Environmental Engineering Reference
In-Depth Information
Theoretical Prediction of Interference Distances and Zones
The
interference zone
around a wind turbine or group of turbines is defined as the
region where the modulation index
m
R
exceeds the long-term tolerance value
m
P,
L
when the
maximum scattered signal is directed toward the receiver [Sengupta and Ferris 1983,
Sengupta 1984]. It should be emphasized that the zone boundary is not sharply defined,
since neither the modulation by the turbine nor the tolerance of the viewer is constant.
Some Practical Aspects of the TVI Problem
Theoretical predictions of interference are usually based on “worst case” assumptions,
so it should be noted that the interference zone is merely the region where video distortion
could possibly
occur. To estimate the actual percentage of viewing time during which
unacceptable TV reception
may
occur within the interference zone, one must take into
account relevant statistical parameters such as wind speed and direction [Sengupta and
Senior 1979b]. In practice, the interference problem could be much less severe than is
suggested by the size of the basic interference zone obtained under the deterministic
assumptions used here. However, it is still true that this zone defines the region where
detailed interference calculations and measurements are justified.
Interference Zone Around a Single HAWT
With
B
E
= B
E,
max
and
F
A, T
=
1, Equations (9-15), (9-26), and (9-28) give
z
D
|
E
R
,
D
|
h
S
|
E
WT
,
D
|
F
E
2
m
R
(9-29a)
L =
=
F
A
,
W
cos(
k
f
RT
)
k
= 0.5
f or
-0.8p £ f
RT
£ 0.8p
0.8p < f
RT
< 1.2p
(9-29b)
k
= 2.0
f or
where L = normalized distance from a HAWT to its interference zone boundary
Figure 9-20 shows the size of the interference zone when
m
R
= 0.15 and the probability of
exceedance is 0.01 [Spera and Sengupta 1994]. The antenna response factor,
F
A,
W
,
has
been approximated by
F
A
,
W
» 10
F
B
/
F
[1- cos(f
S
)]/20
(9-30)
where
F
B/F
=
back-to-front ratio
of the antenna (see Figure 9-3) (dB)
Figure 9-20 clearly indicates the significant reduction of the backward-interference zone
by the antenna's discrimination against signals coming from other than the transmitter.
However, the narrow forward zone is unaffected by the antenna directivity. Since the
blade-scattered signal will be received
via
the side or back lobes of the antenna throughout
most of the interference zone, the desirability of an antenna with a high back-to-front ratio
is now evident. In fact, were a directional antenna oriented incorrectly (
e.g.
,
pointed at the
wind turbine instead of the transmitter), interference could exist well beyond the outer zone
in Figure 9-20.
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