The University of Wisconsin-Platteville
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The Effects of Triplen Harmonic Distortion and Other Electrical Stresses on an
INSTEON Power Line Communications Networks
By: Anthony E. Gregerson, Advisor – Dr. Gang Feng
April 2006
Abstract
In recent years, power line communication networks have been a rapidly developing
technology section. New networks have appeared which make claims of great increases
in reliability over the previous generation. The performance of a power line
communication network based on the INSTEON network architecture was tested in the
presence of triplen harmonic distortion. The network was put through an additional
battery of three qualitative tests to gauge the performance and stability of the architecture
under a variety of electrical stresses. These included presence of capacitive power loads,
distance attenuation, and inter-phase impedance. The experimenter concluded that while
improvements are present, INSTEON does not live up to its reliability claims under
conditions typical of an average home. In particularly, the presence of capacitive loads
proved to be a problem for the network.
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Introduction
What is a Power Line Communication Network?
The term “power line communication network” (hereafter “PLC”) refers to a network of
interconnected devices which use a standard 120 V home power system as their means of
communication. Due to recent advances in PLC technology, networks can now be
separated into two major subgroups: broadband and narrowband. This research concerns
itself with only narrowband, or low bit-rate networks because broadband PLC technology
is not currently commercially available to the general public. In residential settings,
narrowband networks are primarily used for applications such as home automation. Such
networks are not new; the first PLC network, based on the X10 architecture, was
introduced in the late 1970s. Since that time, X10 has found moderate success in niche
markets, but has not penetrated the majority of households. In part, this failure can be
attributed to a number of weaknesses in the X10 infrastructure that made it susceptible to
problems with noise and loading on the power system. During the past few years, there
has been a renaissance in development of PLC technology fueled by promises of
corporate forums such as Broadband over Power Lines (BPL) and the Homeplug Alliance.
Among this new development, there are several new technologies aiming to become the
successor to X10 and become the superfluous communication network in households.
One such successor, SmartLabs’ INSTEON, claims to improve on X10’s stability
problems and achieve 99.97% reliability [1].
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How does a PLC function?
In the United States, the majority of power transmission consists of three conductors,
each carrying a 60-Hz sinusoidal signal, which each conductor, or ‘phase’, separated by
120º from the others. Each power outlet within a home is connected to a single phase.
To communicate to another device plugged into the same phase, an INSTEON device
transmits a digital signal using amplitude-shift-keying on a carrier frequency of 120 kHz1.
Data is transmitted in bursts of no greater than 1 ms2 centered on the zero-crossing point3
of the phase.
Fig. 1 - PLC bursts on the power signal
(Src: http://www.x10.com)
Each INSTEON device is connected to the network though a high-pass filter, which
passes the high-frequency data carrier and blocks the low-frequency power signal.
INSTEON uses a peer-to-peer architecture which is capable of functioning without a
centralized controller. Each device is addressed by a 32-bit ID number and is capable of
functioning as a repeater of INSTEON commands (but not X10 commands).
1
This maintains backwards compatibility with legacy X10 devices.
1 ms burst size is used for legacy X10 commands. INSTEON commands normally use a smaller size.
3
The point in time at which the phase-to-ground voltage is zero.
2
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Why are PLC Networks Important?
Both broadband and narrowband PLC networks have the potential to effect great lifestyle
changes. The power line network in the United States covers much more area than other
possible broadband carriers such as DSL or coaxial cable. In remote locations, power
lines may be the only option to provide “last mile” internet connectivity to homes. Inside
the home, broadband technologies like Homeplug are becoming integrated into high end
electronics like HDTVs. Meanwhile, narrowband technologies provide the sort of home
automation that has been envisioned for decades. Compared to wireless technologies like
Bluetooth or WiFi, PLCs have the potential to provide greater stability for critical
applications.
What is the Purpose of this Research?
The advent of personal computing and increasing number of digital devices in the home
have created a number of new noise and loading problems which didn’t exist when the
original X10 specification was created. These problems were a major contributor to
X10’s difficulty in gaining a foothold in US homes. This research aims to test the
stability claims made by INSTEON to determine whether PLC technology is ready for
consumers.
Triplen Harmonic Distortion Tests
What are Triplen Harmonics?
While the high-pass filters on the INSTEON device prevent the low-frequency power
signal from passing through, this is a highly idealized case. In real power lines, the
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power signal is never a perfect sine wave; rather it is a periodic signal similar to a sine
wave which has some distortion on it. Any periodic signal can be modeled by a Fourier
series of the form:
y (t ) =
∞
∑ a[n] sin(2πnft )
n = −∞
where f is the frequency of the sine wave. It is evident from this equation that for large
values of n, the frequency of the sine wave is also large. At high enough values of n, the
frequency will be too large to be blocked by the high-pass filter on the device.
Frequencies near to 120 kHz will be superimposed on the INSTEON data and may
corrupt the data when present at high enough levels.
Fig. 2 – Distorted power signal made up of fundamental and third harmonic
(Src: http://www.allaboutcircuits.com)
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Each frequency component of the Fourier series is present at different levels. There is a
special class of frequency multiples, or harmonics, that are usually present at high levels
in power systems. These signals are called ‘triplen harmonics’ because of their n values
of 3, 6, 9, 12, 15, and so on. Triplen harmonics are of particular concern because of their
method of generation. There are two major sources of third harmonics on the power
system. The first cause is non-linear effects present in a saturating transformer core [2].
The second cause is a class of power electronics known as inverters. Both of these
elements are present in DC power supplies. This is notable, because the increase in the
number of personal computers, DVD players, and other digital electronics in the home
has led to an increase in the number of large DC power supplies present on the power
system.
At the zero-crossing point, the magnitude of harmonics is theoretically zero, so by
transmitting only in very short bursts near the zero-crossing point, the effects of can be
minimized. Unfortunately, this also places heavy limitations on the bit rate. Hence, the
bit rate of INSTEON, X10, and related protocols is very low.
Testing Methodology
In an attempt to create triplen harmonics at realistic levels, the harmonics were generated
using an adjustable non-linear transformer which was coupled to the INSTEON network
using a high-pass network to isolate the two power systems. A high-power oscilloscope
was connected in parallel with the input to the INSTEON network and configured to
perform a Fast Fourier Transform. A custom program was written for the network which
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flashed a lamp on and off at a 1 Hz frequency. The transformer was adjusted to increase
the triplen harmonic level until the INSTEON controller reported an error rate in excess
of 5%. The amplitude of the third harmonic was measured at this point4. The
transformer was then disconnected and a reference third harmonic level was measured.
Test Results
The reference level test recorded a third harmonic-to-signal level of -32.8 dB. The
breakdown point occurred at a level of -25.2 dB. This is a difference 7.6 dB.
Fig. 3 – Reference third harmonic level
4
The increase in third harmonic should be proportional to that of all harmonics.
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Fig. 4 – Breakdown third harmonic level
Discussion of Results
It should be noted that this test measures the change in amplitude of the third harmonic
with the assumption that the same proportional difference will exist for higher frequency
harmonics. A difference of 7.6 dB corresponds to a factor of 5.8. In other words,
harmonics had to be increased by almost 6 times the base level before they began to
negatively impact performance. It is not likely that such high levels will be reached in a
home environment. The use of a zero-crossing transmission system seems to be very
effective in preventing harmonic problems. Harmonics are not likely to cause a problem
except in areas of very high harmonic contamination, such as large computer data centers.
In such locations, a line filter can be used to reduce the amplitude of harmonics.
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However, INSTEON was not designed to be used in such environments, so some
problems could be expected.
Further Tests
Following the completion of the triplen harmonic test, three additional qualitative tests
were performed to determine whether INSTEON had improved on problems which had
afflicted X10 networks.
Capacitive Load Test
What are capacitive loads?
In general, any load on the power system 5 can be classified as either a capacitive or
inductive load [2]. Most loads, with the notable exception of motors, are capacitive.
Digital electronics, particularly personal computers, act as very strong capacitive loads.
Capacitive loads are a problem for PLC networks, because a capacitor acts like a short
circuit for high frequency signal, such as the PLC data. As capacitors get larger, their
impedance to ground decreases. Capacitors in parallel also add linearly. Thus, when a
large number of computers or other capacitive loads are connected on the same power
system, the communication signal from the PLC devices may be shorted to ground before
it reaches its destination. In its user manual, SmartLabs recommends that INSTEON
devices and personal computers should not be plugged into the same power strip.
5
A ‘load’ is anything drawing power from the system.
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Test Methodology
An INSTEON network was setup in three locations, a home with one computer, a home
with six computers, and a lab with 25 computers, and a test script was run. The script
consisted of commands to turn several appliances on and off. The network was also
tested with an INSTEON controller plugged into the same power strip as a computer.
Test Results
The test script ran without error in the setting with one computer. In the six computers
home, the test script failed twice out of ten trials. In the lab with 25 computers, the
network could not be made to respond in any test. The network also failed to operate
when the controller was plugged into the power strip with the computer.
Discussion of Results
The INSTEON network clearly has some problems on power systems with many
capacitive loads. It is marketed as a home network, so it may not be surprising that it
struggles in a computer lab environment. The results of the six-computer test raise some
concern. The network only achieved 80% reliability, albeit in a small sample test size.
However, as the size and cost of computers and other digital devices drops, it is not
unreasonable to expect that the number of such devices present in the average home will
increase in the future. Without some modification, it seems likely that many INSTEON
networks could begin to fail as owners add more digital devices to their power system.
There is a solution available to reduce the effect of capacitive loads: placing an inductor
or RF choke between the device and the power system can block any high frequency
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signals from being short circuited. Such chokes are commercially available, though their
cost is currently about $20-30 each.
Distance Attenuation Test
What is Distance Attenuation?
All electrical signals traveling in a material experience some attenuation over distance.
The power line that PLC signals travel across can be modeled as having a large number
of small shunt capacitive loads [3]. As discussed in the previous section, a capacitive
load can present a low-impedance path to ground which decreases the amplitude of the
signal. As the signal travels far enough, it will eventually experience so much attenuation
that it will be too small to be detected by the receiver. To alleviate this problem, all
INSTEON devices can act as repeaters, boosting the signal back to a high level and
passing them along.
Test Methodology
An INSTEON network with two devices was set up with device separation distances of
50 ft, 100 ft, and 200 ft. An ‘acknowledge’ signal was then sent from one device to the
other.
Test Results
An acknowledgment reply was received in the 50 and 100 ft tests. There was no
response in the 200 ft test.
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Discussion of Results
For a network targeted at home use, these results are quite good. This test only measured
the separation distance between two nodes on the network. Because INSTEON devices
act as receivers, it would be possible to use several nodes to extend the range of the
network over a very long distance.
High Interphase Impedance
What is Interphase Impedance?
As mentioned earlier, the high-voltage power transmission system utilizes three separate
conductor phases. Most appliances only connect to a single phase, however some heavy
appliances like washing machines require a 2-phase, 240 V power supply. Therefore
most homes have at least two different phases in their power system. These phases
cannot be directly connected. If two PLC devices are connected to different phases, any
transmission between them must travel back to the three-phase transformer that supplies
the house and pass through the windings separating the two phases. These windings tend
to have very high impedances, such that it is not possible for the low power PLC signal to
pass through. This results in some nodes being isolated and unreachable from the rest of
the network.
Test Methodology
Two INSTEON devices were setup such that one was plugged into the ‘A’ phase of a
three-phase power system and the other was plugged in to the ‘B’ phase.
An
acknowledgement request was then sent from one device to the other.
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Test Results
No acknowledgement was received from the device on the different phase.
Discussion of Results
It is not surprising that the INSTEON network failed the interphase impedance test, as its
communication system has no fundamental differences from X10, which also suffered
from this problem. There are some solutions which exist to bypass the problem of
interphase impedance. A solution that has existed since the X10 days is to insert a shunt
capacitor between the phases. As discussed earlier, a capacitor acts as a low-impedance
path to high frequency signals. Thus, the signal can easily propagate between phases
without causing any ill effect to the low frequency power signal. The drawback of this
method is that it requires an electrical technician to install safely and a high-voltage
capacitor may be expensive.
The second method is a new innovation available to
INSTEON, an antenna transmitter/receiver pair that plugs into different phases and
propagates data between phases via a wireless connection. This solution requires no
invasive installation on the power system, however the wireless device currently costs
approximately $80, and foregoes most of the advantages of choosing a wired PLC
network over a wireless network.
Discussion of Overall Results
The INSTEON network showed mixed results across the battery of tests.
Triplen
harmonic and distance attenuation performance were both strong. However, problems
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with capacitive loads and high interphase impedance exist. While fixes exist for both
these problems, they are currently expensive and/or difficult to implement.
The
capacitive load problem is particularly notable, as a filter is required for each new device.
Overall, the performance of INSTEON has increased over that of X10, but SmartLabs’
claims of 99.97% reliability is either inaccurate or based off results of highly-skewed
tests.
Cited Sources
[1] SmartHome. (2006, January 25). What is INSTEON? Retrieved January 25, 2006,
from http://www.smarthome.com/whatisinsteon.html.
[2] Duncan, J. D., & Sarma, M. S. (1994). Power System Analysis and Design (2nd ed.).
New York: Wadsworth.
[3] Johansson, J., & Lundgren, U. (1997). EMC of Telecommunication Lines.
Unpublished masters thesis, Lulea University of Technology, Lulea, Sweden.
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