The following diagrams show the relationship between the tower shadow, wind direction, rotational angle of the rotor, and yaw angle:
Tower shadow is detected by monitoring the instantaneous power produced by the turbine. Tower shadow produces a very short negative spike (drop) in the turbine’s power output. If this negative spike is sharp (very short in duration), and if it occurs when one blade passes the 6 o’clock position, then the turbine is correctly yawed. If the spike occurs “too early” or “too late”, then the turbine is not correctly aligned with wind direction. The lower diagram above shows that when the yaw angle is not correct, the tower shadow for the blade root occurs at a different time than the tower shadow of the blade tip. In this case the duration of the tower shadow increases. Furthermore, since the blade tip is generating torque when the root is in the shadow, and since the root is generating torque when the tip is in the shadow, the magnitude of the negative spike in power should decrease. So the controller knows that the turbine is misaligned whenever:
- the duration of the tower shadow is increased,
- the magnitude of the negative spike in power is reduced, and
- the tower shadow occurs earlier or later than 6 o’clock.
In designing the yaw angle control system, the traditional vane sensor could provide the relative yaw angle when the rotor is not turning or when the wind is coming from behind the turbine.
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