PITCH VERSUS STALL

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Pitch control

This section discusses the two principal means of limiting rotor power in high operational wind speeds - stall regulation and pitch regulation.

Stall regulated machines require speed regulation and a suitable torque speed characteristic intrinsic in the aerodynamic design of the rotor.  As wind speed increases and the rotor speed is held constant, flow angles over the blade sections steepen. The blades become increasingly stalled and this limits power to acceptable levels, without any additional active control.  In stall control, an essentially constant speed is achieved through the connection of the electric generator to the grid.  In this respect, the grid behaves like a large flywheel, holding the speed of the turbine nearly constant irrespective of changes in wind speed.

Stall control is a subtle process, both aerodynamically and electrically. In summary a stall-regulated wind turbine will run at approximately constant speed in high wind without producing excessive power and yet achieve this without any change to the rotor geometry.

Wind turbine

The main alternative to such a stall regulated operation is pitch regulation.  This involves turning the wind turbine blades about their long axis (pitching the blades) to regulate the power extracted by the rotor.  In contrast to stall regulation, pitch regulation requires changes of rotor geometry by pitching the blades.  This involves an active control system, which senses blade position, measures output power and instructs appropriate changes of blade pitch. 

The objective of pitch regulation is similar to stall regulation, namely to regulate output power in high operational wind speeds.  A further option, active stall regulation, uses full span pitching blades.  However, they are pitched into stall in the opposite direction to the usual fine pitching where the aerofoil sections are rotated leading edge into wind direction.  This concept, like the conventional fine pitch solution, uses the pitch system as a primary safety system, but also exploits stall regulation characteristics to have much reduced pitch activity for power limiting.