36
NETWORKING
When designing a PLC network, it's important to create a good signal path,
that suffers from low levels of interference.
Spectrum Master from Anritsu. As well as offering
the capability to measure a frequency spectrum,
this kind of instrument also features a range of
selectable analysis functions, and provides for
data manipulation and storage. When choosing a
spectrum analyser, the key parameters are frequency
range, sensitivity, dynamic range, frequency
resolution and accuracy.
For PLC measurements, the main requirement is
that the lower frequency limit should be no higher
than 10kHz. Sensitivity is an important parameter,
because power lines attenuate PLC signals quite
markedly; high sensitivity enables the instrument
to pick up signals and interference even when they
are far from the measurement point. Resolution
bandwidth (RBW), the scanning ‘window’ which
sweeps across a frequency band, determines the
instrument’s ability to distinguish signals that are at
almost the same frequency. Selectivity, on the other
hand, is the ability to distinguish a weaker signal from
a much stronger signal.
To couple the spectrum analyser to the power
line, the tester must use a probe. Near-field probes
are quick and easy to use, and can isolate the cable
feeding the interfering instrument from other cables,
but offer relatively low sensitivity. Clamp-type current
probes and contacted voltage probes may also be
used in some circumstances.
Interference Hunting in PLC
In normal over-the-air RF communications, locating
interference is often time-consuming, because the
interferer may be intermittent and could be located
anywhere in a 360° radius from the receiver. In PLC
communication the situation is simpler, because there
is always a wired connection between the receiver
and the interferer.
The typical PLC grid topology consists of a hub at
the transformer station, from where power lines are
distributed to multiple distribution boxes. Here, they
are split again and routed to individual buildings, and
possibly again within a building. The most effective
way to find the interferer is to measure the spectrum
from the current – not the voltage – of the circuits.
(The voltage at a given point will be essentially the
same in all of the circuit but the currents will vary.)
The current draw of the interferer will in most cases
contain the interference spectrum and can thus be
pinpointed.
The engineer should start at the transformer and
measure each line (phase) with a current probe. After
finding the line on which the interfering signal has
the highest amplitude, follow it to the next junction.
Here the line will be split: repeat the above procedure,
and keep repeating until the building from which the
interference comes is identified.
At this point, the task becomes more challenging,
but still the same methodology applies: by holding
the probe close to the power supply to the various
devices in the building, the spectrum analyser can
show where the amplitude of the interference is
highest.
This process of triage will narrow down the possible
source to a small number of devices. Now each can
be shut down one by one to view the effect on the
interference signals, and counter-measures (such as
filtering) taken to eliminate or mitigate the effect of
the interference.
Conclusion
PLC communication is a robust and easily deployable
system for smart metering. However, like all
RF communication systems it is susceptible to
interference from devices connected to the power
grid. It is important for the utility company to be
able to diagnose the sources of interference in their
smart metering network quickly and efficiently in
order to avoid the cost and inconvenience of lost or
degraded communication. Anritsu’s line-up of handheld spectrum analysers, when used with the correct
type of probe, provide a simple and economical tool
that makes it possible for engineers to pinpoint the
sources of interference in their network.
www.networkseuropemagazine.com