Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 59

The power curve relates the steady-state output power developed by the wind
turbine to the free wind speed and is generally measured using 10 minute average
data. Below the cut-in wind speed, of about 5 m/s, the wind turbine remains shut
down as the power in the wind is too low for useful energy production. Then, once
operating, the power output increases following a broadly cubic relationship with
wind speed (although modified by the variations in C P ) until rated wind speed is
reached. Above rated wind speed the aerodynamic rotor is arranged to limit the
mechanical power extracted from the wind and so reduce the mechanical loads on
the drive train. Then, in very high wind speeds, the turbine is shut down.
The choice of cut-in, rated and shut-down wind speed is made by the wind
turbine designer who, for typical wind conditions, will try to balance maximum
energy extraction with controlling the mechanical loads (and hence the capital cost)
of the turbine. For a mean annual site wind speed V m of 8 m/s typical values will
be approximately: cut-in wind speed 5 m/s, 0.6 V m ; rated wind speed 12-14 m/s,
1.5-1.75 V m ; shut-down wind speed 25 m/s, 3 V m .
The energy extracted over a year by the wind turbine is, of course, determined by
the power curve and the site wind resource. Figure 3.4 shows binned wind speed data
and a power curve. The wind speed bins are of 1 m/s, e.g. 5.5-6.5 m/s, 6.5-7.5 m/s.
By multiplying the hours per year for each wind speed bin by the binned power
curve of the turbine, the annual energy yield may be calculated. It is necessary to
deduct losses, including electrical losses within the wind farm and aerodynamic
array losses.
The calculation of energy from the binned power curve and wind speed data is
simply:

Energy ¼ X

i ¼ n

H ð i Þ P ð i Þ ð 3 : 6 Þ

i ¼ 1

where H ( i ) ¼ hours in wind speed bin i ; P ( i ) ¼ power at wind speed bin i .

3.4.2 Power regulation
The wind turbine power curve (Figure 3.3) shows that, between cut-in and rated wind
speeds, the turbine operates to extract the maximum power from the wind passing
across the rotor disk. However, at above rated wind speed the mechanical power on the
rotor shaft is deliberately limited in order to reduce loads on the turbine.
Fixed-speed wind turbines, i.e. those using induction (or asynchronous) gen-
erators directly connected to the network, may use one of a number of techniques to
limit power at above rated wind speed:

Pitch regulation: The blades are physically rotated about their longitudinal axis. ●

Stall regulation: The angle of the blades is fixed but the aerodynamic perfor-
mance is designed so that they stall at high wind speeds.

●

Assisted stall regulation: A development of stall regulation where the blades
are rotated slowly about their longitudinal axis but the main control mechan-
ism is stall.

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