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Medium Voltage Overhead Power-line Broadband Communications; Transmission

Capacity and Electromagnetic Interference

P. Amirshahi and M. Kavehrad (FIEEE)

The Pennsylvania State University,

Department of Electrical Engineering,

Center for Information & Communications Technology Research (CICTR)

University Park, PA 16802

E-mail: mkavehrad@psu.edu

Abstract - A channel model suitable for multi-wire

overhead medium-voltage lines is proposed. This model is

then employed in order to evaluate the multipath channel

impulse response and the associated transmission capacity

limit in actual overhead medium-voltage power distribution

networks for broadband power-line communications

applications. Electromagnetic interference limitations of

such systems are discussed, as well.

Keywords: Channel model, impulse response, power-line

communications, medium voltage, capacity, EMC.

I. Introduction

Broadband power-line (BPL) communications systems

have the potential of providing higher data rates compared

to old power-line communications (PLC) systems; due to

progress in PLC modem technology, mainly owed to

advances in signal processing and communications theory.

Last-mile access using Medium Voltage (MV) overhead

power lines is being considered in US, Korea and many

other countries. Although for nearly a century some

elementary transmission models of these lines have been

available, no serious attempt has gone into a comprehensive

BPL channel modeling describing characteristics of

overhead MV lines in terms of magnitude and phase

responses or as an impulse response. Recently, new

modeling of multi-conductor wave propagation in overhead

lines, considering transient effects, has been made available

[1]. In this paper, we present a new transmission channel

model suitable for multi-wire overhead MV power

networks. The proposed model incorporates realistic

ground admittance, appropriate for higher frequencies used

by broadband power-line communications. The suggested

model is more appropriate for higher frequencies than

predicted by the model in [2]. The proposed model is

further used to evaluate the channel impulse response and

transmission capacity in an actual power distribution

network. Interference issue is discussed and remedies are

suggested.

II. Analysis of MTL

Analysis of multi-conductor transmission lines (MTL)

consisting of multiple parallel conductors is a well-

understood topic [3]. For example, in a case involving 3

conductors and a ground return, we can define 3 modes as

shown in Fig.-1 [4]. Using these independent modes, we

can decompose currents I1 through I3 as a linear

combination of 3 modal currents. Common mode (also

called ground mode) is characterized by the highest

attenuation among the modes, and is propagation through 3

phases and a return via the earth. Involving signal

propagation and return only through wires, differential

modes (also called aerial modes) 1 and 2 show a somewhat

lower attenuation than the common mode. While the

common mode current Ic is the same in magnitude and in

direction for 3 lines, the differential mode currents ID1 and

ID2 are the same in magnitude but differ in direction for 3

lines. Common mode currents are much smaller in

magnitude than differential mode currents, but yet

significant. Generally, these modes are not orthogonal

unless the wavelength of electromagnetic wave inside the

conductors is a small fraction of the height of wires and the

spacing between the wires is a small fraction of wavelength

[5]. This condition is satisfied for practical MV power-line

systems up to 100 MHz. Beyond this frequency, the

discrete modes lose orthogonality and continuous modes

start to appear. While the radiated E-field from the

differential mode currents subtract, those from common

mode currents tend to add [3]. This is an important issue in

terms of Electromagnetic Compatibility (EMC) of BPL

systems and potential interference into existing local

communications systems in the shared bands. In PLC,

depending on the way signal is coupled to the lines, either

wire-to-wire (WTW) or wire-to-ground (WTG) injection is

feasible. For WTW injections, differential modes are

mostly excited. For a WTG injection, in the case of

coupling to the middle phase, common mode and

differential mode 2 are excited. Any transmission line is

characterized by its propagation constant. Frequency

response of each transmission line at a distance l from the

source is expressed as:

)()()(

0

VfHlV

=

(1)

flf

eefH

)(

==

lfjl

e

)()()(

β−α−γ−

(2)

in which v(0) is the voltage at the source and γ is the

propagation constant, α, the real part of the propagation

constant, is called attenuation constant and β, the imaginary

part of the propagation constant, is called phase constant.

The first step in finding the MTL propagation constant is to

obtain per-unit-length parameters for the conductors. For

this, Carson [2] suggested incorporating ground impedance.

However, this model, without considering the ground

admittance, is only suitable over low frequency values

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and/or under good conductive ground plane conditions.

As an effort to find a new ground return path model for the

higher frequencies and/or under poor ground conductivity

conditions, a new procedure was suggested. This

methodology by D’Amore et al [1] incorporates per-unit-

length series-impedance and shunt-admittance matrices,

using the curl-Maxwell field equations.

Fig.-1 Modes of three-phase power lines

Following the steps in [1], the real and imaginary parts of

the propagation constant for each mode in a system are

depicted in Fig.-2. This system is composed of three wires

at 1 meter spacing between each pair; so three discrete

modes are defined for the configuration. The diameter of

each wire is 1 cm and they are placed at 10 meters above

the ground level. Earth is characterized by a relative

permittivity of

g

ε =13 and a conductivity of

The phase constants of the 3 modes agree over almost the

entire frequency range. On the contrary, attenuation

constants exhibit different behavior and values. Common

mode shows higher attenuation over the frequency range

and the attenuation factors for the two aerial modes are

close to one another. Common mode attenuation factor

increases up to some frequency and decays beyond it. This

incident is due to a resonance phenomenon in the ground

medium, initially inductive and by increasing frequency

later it becomes capacitive. A more detailed explanation of

this topic is available in [5].

III. Power-line Network Channel Model

Channel transfer function of a matched transmission power-

line follows (2). In the case of unmatched junctions, part of

a propagating signal reflects back to the transmitter at

branch junctions due to impedance mismatch and the

remainder travels through [6]. The propagation along a wire

follows (2), so one can easily express the multipath network

channel model as:

∑

=

i

1

where N is the number of significant arrived paths at the

receiver, di is the length of ith path and gi is the weighting

factor of the ith path. This formulation is basically similar to

what has been mentioned in [7], however, with a model for

propagation constant that is appropriate for overhead MV

power-lines, rather than underground cables in Europe.

Fig.-3a represents frequency response of a matched

transmission channel over a 1 Km span MTL system. As

the system is matched, signal does not get reflected at the

g

σ =5 mS/m.

−−

=

N

dfjdf

i

iieegfH

)()(

)(

βα

(3)

receiver-end and signal path is one straight point-to-point

path. In this case, the only loss comes from MTL path loss.

Fig.-3a depicts frequency response for two coupling

methods: common mode and differential mode-1. Common

mode exhibits more loss than differential mode, especially

at low frequencies. As frequency increases, losses of the

two configurations become comparable. Also, one may

notice that both systems show a very low loss at high

frequencies over a 1 Km repeater span.

(a)

(b)

Fig.-2 Frequency response of (a) Attenuation constants, and

(b) Phase constants of an MTL system

The fact that MV overhead power lines resemble a low loss

transmission system shows promise for data delivery at

high rates. Also, this is a cause for concern, regarding

potential interference into existing services, as elaborated

on extensively in NTIA report volumes [8]. Fig.-3b

illustrates the water filling [9] channel capacity of BPL as it

has been discussed in [10] for matched transmission system

with a 1 Km repeater span at different transmitted power

levels. According to Fig.-3 (a), with an ideal matched MTL

system, over 50 MHz of channel band, we can deliver

almost 600 Mbps by launching 10dBm transmit power. In

reality, this low loss nature of MTL systems changes

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extensively by several factors.

(a)

(b)

Fig.-3 (a) Frequency response of matched MTL transmission

over 1 Km for differential and common modes coupling (b)

Corresponding capacity values for different coupling methods

and transmit power levels as a function of frequency band

Over an actual power line network, there always exist

several branches and junctions between a transmitter and a

receiver. These branches cause nulls in the transmission

channel frequency response due to multipath. To

investigate this, we simulate the complex network shown in

[10]. In this network, we have three branches between a

transmitter and a receiver, which are by 1Km apart. Each

end of these branches is an open-circuit, so reflection factor

at each end is unity. Also, we have assumed that the

transmitter and receiver impedance are matched to that of

the line. Channel impulse response of this system is shown

in Fig.-4 (a). Our simulation results show 12 paths are

dominant and from Fig.-4 (a), 12 pulses with different

arrival times are distinguished. Fig.-4. (b) is an illustration

of the channel capacity values for this complex network.

The average capacity in this network with a 10 dBm

launched transmit power level for a 50 MHz band is about

300 Mbps. Obviously, the junctions and branches between

transmitter and receiver reduce the value of system

capacity, extensively, compared to the ideal point-to-point

case.

(a) (b)

Fig.-4 (a) Channel impulse response of a complex network and

(b) Associated capacity values

IV. Electromagnetic Interference

Another very important factor that should be considered for

evaluating BPL system performance is interference to other

wireless systems. As stated earlier, the NTIA reports [8] on

potential for interference in using BPL are quite extensive.

In displaying channel capacity values, we used the entire

frequency range between 1 to 100 MHz. In reality, FCC has

disallowed use of some frequency bands that are already

occupied by other services, especially the critical ones as,

homeland security, emergency, etc [8]. This should

discourage BPL system designers in considering these

frequency bands. Meaning, higher capacity values at 100

MHz on our figures are by the FCC rules, unattainable.

From the very beginning of BPL experiments, the Radio

Amateur Associations have expressed concern that this new

type of emission will interfere with radio communications

[12]. Recently, NTIA, in their extensive reports [8], made

recommendations to the FCC, so to devise regulatory

methods for radiation measurements, deployment and

simulation of BPL systems. According to this report, there

are some frequency intervals that are dedicated to

emergency services and all BPL systems have to avoid

occupying these frequency intervals. It is also noted in this

report that for evaluating radiation patterns from power-

lines, both far- and near-end fields should be considered

and neither can violate the FCC regulatory limits.

Some example techniques that if applied, could potentially

mitigate the interference are differential-mode signal

injection, power control, filters and signal terminations, and

avoidance of locally used frequencies.

In general, emission from a single line is highly dependant

on the impedance between the line and ground. In theory,

emissions from differential aerial mode currents cancel one

another at far field, given line symmetry (balanced loads).

MV power-lines are not

discontinuities might occur at different locations on

different wires, causing asymmetry. Asymmetry causes

broadening of near-field and far-field patterns.

typically symmetrical;

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For simulation purposes, we used GNEC [13] to evaluate

the far field radiation patterns along the wire for the power-

line system configuration shown in Fig.-5.

1000 m

Fig.-5 Power-line configuration for radiation pattern

simulations

In our simulations, we examined both common and

differential mode injections. In common mode, signal can

be injected either on the middle conductor or on a side

conductor. The far field radiation patterns for differential

and two common modes are depicted in Fig.-6. Horizontal

axis unit is µv/m. The far field radiation pattern has higher

values in both common mode injections than in differential

mode. In differential mode injection, reverse and forward

paths are two current flows with the same amplitude in

opposite directions, as shown in Fig.-1. These currents

travel in two parallel identical conductors with relatively

small separation distance. Due to their opposite direction

and small distance, their radiated electric fields tend to

cancel out one another at the far field region. On the other

hand, in common mode injection, forward path is a current

flow in one phase conductor and the return path is the

ground surface, which is a dielectric with losses. The

distance between one phase and earth is at least ten times

the separation of two phases. Also, in common mode

injection, forward current is traveling in a near-perfectly

conducting material, whereas return current has to go

through a material with loss. Because of these facts, it is

expected that the cancellation of electric fields, emitted

from forward and return paths, at far field, in common

mode injection, is much less than those in differential mode

injection. We should keep in mind that the cancellation

level is reduced in differential mode if the load impedances

between each wire and earth are not equal. As it is seen

from Fig.-6a and b, the radiation pattern from central

common mode injection is symmetric but this is not true for

the radiation pattern of the side injection. It is due to the

fact that the environment around the injected wire in central

injection is symmetrical. This is not the case for side

injection. Fig.-7 shows the far field radiation patterns from

power-line configuration shown in Fig.-5, for differential

modes and three different load mismatches. From these

figures and Fig.-6c, one can conclude that load mismatch

between two lines degrades the far field cancellation in

differential mode injection. Therefore, the radiation pattern

from the differentially injected power-line with load

mismatch has higher amplitude than the radiation pattern of

differentially injected power-line with balanced loads. The

more the mismatch increases, the more radiation is

generated by the power-line system.

V. Conclusions

This research dealt with examining MV overhead power

lines as a communications medium for broadband

transmissions. Available models for overhead power lines

were not suitable at high frequencies with a lossy ground

return. D’Amore et al in [1] have proposed a model for

multi-wires over ground, which is more suitable for

application of BPL systems using overhead MV lines.

Based on this model, we developed a new channel transfer

characteristic function model. Our simulations show ideal

overhead power-lines exhibit a low loss with a capacity

value of about 1Gbps over a 1 Km repeater span, if 10 dBm

transmit power and 100 MHz of channel bandwidth are

available. Junctions and branches in a power-line network

cause signals to reflect and produce a multipath channel.

This causes a reduction in power line system capacity.

Impairments of overhead MV lines are similar to those of

mobile radio systems over metropolitan areas; can suffer

with multi-path fading and are affected by unpredictable

man-made or natural noise. Only measurements can

demonstrate the reality of unguided modes at high

frequencies and over non-flat grounds. Discontinuity

(impedance mismatch) increases

Therefore, discontinuity must be eliminated by matching

the load and the line impedance and by having symmetrical

loads on these lines. Removing discontinuities by adaptive

impedance matching [14] can enhance line data handling

capacity. Use of differential aerial modes along with

symmetric loads on these lines can potentially reduce

interference.

On the economy side, the power grid is a valuable asset, as

it has already been extended to isolated rural areas. The

power poles are omnipresent, thus the poles themselves can

be used to hang fiber over and then bring the fiber close

residential areas. Via the power-lines or by using radio

technology, broadband signals can be delivered to homes,

including those in isolated rural areas that high-speed

service providers do not see a significant return on

investment to offer service to. In the age of gaping

“broadband gap” in the US and other parts of the world, the

power company “poles” can be viewed as a “National

Treasure” in bridging the digital divide between the Have’s

and the Have not’s of broadband services.

Acknowledgements

We would like to thank Dr. Raj Mittra of Electrical

Engineering Dept. at The Pennsylvania State University for

his suggestions regarding EMC issues and Dr. P.S. Henry

of AT&T Research Labs for his insight on BPL

transmissions. We are also grateful for the support of

AT&T Labs in equipment and funding of this project.

EM interference.

σg=0.005, εg=13

500 m

1 Meter

1 Meter

Zl

Zl

Zl

Zl

Zl

Zl

V

500 m

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(a)

(b)

(c)

Fig.-6 Far field radiation from power-line configuration in Fig.-

5, for (a) central common mode, (b) side common mode and (c)

differential mode injections.

References

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(a)

(b)

(c)

Fig.-7 Radiation pattern from an unbalanced power-line with

(a) 10 mH in series difference, (b) 1 mF in parallel difference

and (c) 1 µF in parallel difference.

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[13] http://www.nittany-scientific.com/gnec/T.

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