Unique Method of Yaw Control

Current Methods for Yaw Control

Presently, there are two means for rotating very large three-bladed, horizontal axis wind turbines. The first is use yaw motor(s) to rotate the Nacelle and rotor about the yaw axis. The advantage of this method is that the rotor can be positioned at a desired orientation. The primary disadvantage is that the yaw motor(s) require a significant amount of power and time to rotate both the Nacelle and the spinning rotor. This disadvantage increases with the mass and diameter of the rotor because the yaw motor(s) have to overcome increasing gyroscopic forces. A second means for rotating the Nacelle and rotor is to enable free yawing, where wind forces on the blades cause rotation of the Nacelle and rotor about the yaw axis. This method of yawing does provide a benefit because there is no need for yaw motors, however there are significant disadvantages that overcome this advantage. First, in order to apply sufficient torque to rotate the spinning rotor and Nacelle about the yaw axis, a significant amount of torque has to be applied to the blades. Second, sometimes the wind forces can result in an orientation of the wind turbine that is very different from the desired orientation. Free yaw is typically not used for very large wind turbines.


Unique Method of Yaw Control using Teetering Hub

The ball-and-socket hub provides a new means for changing the orientation of the Nacelle and rotor. Here, the Nacelle is rotated about the vertical yaw axis using yaw motor(s) as is presently done with the first current method. The difference however, is that the rotor does not rotate about the yaw axis along with the Nacelle. Instead, the rotor rotates about a second vertical axis, the Zs axis as shown in the diagram below. The yaw and Zs axes are shown together in a drawing of the wind turbine provided in the Images tab. This arrangement leads to separate rotations of the Nacelle and rotor.


 

Mechanism for Rotation of Rotor about the Zs axis

Experimentation with an actual working model that shows the Nacelle and rotor separately about different vertical axes. Under static conditions, the rotor can be rotated about the Ys and Zs axes without causing a rotation of the Nacelle and conversely, the Nacelle can be rotated about the yaw axis without causing a rotation of the rotor. This demonstrates that the Nacelle and rotor maintain separate rotations as long as the teetering limits are not reached. When teetering limits are reached, the Nacelle and rotor will together rotate about the yaw axis.

When rotation of the blades occurs under actual wind conditions however, rotation of the Nacelle about the yaw axis will lead to follow-up rotations of the rotor about the Ys and Zs axes. These follow-up rotations are initiated by a change in the teetering patterns of the blades as the Nacelle rotates about the yaw axis. This is because the main shaft also rotates with the Nacelle about the yaw axis, and the blades rotate about axes that are perpendicular to the main shaft axis. Hence a change in the orientation of the main shaft will cause a change in the orientation of the teetering axes and a consequent change in the teetering angles, velocities and accelerations of the blades. This change in teetering due to rotation of the main shaft about the yaw axis however will generally be unstable because teetering angles, velocities and accelerations are ultimately determined by imbalances in wind forces on the blades. Over a short period of time, the wind forces will re-establish a stable teetering pattern and in doing so, the rotor will rotate about the Ys and Zs axes.

Computer modeling was performed in order to demonstrate the change in teetering pattern due to rotation of the Nacelle about the yaw axis and the subsequent re-establishment of a stable teetering pattern over a short period of time. A wind file was created with 12 m/s wind speed with no wind shear or turbulence in order to isolate the impact of yaw rotation. Additionally, the rotor shaft tilt angle was set to 0.0° in order to minimize teetering. Changes were made to wind direction according to the following table used as the Hub Height Wind file:




Table I. Hub Height Wind File

Time (Seconds) Wind Speed (m/s)
Wind Direction (degrees) Vertical Speed (m/s) Horizontal Shear Vertical Shear Lin V. Shear Gust Speed (m/s)
0.0 12.0 0..0 0.0 0.0 0.0 0.0 0.0
480.0 12.0 0.0 0.0 0.0 0.0 0.0 0.0
490.0 12.0 20.0 0.0 0.0 0.0 0.0 0.0
520.0 12.0 20.0 0.0 0.0 0.0 0.0 0.0
530.0 12.0 40.0 0.0 0.0 0.0 0.0 0.0
560.0 12.0 40.0 0.0 0.0 0.0 0.0 0.0
580.0 12.0 0.0 0.0 0.0 0.0 0.0 0.0
9999.9 12.0 0.0 0.0 0.0 0.0 0.0 0.0

This hub height file led to the following changes in wind angle between 480 and 580 seconds:


 

The wind angle remained at 0° prior to 440 seconds and after 640 seconds. The figure shows changes occur to the wind angle from 480 – 490 seconds, 520 – 530 seconds and 560 – 580 seconds. In response to these changes in wind angle, the Nacelle underwent rotation about the yaw axis in order to again face directly into the wind. The minimum and maximum yaw rates were set at -1.0 and 1.0 deg/sec.  Hence if the wind angle changes by 20 degrees in 10 seconds, the Nacelle yaw error will be 10 degrees after 10 seconds if the yaw rate is limited to 1 deg/sec.  A chart of the Nacelle yaw error as shown below indicates that Nacelle yaw rotations occur at 480 – 500 sec, 520 – 540 sec, and 560 – 600 seconds.  


 

The charts for the teetering of blades 1, 2 and 3 show Nacelle rotation results in an increase in teetering. The charts show that the increase in teetering occurs during and immediately following Nacelle rotation and then returns to the initial stable teetering state in 5 – 10 seconds. Of significance is that the teetering amplitude after Nacelle rotations becomes equal to the teetering amplitude prior to Nacelle rotations (440 – 480 sec). Since Nacelle yaw error results in an increase in teetering amplitude, a return to the initial teetering amplitude is indicative of the rotor’s return to again facing directly into the wind.  Please note that the data were generated using ideal wind with no shaft tilt in order to detect the unstable teetering.  Under normal circumstances with shaft tilt, wind shear and turbulence, teetering angles of ± 0.7° are very minor and unstable teetering would be indistinguishable from normal teetering.


 

 

 

The next two charts show the positioning of the hub socket about the stationary Ys axis (HubRotPys) and the stationary Zs axis (HubRotPzs). As a background, the relationship between HubRotPys and HubRotPzs and the teeter deflection angles of blades 1, 2 and 3 are given in the Software Modification/Validation tab. Also, the hub socket rotation and the rotor rotation about Ys and Zs axes are equal because the blades are affixed to the hub socket. The charts below show baseline values of -0.02° for HubRotPys and -0.04° for HubRotPzs prior to rotation of the Nacelle. The charts also show significant excursions from baseline for both HubRotPys and HubRotPzs during Nacelle rotation followed by a return to baseline perhaps 5-10 seconds after completion of Nacelle rotation. This return to baseline after rotation of the Nacelle about the yaw axis indicates that the follow-up rotation of the hub socket about the Zs axis is complete and that the hub socket is again aligned with the Nacelle. The return of HubRotPys to baseline is also indicative of alignment of the hub socket with the Nacelle about a horizontal axis, however since the Nacelle does not rotate about a horizontal axis during yaw maneuvers, a return to baseline indicates that there is no net rotation of the hub socket about the Ys axis. Additionally, HubRotPzs provides a direct measurement of the hub rotation about the stationary vertical Zs axis. The wind direction changed three times by -20°, -20° and then +40. After each of these changes ended, HubRotPzs became virtually negligible (-0.04°) within 5-10 seconds, confirming that the rotor became aligned with the Nacelle.


 

 

Conclusion

Based upon these results, it is concluded that a rotation of the Nacelle about the yaw axis is closely followed by a rotation of the hub socket (and blades) about the Zs axis. The mechanism for this is that Nacelle rotation about the yaw axis leads to an unstable teetering state and that wind forces cause rotation of the hub socket about both the Ys and Zs axes in the process of re-establishing a stable teetering state. Once Nacelle yaw maneuvers are complete, there is a net rotation of the hub socket about the Zs axis resulting in alignment of the hub socket with the Nacelle.

Limitation--Not Suitable for Free Yaw

Since yaw rotation results in a temporary unstable teetering state, it would be desirable to minimize unstable teetering. For this reason, it would be undesirable to enable free yaw-- especially with tail furling where significant yaw rotations continually occur.


Benefits

There are several benefits for this unique method of yaw control over the two existing methods. The new method provides a significant advantage over use of yaw motor(s) to rotate both the Nacelle and rotor because the yaw motors are needed for rotation of the Nacelle only. The significant amount of energy required to overcome gyroscopic forces of a very large, spinning rotor is provided by wind forces. Second, other computer simulations for the 1.5-MW and 5-MW turbines comparing fixed and ball-and-socket hubs with yaw control enabled show very considerable reductions in yaw bearing pitch and yaw moments (YawBrMyn and YawBrMzn) for the ball-and-socket hub. Unfortunately, FAST does not provide a direct measure of the power required to rotate the Nacelle and rotor about the yaw axis. The profiles for YawBrMzn, however, are indicative of the significant reduction in power needed for yaw maneuvers. The new method of yaw control also provides a significant advantage over free yaw turbines as demonstrated in a separate computer simulation (15 mps 5MW free yaw comparison). The presentation shows significant Nacelle yaw error with a free yaw turbine, demonstrating that free yaw can be problematic in accurately positioning the wind turbine. The presentation also shows the ball-and-socket hub provides the same load reductions when compared to the free yaw turbine as it does when compared with the fixed hub turbine.