Download BPL - uidaho.edu

Technical Overview of Broadband
Powerline (BPL) Communication
Systems
Robert G. Olsen
School of Electrical Engineering and Computer Science
Washington State University
Pullman, WA, USA
[email protected]
Presented at
University of Idaho
October 4, 2006
Injector
(2 – 30 MHz)
Photo of a Typical BPL Installation (courtesy B. Cramer EPRI)
The BIG Question
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.
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 highly
frequency dependent mismatch to the transmission line
Reflections/Attenuation due to Isolated Junctions
Transmitted Wave
(power line)
Incident Wave
(power line)
Reflected Wave
(power line)
z=0
Transmitted
Wave (tap)
Transmitted Wave
Incident Wave
Z02, 2
Z01, 1
Reflected Wave
z=0
Power Line Tap
T = 2/3
3.5 dB loss
Overhead/Underground
Transition
2 Z02
T
Z01  Z02
Typical Loss = 15 dB
Because the reflections/transmissions from each
junction or connected element interact with each
other, it is necessary to evaluate the entire system
in order to evaluate the system attenuation
Z01
Z02
Consider the simple BPL “system” shown next
ZG=100  VIN
L = 0.5 km
Z0 = 636 
VG
ZS=0
L = 0.5 km
100 pF
Z0
VOUT
ZL=100 
0
Attenuation at
higher frequencies
is very frequency
dependent and may
exceed 30 dB/km
SIGNAL ATTENUATION (dB)
-10
-20
-30
A very messy channel
-40
-50
-60
0.01
0.1
1
FREQUENCY (MHz)
10
100
Broadband Systems for Complex Channels
Modulation schemes (such as Orthogonal Frequency
Division Multiplexing - OFDM) have been developed
for these very complex channels.
These schemes spread the signal among numerous
carriers over a wide bandwidth. They are designed
so 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.
Emission Issues
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 and
these 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.
Emissions from Balanced Systems
Balanced systems (i.e., those with equal and
opposite currents on two conductors such as
most telephone circuits) emit relatively small
fields. This is especially true if the two conductors
are closely spaced.
On Meeting Numerical Emission Limits
For a two wire power line with 1 meter spacing, the sinusoidal
current 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 the
measured emissions since CISPR RX bandwidth is only 9 kHz.
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 further
this return current is from its source current, the greater
the emitted fields.
1. Power lines are multiconductor transmission lines
as shown below. Current on one phase conductor
can return either on the other or on the neutral wire.
“Unbalanced” or “common mode” currents that flow
on the neutral conductor cause greater emissions
I1i
z=0
dpp
PHASE CONDUCTORS
I2i
dpn
NEUTRAL CONDUCTOR
2a
Consider a Typical Excitation System
I1i
I1i
z=0
PHASE CONDUCTORS
I2i
I2i
VS
NEUTRAL CONDUCTOR
By expanding the voltages and currents into common and
differential “modes,” it can be shown that both common and
differential modes are excited and that their amplitudes are
roughly equal.
Tentative conclusion: More complex excitation schemes
that excite only differential modes appear to be warranted.
The reality is not so easy. Consider what happens when
currents are incident on an unbalanced connected element.
I1i
z=0
PHASE CONDUCTORS
I2i
ZL
NEUTRAL CONDUCTOR
Z OD
iCi  iDi 
iCt 
Z OC  Z OD
Z OC
iCi  iDi 
iDt 
Z OC  Z OD
A differential mode current will
create a common mode current
and vice versa
2. Systems with significant displacement currents
Source Current
ZS
PHASE CONDUCTOR
ZL1
VS
NEUTRAL CONDUCTOR
Open circuit current
At low frequency, the open circuit current is nearly zero as
expected. But not at higher frequencies. Here, displacement
currents become important
0.0025
CURRENT MAGNITUDE (AMPS)
0.0020
SOURCE CURRENT
0.0015
0.0010
0.0005
CURRENT AT INPUT TO OPEN
CIRCUITED WIRE NEAR SOURCE
0
0
2
4
6
FREQUENCY (MHz)
8
10
Decay rate of BPL fields away from a power line
Why is this controversial?
FCC Section 15.31(f) says: an attenuation factor 40log10(d2/d1)
should be used to extrapolate BPL field measurements made
at d1 meters to a location d2 meters from the power line.
If the field actually decays more slowly than this, measurements
made close to the line and extrapolated to 30 meters (the
standard distance) will indicate smaller fields than the actual
fields measured.
Measurements have been reported that support a slower decay
rate than suggested by the FCC
Do these measurements make sense?
Decay Rate Calculations
simplified geometry, assume dpn >> dpp
y
dpn
dpp
I3
I2
x
I1
General expression for the magnetic field along x axis

2  [ 2 ( x  d )]
I
H
 1 1
pp





2  2
2  2
 I 2 H1 [
x]  I 3 H 1 [
( x  d pn )]



H 
2
4 j
I1 + I2 + I3 = 0
Messy, but simplified soon! Most important: x, d’s compared to λ
Note: λ = 150 meters at 2 MHz and λ = 10 meters at 30 MHz
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
1
H 
2

d pp
I d
 xx  d pp 
x2d pn  d pp   d pp d pn 
 Ic

xx  d pp x  d pn  
A lot simpler!
At “large” distances”
(i.e., x >> dpp , dpn)
the decay is as 1/x2
Id and Ic are the differential and common mode amplitudes
If the common mode current is zero (i.e., Ic = 0)
1
H 
2

d pp

I d

 x x  d pp







The field decays as 1/x2 as long as x >> dpp (reasonable)
This is the decay rate assumed by the FCC in its Part 15
Rules (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)
d pp
1
H  I d
 Ic
2
x
2x
If, further, the differential mode amplitude is 0, then the fields
decay 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 rates
between 20 and 40 dB per decade.
At higher frequencies different approximations can be made
and
H 
j 2d pp
j  j 2x /  

e
I d
4

 x

 j 2d pn
 1
e

 2I c

 x
x  d pn






The decay rate in this case might be even smaller than
20 dB/decade
The bottom line is that 40 log10(d2/d1) generally does
not properly describe the decay rate of BPL fields.
Harmful Interference Clause
(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 endangers
the functioning of a radio navigation service or other
safety services or seriously degrades, obstructs or
repeatedly interrupts a radio communication service
operating in accordance with this chapter.”
Harmful interference is still not completely defined!!!
Different “World Views” on Defining
Harmful 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 that
results in satisfactory reception in 50% of the locations,
50% of the time. Interference is deemed negligible if certain
D/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 exceed
the noise floor.
*
CISPR QP Receiver – 120 kHz Bandwidth
2. Consider Amateur Radio*
This service is frequency agile and operators often look for
parts 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 level
Therefore, 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
*
World View Comparison
Commercial TV (Ch 2 – 55 MHz)
47 dBμV/m
Amateur Radio - 28 MHz
Grade B signal level
D/U guard
BPL @ 30 m
29.5 dBμV/m
BPL with 20 dB filter
9.5 dBμV/m
Noise Floor
-10 dBμV/m
Acceptable Interference Ranges (Red)
BPL rejected by the ARRL since it
exceeds the noise floor
The Harmful Interference “Wild Card”
Is “harmful interference” any signal that contributes to a
measurable increase in the noise floor or should it be defined
as signals that exceed a defined amount below a specified
protected 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, they
will be answered only as the FCC responds to complaints
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