WIND TURBINE YAW SYSTEM

Abstract: The invention relates to a yawing system for wind turbines for adjusting the wind turbine in the required direction against the wind by turning about the yawing axle (14) of the turbine and for preventing the periodic vibrations in the nacelle about the yawing axle being transmitted as pulsating moments to the tower construction. The invention is characterised by at least one hydraulic motor (34) for turning the nacelle (12) about the yawing axle (14), a controllable throttle valve (50), named a parallel valve, arranged in a parallel line (by-pass line) to the hydraulic motor (34) and means (30) capable of changing the opening diameter of said throttle valve (50) in accordance with the position of the nacelle in relation to the wind direction.

Description: WIND TURBINE YAW SYSTEM TECHNICAL AREA The present invention relates to a yawing system for wind turbines intended for adjusting the wind turbine in the required wind direction about the yawing axis ofthe turbine and for preventing periodic vibrations in the nacelle around the yawing axis being transmitted as pulsating moments to the tower structure. BACKGROUND TO THE INVENTION Hydraulic motors are conventionally used in wind power stations to yaw the turbine nacelle to the required angle against the wind. Once the required position is reached the nacelle is locked with powerful brakes. The turbine is kept in this position until the wind direction has changed by a certain minimum angle, whereupon the brakes are released and the turbine is rotated to a new position where it is again locked etc. The system has a number of disadvantages and limitations; the turbines do not provide optimum efficiency and the stresses on the brakes are very large which often results in slipping and damage. Another problem of wind turbines is that the vibrations occurring in the nacelle as a result of the forces acting on the rotor are not equally large over the entire sweep area of the rotor. For example, the wind speed is often higher at greater heights than nearer the ground. These problems are well known and many systems have been developed to prevent these fluctuations being transmitted as pulsating moments to the power station structure, more particularly the tower. For example, EP-A 1-0 110 807 describes a system in which a controllable flow valve is arranged in a by-pass line to the hydraulic motor which is intended to yaw the nacelle towards the wind, while accumulators are provided to reduce the pressure vibrations in the hydraulic system caused by the nacelle fluctuations about the yawing axis. The inventor ofthe now proposed system have tested systems of this type and have found that in order to achieve an intended effect by the by-pass valve, its throttle must be of such a large diameter that it would need a very large flow from the oil pump to get the turbine to yaw at all This solution is therefore considered to be impossible to use in practice There also remains the problem that the energy which is produced by the nacelle fluctuations is not in itself transmitted to the tower but is retained in the hydraulic system through recharging the accumulators In addition the energy stored in the accumulators acts in the wrong direction, so to speak, as when the nacelle has swung round to an end position and the accumulators have been fully recharged, the pressure is exerted in the same direction as the returning vibrations and so does not damp the vibration but increases it instead The unwanted energy from the fluctuations is therefore not removed to any great extent DESCRIPTION OF THE INVENTION The aim ofthe invention is to reduce the periodic vibrations which arise during operation, to prevent the energy produced as a result of the vibrations being transmitted to the turbine structure, as well as to yaw the turbine continuously dunng operation against the direction ofthe wind This aim is achieved with a wind turbine yawing system for adjusting the wind turbine in the required direction against the wind by rotation about the yawing axle ofthe turbine and preventing periodic vibrations in the nacelle around the yawing axle being transmitted as pulsating moments to the tower structure, which is characterised in that the nacelle is arranged so as to be able to rotate about the yawing axle with the aid of at least one hydraulic motor, that a controllable throttle valve is arranged in a parallel line (by-pass line) ofthe hydraulic motor, and that the opening diameter of said parallel valve is arranged so as to be adjustable in accordance with the nacelle position in relation to the wind direction DESCRIPTION OF THE FIGURES In the following descπption of a preferred embodiment, reference is made to the following figures in which Fig 1 is a side view of the nacelle of a wind turbine in a partially exploded view. Fig.2 is a circuit diagram ofthe yawing hydraulics system, Fig.3 shows variations ofthe nacelle angle in relation to the wind direction in periodic vibrations about the yawing axis ofthe nacelle, Fig.4 shows variations ofthe nacelle vibration speed in accordance with fig. 3, Fig.5 is a control diagram for obtaining and processing of control signals in a parallel valve included in the system, Fig.6 shows signals for a nominal value as input signals for controlling the parallel valve, and Fig.7 & 8 shows changes in the pressure difference in the hydraulic system during periodic vibration for a valve with a fixed throttle and a valve controlled in accordance with the invention. DESCRIPTION OF A PREFERRED EMBODIMENT The wind turbine in which the present invention is intended to be installed is shown at reference number 10 in fig. 1. In a conventional manner the turbine comprises a nacelle 12 which is rotatably mounted on a tower 16 around a yawing axle 14. The nacelle 12 comprises a turbine axle 18, at one end of which there is arranged a rotor 20. The other end ofthe axle 18 is connected to a gear 22 which via gear changes transmits the rotation to a generator 24. A windspeed measurer 26 for measuring the strength ofthe wind, and a wind direction sensor, or wind flag 28 are arranged on the nacelle. These are connected via lines to a controlling and regulating unit 30 which includes a microcomputer. The nacelle 12 is rotatably connected to the tower 16 via bearings 32. Between the tower and the nacelle, two hydraulic motors 34 are arranged in the shown embodiment. A direction sensor 36 is also arranged between the tower and the nacelle. The direction sensor is , connected to the control unit 30 via lines. Referring to fig. 2, the hydraulic motors 34 are connected in parallel to two hydraulic lines 38. The lines 38 are connected to a proportional control valve 40, i.e. a control valve with an opening diameter which is proportional to the apphed voltage, and which in turn is connected partly to a hydraulic pump 42 and partly to a hydraulic oil tank 44. Between the lines 38, two pressure- reduction valves 46 are arranged, i e in parallel with the hydraulic motors 34 and directed in opposite directions In parallel with the hydraulic motors 34 a proportionally controlled regulating valve 50 is also connected, which is hereinafter referred to as the parallel valve A refilling line 52 is connected via an overload valve 54 from the hydraulic pump 42 to a parallel line 56 This line is equipped with a check valve 58 on each side of the connection ofthe refilling line 52 Each of the lines 38 to the hydraulic motors 34 is provided with a pressure sensor 60 for measuring the hydraulic pressure on each side ofthe hydraulic motors 34 The pressure sensors 60 are connected via lines to the regulating unit 30 A temperature sensor 62 is connected to the hydraulic circuit and is connected via lines to the control unit 30. In addition, the regulating valve 40 and parallel valve 50 are also connected to the control unit 30 The invention will be descπbed below in more detail in terms of three different conditions in order to explain the construction, operation and control criteria ofthe control system. Rotation ofthe nacelle at low wmdspeeds When the turbine is active the hydraulic systems is under pressure, the control system is active and all the sensors are continuously monitored At certain windspeeds the turbine does not rotate as the installation does not produce any electricity of value at low windspeeds However, the turbine and nacelle should be kept in the direction ofthe wind in order to be ready to operate if the wind should increase in strength. The wind flag 28 on the nacelle indicates the wind direction and its signal is sent continuously to the control unit 30 If the nacelle is not at the correct angle to the wind direction, the control unit 30 opens the regulating valve 40 and pressurised hydraulic oil flows into the lines 38 to the hydraulic motors 34 which then turn the nacelle The direction sensor 36 checks the direction ofthe nacelle and when this corresponds to the direction indicated by the wind flag 28 the regulating valve 40 is throttled and the nacelle stops. The operation of the parallel valve 50 will be described in more detail below, but is closed during this yawing Increased windspeed from a fixed direction When the windspeed has increased to a certain predetermined value, the wind turbine begins to rotate This can occur by the turbine blades being rotated from a vane position to an operating position and/or the brakes which lock the turbine axle 18 in the parking position being released When the turbine rotates, varying forces are produced on this and on the entire installation These variations are predominantly due to the fact that the windspeed is different at different heights, usually being greater at greater heights, so that the turbine blade which is higher is under greater load than the other blade dunng rotation The vaπations produce periodic vibrations ofthe nacelle 12 about its yawing axle 14 Fig 3 shows the yawing angle as a function of time and fig 4 shows the yawing angle speed Yawing about the yawing axle results in the hydraulic motors starting to rotate, and depending on the direction a pressure is built up in one or the other line and hydraulic oil flows The flow from the hydraulic motors and the pressure vary with the same phase as the yawing speed, i e when the speed is greatest the pressure difference is greatest The control unit 30 continuously receives information about pressure levels Pi and P 2 in both lines and calculates the differential pressure as the difference in pressure in both lines The differential pressure enters the control circuit 70, fig 5, as an actual value, hereinafter called P d A nominal differential pressure value is entered into the control circuit, hereinafter called P dr How the value of P Jr is obtained will be desenbed below Before the P r value and the P d value are introduced into the control circuit, the absolute value is taken In the control circuit these are compared and the control error leaves the circuit as a value P_ c The P dc value is first processed in a so-called lag filter 72 which is a type of low-pass filter where lower frequencies are amplified more than are higher frequencies From the lag filter 72 a signal d ps is obtained which shall be sent to the parallel valve 50 and which corresponds to the throttling, i e the greater the control error the greater the opening As the parallel valve 50 generally operates in reverse, I e the greater the control error the smaller the opening of valve 50, the value from the lag filter 72 has to be inverted This is done in a circuit 74 which inverts the signal d ps with a conection factor From circuit 74 a signal d p is obtained , the magnitude of which corresponds to the diameter of the parallel valve, I e a voltage of a certain value is applied to the valve control which opens to the corresponding extent. When the parallel valve 50 opens, oil flows from the line at higher pressure via the valve 50 to the line at lower pressure. As stated, the pressures are measured continuously and the differential pressure P is thus continuously compared with the nominal value P dr and the regulating error Pde continuously controls, after processing, the diameter d p ofthe parallel valve. Determination ofthe nominal differential pressure value As described above, the parallel valve is thus controlled by comparing the differential pressure with a nominal value and the valve is opened and closed until the control error is minimised. The nominal differential pressure value is obtained as follows. The wind flag 28 continuously indicates the wind direction, i.e. the nominal value ofthe yawing angle which the nacelle should have, hereinafter called THr. At the same time the direction sensor 36 indicates the actual value o the nacelle yawing angle, called TH. Both of these signals enters into a control circuit 76 and the control error THe is obtained as the output signal. As the nacelle experiences periodic vibration about the nominal value THr, the control error THe will continuously change its sign. The control error is partly entered into a circuit which includes a lag filter 78 for calculating the required mean differential pressure Pd nιr . The control error THr further enters as a signal into another lag filter 82 for calculating the mean nominal speed ω rg . i.e. the yawing speed which the machine should have. The signal from the direction sensor 36 is also taken and derived in a circuit 84 in order to obtain the actual value ω ofthe rotation speed. Due to the periodic vibrations this signal also changes its sign. The signals Pdmr, ω rg and ω are input values in a circuit 86 for calculation ofthe actual value ofthe damping pressure P dr d. The signal Pdmr which leaves the circuit 78 is processed in such a way that the filter amplifies low frequencies and removes higher frequencies. This means that in the event of the wind being from a constant direction and the nacelle periodically turning about the yawing axle, the frequency ofthe control error THe obtained by comparing THr and TH will be filtered out and P dmr is equal to zero. If Pdmr is zero, the damping pressure P drd is set just below the maximum the hydraulic system can withstand. The signal from the rotation speed T is used in such a way that the sign is used to indicate whether the damping pressure should be positive or negative. As the nacelle vibrates periodically, the sign changes periodically A damping pressure P rd output signal is thus obtained as a square wave of constant amplitude which is symmetrical about zero The signal is then processed in a circuit 88 known as the energy zero setter, which will be described in more detail below The output signal from this circuit 88 is now the nominal value P dr ofthe differential pressure and is then entered in the form of a symmetrical square wave, part I of the curve in fig 6, into the control circuit 70 where it is first absolute value processed As the signal before this is a symmetrical square wave, the signal has a constant positive nominal value after absolute value conversion As stated above, the actual alue Pd ofthe differential pressure enters the control circuit 70 and is compared with the constant nominal value The control error signal P de therefore fluctuates with the ibrations and P d will be smaller the higher the differential pressure is As P d . is inverted, the signal therefore opens the parallel valve d p , said signal being essentially proportional to the opening diameter of the parallel valve, being greater the higher the differential pressure This means that at the nacelle's end positions for the vibrations the differential piessure is at its lowest and the parallel valve opening diameter as large as possible When the nacelle then turns back from the end position, the differential pressure increases as the hydraulic motors start to rotate This is detected by the pressure sensors 60 and the calculated diffeiential piessure enters the control circuit and is compared with the nominal value The output signal from the control circuit will thus become smaller and the opening diameter ofthe valve will thus become smaller The control circuit thus controls the valve so that it operates in such a way as to keep the differential pressure at a relatively constant level The differential pressure acts as a damping force to damp the vibrations at the same time as the energy produced by the vibr tions is transmitted to the hydraulic system and given off as heat when the hydraulic oil passes through the parallel valve The damping force is obtained as the area between the curves for the differential pressure and the x-axis As can be seen m figures 7 and 8 if a parallel valve with a fixed throttle is compared with a parallel valv e controlled in accordance with the invention, a much greater damping force is obtained with a throttle in accordance with the invention Relatively strong wind which changes direction If the wind changes direction during operation, the nacelle has to be turned into the wind in order partly to achieve optimum operation and partly to avoid oblique stresses on the installation When the wind changes direction this is registered by the wind flag and THr is changed This change is introduced into control error TH e as a low frequency signal change which is amplified in the lag filter A value of Pd πιr is thus obtained, see the lower curve in fig 6, which is used for calculating the damping pressure Pdrd For the nacelle to turn in the required direction, P dr d is reduced by the appropriate value and with the appropriate sign P dr d thus obtains a signal which looks like a non-symmetrical square wave, ι e w ith a lower vibi ation amplitude on one side of the x-axis, see part II of he curve in fig 6 P drd is entered as a nominal value into control circuit 70 for the parallel valve and is processed in the same way as descπbed above What now occurs in the regulation ofthe parallel valve is that the valve fully damps one direction and somewhat less the other direction, l e the direction towards which the nacelle has to be turned The regulation achieves a successive movement ofthe nacelle a small step in the required direction during each periodic vibration around the yawing axle at the same time as it is damped by the system The periodic vibrations are thus utilised for turning the nacelle and correcting for changes in the wind direction during operation The control valve 40 does not thei efore have to be used to change the direction of the nacelle during operation As stated above, the signal P dr is entered into and is processed in a so-called energy zero setter 88 Signals P dmr , Pd and P dr d are entered into this circuit The hydraulic system in accordance with the invention operates at a system pressure of 350 bar as a high pressure is an advantage for attaining a high moment on the hydraulic motors 34. A disadvantage of this high system pressure is that there is a resilient effect in the hydraulic oil as the oil is slightly compressible The resilience effect is shown in a differential pressure which is remains at the end position ofthe periodic vibrations, as the differential pressure should be zero and should thus counteract damping The energy zero setter 88 gives the parallel valve a nominal value when the speed is zero so that it opens fully, allows oil to pass through and thus reduces the differential pi essui e at this point If the wind direction is fixed, the differential pressuie is preferably lowered to zero, but if correction ofthe nacelle is necessary due to changes in the wind direction, the pressure is lowered to a certain value The regulation in the energy zero setter does this if the signal from P dmr has a value other than zero Throttling of the hydraulic oil through the parallel valve 50 produces a large amount of heat in the hydraulic oil as the energy from the periodic vibrations in the valve is converted to heat In order to keep the temperature at a normal operating temperature it must be replaced and cooled continuously This is done by the control valve 40 continuously supplying new oil and removing heated oil The regulation ofthe control valve 40 as descπbed above can always take place, even when the turbine is operating The flow from the latter is compensated directly by the parallel valve 50 and the flows from the yawing valve are generally set much lo er than the flows arising during the damping of the period vibrations The temperature in the hydraulic circuit is also monitored continuously by means of tempeiatui e sensoi 90, fig 2 With a system of damping and yawing in accordance with the invention several advantages are obtained On the one hand the existing hydraulic system and hydraulic motors are used, and on the other hand, through the active throttling ofthe parallel valve, a much better damping is achieved than ith existing systems at the same time as the unwanted energy from the vibrations is removed from the system instead of being transmitted to the structure or kept within the s stem, and, in addition, throttling ofthe parallel valve allows the nacelle to be yawed at the same time as it is damped It should be understood that the invention is not restπcted to the above description and to the embodiments shown in the figures, but can be modified within the framework of the following claims

Claims: PATENT CLAIMS 1. A yawing system for wind turbines for adjusting the wind turbine into the required direction against the wind by irning about the turbine ' s yawing axle (14) and for counteracting periodic vibrations in the nacelle around the yawing axle being transmitted as pulsating moments to the tower structure, characterised by at least one hydraulic motor (34) for turning the nacelle (12) about the yawing axle (14), a controllable throttle valve (50), named the parallel valve, arranged in a parallel line (by¬ pass line) to the hydraulic motor (34), as well as means (30) capable of changing the opening diameter of said throttle valve (50) in accordance with the position ofthe nacelle in relation to the wind direction. 2. A system according to claim l,characterisedin that the periodic vibrations in the nacelle produce pressure differences in the hydraulic lines (38) which are connected to the hydraulic motors, that means (36, 28) are arranged for continuously measuring the actual value (TH) and nominal value (TH r ) ofthe position ofthe nacelle, that means (76) are arranged for continuously comparing the difference between these (TH c ), where the difference involves the nominal value (P dr ) which said pressure difference should have, that means (60) are arranged for continuously measuring the actual value (P d ) of said pressure difference, and that means (70) are provided for comparing the actual and nominal values of said pressure difference, where the difference between these (P d(: ) constitutes the control signal for said parallel valve (50). 3. A system according to claim 2, characterisedin that the parallel valve (50) is arranged to be controlled in such a way that said difference between the actual and nominal values of said pressure difference is rninimised. 4. A system according to claim 3, characterisedin means (36) for continuously measuring the speed (ω) ofthe nacelle during the periodic vibrations, and means (30) for adjusting the nominal value (P dr ) of said pressure difference to the same sign as the sign of said speed, i.e. so that said nominal value is periodically varied between a positive and a negative value. 5 A system according to claim 4, charactei lsedby means (30) capable of setting the nominal value (P dr ) of said pressure difference to a value just below the maximum loading value ofthe hydraulic system for yawing the nacelle, if the difference between the actual and nominal values of nacelle position vanes symmetrically, i e if the nacelle periodically turns about the yawing axle with an amplitude which is equally large on both sides ofthe nominal value of this position 6 A system according to claim 4, characterised by means (30) capable of changing the nominal value (Pd r ) of said pressure difference with said amplitude difference, if the difference between the actual and nominal values ofthe nacelle does not vary symmetrically, I e if the nacelle periodically turns about the yawing axle with an amplitude which is greater on one side ofthe nominal lue of this position 7 A system according to claim 6, characterised by means (30) capable of reducing the positive value ofthe pressure diffei ence's nominal value by a corresponding amount if the sign ofthe amplitude difference is positive, and vice-versa

Inventors: HILDINGSSON STEN, WESTIN TORBJOERN, HILDINGSSON, STEN, WESTIN, TORBJOERN
Applicants: HILDINGSSON STEN, KVAERNER TURBIN AKTIEBOLAG, WESTIN TORBJOERN, HILDINGSSON, STEN, WESTIN, TORBJOERN
Application year: 1996
Main classification: F03D7
IPC codes: F03D7