Experimental Verification of Common

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Experimental Verification of Common-Mode Current Generation in Home Electrical
Wiring in the Poweline Communications Band
A. Rubinstein1 , A. Vukicevic2, M.Rubinstein1, F. Rachidi2
1
2
University of Applied Sciences of Western Switzerland, Yverdon, Switzerland.
[email protected], [email protected]
Swiss Federal Institute of Technology (EPFL), EMC Group, Lausanne, Switzerland.
[email protected], [email protected]
Abstract
This paper presents a test of the relation between the common mode current in the cabling inside a home and the
common and differential mode signals injected into any other part of the cabling in the house. The results are in
satisfactory agreement with the hypothesis that the common mode current at any point inside the house can be
calculated by computing the linear combination of the effects of the two input modes.
1.Introduction
Powerline communications (PLC) or Broadband Powerline (BPL) is a technology for data transmission in the
last mile and in-home. Since it uses existing electrical cabling as a transmission medium, and since such cabling is
present in most households, PLC exhibits great a priori potential and in may become a competitor to other technologies
such as DSL or wireless. In addition, whereas the number of telephone outlets per household tends to be low in many
countries (one or two per home), there is usually at least one power outlet in every room. On the other hand, EMC
issues, achievable throughputs and proper marketing are likely to play a decisive role in the commercial success the
technology.
Emissions from PLC transmissions inside the home are due essentially to common mode currents in the
intrinsically asymmetrical electrical cabling. The common mode appears to be related to asymmetries in the home
wiring, such as unequal distances of conductors to the ground, open switches, etc [1].
In this paper, we test a simple formula that assumes that the common mode any place in the home can be
calculated by adding the contributions to it due to the differential mode and the common mode injected elsewhere in the
house. This simple approach can be extended to calculate the common mode by superposition of all of the differential
and common mode sources around the house, although the required transfer functions, which we obtained
experimentally in the present paper, would need to be obtainable through theoretical modeling for the technique to
become widely applicable in practice.
2. Experimental Set-Up
We installed cabling and fixtures as needed on a wooden structure in the shape of a small house having
dimensions comparable to those of a room. The structure is shown in Figure 1. The installed circuit, shown in Figure 2,
was composed of typical electrical components, such as outlets and light sockets similar to those found in domestic and
small office installations.
The input signal was produced by a signal generator at frequencies within the PLC band. Three different baluns
were used in order to inject symmetrical and asymmetrical signals (Figure 3).
The common mode current was measured at a point along the line, using a high frequency current clamp. The
differential mode input voltage was directly measured using a oscilloscope channel in high-impedance input mode. The
common mode current was also recorded at the input with a current clamp when using asymmetrical injection.
Figure 1 - Wooden Structure
Figure 2 – Wiring used in the wooden model
Figure 3 - Injection and Measurement System
3. Theory
B
at an arbitrary point B in the low voltage
ICM
A
and common mode current ICM at an injection
We analyze here the relation between the common mode current
A
wiring inside a house and the injected differential mode voltage VDM
point A in the the same house wiring. We postulate that the relation between these three signals is given by
!
B
AB A
AB
A
ICM
= kCM
ICM + k DM
VDM
where
!
!
!
AB
AB
and k DM are frequency-dependent, complex transfer functions.
kCM
!
According to this relationship, the common mode at point B stems a) from direct propagation of an injected common
mode!signal and b) from the conversion of a part of the injected differential mode into the common mode.
AB
AB
A
To find the complex constants kCM and k DM , one can set the inputs to zero one at a time. Setting ICM to zero, we
obtain
AB
kDM
=
B
ICM
A
VDM
Similarly, setting
!
(1)
AB
kCM
=
!
!
(2)
!
A
equal to zero, we have
VDM
B
ICM
A
ICM
(3)
!
With the values of the two constants evaluated, we can rewrite Equation (1) in the time domain as
!
!
B
AB
A
AB
A
ICM
(t) = kCM
ICM
sin("t + #CM ) + k DM
VDM
sin("t + #DM )
(4)
4. Measurement results
AB
AB
To verify Equations (1) and (4), we first estimated the values of the constants kCM and k DM using different types of
baluns that allowed us to apply equations (2) and (3).
A
Using a symmetrical balun at the input, we forced ICM to be essentially zero and we measured the output common
B
A
and the input differential mode voltage VDM . The results for a frequency of 20 MHz are plotted in
ICM
!
!
Figure 4 and Figure 5. By taking the ratio of the peak amplitudes and measuring the phase shift between the two
AB
signals, we estimated the value of the k DM constant to be
!
! = 2.6 "10#4 e# j 0.33
!
(5)
k AB
mode current
DM
!
!
Figure 4. Output CM current for the case of a symmetrical
input signal
Figure 5. Input DM voltage that produced the CM current
presented in Figure 4
A
and we measured
VDM
B
A
the output common mode current ICM and the input common mode current ICM . The results are plotted in Figure 6
AB
and Figure 7. We then used Equation (3) to estimate the value of kCM to be
AB
(6)
kDM
= 7.5 "10#1 e# j
!
Similarly, by using an asymmetrical balun, we injected a common mode current with negligible
!
!
!
!
Figure 6. Output CM current for an asymmetrical input
current
Figure 7. Input CM current that produced the output CM
current presented in Figure 6
With the values of the two constants, we can use Equation (4) to plot the output common mode current in the time
domain and to compare it with the corresponding measured common mode current at point B. The results are plotted in
Figure 8, from which we can observe a good agreement.
Figure 8. Comparison of the measured output CM current and the theoretically estimated CM current
5. Discussion and Conclusions
The estimation of the common mode current in in-house electrical wiring is important since it is this mode that
produces most of the radiated emissions. We have shown in this paper that the common mode can be estimated as a
linear combination of the effect of a) the injected differential mode, which converts partially to the common mode, and
b) the common mode that is applied directly at the injection point and propagates through the wiring with relatively low
attenuation.
Although we worked with common and differential modes injected at the same input point, the results of this
paper should in principle be applicable to the case of multiple common mode sources and multiple differential mode
sources spread across the low voltage network. Measurements are required to confirm this hypothesis.
The overall agreement between measured and calculated common mode currents was good but differences of the
order of 20% or so were observed in some cases between the estimated and the measured common mode currents. It is
important to make further measurements to determine whether this discrepancy was caused by measurement error or if
the simple model presented here needs to be improved.
6. Acknowledgments
This work has been financially supported by the OPERA project (http://www.ist-opera.org)
7. References
[1] K. Dostert, Powerline Communications, Prentice-Hall, 2001.
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