An Introduction to Communication Methods Across Power Distribution Systems and Their Effects

An introduction to Communication Methods Across Power Distribution Systems and Their Effects.

By Samuel Nyall Stearley

type of devices do not need to be constantly supplying heat Abstract—This Paper is intended as an introduction to and can afford to take a break every few seconds. [1] methods that use existing power distribution systems to Through the use of ripple control, certain German cities communicate with devices. It will go over the other effect have been able to keep their energy use sustained at an almost that such communications have, namely the creation of constant draw. This means that more power is being used noise and the effect that such noise has on other forms of during what were previously slump periods. So more power is communication. There is an emphasis on the modeling of being consumed overall and that means higher sales. Used in this noise. such a manner, up to 30% of the costs of installing ripple control can be derived back. But there are other advantages Index Terms—Broad band over power lines, Ripple like the control of meters. [1] Control, Load control. Signal injection is a critical issue and must be done properly. The signal must be added in a synchronous manner at the appropriate timing with the existing power. It is often done at the medium voltage level (10 to 35kV) to all three I. INTRODUCTION phases; however it can be done at other levels as desired. The higher voltage levels are distributed to farther reaches, and if This paper is a discussion of methods and issues whereby the signal needs to go farther then injection should be done at power lines are simultaneously used to deliver power and these higher levels. information to their target systems. The level of information There are two methods for injecting data into a power can vary from complex data to simpler Boolean signals. And network. The network can be intercepted serially. With this the data being transmitted can move from a single source to method power supply can be briefly stopped and data is sent single or multiple destinations. This paper will go into the instead. The advantage of this method is that data does not details of signal injection, their effectiveness and noise jump across transformers onto the higher voltage level. consequences. However adding hardware in series with existing power lines can be dangerous. If the data transmitter fails it could bring down the entire network. [1] II. DYNAMIC LOAD CONTROL The second method is to add the signal in parallel with the One of the simplest forms of communication over a power power. Meaning it is layered on top of the existing three system is that of adding a ripple voltage. The purpose of this phase sinusoidal wave. The disadvantage of this is that the signal is the control of critical loads that are being powered. data can get through the transformer to the higher voltage The ripple voltage is an indicator that tells these loads to lines. For high frequency signals this is minimized as the switch on or off. This is often used as a safety feature; if too impedance the transformer applies to the signal is proportional much current is being drawn and the source will soon give out, to frequency (ZInductor = w*l*j) and prevents power being the signal can be sent telling the loads to shutdown. Thus delivered to the high voltage lines. If the data will be giving the load the chance to shutdown safely. transmitted above 210 Hz then parallel distribution is fine, else There are several methods available for load control. serial is used. Switch over meters are dumb terminals by themselves and In practice most frequencies used fall between 110 to 750 cannot be controlled dynamically. They have been used in the hertz. Particular values used depend on the system. It is US before World War II. Or instead of injecting a ripple the possible that resonant effects can occur with power factor frequency of the power is reduced from sixty to fifty hertz. A correction capacitors and help propagate the signal. However third popular method in the US used for large scale load in general resonance with series impedance is something that management systems is the use of a separate radio controller. needs to be avoided. Frequency of injection also affects the 1All these methods are considered to be a “direct control” of current that makes its way into the high voltage supply the loads and are used to control loads that have large storage network, and from this network into neighboring networks. characteristics. This means that they do not require a steady The chosen frequency should not overlap with possible power source to continue functioning. Examples include harmonic frequencies that could exist in the network, else a heaters (space or water) washing machines, driers, etc. These fake signal could be detected as the actual signal.[1] Receivers involve a band pass filter that checks for the 1Samuel Stearley is a student of Electrical Engineering at desired frequency. To prevent accidental detection the desired Calvin College, Grand Rapids, MI, 49546, US; email: frequency must be maintained for a set period of time before [email protected] the system judges the signal is valid. Standard ripple receivers also feature three relays for guiding power about but custom ripple receivers exists for specific applications, like the street II. DIGITAL COMMUNICATION lighting receiver shown in Fig 1. A. Networking the home A form of residential network communication method being promoted currently by the “Home Plug Alliance” is deemed the HomePlug specification. The idea of interfacing to the power network via the house plugs to provide networking capability is not a new idea and has been done in the past. But these past advances have not lived up to expectations and did not perform as well as hoped. Recently filtering technology has taken a large enough leap to allow a newer generation to arrive. Home power networks vary considerably depending on the regional wiring codes, age, and size of the house. Extensive testing has been done to verify that modern in house networking is universally viable for the consumer. For example it is not a good idea to create consumer electronics that people in New York can use, but not people in Texas. [2] Figure 1

Figure 2 The results showed that speeds of 1.5 Mbps where produced achieve five Mbps transfer speeds. This test involved simple in ninety-eight percent of connections tested. (Out of 6690) communication between two devices. More advanced testing Seventy-seven percent of these connections where able to among five devices. showed that ninety-three percent of interference can blanket wide neighborhoods or entire cities. houses where able to sustain talking speeds of three Mbps. [8] While these tests left out houses found in the Midwest states The discussion of noise so far has only addressed one they are quite convincing that home networking via the power component and does not consider further complications with network is feasible. [7] the geometry of the power networks. There are issues with the power lines being designed for operation at sixty Hz. And there are issues with the buildings the power lines interface to. B. Internet Delivery The electrical wirings of buildings result in a good transmission line, but there are other aspects involved. For Of significant interest lately and a hotly debated political example when a light switch is opened (ie the light is turned issue is the emerging technology of internet providers off) the result is an antenna configuration that radiates nicely. providing broad band internet connections through power And there might be many other loads the building is connected lines. However it generates a large amount of interference to that also radiate. with Ham radios (among other things) so many Hobbyists are Furthermore there are odd loopholes in the FCC regulations up in arms over this potential threat. that are used inappropriately. When doing actual testing it is The idea is that the majority of the internet transfer is done permitted to do measurements at three meters and extrapolate through the normal means. However the “last mile” delivery to thirty meters. In actuality this underestimates the noise by whereby an individual’s home is connected to the internet is 20 dB. The FCC rules do specify conditions in using this done via the power lines (Fig 2). The data is traveling at such approximation, like when measuring at a distance of thirty a high rate that it cannot get through the inductors of the meters is impractical. But for power lines it simplicity to power transformers so the transmission cannot be entirely setup equipment thirty meters from a power line. done via the high, medium and low transmission lines. The modeling and comparison of E field noise produced by The FCC regulations that govern unintentional emitters are a high frequency to that of a low frequency is important in partly to blame for the cause of the scandal. They require that understanding what is occurring. For a case study a twenty current-carrier systems be tested in three “typical” real world meter two line power transmission cable delivers power to a installations. The definition of “typical” is not standardized, house. One of the lines has a 120 volt AC source at 60 Hz or that there even exists three distinct kinds of “typical” while the other is ground. The lines are made out of copper applications. with a radius of 1 centimeter. Using transmission line However this works out to the advantage of a company modeling methods described in Fig 3. The voltage at any developing hardware. If passing the FCC’s rules were as point z in the line at any time t can be found. simple as doing white lab coat experiments to measure generated electromagnetic fields, then PLC hardware would not be legal to use. If the FCC judged PLCs based on emissions then a double standard would result. There would be one standard that was lenient and allowed PLC operation, while another would disallow other class of devices. When doing lab coat experiments the standard is when at a distance of 30 meters from the unintentional emitter (whose frequencies are below 30 MHz) it should not create an electric field greater than thirty micro volts per meter. The testing is done at a bandwidth of 9 kHz. Figure 3 However this does not provide enough protection of ham radio signals. A 30 uV/m field will create a noise on a half With the voltage in the transmission line, the E-field internal wave dipole of -86.4 dBW which is equivalent to 338 micro to the line may be found. If the voltage did not vary with the volts. However when comparing 338 uV to existing radio length of the transmission line in the z direction then the E- signal transmissios (Which are much smaller by design) the field would be zero. From the E-field the electric flux density noise is 16 dB, this is very harmful. [8] is derived. The electric flux density is multiplied by a differential cross sectional area to find the differential charge This is not to say that the “thirty micro volt per meter when it contains. Then from the differential charge a differential at a distance of thirty meters” rule is invalid, but that it was external E-field to a point P can be calculated. After designed for the analysis of point sources. With Power lines Integrating all the differential E fields the total E field is the source is as long as the carrying part of the wire. For areas found. These dimensions and what is being integrated are that Broadband over Power lines is implemented, the described in fig 4. Figure 4

The calculations are now given. The principle of superposition is used to separately examine a 120 volt source at a frequency of 60 hertz and a 1 volt signal at 3.5 mega Hertz. The formulas used to gauge the transmission line parameters are different at high frequencies than they are at lower frequencies and they are changed for each of the Knowing the system parameters the electric length () of frequencies considered. For calculation sake the final the transmission line are found. See equation 5. transmission line delivering power to a home will be a twenty meter copper cable with a radius of one centimeter. It will be composed of two lines, a ground line and power line.

At the beginning the initial inputs are given.

Lcable  20 Rcable  0.01 Seperationcable  .021 Finally the fully developed formula for Voltage in the 7  7 transmission line at a given point and time is written in c  5.810 air  410 air  1.005 equation 6.

 12 v0  120   602 air  1.510

At this point the low frequency model (equations 1 to 4) Fig xyz is a three dimensional plot of this voltage. Time is must be used to estimate the parameters R, L, G, and C (Fig on one axis and distance down the transmission line is on the 2) of the two line system. other. Fig 5 displays the sixty hertz source voltage is able to maintain a magnitude of a 120 volts down the entire transmission line. Fig 6demonstrates the amount of E-field noise the sixty hertz, 120 volts source creates at a point thirty meters from the halfway point of the line. The normal source voltage creates an E-field signal that is less than five nano- volts/meter.

Figure 5

The fact that voltage barely varies down the transmission line is going to have a significant effect. The E- field inside the transmission line is the derivative of the voltage (Equation 7) Figure 6

Having calculated the noise from the power line the E-field This derivative expands into a much larger expression. noise that results from a 3.5 MHZ wave placed on top of the power signal needs to be found for comparison. The process is similar to what has already been done with a few exceptions. The first being that the high frequency model (Formulas 13-16) must be used for estimating the C, R, L and G parameters of the system. From this point on variables will have a ‘_2’ appended to their names. The electric flux density comes from multiplying the E- field by 0 (Formula 9). This electric flux density is multiplied by a differential surface area as described in fig 3 to get the charge it contains. Finally this differential charge divided by a 4R^2 is integrated the length of the transmission line to calculate the E-field external of the transmission line. (Formulas 10 -12) This E-field is symmetric about the transmission line. It is this E-field that will have an impact on antennas and other devices: Note that z no longer refers to distance down the transmission line, but the axis drawn in fig 3.

From the transmission line parameters to the electric length in equation 17. A graph of the magnitude of this E-field (Fig xyz) shows a signal that peaks at 3 micro volts per meter. The goal was to get a 30 micro volt per meter signal that is known to be produced. However a factor of ten can be explained away as the result of the initial parameters of the system. From these constants the voltage as function of time and position can be graphed. The bold line represents voltage at a distance of zero into the transmission line, it is part of a one volt sinusoid. As the voltage moves into the transmission line the peak of this sinusoid can be seen to drop down.

Figure 4

This three micro volt per meter E-field is six hundred times larger than the E-field produced by the sixty hertz sinusoid. Furthermore if there was an antenna that was tuned for this frequency sitting in a thirty micro volt per meter field it would pick up 2.2 nano-watts of power, when real signals deliver pico-watts of power. However it seems that the FCC is ignoring these issues in the push to get high speed internet access to the masses. Figure 3

[1] HD-Comsys & Itd tim, “Ripple Control Systems,” http://www.tel.hr/hdc- itd/Engleski/RCS.htm. 2004. The internal E-field (equation 18) that results from this [2] Home Plug Power Alliance, “Home Plug Field Test Results,” 2003 [3] Ed Hare. “Calculated Impact on PLC Stations Operating in the Amateur voltage is of a significantly larger magnitude than the internal Radio Service,” 2002. E-field created by the sixty hertz source. Samuel S. Author was born in Missouri in 1982. He is currently a senior EE student and will graduate in 2004 with a BSE with an emphasis in Electrical Engineering from Calvin College, Grand Rapids Mi.. He has previously worked for Delphi Automotive where he maintained and developed software than modeled how parts of an engine’s valve train operate.

With the internal E-field, Internal Electric Flux and the external E-field are found in equations 19 -22.