Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 105

By increasing the pitch angle as rated power is reached, the angle of attack can be reduced. A reduced angle of attack will reduce the lift force and the torque. The flow around the blade remains attached. Figure 3.59 is for the same turbine as Figure 3.58, but only the zero degree power curve is relevant below the rated level. Above the rated level, fragments of power curves for higher‐pitch angles are shown as they cross the rated power line: the crossing points give the necessary pitch angles to maintain rated power at the corresponding wind speeds. As can be seen in Figure 3.59, the required pitch angles increase progressively with wind speed and are generally much larger than is needed for the pitching to stall method. In gusty conditions, large pitch excursions are needed to maintain constant power, and the inertia of the blades will limit the speed of the control system's response.

Figure 3.59 Pitching to feather power regulation requires large changes of pitch angle.

Because the blades remain unstalled if large gusts occur at wind speeds above the rated level, large changes of angle of attack will take place with associated large changes in lift. Gust loads on the blades can therefore be more severe than for stalled blades.

The advantages of the pitching to feather method are that the flow around the blade remains attached, and so well understood, and provides good, positive damping. Feathered blade parking and assisted starting are also available.

Pitching to feather has been the preferred pitch control option mainly because the blade loads can be predicted with more confidence than for stalled blades.