Environmental Engineering Reference
In-Depth Information
Laboratory Simulation of Television Interference
Full-scale field measurements have verified that a wind turbine can distort the video
portion of a TV signal and have shown how this distortion depends on some of the
parameters involved. Because of environmental conditions near the wind turbine and the
lack of a full range of receiver sites, field test results are necessarily limited in scope.
However, information gained from these measurements makes it possible to simulate the
interference, and to examine distortion under the controlled conditions that only a laboratory
can provide.
Threshold of Perception and Tolerance of TV Interference
Laboratory simulations of HAWT-induced TVI have been conducted to establish the
following three levels of the modulation perception index , defined previously as m P in
Equation (9-13):
-- m P, T Threshold of perception: Interference is first perceived on a TV screen. This
is defined as the smallest modulation which produces video distortion detect-
able by an observer viewing the picture from a distance of about 1.5 m.
-- m P, L Long-term tolerance: Interference is no longer acceptable for most prolonged
viewing purposes. This modulation level usually dictates the maximum
allowable extent of the interference region around a wind turbine.
--
m P, S Short-term tolerance: Interference at this level is no longer acceptable even
for short viewing times. Disruptive distortion occurs at larger modulations,
occasionally resulting in picture breakup caused by loss of vertical hold.
Although these criteria are obviously subjective, results are found to be reproducible when
the tests are repeated at a later time with the same observers.
Laboratory Testing Procedure
Details of the test equipment and procedures that have been used for laboratory
simulations of TVI are described in [Sengupta and Senior 1978 and 1979a]. Figure 9-19
shows a block diagram of this equipment. The TV signal is received with a commercial,
roof-mounted log-periodic antenna. With the weaker UHF channels a preamplifier is
desirable, but this can be bypassed for VHF channels with strong signals. The signal is
taken through a set of variable attenuators to a coaxial T-junction where it is split into two
branches: a direct line representing the primary signal from the transmitter to the receiver,
and a multipath line representing the signal scattered by the wind turbine. The two
branched signals are then combined and fed to a receiver and a spectrum analyzer. The
latter is tuned to the channel's audio carrier frequency, and its vertical output is fed to a
strip chart recorder. The strip chart is used to monitor the effective modulation level which
can be set to a desired value with the attenuators and the DC bias at the terminals of the
receiver's tuner.
Signal scattering can be simulated in the multipath line by a time delay followed by a
repetitive pulse amplitude modulation, producing waveforms typical of those obtained
during the full-scale measurements with HAWTs. In the studies cited, laboratory
simulations of the scattered signal were all made with a pulse repetition period of 0.5 s,
which is representative of two-bladed rotors about 30 m in diameter. Although the field
tests were conducted on larger HAWTs with longer repetition periods, this change in
frequency did not affect the nature of the interference phenomenon.
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