[Fwd: [LW] Power Line Carrier Systems de VY1JA]
andre.kesteloot at verizon.net
Sat Feb 5 16:10:04 CST 2005
-------- Original Message --------
Subject: [LW] Power Line Carrier Systems de VY1JA
Date: Sat, 05 Feb 2005 21:34:13 +0000
From: J. Allen <vy1ja at northwestel.net>
Reply-To: lowfer at lwca.org
To: lowfer at lwca.org
References: <4202CF95.2020408 at capecod.com> <4203A5EA.1B50 at pacbell.net>
The recent thread on PLC systems has made interresting reading. Before my
retirement as an electrical engineering technologist, maintenance of PLC
systems was part of my hands on work or under my supervision. Most power
system electrical engineers are unfamiliar with radio communication, so I
can fully understand the concerns of the power utilities, who fear what they
do not understand.
PLCs have the simple breaker trip schemes that have been described, but many
can and do have more complex systems with several channels. These channels
often contain SCADA information (Supervisory Control And Data Acqisition).
SCADA provides the two-way information exchange which allows a remote
control room operator to control and see the status of the power line and
all operating parameters, including the start-stop control of generators,
and massive circuit breakers and switches controling power transmission and
distribution via the power lines. PLC's also contain voice channels used
for system maintenance, and telemetry for revenue metering.
More and more, the power grids in North America have had massive outages,
which to this point have had natural or human-error causes. With increasing
threat of terrorist activities, everyone will appreciate the utilities
wanting to keep system information from public hands. As a previous
employee of a power company I will not release that kind of data to anyone,
but will give a general outline so that you can set aside fears...
Basically, most PLC systems act as an RF source connected through a parallel
wire transmission line to a receiver. The power lines are used for the
parallel wire transmission line. The RF source will generate signals which
are measured at the receiving end in terms of VOLTS not millivolts. Each
system is set up for efficient transfer, so that the transmission line
impedance is matched at each end. This means that the line will radiate and
receive as little to or from the outside as possible. The little that the
lines do leak it is horizontally polarized.
Our systems are essentially vertically polarized and most of them are 1W
erp. Cross polarization between vertical and horizontal waves generally is
accepted to be 20 to 30 dB down. If we allow that 20 or 30 dB for near
field effects possible from someone crazy enough to locate the end of their
antenna near the power line, it is very easy to see that we cannot generate
a signal of significant level to be received by, much less interfere with
the PLC systems. A 138,000 Volt line which has PLC on it passes by the
front of my property. The noise generated by that power line has made me
locate my tower nearly 100 meters from that power line so that I can receive
during times of dew, frost, rain, and poor powerline maintenance. This
means that the coupling from my antenna system will not have near-field
coupling into the power line. By my experience, I cannot imagine a
practical amateur station with enough coupling to cause any kind of problem,
or even enough to be heard in a PLC SSB voice channel.
I am strongly convinced that our operation on LF even if there is a channel
using 137 in the area, is extremely unlikely to cause any disruption of
these PLC services, especially that of tripping a breaker or shutting down a
generator, without someone endangering their life to get a higher signal
level into their systems...
In months ahead, I hope to provide LF with reproducable, scientific data,
from experiments with a 1 W eirp station.
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