Anyone familiar with automatic meter reading (AMR) systems knows that power line communications have proven to be a reliable, efficient and economical approach. The technology serves well for moving metering data between the customer’s meter and the data insertion/extraction point – usually a ‘grey box’ in the utility substation. Several of these systems have been commercially available since the 1970s. The DCSI TWACS™ AMR system and the Cannon Emetcon™ system are examples, joined later by the Hunt Turtle™ and TS-2™ systems.
The so-called ‘ripple’ system for direct control of loads has been used around the world since the 1940s, but today one-way ripple has mostly been supplanted by other technologies. Power line communications for distribution system control were in commercial use by 1949 in vacuum tube versions. Transmission powerline carrier designed for use on transmission lines, and capable of carrying both voice and data, has been widely used for decades.
If power line communications is no newcomer to the electricity utility landscape. What then is all the buzz about Broadband Over Powerline (BPL)?
All the technologies and applications mentioned above are solid, commercially available, well-proven – and very slow by the standards of today’s data communications systems! But slow isn’t necessarily bad. These systems are fast enough for the meter data acquisition job they have to do, and they do that job very well. They operate slowly for a reason. Low data rates require low bandwidths, and low bandwidths can be supported by low carrier frequencies, and low carrier frequencies go much farther and get through transformers much better than high carrier frequencies. This is because low frequencies get closer to the 50 or 60 Hz power frequencies that the distribution system was designed to carry. The PLC AMR systems are designed to economically communicate over many kilometres of overhead and underground distribution wiring, through the distribution transformers, and then down onto the distribution secondaries serving the customer’s premises.
Figure 1 - Basic BPL with different system architectures
These AMR systems provide data rates ranging up to 120 bits per second, compared to 750,000 bits per second and much higher in broadband data systems. But many portions of the systems can operate concurrently, thus magnifying the effective data rate when considered on a system-wide basis. This was good enough in the past, and it is good enough now for most AMR applications. And the signals do go where we want them to. So what is this excitement about broadband over power lines?
The key word is broadband. 25 years ago dial-up telephone modems supported 300 baud data rates. Technology progressed and appeared to top out at 56 kilobaud. Then came digital subscriber line (DSL) with data rates of hundreds of kilobits per second and into megabits per second. Other wired and wireless data com-munications technologies continuously evolve and support many tens and hundreds of megabits as the quest for speed continues.
In the meantime, most businesses and many homes have found broadband data communications to be essential. Fibre, cable, DSL, local and wide-area radio frequency (RF) and satellite have been developed to provide competitive broadband access to meet this need. What if utilities could accomplish this too? Then they could:
- Harvest value from their electricity distribution system assets by using them to disseminate broadband services, including high speed Internet access to consumers. Is there a business case that would allow them to compete commercially with cable and DSL, make some money, and provide broadband access to areas that are un-served or under-served by competitive offerings?
- Propagate high speed, two way data communications throughout their distribution system for the purpose of automating their distribution system and substations, and providing access ‘hot spots’ through which their field personnel could communicate, acquire maps and data, receive work orders and track assets.
- Or both, providing consumers with broadband access and providing data communications backbone support to internal utility applications.
BENEFITS FOR UTILITIES
The technology could be attractive to a utility, especially now with the rapid evolution of Voice over Internet Protocol (VoIP) that allows the broadband data system to supplant connection-based copper wire telephone service – another source of value for many consumers.
In the mid 1990s a firm named Media Fusion burst onto the scene, claiming that its technology could communicate for thousands of miles at high data rates over “the utility grid.” The firm and its questionable offerings were later discredited by many technically qualified observers. In the same period, the NORWEB trial installations of high speed broadband over power line were announced with much fanfare before being summarily shut down, citing concerns with interference and the overall business case. Unfortunately, the early Media Fusion fiasco had cast a long shadow over the nascent BPL industry, and tainted the credible developments of many highly qualified firms. Today there are more than 20 developers of high speed power line communications systems, system vendors, chip suppliers, and system integrators.
Questions about BPL’s technical performance and its potential for interference with other communications have surfaced in the past. Advanced generation chip sets and the products they go into have appeared. In the US, BPL is classified as an unintentional radiator. This classification of an unlicenced system establishes that it may not cause harmful interference with licenced uses, and must tolerate any interference it receives from licenced uses.
The amateur radio community has been especially vocal about the effects of BPL on shortwave radio communications. The mechanisms of interference have been identified, and mitigation techniques have been demonstrated, usually by ‘notching out’ use of frequencies that potentially interfere. Some BPL systems operate entirely at frequencies higher than the 1.7 to 30 MHz range in which most interference problems were alleged. In October 2004 the US Federal Communications Commission (FCC) established rules and procedures for dealing with concerns about interference, allowing this promising technology to move forward. The regulatory bodies of other countries are also addressing this matter.
Other regulatory challenges exist, especially for investor-owned electric utilities (IOUs). These challenges stem from financial treatment and proper compensation for the use of electricity utility distribution system assets for non-utility purposes. Utilities must be careful to structure their activities to avoid allegations of cross subsidisation between the regulated and unregulated activities of their companies.
Perhaps the biggest challenge for any advanced utility technology, including AMR, is the development of a solid and positive business case. This applies, in spades, to BPL. BPL depends upon communication infrastructure that must be purchased, installed, powered and maintained – just like fixed network AMR! Sometimes the business case is positive; sometimes it isn’t.
The business case for BPL has some special challenges, just as AMR does. They are both challenged in geographic areas of very low customer density, where the cost of the communications infrastructure gets spread over just a few customers. This often makes the solution uneconomic. In AMR we presume that we would like to read all the meters, but it is unlikely that all the customers potentially served by BPL will want the high speed access. Some will be satisfied with competitive offerings. Others simply don’t want or don’t wish to pay for broadband service. So in BPL we must always consider ‘take rate’, which is the measure of how many potential customers will sign up for the broadband service over time.
The second crucial ingredient of the business case is the average revenue per user (ARPU). How shall we price our broadband offering? What does the competition charge? Can we make a profit at that price? It isn’t perfectly clear, and it will be very much a case-by-case matter. There are no large BPL systems that have years of experience upon which to make solid comparisons. Fortunately, many vendors, consultants and service providers have very refined modelling tools that can explore the business case with a wide range of assumptions, eliminating many uncertainties.
Figure 2 – Categories of Power Line Communications
There are fundamentally three levels in most BPL systems. There is the system head end and the wide area infrastructure that brings high speed, bi-directional access, services and content to the utility distribution system, usually by optical fibre. It is most economic to inject and extract BPL signals at a location, not necessarily the substation, where the fibre and distribution system cross paths. The high speed communications are coupled onto the distribution primaries or medium voltage (MV). These communications then propagate along the feeder, eventually reaching the distribution transformer that drops the medium voltage down to the service voltage entering the customer’s premises. In the US, 1 to 8 customers are typically served by a distribution transformer. In Europe, parts of Asia and elsewhere, there may be 100 to 600 customers served by a single distribution transformer. This difference can have a dramatic effect on system economics.
DEALING WITH DISTRIBUTION TRANSFORMERS
Distribution transformers are notoriously hostile to power line communications. They are, after all, designed to efficiently pass 50 Hz or 60 Hz frequencies, but not 60,000 Hz or 60,000,000 Hz! The developers of power line communi-cations for AMR systems 30 years ago learned that North American distribution transformers started to severely attenuate frequencies at 15 kHz to 20 kHz. Since these systems need to communicate through transformers, frequencies below 15 kHz were chosen by all of the commercially successful suppliers. But that choice is not available to BPL systems that must support vastly higher data rates than are needed for AMR.
BPL systems must deal with the distribution transformer. Most vendors extract the BPL communications from the MV distribution primary line, ahead of the transformer, using an inductive coupler with appropriate cutouts and fusing. The signal is then processed in another grey box and sent to the consumer’s premises.
How? That last few hundred metres into the customer’s premises can be achieved using several different technologies. Some BPL vendors use a different short distance powerline communications method, such as HomePlug®, in which the communications are placed onto the low voltage secondary side of the distribution transformer, and will thus appear at the consumer’s wall outlets. Other vendors have chosen to use the popular 802.11 WiFi short range radio. At least one vendor does communicate through the distribution transformer, using various techniques to mitigate the usual signal attenuation. There are merits to each approach.
So, to review, the three levels of a typical BPL system are:
- The head end and wide area infrastructure, usually optical fibre.
- The BPL communications infrastructure on the MV distribution primaries, including the injection/extraction equipment, repeaters/regenerators (if needed) and couplers.
- The short distance communication from the distribution transformer into the customer’s premises.
Utilities that are seriously examining BPL tend to fall into either of two camps. So far, most utilities have been intrigued by the idea of using their distribution systems as the backbone of a communications architecture for providing competitive broadband service and Internet access to consumers. Other utilities see BPL as the enabling technology for their vision of the smart grid in which high speed data communication supports a host of new information and automation possi-bilities that could enhance quality of service and reliability.
And, as you would expect, many utilities are potentially interested in both, but are characteristically conservative in making bold pioneer moves into a new technology. So the adoption of BPL has taken the path of many pilot system deployments accompanying a smaller number of commercially significant installations.
Now let’s explore that potential intersection between BPL and AMR. Assume that an electric utility has now installed BPL coverage of its entire service territory. BPL is established everywhere on the medium voltage and all distribution transformers have couplers and some ‘grey box’ means of communicating into the customer’s premises. Can’t we now simply build a Wi-Fi module or HomePlug module into a meter, and let the meter communicate as a node on the BPL system, just as if it were a customer’s home computer? Yes, we can – it is relatively straightforward, and is being done in some BPL trials. But we had a lot of assumptions that were in our favour.
Many BPL scenarios lean toward a ‘smart build’ strategy. With smart build you only incur the costs of building out the BPL system where there will be an adequately high take rate by consumers to justify the expense. This means that you may not build out in an urban environment, where there are many low cost and readily available broadband alternatives. And you may not be able to justify build out in the deep rural environment, where there is only one customer per transformer, and where a cascade of repeaters is needed to reach that customer. And you may choose not to build out unless you can be assured of at least two BPL customers or more per distribution transformer.
It is all about the economics. In this scenario we don’t have anything like ubiquitous coverage. Is that adequate for AMR? In many cases it won’t be, at least with short range Wi-Fi or HomePlug embedded in the meter.
Fixed network AMR systems exist in a hierarchical structure, as do BPL systems. Familiar AMR-equipped meters have a low cost communications device that typically communicates with a data collector/data concentrator a short distance away. This data concentrator then aggregates the metering data from all its nearby sites, and often adds extra functionality such as time tagging and buffering.
Next it hands the data off to another technology, moving the data further up the food chain. Now, if this data concentrator is tied into the BPL system on the MV primaries, we make good use of the BPL infrastructure. This contrasts with the questionable approach of having individual residential meters communicating into the BPL system. Now the economics start to look a lot better, and we retain the use of proven AMR devices, already manufactured in high volumes. Now we are preserving the proven low cost AMR communication devices in the meter. Now we are not so concerned with the economics of whether two customers per transformer are signed up for BPL broadband service. Now we are retaining the use of the neigh-bourhood data concentrator and the ‘intelligent’ functions it performs. Now we are using the medium voltage BPL as the backhaul for the AMR system data, but not as the direct link to the meters.
Each new technology challenges us to think how it can be used to best advantage. Sometimes we confuse what can be done with what should be done. BPL is an interesting and promising new arrow in the utility’s quiver of telecommunications and business alternatives. The challenge is to apply all new technologies when and where they are better suited to the task than alternatives.
At Plexus Research, our consulting engineering team often begins a new AMR project confronted by utility questions about BPL, wondering “Why don’t we just do the AMR on a BPL system?” Or, “If we can provide broadband communications throughout our distribution system, why wouldn’t we use that for everything – including AMR?” These are good questions, and that time may eventually come. But even now, AMR and BPL can complement one another, as we have seen here. Utilities that can justify AMR should proceed to deploy AMR systems, just as they have been doing. If these utilities eventually also deploy BPL, either selectively or ubiquitously, the option will always exist to use BPL for efficient backhaul of metering data.