Technical Overview of Broadband Powerline (BPL)

Communication Systems

Robert G. OlsenSchool of Electrical Engineering and Computer Science

Washington State UniversityPullman, WA, USAbgolsen@wsu.edu

Presented atUniversity of Idaho

October 4, 2006

Photo of a Typical BPL Installation (courtesy B. Cramer EPRI)

Injector(2 – 30 MHz)

Do attenuation, background noise and restricted input power due to emission limits result in the need for financial investment (i.e., for additional equipment, system conditioning, or maintenance)that is incompatible with the requirement that a BPL system be profitable and/or useful to the electric utility?

Here, we will look at attenuation and emission issues.

The BIG Question

Sources of Channel Attenuation

1.Ohmic Absorption – generally small

2. Reflection/Transmission “loss” (By far the largest component of “loss”)

Reflection/Transmission loss will be examined

HF Transformer Equivalent Circuit

M

M

M

At BPL frequencies the transformer presents a highlyfrequency dependent mismatch to the transmission line

Reflections/Attenuation due to Isolated Junctions

Incident Wave(power line)

Reflected Wave(power line)

z = 0

Transmitted Wave(power line)

TransmittedWave (tap)

Incident Wave

Reflected Wave z = 0

Transmitted Wave

Z01, 1

Z02, 2

T = 2/33.5 dB loss

Power Line Tap

Overhead/UndergroundTransition

0201

02

ZZ

Z2T

Typical Loss = 15 dB

Because the reflections/transmissions from eachjunction or connected element interact with eachother, it is necessary to evaluate the entire systemin order to evaluate the system attenuation

Z02Z01

Consider the simple BPL “system” shown next

ZG=100

Z0VG Z0 = 636

L = 0.5 kmZS=0 VIN VOUTL = 0.5 km

100 pF ZL=100

-60

-50

-40

-30

-20

-10

0

0.01 0.1 1 10 100

FREQUENCY (MHz)

SIG

NA

L A

TT

EN

UA

TIO

N (

dB

)

Attenuation at higher frequenciesis very frequencydependent and mayexceed 30 dB/km

A very messy channel

Modulation schemes (such as Orthogonal FrequencyDivision Multiplexing - OFDM) have been developedfor these very complex channels.

These schemes spread the signal among numerouscarriers over a wide bandwidth. They are designedso that carriers can either be “turned off” or not available due to attenuation without losing the signal.

The cost of shutting off carriers to protect licensed operations is a sacrifice in channel capacity.

Broadband Systems for Complex Channels

Enough power must be used to overcome the attenuation so that the signal to noise ratio is sufficiently high for satisfactory communication.

But, larger power causes larger emissions andthese must be controlled so that:

1.FCC Numerical limits are satisfied

2. No harmful interference is created.

Balancing attenuation, input power and emissions(assuming Part 15 as is) results in repeater spacing of less than 1 km.

Emission Issues

Emissions from Balanced Systems

Balanced systems (i.e., those with equal andopposite currents on two conductors such asmost telephone circuits) emit relatively smallfields. This is especially true if the two conductorsare closely spaced.

For a two wire power line with 1 meter spacing, the sinusoidalcurrent required to produce a 50 dB V/m equivalent electric field(i.e., the FCC limit in a CISPR QP Rx) at x = 3 meters is 47 A!

If Z0 = 550, then the maximum power flowing (without violating

the FCC limit) on the transmission line is 1.21 W or –29 dBm! This is not much!

It does not take much power to violate the FCC limits. Any unbalance in the currents will increase the fields.

Note: Spreading the signal over a wide bandwidth will reduce themeasured emissions since CISPR RX bandwidth is only 9 kHz.

On Meeting Numerical Emission Limits

Systems that are likely to be Unbalanced

1. Systems with more than one return path

2. Systems with significant displacement currents (i.e., stray capacitance)

Fundamental Problem: Since currents are continuous, every current must “return” to its source. The furtherthis return current is from its source current, the greaterthe emitted fields.

I1i

I2i

z = 0

PHASE CONDUCTORS

NEUTRAL CONDUCTOR

dpp

dpn

2a

I1i

I2i

z = 0

PHASE CONDUCTORS

NEUTRAL CONDUCTOR

dpp

dpn

2a

1. Power lines are multiconductor transmission linesas shown below. Current on one phase conductor can return either on the other or on the neutral wire. “Unbalanced” or “common mode” currents that flowon the neutral conductor cause greater emissions

I1i

I2i

z = 0

PHASE CONDUCTORS

NEUTRAL CONDUCTOR

I2i

VS

I1i

Consider a Typical Excitation System

By expanding the voltages and currents into common and differential “modes,” it can be shown that both common anddifferential modes are excited and that their amplitudes areroughly equal.

Tentative conclusion: More complex excitation schemesthat excite only differential modes appear to be warranted.

z = 0

PHASE CONDUCTORS

NEUTRAL CONDUCTOR

I2i

I1i

ZL

DiCiODOC

OCDt

DiCiODOC

ODCt

iiZZ

Zi

iiZZ

Zi

The reality is not so easy. Consider what happens whencurrents are incident on an unbalanced connected element.

A differential mode current willcreate a common mode currentand vice versa

ZL1VS

ZS PHASE CONDUCTOR

NEUTRAL CONDUCTOR

2. Systems with significant displacement currents

Source Current

Open circuit current

0

0.0005

0.0010

0.0015

0.0020

0.0025

0 2 4 6 8 10

CURRENT AT INPUT TO OPEN CIRCUITED WIRE NEAR SOURCE

SOURCE CURRENT

FREQUENCY (MHz)

CU

RR

EN

T M

AG

NIT

UD

E (

AM

PS

)

At low frequency, the open circuit current is nearly zero asexpected. But not at higher frequencies. Here, displacement currents become important

Decay rate of BPL fields away from a power lineWhy is this controversial?

FCC Section 15.31(f) says: an attenuation factor 40log10(d2/d1)should be used to extrapolate BPL field measurements madeat d1 meters to a location d2 meters from the power line.

If the field actually decays more slowly than this, measurementsmade close to the line and extrapolated to 30 meters (the standard distance) will indicate smaller fields than the actualfields measured.

Measurements have been reported that support a slower decayrate than suggested by the FCC

Do these measurements make sense?

dpn dpp x

y

I1I2I3

simplified geometry, assume dpn >> dpp

General expression for the magnetic field along x axis

)](2

[]2

[

)](2

[4

2

213

212

211

pn

pp

dxHIxHI

dxHIj

H

I1 + I2 + I3 = 0

Messy, but simplified soon! Most important: x, d’s compared to λNote: λ = 150 meters at 2 MHz and λ = 10 meters at 30 MHz

Decay Rate Calculations

Suppose x, (x-dpp), (x+dpn) << λ This is a “low frequency” or “near field” approximation

Then the Hankel functions can be approximated using“small argument” approximations. Then

pnpp

pnpppppnc

pp

ppd

dxdxx

ddddxI

dxx

dIH

2

2

1

Id and Ic are the differential and common mode amplitudes

A lot simpler!

At “large” distances”(i.e., x >> dpp , dpn)the decay is as 1/x2

If the common mode current is zero (i.e., Ic = 0)

pp

ppd dxx

dIH

2

1

The field decays as 1/x2 as long as x >> dpp (reasonable)

This is the decay rate assumed by the FCC in its Part 15Rules (i.e., 40 log10(d2/d1) or 40 dB/decade). …… But, the common mode is not always zero, the frequency is not always low and near field approximations may not always be allowable.

If the return currents are very far away (i.e., dnp >> x)

xI

x

dIH c

ppd

1

2 2

If, further, the differential mode amplitude is 0, then the fieldsdecay as 1/x (i.e., 20log10(d2/d1) or 20 dB/ decade).

Since the reality is probably in between these extreme cases,it is no surprise that measurements indicate decay ratesbetween 20 and 40 dB per decade.

At higher frequencies different approximations can be madeand

pn

dj

cpp

dxj

dx

e

xI

x

djIe

jH

pn

2/2 1

22

4

The decay rate in this case might be even smaller than20 dB/decade

The bottom line is that 40 log10(d2/d1) generally doesnot properly describe the decay rate of BPL fields.

(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused….

Harmful interference is defined as

“any emission radiation or induction which endangersthe functioning of a radio navigation service or othersafety services or seriously degrades, obstructs orrepeatedly interrupts a radio communication serviceoperating in accordance with this chapter.”

Harmful Interference Clause

Harmful interference is still not completely defined!!!

Different “World Views” on DefiningHarmful Interference

1. Consider Broadcast TV for example:

For Grade B, UHF television service (Channel 2 – 55 MHz)a signal strength contour (i.e., 47 dB μV/m*) is defined thatresults in satisfactory reception in 50% of the locations, 50% of the time. Interference is deemed negligible if certainD/U (desired to undesired signal) ratios defined by the FCC are exceeded.

Note: There is no mention here of the noise floor. Therefore,it may be permissible for an interference signal to exceedthe noise floor.

* CISPR QP Receiver – 120 kHz Bandwidth

2. Consider Amateur Radio*

This service is frequency agile and operators often look forparts of the frequency spectrum with “low noise” (includes man-made interference). Measurements at 28 MHz in a quiet area indicate that a noise floor of -10 dBuV/m** is not uncommon.

Note: The noise floor defines an acceptable interference levelTherefore, it is not acceptable for an interference signal to exceed the noise floor.

* Position of the ARRL – not endorsed by the FCC** CISPR QP Receiver – 9 kHz Bandwidth

Commercial TV (Ch 2 – 55 MHz) Amateur Radio - 28 MHz

Grade B signal level

D/U guard

Noise Floor

Acceptable Interference Ranges (Red)

47 dBμV/m

-10 dBμV/m

29.5 dBμV/mBPL @ 30 m

9.5 dBμV/mBPL with 20 dB filter

World View Comparison

BPL rejected by the ARRL since it exceeds the noise floor

Is “harmful interference” any signal that contributes to a measurable increase in the noise floor or should it be definedas signals that exceed a defined amount below a specifiedprotected signal level?

At how far from the source must the noise be deemed acceptable (i.e. should points further than 10 meters be protected? 30 meters?)

These are questions that only the FCC can answer. And, theywill be answered only as the FCC responds to complaints

The Harmful Interference “Wild Card”

Where are we now?

• BPL has not grown as quickly as its proponents thought. Some utilities have dropped BPL programs.

• I know of no one who claims to be making a profit on BPL.

• In some cases, there have been no interference complaints, but the business case still has not been satisfied.

• Independent economic analysis suggests a difficult business case.

• Utilities have many applications that are narrowband. It is possible that narrowband power line communication (i.e., the old PLC) could be used on medium voltage and low voltage systems