Electromagnetic Interference from Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 554

Idealized Scattering Models of Wind Turbine Rotors
The potential scattering of an electromagnetic signal by wind turbine blades necessarily
depends on many factors, such as the three-dimensional geometry and the material of the
blades, the blade rotation frequency, polarization of the direct signal, the directions and
elevations of the transmitter and receiver relative to the blades, the terrain, and the medium
of transmission. An exact mathematical description of the scattered signal would therefore
require the solution of a rather complicated boundary-value problem. In principle, it would
be feasible to obtain a numerical solution for a sequence of incremental positions of the
blade, but it is doubtful whether this type of solution would bring out the features that are
common to all wind turbines. Moreover, the scattering of electromagnetic waves by even
a stationary blade is a complicated problem by itself.
Instead, we will develop simplified, idealized models of HAWT and VAWT rotors and
compare model predictions of signal scattering with measured scattering. This approach
will illuminate the basic principles of EMI by wind turbines and provide us with empirical
equations useful for estimating the magnitude of potential interference in practical situations.
Flat Plate Model of a Single HAWT Blade
It has been found that the main scattering characteristics of a rotating HAWT blade can
be adequately analyzed with the help of an idealized model consisting of a rotating flat plate
[ e.g. , Sengupta et al. 1981a]. Consider the rectangular flat plate of length l and width w
shown in Figure 9-6, lying in the y-z (vertical) plane of a Cartesian coordinate system and
rotating about the x axis. The origin of the x-y-z coordinates is also the origin of a spher-
ical coordinate system ( r , q, f). The transmission medium is assumed to be free space
Figure 9-6. Flat plate model of a rotating HAWT blade and its position relative to the
transmitter and the receiver.