Power curves for wind turbines - Wind Power Generation and Wind Turbine Design - page 601

(based on the standard deviations). A normalization must then be applied to the
measurement data. Depending on the type of turbine, either the means of wind speed
(for turbines with active power control) or of power output (for stall-regulated
turbines) must be normalized to a reference air density.
The IEC power curve is then derived from the normalized values using the
so-called method of bins, i.e. the data is split into wind speed intervals of a width
of 0.5 m/s each. For each interval i , bin averages of wind speed u i and power output
P i are calculated according to

N N

1 1

i i

∑ ∑

u = u and P = P

( 4)

i norm, , i j i norm, , i j

N N

i i = 1 i i = 1

where u norm, i , j and P norm, i , j are the normalized values of wind speed and power aver-
aged over 10 min, and N i is the number of 10 min data sets in the i th bin.
For the power curve to be complete, or reliable, each bin must include at least
30 min of sampled data and the entire measurement must cover a minimum period
of 180 h of data sampling. The range of wind speeds shall extend from 1 m/s below
cutin wind speed to 1.5 times the wind speed at 85% of the rated power P r of the
wind turbine. Such power curve was represented in Fig. 4 for a multi-MW wind
turbine. Error bars were included following the recommendations below.
The standard also provides a description of the evaluation of uncertainty in the
power performance measurement [3]. In a fi rst step, the respective uncertainties
are obtained from the measurement as the standard error of the normalized power
data. Additional uncertainties are related to the instruments, the data acquisition
system and the surrounding terrain.

Figure 4: Power curve (line) obtained from the IEC standard procedure for a
multi-MW wind turbine. Corresponding error bars were displayed. The
grey dots represent 10-min averages. The power output is normalized
by its rated value P r .