Geoscience Reference
In-Depth Information
3.3.2 Models
A few methods have been proposed to model the frequency spectra generated by wind
turbines (Gallardo-Hernando et al., 2010; Hood et al., 2009; 2010; Kong et al., 2011; Zhang
et al., 2011). In these models clutter from the tower, hub, and rotor blades are included. The
model results show the zero-velocity returns of the tower, near-zero returns of the hub, as well
as spectral broadening of the blades.
3.3.3 Mitigation concepts
Most concepts proposed to prevent or reduce clutter (see Section 3.1.3) are also valid for
erroneous wind measurements.
Adaptive filters, suggested for removing wind turbine clutter, can also help mitigate
erroneous wind measurements. If clutter is removed from the signal, the average wind
velocity as well as the spectrum width can easily be estimated. Suggestions for adaptive
wind filters can be found in, e.g., Bachmann et al. (2010a;b); Gallardo et al. (2008); Isom et al.
(2009); Nai et al. (2011). As for the adaptive clutter filters, all these methods use raw I/Q data
as input.
4. Conclusions
In many countries the number of wind turbines is growing rapidly as a response to
the increased demand for renewable energy. As wind turbines grow larger and more
numerous potential conflicts with other interests are emerging. Doppler radars, for example,
are easily disturbed by wind turbines. In this chapter we have presented an overview
on wind turbine-related problems experienced by Doppler weather radars. Three main
wind turbine-related problems have been identified: clutter, blockage, and erroneous wind
measurements.
Clutter — unwanted radar echoes — are generated by all obstacles in line of sight of a
radar. Static clutter, i.e., echoes with no or low radial velocities are easily removed by the
Doppler radar's built-in clutter filter. However, the moving blades of a wind turbine generate
dynamic clutter which displays a wide range of radial velocities that cannot be removed by
a conventional clutter filter. In this chapter it has been shown that wind turbine clutter can
be problematic for weather radars since such echoes are interpreted as precipitation. Wind
turbine clutter can display a large variation in strength, ranging from barely visible to near
saturation levels of the radar. Behind wind turbines a tail of clutter can often be seen. This
phenomenon is believed to be the result of multiple scattering effects. In an example shown
in this chapter, clutter tails were seen to decrease exponentially behind the wind turbines.
Cross-range clutter has also been observed. This can be caused by differences in azimuthal
resolution of the actual radar measurements and azimuthal spacing of the radar data matrix
but it has also been suggested that it may occur as a result of the radar sidelobes.
The magnitude of wind turbine clutter is often estimated by calculating the radar cross section
of wind turbines. Such models may lead to results inconsistent with observations. Effects from
the ground and terrain should be taken into account, otherwise a calibration of the model may
be necessary.
