Key points: New power-line modems use multiple carriers to avoid transmission problems that beset older single-carrier communication gear. • Smart-grid applications for power-line communications could pave the way for using the technology as a less-expensive fieldbus networking scheme.
A fast-food chain recently began installing control networks running among its steamers, toasters, and other food-prep equipment at its restaurants. But you’d never know it from just eyeballing their kitchens. There are no telltale coax lines or other network gear. That’s because all communication takes place over the ordinary ac-power lines.
It looks as though these fast-food kitchens could be harbingers of the future for factories and plant floors of all kinds. The same communication electronics that makes possible demand-response interactions between utilities and power consumers could eventually do away with the need for installing special factory networks.
Modern power-line communication (PLC) schemes are a far cry from predecessors first devised in the 1970s for home automation. Early systems such as the well-known X10 typically communicated over a single carrier frequency superimposed onto power lines. One of the problems with this idea was that power-line noise at the carrier frequency could garble signals to the point where receivers couldn’t decipher them. Moreover, carriers superimposed on one branch of wiring couldn’t reach units sitting on another branch unless a bridging capacitor was installed at the breaker panel to connect the two.
These issues could make home installations of the X10 and systems like it problematic. Industrial or commercial settings — where power-line noise could be orders of magnitude worse than anything seen in houses — were more or less out of the question.
Modern PLC systems avoid such difficulties through use of multiple carriers of different frequencies, on the theory that power-line noise that blocks some carriers won’t block others. Moreover, they can get around the problem of jumping from one wiring branch to another through use of sophisticated error-correction routines that dig out information from carriers that get severely attenuated as they jump between branch lines.
The technique used to encode information in modern PLC systems is called OFDM, for Orthogonal frequency-division multiplexing. It uses a large number of closely spaced orthogonal subcarriers to carry data. (Here the term orthogonal refers to the use of a set of signals with the exact minimum frequency spacing that ensures they do not interfere with each other. Orthogonality arises from the fact that one can model the effect of each subcarrier separately, then combine the models to give a model which predicts the combined effect of varying them jointly.)
In OFDM the data is divided into several parallel streams or channels, one for each subcarrier. Each subcarrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying) at a low symbol rate, maintaining total data rates similar to those of conventional single-carrier modulation schemes in the same bandwidth.
Though PLC systems using OFDM are robust enough to handle industrial duties, they haven’t been widely deployed there partly because, “power-line modem suppliers have focused on homes and on extending WLANs,” says Maxim Integrated Products Inc. Business Director Bart DeCanne. Maxim, Sunnyvale, Calif., makes power-line modem chips that implement OFDM. DeCanne says industrial firms are increasingly looking for ways to get equipment up and running quickly. With PLCs, companies can “reconfigure the floor plan of a job shop without having to rewire the building. And there is a cost advantage from not having to run dedicated wiring,” he explains.
Another reason PLC systems have been slow to catch on in industrial uses is a lack of communication standards. So the technique tended to be deployed only in stand-alone facilities where standards were less of an issue. However, this is changing: Next-generation PLC systems have adopted OFDM. An example is Maxim’s G3-PLC spec which has been endorsed for a national rollout of the automatic meter management (AMM) infrastructure in France by French power utility ERDF. G3-PLC spec is an open specification which is being submitted to IEEE and ITU for a low-frequency narrowband PLC standard.”
Despite a lack of standards, PLC is already used for handling the automation of street and other kinds of lighting. In one case, it was used as an inexpensive way to control a lighting system involving thousands of LEDs. “The legacy system used an RS-485 interface and a BMS protocol to individually address the LEDs in the system. But they had to run dedicated wires to each LED in addition to those for the power supply. When you are talking about thousands of LEDs, that is a tremendous installation cost,” explains DeCanne.
One area of ongoing development for PLC is in facilities for handling super-distributed networks, as might arise with street lights going for miles. According to DeCanne, new power-line modem chips will incorporate repeater functions so they can handle these kinds of kilometer-scale distances.
“Power-line modems can make particular sense where the topology of the network is not a star,” says DeCanne. “In a mesh network, for instance, individual modems must become repeaters to let other modems communicate with those farther out. So current development focuses on implementing repeater functions. Further out, we are working on the ability to hop over repeaters to reach the final destination. We see applications for this technology but we haven’t been able to address them because the necessary firmware has only just become available.”
The difficulty in implementing particularly lengthy strings of communication nodes stems from hopping delays that arise every time a communication packet hits a repeater. Too many hops between talkers and receivers on the line can garble the information transmitted. So the topic is a source of research.
One other factor that has slowed the use of PLC in electrical systems is the fact their signals couldn’t jump past medium (10-kV) and low-voltage (120-V) transformers on the utility power network. This is also changing. “Right now, we feel our parts are the only ones able to cross transformers,” says DeCanne. “You can see a 20-dB attenuation at some frequencies when you go across, and other OFDM modems still can’t overcome this problem because they don’t have sufficiently robust error correction. They can’t correct for the loss of individual carriers that happens when you jump a transformer.” The ability to cross transformers allows for a more-economical rollout as compared with legacy PLC systems because it lowers the number of data concentrators that “back haul” the data via cellular or optical fiber to the power utility.
Maxim claims its robust OFDM modulation (which allows for frequency selective fading), G3-PLC’s strong FEC, and several other provisions, let the modems cope with strong attenuation and interferers found in applications inside plants (like voltage-spike-inducing inductive machinery) as well as MV/LV transformers.
And there are data-rate limitations when PLC is used on power lines that run outdoors. “The bandwidth you are allowed to occupy on the power line is quite limited. Utilities are scared that putting signals on the line that occupy tens of megahertz would turn the power line into a gigantic antenna,” says DeCanne. In the U.S., PLC communication is not allowed in the AM radio band. And above 1.5 MHz, in the various short-wave-radio bands used by Ham radio broadcasters, PLC is a thorny issue. In Japan, stricter rules are in place and any PLC above 500 kHz is illegal.
PLC modems aimed at industrial uses work at higher data rates. One in this category works over a 4.49 to 20.7-MHz frequency band and provides up to a 14-Mbps data rate.
Enter the smart grid
One big impetus behind PLC is the smart grid. OFDM techniques are a way to get control and status information between the power meter and appliances. At least in the U. S., though, wireless techniques rather than PLC have been the main way of handling these chores. The reason is that standards organizations have been slow to settle on standards for smart-grid PLC. Wireless smart-grid schemes generally just use WiFi connections.
Wireless has not been as widely used outside the U. S., however. In Europe and Asia, wireless smart-grid schemes tend to have transmission problems because residential walls generally are thicker and utility meters often sit in basements where it’s hard to get a strong signal. Denser populations in these areas also makes conflicting wireless signals more of a problem. Consequently, PLC modems for these overseas markets have been selling in relatively high volumes. The high-volume sales, in turn, are helping to bring down the price of the ICs.
The dropping chip costs are likely to help make future PLC installations more economical. Costs are also likely to stay reasonable thanks to efforts aimed at devising chips designed to keep costs down within smart-grid facilities in the home. A case in point: A standard called HomePlug Green PHY targets such smart-grid uses as home appliances, smart meters, and HVAC. Where G3-PLC was specifically designed from the ground up for low-frequency narrowband communication, Green PHY is basically a slower and simpler version of HomePlug AV, a PLC scheme for home-entertainment uses. HomePlug devices now account for over 80% of the broadband power-line communication equipment, and over 45 million HomePlug devices have shipped to date, according to the HomePlug Powerline Alliance. The recently created Green PHY spec allows for PLC devices that consume 75% less power than current HomePlug AV gear and reduce the total bill of materials by 75% as well. Messages pass between Green PHY equipment using IP networking (802.2, IPv6) at a 256-Kbps minimum throughput rate. Peak rates hit 10 Mbps.
As with other types of PLC, HomePlug is more widely used outside the U. S. “Europe is the biggest market. HomePlug AV is used for IPTV and video distribution because wall construction over there makes a challenging environment for wireless schemes,” says, Atheros Communications Inc. Senior Product Marketing Manager Jim Zyren. Atheros, Santa Clara, Calif., makes the power-line modem chips upon which HomePlug AV and Green PHY equipment is based and recently got $4.5 million in DoE funding to develop the chips. (Internet Protocol television, or IPTV, is a system through which TV is delivered using the Internet Protocol Suite over a packet-switched network rather than through cable or the airwaves.)
Perhaps the most visible role for HomePlug devices in the U. S. so far has been as a means of feeding Internet signals over home power lines. Electronics stores sell HomePlug-based power-line bridges that are typically used to bring Internet signals from one room into another. The typical configuration is to plug one power-line bridge module into an outlet near where the Internet connection resides, and its mating module into an outlet in another room where resides an HDTV, Blu-ray player, or some other home-entertainment device.
As with Maxim’s power-line modems, HomePlug gear operates at frequencies that can pass via the inherent capacitive coupling between branches of wiring. “You don’t need a pro to install a special coupling capacitor at the breaker panel,” says Zyren. “We get good coupling on power lines because they are long runs of unshielded copper. We’ve found through testing that home wiring talks to itself effectively at 2 to 28 MHz thanks to capacitive coupling between the wires.”
Zyren also says the biggest question for HomePlug connections was whether signal strength was adequate to jump the 24-V step-down transformers used for running HVAC thermostats. But tests show that’s not an issue. “We’ve shown on the bench that you can lose about 40 dB through the transformer. In the field, we’ve found the results to be much better than this when we measured the insertion loss with a network analyzer. We think the signal is propagating around the transformer rather than going through it,” he says.
The latest Green PHY flavor of HomePlug is designed to address a need for smart-grid electronics that don’t consume much power. “The utilities said the original HomePlug chips were costly and consumed too much power for use in electric meters that are sealed in glass. They also didn’t care that HomePlug could hit 200 Mbits/sec,” says Zyren. The answer was to operate Green PHY at a data rate low enough to be considered a high-reliability mode in the normal spec. Even this lower spec provides a 4 to 10-Mbit/sec throughput and will work with conventional HomePlug devices. “We’ve restricted Green PHY to between 7 and 10% of the time on the wire so HomePlug AV networks will coexist with it,” says Zyren.
Indications are that Green PHY won’t just be a smart-grid phenomenon. “I can’t help but think it will find use in other markets as well. There’s nothing that restricts it to smart grid,” says Zyren. Standards are being developed for message protocols for Green PHY. Moreover, HomePlug developers are working on a version of the spec that would increase transmission speeds by a factor of five. Zyren says this should be ready by the first quarter of next year.