Geoscience Reference
In-Depth Information
turbine towers may be reduced, but blockage caused by rotor blades will persist since there
is no synchronisation of their movements.
• Use of a gap-filling radar. A way to remove or reduce blockage is to place an additional
radar to cover areas affected by blockage.
• Adapting the radar scan strategy so that the radar beam passes over areas with wind
turbines. This method ensures that measurements are not affected by blockage but in
return data will be gathered from higher altitudes.
3.3 Wind measurements
A Doppler radar measures frequency shifts of the received signals and translates the shifted
frequencies to radial velocities. A conventional clutter filter removes echoes with low or zero
frequency shifts and thereby prevents static clutter from entering the radar products.
Signals scattered from rotating blades of a wind turbine are shifted in frequency and thereby
interpreted by the radar as moving objects, escaping the clutter filter. The tip of a rotor blade
can move with a velocity up to 100 m s
−
1
whereas close to the hub the blade velocity is close to
zero. The scattered signals will therefore display a broad distribution in frequency space. The
wind velocity is normally estimated as the strongest (non-zero) frequency component. Since
echoes from wind turbines often are stronger than weather echoes this can lead the weather
radar to display erroneous wind measurements.
3.3.1 Observations
There are many observations of wind turbines causing erroneous wind measurements in the
literature (see, e.g., Burgess et al. (2008); Cheong et al. (2011); Crum et al. (2008); Haase et al.
(2010); Isom et al. (2009); Toth et al. (2011); Vogt et al. (2007a)). One such example from the
weather radar in Dodge City, Kansas, is shown in Fig. 10. From this figure it is clear that
at the time of the measurements the overall wind direction was to the northwest but signals
from radar cells containing a large wind farm, approximately 40 km to the southwest, show
up as having close to zero velocity. In Fig. 11 is shown the spectrum width of the velocity
measurements and from this figure it is clear that there is a significant broadening of the
frequency spectra over the wind farm.
These observations can be understood by examining the raw I/Q data from the radar.
Spectrograms of I/Q data, containing echoes from wind turbines, show highly complex
and richly structured patterns. Examples of such spectrograms are given by, e.g.,
Bachmann et al. (2010a); Gallardo et al. (2008); Gallardo-Hernando & Pérez-Martínez (2009);
Gallardo-Hernando et al. (2009); Hood et al. (2009); Isom et al. (2009); Nai et al. (2011); Poupart
(2003); Vogt et al. (2007a;b). From these and other studies it is clear that echoes from wind
turbine rotor blades in different positions result in broad distributions in frequency space
even though the average velocity estimate is often close to zero.
As for wind turbine clutter there are observations showing tails of erroneous wind
measurements behind the wind turbines (Burgess et al., 2008; Seltmann & Lang, 2009; Vogt
et al., 2007a; 2009).
