# Limiting power output and storm protection

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The power that can be taken from the wind varies with the wind speed. After reaching the nominal power, the power should remain constant for wind speeds greater than the nominal wind speed because the turbine and generator cannot handle more power. Therefore, a wind power plant must limit the power with one of the two following methods:

• stall control

Figure 5.14 Stall Effect at Higher Wind Speeds

Stall control takes advantage of the stall effect that occurs at high angles of attack (see Figure 5.14). This effect destroys the buoyancy force and thus limits the power that the wind transfers to the rotor blade. The rotor's rotational speed n and the circumferential speed u remain constant for stall-controlled wind generators. Stall control is achieved by increasing the angle of attack at higher wind speeds vW. The rotor blades do not pitch, i.e. the pitch angle remains constant; stall control can be realized by construction measures without advanced technical requirements. The disadvantage of stall control is the low possibility to influence operation because stall control is purely passive. The maximum power of a newly designed rotor blade is not easy to estimate because a mathematical description of the stall effect is rather difficult. After reaching the maximum power, the power output of stall-controlled wind turbines decreases again and does not remain on a constant level as shown in Figure 5.13.

Many manufacturers of wind turbines prefer pitch control, although the technical effort is much higher than for stall control. However, since pitch control is an active control, it can be adjusted to suit the conditions, in contrast to stall-controlled systems. Pitch control directly increases or lowers the pitch angle of the rotor and therefore the angle of attack. The rotor blade is turned into the wind at higher wind speeds (see Figure 5.15), lowering the angle of attack and actively decreasing the power input of the rotor blade. Pitch-controlled wind turbines are more difficult to manufacture because the rotor blades must twist inside the rotor hub. Small systems often use mechanically controlled pitch mechanisms using centrifugal forces. An electric motor moves the rotor blade to the desired position in large systems.

If the wind generator is stopped due to storm protection, the pitch control can pitch the rotor blade towards the feather position. This reduces the power input and avoids damage to the wind turbine. Stall-controlled systems often have additional aerodynamic brakes. For instance, the rotor tip can bend. During storms, the tip bends by 90° and slows the wind turbine.

Figure 5.15 Rotor Blade Positions for Different Wind Speeds for a Pitch-controlled System