Environmental Engineering Reference
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Figure 5: The effects of non-linearity of the power curve for turbulence intensities
I
= 0.1, 0.2, 0.3. The full line is the theoretical power curve
P
(
u
) and the
dotted line is the standard power curve given by the IEC procedure. The
data has been obtained from numerical model simulations [10].
during the measurement [10]. Figure 5 illustrates this mathematical limit. The IEC
power curve fails to characterize the wind turbine only, as the fi nal result also
depends on the wind condition during the measurement. For this reason, the IEC
procedure cannot be fully satisfactory as a power performance procedure. The
requirements for a measurement of power performance are well defi ned, but it is
necessary to introduce a new method to process the measured data
u
(
t
) and
P
(
t
).
2.4 Dynamical or Langevin power curve
The averaging procedures within the IEC standard [3] induce the problem of sys-
tematic errors because of the non-linear dependence of the power
P
on the wind
speed
u
in a wide range of
u
. Thus the standard power curve will depend not
only on the characteristics of a turbine, but also on the wind situation, and on
the conversion dynamics of a turbine. On the other hand, if no averaging is per-
formed, the power conversion is discovered to be a highly dynamical system even
on very short-time scales, as it can be seen in Fig. 6. Recently it could be shown
that the statistics of the electrical power output of a wind turbine is close to the
intermittent, non-Gaussian statistics of the wind speed [1].
2.4.1 Obtaining the Langevin power curve
To derive the power characteristic of a wind turbine from high-frequency measure-
ments without the use of temporal averaging, one can regard the power conver-
sion as a relaxation process which is driven by the turbulently fl uctuating wind
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