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Fig. 1
NETWORKING
Topology of a typical PLC network.
"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."
only RF transmitter on the power line should be the
PLC transmitter. Unlike the air interface, the copper
cabling is a closed network. How can interference get
in to the system?
PLC System Principles
In a smart grid system connected via PLC, each
user will have their own energy meter which will
communicate with a hub, usually located at the
transformer (see Figure 1). The hub will then use a
radio modem operating over a cellular telephone
network to transmit data to the utility company (via
an air interface).
The number of installed meters per hub can be in
the order of hundreds; a utility might have anywhere
between thousands and millions of metering units
installed in the field. A number of different standards
govern the implementation of PLC around the world;
most technologies use a frequency range of
10-150kHz (in Europe), or 150-400/500kHz (in Japan/
USA).
There are many potential sources of interference, but
the most common are:
• Switch-mode power supplies
• Electrical motors and their drives, such as
frequency converters
• Other types of switching and dimming devices
Many other types of device, especially if they
are faulty, broken or poorly designed, can cause
interference. The interference can be a signal at a
particular frequency, or a number of frequencies
(as is the case with switch-mode power supplies),
or wideband noise (often the case with frequency
converters), or even a combination of specific
frequencies and wideband noise.
Unfortunately, Electromagnetic Compatibility
(EMC) regulation normally only limits interfering
emissions from electronic devices at frequencies
above 150kHz. This means that the European PLC
frequency band is largely uncontrolled when it comes
to conducted emissions from connected equipment.
What Does Interference Look Like?
The frequency of an interfering signal is the most
common means of identifying its source. The ability
to see the frequency characteristics of the signals
carried on the power line is therefore essential to the
engineer’s attempt to find and deal with interference.
Thus, the first step in recognising whether
interference is degrading a receiver’s performance is
to study the characteristics of the wanted signal. By
analysing the frequency domain using a spectrum
analyser the signal’s frequency, power, harmonic
content, modulation quality, distortion and noise or
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