Wind Power in Power Systems

Wind Power in Power Systems 35
Power output (kW)
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Wind speed (m s –1 )
Figure 3.5 Typical power curve of a 1500 kW pitch regulated wind turbine with a cut-out speed
of 25 m s 1 (the broken line shows the hysteresis effect)
At wind speeds higher than the rated wind speed, the maximum power production will
be limited, or, in other words, some parts of the available energy in the wind will be
‘spilled’. The power output regulation can be achieved with pitch-control (i.e. by feathering
the blades in order to control the power) or with stall control (i.e. the aerodynamic
design of the rotor blade will regulate the power of the wind turbine). Hence, a wind
turbine produces maximum power within a certain wind interval that has its upper limit
at the cut-out wind speed. The cut-out wind speed is the wind speed where the wind
turbine stops production and turns out of the main wind direction. Typically, the cutout
wind speed is in the range of 20 to 25 m s 1 .
The power curve depends on the air pressure (i.e. the power curve varies depending on
the height above sea level as well as on changes in the aerodynamic shape of the rotor
blades, which can be caused by dirt or ice). The power curve of fixed-speed, stall-regulated
wind turbines can also be influenced by the power system frequency (see also Chapter 15).
Finally, the power curve of a wind farm is not automatically made up of the scaled-up
power curve of the turbines of this wind farm, owing to the shadowing or wake effect
between the turbines. For instance, if wind turbines in the first row of turbines that
directly face the main wind direction experience wind speeds of 15 m s 1 , the last row
may ‘get’ only 10 m s 1 . Hence, the wind turbines in the first row will operate at rated
capacity, 1500 kW for the turbine in Figure 3.5, whereas the last row will operate at less
than rated capacity (e.g. 1100 kW for the same turbines).
3.6.3.2 Hysteresis and cut-out effect
If the wind speed exceeds the cut-out wind speed (i.e. 25 m s 1 for the wind turbine in
Figure 3.5) the turbine shuts down and stops producing energy. This may happen during
a storm, for instance. When the wind drops below cut-out wind speed, the turbines will
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