The Effects of Triplen Harmonic Distortion and Other Electrical Stresses on an INSTEON Power Line Communications Networks By: Anthony E., 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. The Big M, Vol II, 2006 26
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]. The Big M, Vol II, 2006 27
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 khz 1. Data is transmitted in bursts of no greater than 1 ms 2 centered on the zero-crossing point 3 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. 2 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. The Big M, Vol II, 2006 28
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 The Big M, Vol II, 2006 29
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: n= y( t) = a[ n]sin(2πnft) 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) The Big M, Vol II, 2006 30
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 The Big M, Vol II, 2006 31
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 point 4. 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. The Big M, Vol II, 2006 32
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. The Big M, Vol II, 2006 33
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. The Big M, Vol II, 2006 34
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 The Big M, Vol II, 2006 35
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. The Big M, Vol II, 2006 36
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. The Big M, Vol II, 2006 37
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 The Big M, Vol II, 2006 38
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 (2 nd ed.). New York: Wadsworth. [3] Johansson, J., & Lundgren, U. (1997). EMC of Telecommunication Lines. Unpublished masters thesis, Lulea University of Technology, Lulea, Sweden. The Big M, Vol II, 2006 39