DESIGN OF BROADBAND COUPLING CIRCUITS FOR POWERLINE COMMUNICATION
Osama Bilal, Er Liu, Yangpo Gao and. Timo O. Korhonen
Communications Laboratory,
Helsinki University of Technology
P.O. Box 3000, FIN-02015 HUT, Finland
Phone: +358-9-4514905, Fax: +358-9-4512345
E-mail: {osama, liuer, gyp}@cc.hut.fi, [email protected]}
Abstract
One of the most critical components of any
Power Line Communication (PLC) system is its
interface circuit (or coupling circuit) with the
power distribution network. This is by no
means a simple unit considering the challenging
characteristics of the PLC channel. Due to high
voltages, varying impedances, high amplitudes
and time dependent disturbances, coupling
circuits need to be carefully designed to provide
both the specific signal transmission with the
appropriate bandwidth, and the safety level
required by the applicable domestic or
international standard. This paper presents
various aspects on practical coupling circuit
design. We investigate inductive coupling,
capacitive coupling and some hybrid designs.
We present measurements of the coupling
circuits in terms of transfer function and PLC
channel measurements under practical power
line noise and impedance loading. We
demonstrate the influence of coupling circuit in
measurements and show how to compensate its
effects in order to measure the actual scattering
parameters of the power lines. We compare and
comment the various coupling circuit designs in
order to define their applicability in practical
power line communication systems.
1. Introduction
The superimposing of a PLC signal on a power
waveform implies that the coupler circuitry and
power circuitry would have to be carefully
designed
and
interfaced
for
optimal
compatibility between the two systems. Power
system and the communication system operate
at the two extremes – power system at very low
frequency and very high power, current and
voltages levels and communication systems at
much higher frequencies and very low power,
current and voltage levels. To be able to design
PLC systems as well as to supply a proper
interface between power and communication
system the coupling circuitry must be clearly
understood. Coupling of the communication
signal on the PLC channel can be achieved
using several closed current paths [1]:
-
Differential mode coupling: In this case the
‘line’ wire is used as one terminal and the
‘neutral’ wire is used as the second
terminal.
-
Common mode coupling: In this case the
‘line’ and ‘neutral wires are used together,
forming one terminal, and the ‘ground’
wire serves as the second terminal. This
coupling mode is known to yield up to 30
dB better coupling than the differential
coupling. In some countries, common
mode coupling is not allowed on the low
voltage networks, due to potential danger
for the customers.
Coupling circuit has to provide the necessary
galvanic isolation of the PLC system from the
power line, which can be achieved through
inductive or capacitive coupling. Inductive
coupling is known to be rather lossy up to
several decibels. However, it avoids physical
connection to the network, which makes it safer
and often easier to install than the capacitive
coupling. Capacitive coupling, on the other
hand, realizes the required high-pass filtering
with a straight-forward electronics that is easy
and compact to design. Practical coupling
circuits often apply a combination of both
techniques.
2. Coupling Circuit Components
To be able to design an optimum coupling
circuit, appropriate components must be chosen
and their operation must be understood:
-
Coupling capacitors: These are extensively
used in power line communications, most
commonly to couple the PLC signal to the
power line [6], but also as a part of more
sophisticated, higher-order filters [5]. The
requirements and essential characteristics
of coupling capacitors have been
standardized in ANSI C93.1-1972, [7].
Coupling
capacitors
carry
the
communication current and thus have to be
high-frequency capacitors (self -resonant
frequency has to be higher than the
modulation frequency [4]). Conversely,
they have to filter the power voltage
(dropped across the component), as well as
voltage surges and therefore need to be
high-voltage capacitors. The filtering
characteristics of the coupling capacitors
are quite dependent on the load onto which
the waveform terminates [3].
-
-
-
Coupling transformers: The main function
of the coupling transformers is to provide
galvanic
isolation
and
impedance
adaptation, but the coupling transformer
has also to pass the high-frequency
communication signal and it has to be
designed as such. The power waveform
has a much lower frequency and much
higher voltage level, and the power
waveform has a saturating influence in the
order of at least 105 compared to the
communication waveform [4]. Therefore,
the power waveform is typically first
lowpass filtered before entering the
coupling transformer.
Blocking inductors: These have to be
designed for the power frequency (to
prevent saturation) and for the power
current (to prevent voltage-drops).
Blocking inductors need to block the
modulation frequency, and therefore the
self-resonant point needs to be above that
frequency [4]. Air-core inductors are well
suited to this application.
Resistors: For power-line coupler circuits,
in general, one strives to avoid using
resistors, as a resistor, in essence, implies a
loss of power, either of the communication
signal or the power waveform.
3. Coupling Circuit Example
A typical coupling circuit employs generally
both coupling capacitors and a coupling
transformer. Fig. 1 shows a circuit diagram of a
broadband coupling circuit designed for our
PLC channel measurements. The circuit
employs high voltage capacitors to filter out the
50/60 Hz high voltage waveform, a broadband
transformer, and a combination of diodes for
over voltage protection.
Fuse
Capacitor
Diodes
Mains
Transformer
Figure-1. A broadband coupling circuit.
Figure-2.Transfer function of the coupling
circuit of Fig. 1.
Fig.2 presents the transfer function of this
coupling circuit. The coupling circuit should
not influence the actual scattering parameters of
the PLC channel during measurements. The
effect of coupling circuit needs to be
compensated in the measured data. This can be
done by post processing of the measurement
data, once the transfer function of the coupling
circuit is known. Another way is by calibrating
the measuring equipment (in our case Network
Analyser S200) in such a way that the effect of
coupling circuit is compensated. Fig. 3 and 4
present a sample of PLC-channel measurements
obtained by using this coupling circuit. Fig. 3
presents the measured frequency response of a
power cable with a length of approximately 20
m and Fig. 4 shows the frequency response of
the PLC channel under practical power line
noise and impedance loading.
Ferrite
Mains
Figure-5. Inductive coupling using ferrites.
Figure-3. Attenuation in the 20 m power cable.
The inductive coupling is often the preferred
method for coupling due to its better
performance in low impedance situations, lower
radiation from power mains and its simplicity
to use [2]. Inductive coupling employs ferrite
rings (acting as transformers) to inject the
communication signal into the mains. In this
case, there is no galvanic connection between
the power grid and the PLC equipment, which
is handy, and also safe from the practical point
of view. Selection of the ferrite depends on:
-
Figure-4. Frequency response of PLC channel.
4. Inductive Coupling
-
-
The cut-off frequencies of the ferrite. (Fig.
6 shows the frequency response of the
applied ferrite.)
The current rating of the ferrite ring. (The
current in the conductor, which passes
through the ferrite, should not exceed this
current rating.)
Choose the ferrite with a smaller diameter
that can alleviate installation work.
In the inductive coupling, PLC signal current is
injected into the power distribution lines. This
is achieved through an inductive transformer
coupler using appropriate high-frequency
ferrites. The inductive injection method is most
effective when the mains impedance is low at
the signal injection point. This is typically the
case when injecting the signal into a bus
network where several power cables are
connected together. Connecting several power
cables to a single point or bus effectively results
in a parallel connection of the individual cable
impedances. This results in low input
impedance.
Figure-6. Frequency response of the ferrite.
The coupling capacitors are used to lower the
impedance at the coupling point, resulting in
increased coupling efficiency, and also to limit
signal propagation in an unwanted direction.
The coupling capacitors act as a signal shortcut
for the injected communication signal.
Therefore, the signal current flows mainly
through the coupling capacitors. Two schemes
can be used to inject the PLC signal on the
power distribution lines as shown in Fig.7. The
first scheme uses one ferrite and the second
scheme uses two ferrites. The second method
enhances coupling efficiency. Fig. 8 presents a
measurement of a 20 m power cable with the 50
ohm termination at both ends, employing
inductive coupling with ferrites. The second
scheme, employing two ferrites (Fig. 7b), yields
an improved coupling of up to 8 dB (Fig. 8 ).
Figure-8. Cable measurements using ferrites.
5. Conclusion
Coupling circuit is an important component in
power line communication systems. To be able
to design an optimised interface between the
power and PLC system, the components of the
circuitry must be carefully chosen.
Ferrite
Mains
Capacitor
a)
In this paper, we have presented a generic
design for a broadband coupling circuit suitable
for PLC channel measurements, and included a
set of measurements of PLC channel under
practical power line noise and impedance
loading. We have also investigated inductive
coupling using ferrites, and pointed some
practical guidelines for their design practises.
Acknowledgement
This project was financially supported by
TEKES, the National Technology Agency of
Finland.
Ferrite
Mains
Capacitor
References
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Vinck, “Powerline Communications: An
Overview,” Africon 1996, Stellenbosch,
pp. 558-563.
b)
Figure-7. Inductive coupling schemes.
[2] Walter Hagmann, “Installation and Net
Conditioning Manual for Powerline
Infrastructure Units,” Ascom Powerline,
pp. 8-13, 2000.
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Ferreira,
Understanding
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V. Rensburg and H. C.
“Coupling
Circuitry:
the Functions of Different
Proc. of 7th ISPLC-2003,
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2003.
[4] Petrus A. J. V. Rensburg and H. C.
Ferreira, “Practical Aspects of Component
Selection and Circuit Layout for Modem
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March 26-28, 2003.
[5] “IEEE Guide for Power-Line Carrier
Applications,” IEEE Standard 643-1980.
[6] H. –K Podszeck, “Carrier Communication
over Power Lines,” 4th Edition, NewYork:
Spinger-Verlag, 1972.
[7] ANSI C93.1-1972, Requirements
Power Line Coupling Capacitors.
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