Cellular Access for Substation Distribution Automation
by Tom Trexler, Teltone Corporation
As a result of deregulation within the electric utility industry and the ensuing increase in competition, all utilities–whether they be investor-owned, municipals, co-ops or government entities–are finding themselves in a position of either differentiating their product, attempting to reduce costs or both. The laws of the free market dictate that only the most efficient will survive, and in fact thrive, at the expense of the less-efficient competition. The recent wave of mergers and acquisitions is an attempt to forge strategic alliances and realize advantages through economies of scale and tactical positioning. While mergers and acquisitions are usually reserved for the boardroom, providing better customer service and cost-saving ideas is within the reach of most utility employees.
Differentiating your kilowatt from the guy`s down the street is not the easiest of exercises. Mr. Customer has a tendency to focus on price, and to some extent that will never change. However, there are many customers–typically large commercial and industrial customers–who also demand dependable, flexible and intelligent solutions to their energy needs. These customers make up, if not a majority of utility`s revenues, certainly a large portion of it`s income. So it is not a coincidence that when competitors decide to invade a neighboring market, the first target is the large energy user.
As a result, there has been an increasing amount of capital spending directed towards substation automation as utilities strive to provide better customer service to the large energy user. This will promote brand loyalty and enable the utility to not only protect, but possibly invade competitive markets. Intelligent Electronic Devices (IED), such as electronic meters, microprocessor-based relays and digital fault recorders, have been steadily finding their way into both transmission and distribution substations due in part to this phenomenon.
Once installed, these IEDs–many of which may be from different manufacturers–must be tied together into an enterprise-wide information network. The information needs to be accessed, downloaded and transported over some means of communication medium to various host/polling locations.
More and more utilities are turning to cellular communication in the substation as a reliable, cost-effective and quick method of linking these IEDs to their information network–and there is good reason. Three major factors have been changing in the last few years that have dramatically altered the economics and performance characteristics of using cellular service.
The first factor is the increase in the overall number of cell sites. Cell sites have exploded in most of the medium and highly populated areas of the United States. This provides excellent coverage in areas that a few years ago were either too saturated during peak demand times, or too remote to provide signal strength to reliably connect and transmit voice and data.
The second factor is the cost of cellular service. Cellular providers have dramatically reduced the price of cellular service across the board to attract new customers to offset high capital investment and to stay competitive with the other cellular providers in the area. Most utilities can obtain very favorable cellular rates that individual consumers can only dream about.
The third factor is advances made in technology and, in particular, advances made in cellular transmission rates. Several years ago, you would be lucky to attain 2,400-bps transmission rates over the slightest impaired cellular circuit. Today, advances by cellular modem manufacturers permit transmission rates of 9,600 bps or more through advanced-level algorithms and proprietary protocols that do an excellent job of error correcting and retraining. This improved speed can save substantial money by lowering the air time required to download data. This is especially important as the amount of data being stored in IEDs continues to increase.
Cellular service also compares favorably when stacked up against dealing with the local exchange carrier. For one, there is no waiting on the telephone company to come out and install the line. Most cellular service can be activated in about an hour, compared to days, weeks or even months for the telephone company. Second, since there is no telephone company installing a new line, there is no telephone company installation charge that, in some cases, can be very expensive. Finally, many telephone companies require expensive telephone isolation equipment to protect their lines from large ground potential rise. Charges can range between leasing equipment from the telephone company for up to $100 per month per line, to buying the equipment for thousands of dollars. Naturally, with cellular service this consideration is completely eliminated.
AEP Uses Cellular Technology
American Electric Power (AEP), based in Columbus, Ohio, has begun utilizing cellular technology to gain access into substations. AEP currently utilizes cellular communication to gain access to their Davidson substation, located in the fast-growing suburb of Hilliard, and to a substation located in New Philadelphia, Ohio.
Dave Augenstein, senior engineer at AEP, said that the Davidson location is a “high-tech, automated substation” where AEP uses a substation-hardened, cellular-communication interface unit that feeds into a telephone-line-sharing switch. From the telephone switch, AEP runs a phone from port 1 for voice, a data concentrator (PC) that has an RS-485, daisy-chain bus running throughout the substation to many different IEDs from port 2, a GE/DR 87 meter from port 3 and a closed-loop heat pump from port 4. AEP utilizes cellular modems with this setup and has consistently been able to attain 9,600 bps throughput.
The data concentrator, the heart of the substation automation, is a Tasnet, HMI with a 17-inch SGVA monitor. The data concentrator has two RS-485 serial connections that interface with AEP`s LAN and a backup LAN. The data-highway connects with network-interface modules (NIM). In this application, two NIMs act as gateways to ABB relays, another to a back-up master, and yet another to AEP`s SCADA, Multiple Address Radio System.
The New Philadelphia location also utilizes a substation-hardened, cellular-interface unit, and was installed primarily to provide voice communication at the substation for safety reasons. In both cases, Augenstein indicated that the decision to use cellular instead of landlines was based on total cost and installation-time considerations.
Dave Augenstein, senior engineer at AEP, uses a cellular handset at the Davidson substation
AEP`S CELLULAR COMMUNICATION SYSTEM AT THE DAVIDSON SUBSTATION