Power curves for wind turbines - Wind Power Generation and Wind Turbine Design - page 600

In partial load u > u cutin , where u cutin is the minimum wind speed for power pro-
duction, the wind turbine yields the maximum wind energy extraction by power
optimization operation. This is achieved by an effective power control system
which adjusts to the desired pitch angle q , at a given wind speed u , in order to
optimize the power coeffi cient c p ( u ) and hence the power production. In practice a
simple lookup table is the most used method for this operation [9]. The partial load
area of the power curve is limited to the range u cutin < u < u r where u r is the rated
wind speed.
In full load u r < u < u cutout , where u cutout is the maximum wind speed (or shut-
down wind speed) for power production, the wind turbine power output is limited
to nominal or rated power. In this power setting typically the pitch angle q is
adjusted to control the power output to its rated power value P r .
For u > u cutout the pitch angle q is maximized (minimizing the angle of attack) to
the feathered position in order to eliminate the lift forces on the rotor blades. As a
consequence power generation is switched off (stopped).
The main properties of wind turbine power curves have been introduced so
far. However, the theoretical power curve is derived from a laminar wind fl ow,
which never occurs in real situations. The complexity of the wind, i.e. the turbu-
lence needs more complex models to analyze power performance. Following the
path of turbulence research, statistical models to deal with complexity will now
be introduced.
2.3 Standard power curves
The power performance procedure for wind turbines defi ned by the IEC in 2005,
and labeled IEC 61400-12-1 is now introduced. For a detailed description of the
procedure, please refer to the complete proceeding [3]. This procedure provides a
common methodology to ensure consistency, accuracy and reproducibility in the
measurement and the analysis of power performance of wind turbines. It consists
of the minimum requirements for a power performance test, as well as a procedure
to analyze the measured data that can be applied without extensive knowledge.
The standard procedure fi rst describes the necessary preparations for the perfor-
mance test, such as criteria for the test equipments, guidance for the location and
setup of the meteorological mast that will be used to measure the wind speed and
other parameters like the wind direction, the temperature and the air pressure. The
measurement sector is also described, as the range of wind directions that are valid
for a representative measurement. Wind directions in the wake of the wind turbine
must be excluded. A more detailed assessment of the terrain at the test site is pro-
vided in the optional site calibration procedure that reports for additional obstacles
in addition to the wind turbine itself. The measurement procedure must be per-
formed for the different variables, so that the data collection displays a suffi cient
quantity and quality to estimate accurately the power performance characteristics
of the wind turbine.
The measured data is then averaged over periods of 10 min. These averaged
values are used for the analysis, together with their corresponding standard errors