force of the rotor remains unchanged.

Correspondingly, for an increase of 10%> of the wind speed, the pitch angle of the blades will change less than with purely power-controlled pitch regulation. This has the result that the rotor's axial force remains unchanged, but that the effect on the generator becomes somewhat greater than the nominal effect. A 10% increase in the instantaneous wind speed gives about a 10% increase in the power, while the rotor's axial force remains unchanged. By combining the two effects as described above, they will collectively result in the instantaneous wind speed varying by typically +/- 20% without the rotor's axial force varying. Since the associated generator power variations will be short-lived and fluctuate around the rated power of the generator, the average generator power will be approximately unchanged, i.e. equal to the nominal power (rated power), while the axial force for a given average wind speed can be kept constant or nearly constant within typical + /- 20% variation of the instantaneous wind speed.

For a given average wind speed, the rotor's axial force (target value) can be calculated which corresponds to the nominal power on the generator. Acceptable maximum and minimum values for the generator's power variations around a mean value can be pre-programmed and the pitch controller unit will then calculate optimal instantaneous pitch angles so that the rotor's axial force is kept as constant as possible around the aforementioned calculated target value while the generator power is kept within the pre-programmed bandwidth.

The calculated target value for the rotor's axial force will therefore vary with different average wind speeds. Within each average wind speed, the axial force will then be tried to be kept approximately constant by means of the pitch regulation. The average wind speed can, for example, be the average of the last 10 minutes. If necessary, pre-calculated values can be used for the axial force's target values for given average wind speed intervals, for example divided into intervals with 0.1 m/s differences.

For instantaneous wind speed variations beyond approx. +/- 20%, the pitch regulation can be carried out with the priority of not varying the generator power beyond the typical approx. +/- 10%) as described above. In the event of such larger variations in the instantaneous wind speed, the axial force of the rotor will begin to vary, but this variation will also in these cases be substantially smaller than for pitch regulation according to known techniques.

Since an average value of the wind speed over a longer period of time, e.g. 10 minute average, varies much less than the instantaneous wind speed variations, the described method will ensure that the rotor axial force variations are substantially reduced with its positive effect on the output loads on the tower and rotor.

The same method as described above can also be used to actively regulate the rotor's axial force in relation to a given mean value. If the rotor's axial force is actively controlled in this way with a varying value, this can be used for e.g. to apply forces to the tower in anti-phase with its movements so that the tower's movements are damped.

The tower's movements can e.g. recorded with an accelerometer.

Furthermore, in a similar way, the axial force can be actively used to counteract any forces that try to turn the rotor out of the wind. This can be done by the individual force of the rotor blades in the wind direction being controlled so that any torques that try to turn the rotor and/or nacelle and/or tower out of the wind are counteracted, reduced or eliminated by cyclically changing the blades' individual pitch angle depending on physical position of each individual blade at all times so that the axial force on the rotor is greater on one or the other side of the rotor's vertical axis as required. When a given rotor blade passes one side of the tower's vertical axis, e.g. pitch angle by 0.5 degrees and when the same blade passes the opposite side, the pitch angle is reduced accordingly. This therefore does not need to have an impact on the rotor's overall power or the rotor's overall axial force. This extra cyclical pitch variation is simply superimposed on the calculated pitch angle according to the above-described method to control the rotor's overall axial force. This described cyclic pitch regulation can and is used to actively control the rotor so that parts of, or possibly the entire wind power plant in the case of a floating plant, can be kept in the desired position in relation to the wind direction. In this way, one can eliminate or reduce the size or number of the motors which, according to known techniques, turn the mill house, or possibly the entire tower in the case that the mill house is rotatably mounted on the tower for a floating plant, in the desired position in relation to the wind.

Furthermore, the thrust variations in the direction of the wind on each individual blade can be reduced by changing the pitch angle according to the above-described method to control the momentary thrust of the blade in the direction of the wind.

The blade can then be controlled individually in relation to its position in its orbital path and measured values of the wind speeds in different positions in or around the rotor's swept area.

The measured axial force will be recorded and included in the pitch control unit for calculating the optimal pitch angle at all times according to the described procedure.

Instead of just using measured wind speed and pitch angle to calculate the rotor's axial force, one or more other direct or indirect methods can be used.

By mounting the blades backwards in the pitch bearing, i.e. the longitudinal axis of the blades deviates somewhat from the axis of the pitch bearing so that the longitudinal axis of the blades does not cross the axis of rotation of the rotor and the pitch torque that then occurs can be measured via hydraulic pressure via the pitch control system of the blades and the axial force can then be calculated or;

By using tension flaps on the blades and/or on the rotor's main shaft and/or on other parts of the wind power plant or;

Indirectly by measuring the pitch angles of the blade(s) and the rotor's torque directly or by recording other parameters such as the generator's torque, power etc. and then the rotor's corresponding axial force can be calculated.

By measuring deflection of the blades using a mechanical or electronic measuring system.

Example of preferred method.

1 the following describes a non-limiting example of a preferred method which is visualized in the accompanying drawings, where: Fig. 1 shows a floating wind power plant 1 with rotor 2 which has a horizontal or essentially horizontal mounted rotor axis 11 mounted downwind in relation to tower 4 , further shows the mill house 3, anemometers 5, anchor connection 6 and anchor 7. Fig. 2 shows a wind turbine 1 which is placed on land or in shallow water with a rotor 2 which has a horizontal or essentially horizontal mounted rotor axis 11 mounted upwind in relation to the tower 4, the mill house 3 and anemometers 5 are also shown, Fig. 3 shows a wind turbine 1 which is placed either on land or in shallow water or floating in water with rotor blades 13 which are rotatably mounted about their longitudinal axis or mainly about their longitudinal axis 14 with pitch bearings 10. Fig. 4 shows a flow diagram illustrating the method according to the invention. Fig. 5 shows a flow diagram for an optional part of the method according to the invention.

A wind turbine 1 with a horizontal or substantially horizontal rotor axis 11 consists of one or more rotor blades 1) which together form a rotor 2 where the rotor blades can be coordinated or individually rotated (pitches) around their own longitudinal axis or substantially around their own longitudinal axis 14 for mainly to control the rotor's 2 power into the generator (not shown) and where the rotor's shaft is held firmly in a mill housing 3 and the rotor's shaft is connected to a generator possibly via an exchange system (gear). The pitch regulation of the rotor blades is carried out by a pitch control unit which, on the basis of various recorded operating information, wind measurements etc., sends a signal to the pitch motors about how much the pitch angles of the blades should be changed at any given time.

The mill house can be mounted on a tower 4 which is permanently mounted on land 9 or on the seabed 8 or which is part of a floating device or which itself constitutes a floating device with possibly one or more anchor connection(s) 6 to anchorage 7 on the seabed 8. Design of the anchor system 6,7 is not essential for the described method.

The method described below aims to reduce the variations of the rotor's axial force in relation to known technology, at the same time that the resulting effect on the generator is not significantly affected or is kept within acceptable limits in relation to limitations on the drive, generator and electrical network. The method also aims to use the axial force of the rotor to actively counteract the movements of a floating wind turbine. Furthermore, the described method aims to control and counteract rotational forces about the tower's vertical axis 12 and to reduce the aerodynamic force variation on each individual blade throughout a complete rotation cycle as a result of different wind speeds at different heights (vertical wind shear) and in the horizontal direction parallel to the rotor plane (horizontal wind shear).

One or more anemometer(s) 5 are placed at favorable location(s) on the wind power plant 1 so that the spatial distribution of the wind speed can advantageously be recorded and interpolations between the different anemometers can be made to form a picture of the wind's distribution over the rotor's swept area. This can be done by placing anemometers at significantly different heights and significantly different horizontal positions. This spatial distribution of the instantaneous wind speed can then be used to individually regulate the pitch of the rotor blades, or all the blades can be pitch regulated together.

The rotor 2 can advantageously be placed downwind in relation to the tower 4 so that the anemometers record the speed of the wind before it hits the rotor. In this way, the blades' optimal pitch angle can be pre-calculated so that there is little or no delay between aerodynamic forces and the pitch response of the rotor blades. Thus, sudden changes in the instantaneous wind speed can be predicted. Using the horizontal distance between the wind gauges mounted upwind and the rotor's vertical plane and the wind speed, the time delay can be calculated from the time the measurements are made until the effect of the relevant measured wind speed will appear in the rotor. The controller unit (not shown) which controls the pitch regulation is given access to all these measurements and can at any time use this information to optimize the blades' 13 pitch angles. In this way, especially the large rotor axial force reductions that occur with sudden instantaneous reductions in the instantaneous wind speed can be avoided because the pitch regulation for known technology has a time delay.

In cases where the average speed is above the nominal wind speed for the wind power plant 1 and the instantaneous wind speed then increases beyond the given average wind speed, the rotation speed of the rotor 2 according to this method will be increased by means of a reduced pitch response in relation to known technology, while the generator's torque may be simultaneously reduced following input from control the unit, which will also help to increase the rotor 2 rotation speed. Since rotor axial force is generally increased with increased rpm for a given rotor power, the reduced rotor axial force as a result of the pitch turning of the blades due to the increased instantaneous wind speed can be compensated. The result is that within a small increase in wind speed, both the rotor's axial force and the effect on the generator can be kept approximately constant by increasing the rotor's rotation speed and with an optimal pitch angle. With a wind increase of approx. 10%, the rotation speed must be increased according to this method by approx. 10%> in order to achieve an unchanged rotor axial force and effect on the generator. The pitch angle must be changed at the same time.

A similar procedure is used when reducing the instantaneous wind speed, but in this case the rotor's rotation speed is reduced while the generator's torque is possibly simultaneously increased following input from the control unit.

For larger changes in the instantaneous speed of the wind, the following is done:

When the instantaneous wind speed is reduced in relation to the average wind speed measured over a longer period than the update frequency of the pitch regulation, for example over a 10 minute period, the pitch angle of the blades changes less than with purely power-controlled pitch regulation. This has the result that the rotor's axial force remains unchanged, but that the effect on the generator is somewhat less than the nominal effect. 10%) reduction of wind speed gives approx. 10%> reduction of the effect while the axial force of the rotor remains unchanged.

Correspondingly, for an increase of 10%> of the instantaneous wind speed, the pitch angle of the blades will change less than with purely power-controlled pitch regulation. This has the result that the rotor's axial force remains unchanged, but that the effect on the generator becomes somewhat greater than the nominal effect. A 10% increase in the instantaneous wind speed gives about a 10%> increase in the power while the rotor's axial force remains unchanged. By combining the two effects as described above, they will collectively result in the instantaneous wind speed varying by typically +/- 20% without the rotor's axial force varying. Since the associated generator power variations will be short-lived and fluctuate around the nominal power of the wind power plant or the rated power of the generator, the average generator power will be approximately unchanged, i.e. equal to the nominal power (rated power), while the axial force for a given average wind speed can be kept constant or approximately constant within typical +/- 20% variation of the instantaneous wind speed.

For a given average wind speed and rotor rotation speed, the rotor's axial force (target value) which corresponds to the nominal power on the generator can be calculated. Acceptable maximum and minimum values for the generator's power variations around a mean value can be pre-programmed and the pitch controller unit will then calculate optimal instantaneous pitch angles (in response to the torque wind speed) so that the rotor axial force is kept as constant as possible around the aforementioned calculated target value while keeping the generator power within the pre-programmed bandwidth .

The calculated target value for rotor axial force will vary with different average wind speeds. Within each average wind speed, the axial force will then be tried to be kept approximately constant around this target value using the pitch regulation. The average wind speed can e.g. be last 10 minutes mean. If necessary, pre-calculated values can be used for the axial force's target values for given average wind speed intervals, e.g. divided into intervals with 0.1 m/s differences.

For instantaneous wind speed variations beyond the approx. +/- 20% as described above, the pitch adjustment can be made with the priority of not varying the generator power beyond the typical approx. +/- 10% > bandwidth as described. In the event of such larger variations in the instantaneous wind speed, the axial force of the rotor will begin to vary, but this variation will also in these cases be substantially smaller than for pitch regulation according to known techniques.

Since an average value of the wind speed over a longer period of time, e.g. 10 minute average, varies much less over time than the instantaneous wind speed variations, the described method will ensure that the rotor axial force variations are significantly reduced with its positive effect on the output loads on the tower and rotor.

The same method as described above can also be used to actively regulate the rotor's axial force around a given mean value. If the rotor's axial force is actively controlled in this way with a varying value, this can be used for e.g. to apply forces to the tower 1 in anti-phase with its movements so that the movements of the tower are damped. This is particularly advantageous for floating wind power plants.

In this case, the control unit will also have access to the tower's movements.

The tower's movements can be recorded with an accelerometer or other suitable measurement method.

Furthermore, in a similar way, the axial force can be actively used to counteract any forces that try to turn the rotor out of the wind. This can be done by the individual force of the rotor blades in the direction of the wind being controlled so that any torques that try to turn the rotor and/or mill house and/or tower out of the wind are counteracted, reduced or eliminated by cyclically changing the individual pitch angle of the blades depending on physical position of each individual blade at all times so that the axial force on the rotor is greater on one or the other side of the rotor's vertical axis as required. When a given rotor blade passes one side of the tower's vertical axis, for example, the pitch angle is increased by 0.5 degrees and when the same blade passes the opposite side, the pitch angle is reduced accordingly. This therefore does not need to have an impact on the rotor's overall power or the rotor's overall axial force.

This additional cyclic pitch variation is superimposed on the calculated pitch angle according to the above described procedure to control the rotor's

total axial force. This described cyclic pitch regulation can and is used to actively control the rotor 2 so that parts of, or possibly the entire wind power plant in the case of a floating plant, can be held in the desired position in relation to the wind direction. Thus, one can eliminate or reduce the size or number of the motors which, according to known techniques, turn the mill housing 3, or possibly the entire tower 5 in the event that the mill housing is rotatably mounted on the tower for a floating plant, in the desired position in relation to the wind.

Furthermore, the thrust variations in the flap direction (normally approximately equal to the wind direction) on each individual blade (which together make up the rotor's axial force) are reduced by changing the pitch angle according to the above-described procedure to control the blade's instantaneous thrust in the wind direction.

The blade can then be controlled individually in relation to its position in its orbital path and directly or indirectly measured values of the wind speeds in different positions in or around the rotor's sweep area.

The measured or calculated axial force will be recorded and included in the pitch control unit for calculating the optimal pitch angle at all times according to the described procedure.

Instead of just using measured wind speed and the pitch angle of the blades to calculate the axial force of the rotor, one or more other direct or indirect methods can also be used.

By having the blades 13 mounted backwards in the pitch bearing 10, i.e. the longitudinal axis 14 of the blades deviates somewhat from the axis of the pitch bearing so that the longitudinal axis 14 of the blades does not cross the axis of rotation 11 of the rotor and the pitch torque that then occurs can be measured via hydraulic pressure via the pitch control system of the blades and rotor blade thrust in the wind directions for each individual leaf can then be calculated or;

By using tension flaps on the blades 13 and/or on the rotor's main shaft and/or on other parts of the wind power plant or;

Indirectly by measuring the blade(s) 13 pitch angles and the rotor's 2 torque directly or by recording other parameters such as the generator's torque, power etc and then the rotor's corresponding axial force can be calculated.

By measuring deflection of the blades using a mechanical or electronic measuring system.

In fig. 4 is an embodiment of the method according to the invention illustrated by means of a flow diagram. The procedure is based on the determination in 40 of whether the instantaneous rotor speed, or possibly the output power of the generator, lies within an interval of the nominal value for the wind power plant, in the example by determining whether the rotor speed or possibly the output power of the generator lies within ±10% of nominal value.

If the instantaneous/instantaneous rotor speed, possibly output power on the generator, lies within the interval, an attempt must be made to minimize the rotor's axial force variations, possibly each blade's thrust in the wind direction individually, by regulating towards a target value for the axial force. In 44, it is then determined whether the average rotor speed or average generator output power for a given time t, for example the last 10 minutes, is above or below the nominal power for the wind power plant. According to this, the target value for the axial force of the rotor is adjusted in 45, 46. As previously described, a new target value for the axial force can be calculated on the basis of the average value of the axial force over a given period of time t, for example in 10 minutes with a given incremental increase or decrease of the target value for the axial force depending on whether one wants to increase or decrease the average power on the generator. The target value for the rotor's axial force can also optionally be a pre-calculated quantity linked to the average wind speed. The instantaneous value for the rotor axial force is then compared in 47 with the target value for the axial force obtained in 45/46 and the pitch angle of the rotor blades is then changed in 48 and 49 according to this comparison.

If, on the other hand, the instantaneous/instantaneous rotor speed, or possibly the output power of the generator as calculated in 40 lies outside the given interval, an attempt is made to get within this interval by adjusting the pitch angle correspondingly as in known techniques to bring the rotor speed, or possibly the average output power of the generator within desired interval, e.g. within +/- 10% of the nominal. The pitch angle is adjusted in 42/43 according to the calculation in 41 of whether the instantaneous rotor speed, or possibly output power on the generator is above or below the desired interval. In this way, the pitch angle is adjusted primarily with regard to maintaining a constant rotor axial force regulated against a slowly-varying target value and, in contrast to known technology, the pitch angle will only be adjusted to the extent necessary to bring the generator output power or rotor speed within the desired the interval. You will thus get a smaller adjustment and smaller rotor axial force variations than with known techniques where you regulate towards a constant value for the generator power with large rotor axial force variations.

In the example in Figure 4, the pitch angle of the rotor blades can be adjusted either for all rotor blades together, or for each individual rotor blade. For systems where it is possible to adjust each individual rotor blade, in addition to the points mentioned above, directional information for the wind turbine can be taken into account with a view to keeping the wind turbine in a stable position. This information 50 is taken into account in step 47 in the figure. Figure 5 illustrates in more detail the steps for providing this directional information.

In step 51, sine(a) is calculated or recorded, where a is the blade's rotational position, i.e. describes where in the rotation the individual rotor blade is located. In step 52, it is determined whether the direction error of the wind turbine's direction in relation to the wind direction lies outside a given interval, here +5°. If the direction error lies within the interval, no action is taken, but if the direction error lies outside the interval, a rotation mechanism for the tower is possibly triggered and a signal is calculated in 55, 56 depending on which side of the interval the direction error lies. The information provided in 55 or 56 is superimposed on the control signals provided to adjust the pitch angle with respect to rotor axial force variations, possibly each individual blade's downwind thrust. The direction error information includes information about rotational position, i.e. the pitch angle is adjusted individually for each blade according to the instantaneous rotational position. This means that the thrust force of each individual blade in the direction of the wind is adjusted differently for the various rotational positions so that a force effect is obtained which counteracts the directional error of the wind power plant.

Claims (12)

1. Procedure for checking the effect of a wind power plant (1) comprising a conversion unit, characterized in that when the output power of the conversion unit lies within a given interval, the pitch angle of the rotor blades (13) is changed with regard to minimizing variations in the thrust of the rotor blades (13) in the wind direction individually or collectively, and when the output power of the conversion unit lies outside this interval, the pitch angle of the rotor blades (13) is changed by consideration to bring the output power within the interval. 2. Procedure according to claim 1, characterized in that minimization of variations in the thrust of the rotor blades (13) in the direction of the wind is done by regulating against a calculated target value for the thrust of the rotor blades (13) in the direction of the wind, as the target value for the thrust in the direction of the wind is different for different average wind speeds. 3. Procedure according to claim 2, characterized in that the target value for the thrust of the rotor blades (13) in the wind direction is adjusted in relation to the average conversion unit power or rotor speed over a given period of time. 4. Procedure according to claim 2, characterized in that the target value for the thrust of the rotor blades (13) in the wind direction is predefined linked to given average wind speeds. 5. Procedure according to claim 1, characterized in that the thrust of the rotor blades (13) in the wind direction is additionally adjusted by changing the speed of the rotor (2) by adjusting the generator's torque and/or rotor brakes. 6. Procedure according to claim 1, characterized in that the momentary thrust of the rotor blades (13) in the wind direction can be determined directly or indirectly by means of strain gauges, wind speed measurements, by measuring geometric deflection of the blades, measurement of the generator's torque and/or measurement of the generator's power together with simultaneous measurement of the blade(s) s pitch angles, and/or by measuring or using the pitch moment of the blades around the axis of rotation of the pitch bearing (10) by either mounting the blades (13) leaning backwards in the pitch bearing (10) or shaping the blades (13) so that the center of gravity of the wind pressure on the blade is behind the pitch bearing (10 ) axis of rotation in relation to the direction of rotation of the rotor (2). 7. Procedure according to claim 1, characterized in that the pitch angle of the rotor blades (13) is also changed with regard to minimizing directional errors for the wind power plant. 8. Procedure according to claim 7, characterized in that the direction error is corrected if it lies outside a given interval. 9. Procedure according to claim 1, characterized in that the pitch angle of the rotor blades (13) is adjusted differently for different rotational positions. 10. Procedure according to claim 1, characterized in that the pitch angle of the rotor blades (13) is adjusted individually and/or independently of each other. 11. Procedure according to claim 1, characterized in that the wind field in a plane which is mainly perpendicular to the wind direction is predicted by using directly or indirectly measured values of the wind forces acting on the rotor blade(s) (13) which are in front of the rotor (2) in the direction of rotation. 12. Procedure according to claim 1, characterized in that the thrust of the rotor blades (13) in the direction of the wind is used actively to counteract movements of the wind power plant's tower by regulating the pitch angles of the rotor blades (13).