Yawing system for adjusting a wind turbine into a required wind direction by turning the turbine about a yawing axle

A yawing system for a wind turbine is provided. The system adjusts the wind turbine into a required wind direction against the wind by turning the turbine about a yawing axle (14) and counteracts periodic vibrations in a nacelle of the turbine as it turns around the yawing axle from being transmitted as pulsating moments to a tower structure. At least one hydraulic motor (34) turns the nacelle (12) about the yawing axle (14). A controllable throttle valve (50) is disposed in a parallel line to the hydraulic motor (34). A regulator (30) is capable of changing an opening diameter of said throttle valve (50) in accordance with a position of the nacelle in relation to the wind direction such that the periodic vibrations are utilized for turning the nacelle and correcting for changes in the wind direction during operation by a successive movement of the nacelle in small steps in the required wind direction during each periodic vibration around the yawing axle and at the same time as the nacelle is dampened.

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 of the 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-A1-0 
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 inventors of the 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 of the 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 during operation against the direction of the 
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 of the 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) of the 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 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 of the 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 of 
the 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 applied 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 of the 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 of the 
hydraulic motors 34. The pressure sensors 60 are connected via lines to 
the regulating unit 30. A temperature sensor 90 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 described below in more detail in terms of three 
different conditions in order to explain the construction, operation and 
control criteria of the control system. 
Rotation of the Nacelle at Low Windspeeds 
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 of the 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 (regulating 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 of the 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 of rotor 20 which is 
higher is under greater load than the other blade during rotation. The 
variations produce periodic vibrations of the 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 P.sub.1 and P.sub.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.sub.d. A nominal differential 
pressure value is entered into the control circuit. hereinafter called 
P.sub.dr. How the value of P.sub.dr is obtained will be described below. 
Before the P.sub.dr value and the P.sub.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.sub.de. 
The P.sub.de 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.sub.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.sub.ps with a correction 
factor. From circuit 74 a signal d.sub.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.sub.d is thus continuously compared with the nominal value 
P.sub.dr and the regulating error P.sub.de continuously controls, after 
processing, the diameter d.sub.p of the parallel valve. 
Determination of the 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 of the yawing angle 
which the nacelle should have, hereinafter called THr. At the same time 
the direction sensor 36 indicates the actual value of 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 P.sub.dmr. The control error THr 
further enters as a signal into another lag filter 82 for calculating the 
mean nominal speed .omega..sub.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 .omega. of the 
rotation speed. Due to the periodic vibrations this signal also changes 
its sign. 
The signals P.sub.dmr, .omega..sub.rg and .omega. are input values in a 
circuit 86 for calculation of the actual value of the damping pressure 
P.sub.drd. The signal P.sub.dmr 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 of the control error THe obtained by comparing THr and 
TH will be filtered out and P.sub.dmr is equal to zero. If P.sub.dmr is 
zero the damping pressure P.sub.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.sub.drd 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.sub.dr of the 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 value P.sub.d of the differential pressure 
enters the control circuit 70 and is compared with the constant nominal 
value. The control error signal P.sub.de therefore fluctuates with the 
vibrations and P.sub.de will be smaller the higher the differential 
pressure is. As P.sub.de is inverted, the signal therefore opens the 
parallel valve d.sub.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 pressure 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 differential pressure 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 of the 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 vibrations 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 in FIGS. 7 and 8, if a parallel valve with a fixed throttle is 
compared with a parallel valve 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.sub.e as a low frequency 
signal change which is amplified in the lag filter. A value of P.sub.dmr 
is thus obtained, see the lower curve in FIG. 6, which is used for 
calculating the damping pressure P.sub.drd. For the nacelle to turn in the 
required direction, P.sub.drd is reduced by the appropriate value and with 
the appropriate sign. P.sub.drd thus obtains a signal which looks like a 
non-symmetrical square wave, i.e. with a lower vibration amplitude on one 
side of the x-axis, see part II of the curve in FIG. 6. P.sub.drd is 
entered as a nominal value into control circuit 70 for the parallel valve 
and is processed in the same way as described above. What now occurs in 
the regulation of the parallel valve is that the valve fully damps one 
direction and somewhat less the other direction, i.e. the direction 
towards which the nacelle has to be turned. The regulation achieves a 
successive movement of the nacelle in small steps 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 therefore have 
to be used to change the direction of the nacelle during operation. 
As stated above, the signal P.sub.drd is entered into and is processed in a 
so-called energy zero setter 88. Signals P.sub.dmr, P.sub.d and P.sub.drd 
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 of the 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 pressure at this point. If the wind direction is fixed, the 
differential pressure is preferably lowered to zero, but if correction of 
the 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.sub.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 of the control 
valve 40 as described 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 lower than the flows arising during the damping of the period 
vibrations. The temperature in the hydraulic circuit is also monitored 
continuously by means of temperature sensor 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 of the parallel valve, a much better damping is achieved 
than with 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 system, and, in addition, throttling of 
the 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 restricted to the above 
description and to the embodiments shown in the figures, but can be 
modified within the framework of the following claims.