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PLC communication on Egypt’s
LV grid for AMR
by A Helmy , M Abdel Rahman and MM Mansour, Ain Shams University,
Electrical energy consumers are demanding better customer service, higher power quality, higher energy measurement accuracy and more timely
data. Utility companies all over the world are being forced to find solutions giving greater information on the population’s power consumption.
The automatic meter reading system (AMR) is one of the ways in which
utilities are go-getting to achieve these goals. Power lines are one of the
communication mediums used in the AMR system. However, power lines
are a hostile communication environment.
This paper describes a prototype for AMR system using power lines showing
performance and reliability tests. The measurements are carried out on
a sample of Egypt’s low voltage network.
Considerable savings can be achieved by the utility companies by
removing the need for a visual inspection of the meter at each billing
term. The process is labor intensive, time consuming and prone to
human error. A study shows that on average a meter reader achieves
an information rate of only around 1 bps [1], which is highly inefficient in
terms of modern standards.
AMR also reduces the problems and costs associated with reading
meters at hard to access locations. Additional significant cost savings
can be achieved by identifying the tampering of meters These can be
detected because most of these AMR systems offer bi-directional
communications, which allows the current meter data to be
checked against the historical data for any suspicious divergence.
Several communication technologies are currently in use to
achieve automated meter reading. Radio, telephone lines and
power lines are the most used mediums where each system has
its own advantages and disadvantages.
This paper focuses on power line communication (PLC) and its
performance as a channel for AMR system, statistical analysis and
suggestions for reliable communication is presented.
AMR system structure
The main parts of an AMR system as shown in Fig. 1 are:
Meter interface unit (MIU): Each meter at the consumer side is fit
with an interface unit which acts as a transceiver for meter data
on the low voltage power lines using a single phase power line
modem (PLM).
Data concentrator unit (DCU): The data sent by the MIU is
received by the data concentrator unit located at the distribution
transformer. DCU is fit with three phase PLM receiving data from
all meters on different phases.
Utility central unit (UCU): The data received by the DCU is then
transmitted to the utility central unit. The communication link in this
case may be PSTN, RF, GSM or Ethernet which is out of the paper
scope. As mentioned before the communication links either
between the MIUs and their DCU or between the DCUs and the
UCU are bi-directional links.
PLC benefits, challenges and regulations
Benefits
By using existing cable infrastructure PLC system eliminates the need
for installation and maintenance of dedicated communication
links. Already every building or household is connected to the
electrical power grid and moreover; every room has power line
contact points installed. Without doubt the extent of this existing
infrastructure cannot be matched by any other telecommunication
technologies that are available today.
Fig. 1: AMR system structure.
The challenge
As the electric power distribution lines were not originally designed
for communication purposes as a result they exhibit highly variable
and unpredictable levels of channel noise, signal attenuation and
distortion [8].
Standards and regulations
Fig. 2: Frequency bands for CENELEC EN50065-1 standard.
There are many established standards that provide regulations
on the operating specifications of PLC systems. Federal
Communications Commission (FCC) and European Committee
for Electro-technical Standardization (CENELEC) govern regulatory
rules in North America and Europe respectively. For Europe the
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regulations concerning low voltage signaling are described in
CENELEC standard EN50065. In part 1 of this EN-standardization
paper the allowed frequency band and the output voltage for
communication is indicated [2].
The frequency range allowed for communication is subdivided
into five sub bands as shown in Fig. 2 ranges from 3 kHz to 148,5
kHz, with transmitted power not more than 500 mW.
Frequency ranges between 9 – 95 kHz, which is allocated for
electrical utility use for applications such as AMR and load
management; there is no need for access protocol when
operating in this band. The rest of the frequency range 95 – 148,5
kHz comprises of the B, C, and D frequency bands and is reserved
for end-user applications.
Focus on PLC challenges
Noise and disturbances
Common causes of noise on the high voltage electrical power
networks include corona discharge, power factor correction banks
and circuit breaker operation. On the low voltage network, much
of this noise is filtered by distribution transformers, so the most
common interference in low voltage domestic networks can be
attributed to the various household devices and office equipment
connected to the network.
Noise on the low voltage network is classified to [1], [4]:
• Noise having line components synchronous with power system
frequency: The usual source of this noise are Triacs or silicon
controlled rectifiers (SCRs), found domestically, for example,
in light dimmers or photocopiers.
• Noise with a smooth spectrum: caused by universal motors
that can be found in a lot of household appliances such as
blenders and vacuum cleaners.
• Single event impulse noise: usually caused due to switching
events as closing of contacts.
Signal attenuation
Reference [3] states that the attenuation between homes in the U K
ranges from 15 dB to 25 dB per 100 m, for the frequency range 3 –
150 kHz. The corresponding value for homes in Zurich, Switzerland, was
in the range 3 – 13 dB. In [3] analysis of signal attenuation is introduced
giving some factors affecting signal attenuation as time dependency
where there is a strong day and night sensitivity.
Communication system
Noise, fluctuated channel impedance, signal attenuation and distortion
are big challenges facing selecting an efficient communication system
to insure reliable data transmission through this hostile environment.
However, remote meter reading and load control are examples of PLC
applications with rather low demands on the communication system.
This type of applications has two important characteristics: firstly, the
amount of information that is transferred is small, and therefore the
information bit rate (in bit/second or bps) is low.
Secondly, they do not have high real time demands therefore a relatively
large communication delay is acceptable. These two characteristics
make it easier to establish reliable communication over the power line
than other PLC high speed and real time demand applications.
Many researchers and investigators have been performing important
works to select a proper modulation technique, frequency shift keying
(FSK) and spread spectrum (SS) are the most commercially applicable
by Echelon, Intellon, ST, Itran, Phillips, Archnet the most famous PLM
producers.
Spread Spectrum uses wide band, noise-like signals. Because Spread
Spectrum signals are noise-like, they are hard to detect. Spread
Spectrum signals are also hard to Intercept or demodulate. Further,
Spread Spectrum signals are harder to jam than narrowband signals.
These low probability of intercept anti-jam and high noise immunity
features [6] are why the military has used Spread Spectrum for so many
years and it’s widely used now commercially in PLC systems.
• Non synchronous noise: Is characterised by periodic
components that occur at frequencies other than harmonics of
the mains frequency. Major sources of this noise are televisions
and computer monitors.
Proposed system configuration
Fluctuated channel impedance
• Remote controller Units (RCU)
Unfortunately, a uniform distributed line is not a suitable model
for PLC, since the power line has a number of loads of differing
impedances connected in parallel to it for variable amounts of
time. It can be seen that the channel impedance is a strongly
fluctuating variable that is hard to predict.
The proposed system shown in Fig. 3 consists of:
• Master controller Unit (MCU)
• Laptop
The main aim of this prototype to have a point-to-point system where
the MCU (acting as DCU) will communicate with RCU (acting as energy
meter) connected to the same phase at different distances from the
The overall impedance of a low voltage network as discussed
in [1] and [5] results from three main sources, impedance of the
distribution transformer, characteristic impedance of cables and
impedance of the devices connected to the network.
Various measurements and tests were done to evaluate the
impedance of the low voltage network, in [7] the CENELEC A-band
maximum absolute impedances are reported increasing from 20
Ω at 20 kHz to 80 Ω at 100 kHz. Mean values increase from 5 Ω
to 17 Ω. Clearly, channel impedance is very low. This presents
significant challenges when designing a coupling network for
PLC communications. Maximum power transfer theory states
that the transmitter, receiver and the channel impedance must
be matched for maximum power transfer. With strongly varying
channel impedance, this is very difficult.
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Fig. 3: Proposed system for PLC testing.
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MCU. All data transactions on the same phase are recorded with date
and time stamp where various performance and reliability tests are
carried out to examine the power line channel as a communication
medium for the AMR system.
Master control unit (MCU)
MCU is shown in Fig. 4 where it mainly consists of a single phase PLM from
Archnet [10] communicating with a Laptop through RS232 serial port .The
Archnet PLM ATL90115-1 is based on the direct sequence spectrum (DSSS)
and it provides bi-directional half duplex data communication over the
low voltage grid at baud rate 300 bps and it complies with CENELEC
standard EN50065-1 A band. Thus, the data could be transferred in bidirectionally between the Laptop and the power lines. Fig. 4 shows the
MCU prototype.
Remote control unit (RCU)
RCU is shown in Fig. 5 where it consists as MCU of PLM communicating
serially (RS232) with a microcontroller which is responsible for
performing all the calculations required for electrical power
quantities measurements besides receiving and sending data from
and to PLM. Thus, the data could be transferred in bi-directionally
between the microcontroller and the power lines. Fig. 5 shows the
RCU prototype.
The microcontroller is PIC18F258 from microchip [9] fit with 10 bit
analog to digital converter (ADC) required for accurate current and
voltage signals sampling, serial port required for interfacing with PLM,
high internal bus speed up to 10 MHz and other many advanced
features.
Laptop
The laptop computer is responsible for: firstly, sending control signals
to all RCUs or one of them. Secondly, receiving and storing all data
transactions with date and time stamp for analyses.
Communication datagram
As the system designed and constructed is still in the prototyping
stage, the communication datagram used in the power line
communication was designed initially without any concern for data
security and improving data reliability the main aim is to find out the
performance of the proposed system in the hostile environment.
The data request packet is designed for the purpose of collecting
meter readings. MCU sends out this packet to ask all RCU within the
network for meter readings at a certain interval. All RCUs respond
to this by sending back a data send packet to the source, which
contains the information requested.
Fig. 4: MCU structure diagram and prototype.
For simplicity the data request packet sent by MCU to all RCU is
one byte which is the ASCII value of letter “t” (stands for transmission
request).While the data send packet sent by each RCU is 16 bytes
with the first two bytes representing the RCU ID which is unique ID for
each RCU and stored on the microcontroller. The other 14 bytes are
the data reading requested of the specified RCU.
Performance and reliability site tests
Test method
The MCU and a single RCU is communicating on the same phase
at a distance 50 m apart. Tests are done at a dense residential area
in Cairo the capital of Egypt. The tests involves putting the system
under hard test continuously for three weeks where the laptop is
programmed to send the data request packet every 60 s and
waits for the data send packet sent back from the RCU. The laptop
stores all the transactions done either transmitted or received this
data is then checked for accuracy of response. 1440 data request
packet/day (1 packet/min) were sent and the results were reported
for three weeks.
Test objectives
The site tests focused on three main objectives:
• Report the general performance of PLC for a medium distance
(50 m)
• Report the best day of the week and the best hour of the day
for AMR system data collection
Fig. 5: RCU structure diagram and prototype.
• Report the influence of large number of TV sets loading the
power lines where some results are reported during the African
cup of nations for football (Egypt ACN 2006) which reported a
large number of spectators (especially for the Egyptian team
matches).
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Test results
The results are based on the accurate packets received by the MCU
from the RCU. If an inaccurate packet is received by the MCU, a
checksum error will be reported. However, if the MCU does not receive
any reply from the RCU after a certain allocated time, a timeout error
is reported. Both the checksum and timeout errors constitute the failure
of the proposed system.
Test results are collected and subdivided to:
• Test results based on a day of the week: samples of these results
are shown in Table 1.
• Test results based on the same day of the week: where results from
1 are averaged for the same day of the week for the whole three
weeks as shown in Table 2.
• Test results based on the hour of the day: Fig. 6 shows histogram
samples of these results.
Success
trials
CS
errors
TO
errors
Success
%
Failure
%
Fri
17/2/06
1191
46
203
82,7
17,3
Sat
18/2/06
1311
3
126
91
9
Sun
19/2/06
1037
7
396
72
28
Mon
20/2/06
1104
21
315
76,7
23,3
Tue
21/2/06
1033
14
393
71,7
28,3
Wed
22/2/06
983
5
452
68,3
31,7
Thu
23/2/06
1182
26
232
82,1
17,9
Table 1: Day of the week reliability results.
Observations
Based on the results obtained, the observations lie in the following
areas:
Suggestions
• Reliability: As shown in Table 1 successful attempts in any day do
not exceed 91%
All performance and reliability tests show fluctuating results varying
with time. Thus, to overcome this we suggest the following:
• Timeout: High timeout (TO) error is noted while checksum (CS) errors
is so rare compared to the timeout error forming a total failure
percentage up to 35% and increase to 49% under heavy loading
conditions
• Initiate communication at specific times, Thursdays and
Saturdays between 03h00 am to 09h00 are reported the best
results.
• Correlation with day of the week: As shown in Table 2 the average
successful attempts vary from 67% to 83% depending on the day
of the week. Best average results are reported on Thursdays and
Saturdays. However, worst results are reported on Sundays and
Wednesdays. Fig. 6b and Fig. 6c show a histogram samples for
PLC performance on Sunday and Thursday respectively
• Using repeaters to reduce the timeout errors and one of the
error correction methods to reduce the checksum errors.
• Correlation with time of day: The histogram of the first week of
the tests reports very high failure percentage up to 100 % on the
hours where there is a match for Egypt football team in the ACN
2006 due to very high loading effects by TV sets, this is noted on
Tuesday 7 February 2006 from 19h00 to 21h00 and Friday 10
February 2006 from 18h00 to 21h00.
Fig. 6a: The histogram of the other two weeks reports very high failure
rates up to 100% on the hours of the day between 11h00 to 17h00
and high successful rates up to 100% on the hours between 01h00
to 09h00 as shown in Fig. 6b and Fig. 6c.
• Avoid times which record high TV sets audience density.
Success
trials
CS
errors
TO
errors
Success
%
Failure %
Mon
1069
14
Tue
1067
27
356
74,2
25,8
346
74,1
25,9
Wed
995
7
438
69,1
30,9
Thu
1202
14
224
83,5
16,5
Fri
1109
34
297
77
23
Sat
1174
16
250
81,5
18,5
Sun
977
11
452
67,8
32,2
Table 2: Same day of the week reliability results.
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Conclusion
The power lines are a hostile environment for communication with
channel parameters highly fluctuating with time. Thus, a special care
about choosing the right time for data transmission must be considered
by the utility central unit to acquire the meters readings through the
AMR system.
In spite of the satisfactory performance of the prototype system,
there is still work to be carried out. It would be useful to examine the
reasons for timeout error and also to determine the actual cause of
all the errors. Error correction should be added to the system in future
to increase the reliability of the system and to prevent tampering.
Acknowledgement
This paper was presented at the IEEE PES Power Africa 2007
conference and exhibition in Johannesburg, and in republished with
persmission.
References
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[10] Archnet Corporation, “Embedded Power Line Modem ATL90115-1
Fig. 6: PLC performance histogram.
and ATL90115-3 Manual ” http://www.archnetco.com v
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