Mecklenburg Electric Cooper-ative is one of many rural cooperatives in the United States that came into being in the late 1930's to supply electricity to customers too few and too remote for regular power companies to serve. Like many established utilities, Mecklenburg adopted a Supervisory Control and Data Acquisition (SCADA) system for handling its telemetric needs. Mecklenburg operates 22 substations that are managed remotely through radio modem links. Mecklenburg also utilizes a Load Management system that has saved the Cooperative a great deal of money since its inception in 1985. However, the hardware that supported the SCADA system was old and outdated.
" We were trying to keep our old system going as long as we could, and we managed to do that for a couple of years," said Kim LaTulipe, electrical engineer with Mecklenburg. The SCADA system needed to be totally upgraded, but there was a problem involved. The upgrade would mean separating out the SCADA system from the Load Management System that managed power during peak operating load times. Mecklenburg would have two systems that would be virtually incompatible with each other, and only one communications path. The choices to resolve the problem were limited.
" We did not want to add a second communications path," said LaTulipe. " The licensing of the frequency range we were using was frozen a couple of years ago by the FCC, so we couldn't add an additional radio line. Leasing telephone lines was far too costly. Installation for the leased telephone line was estimated at $200,000 and the monthly costs of the leased lines would have amounted to $6,000 per month for the 20 substations, or over $70,000 per year. Mecklenburg passes costs directly to our consumers, so we could not afford an expensive project. We had to find some way for the two systems to share the same communication path, in an application with a very low margin for error."
One solution seemed possible: using statistical multiplexers (MUX) to run both the SCADA system and the Load Management System over the single radio line.
" The engineer for the manufacturer of our SCADA system mentioned statistical multiplexers," LaTulipe said. " They told me that they did not work well, and that they weren't interested in helping us because they couldn't guarantee that a statistical MUX setup would work."
LaTulipe refused to give up and she started searching on the Internet for information on statistical MUXÕs. She came up with a company called Data Comm for Business (DCB), a manufacturer and distributor of a diverse line of data communication devices. From the first phone call to Russ Straayer, president of DCB, LaTulipe knew she had found her solution.
" Russ told me that there was no problem, they could help us use statistical MUXs to run both systems over the same radio line." LaTulipe said. " All I was concerned with then was how much it was going to cost."
LaTulipe found that it was going to cost only about a quarter of the installation charge to lease telephone lines, and there would be no continuing cost burden. Not only had LaTulipe found a technological solution to Mecklenburg's problem of running two incompatible systems over the same communications path, but it was going to save the Cooperative over $70,000 in leased line costs per year. It seemed the ideal solution. Now it had to be implemented.
The overall project was complicated and involved much more than just getting statistical multiplexers.
" We were doing radio upgrades, Remote Terminal Unit (RTU) upgrades and additionally we were implementing a new communications system withe the statistical MUXs. We had a lot going on at the same time and we were trying to make this all work together." said LaTulipe. " Having one system work is pretty impressive, but getting three to work at the same time is a little hairy."
The overall project for installing the statistical MXUs was broken down into two parts: the one substation that did run on a leased line was to be installed first, with the remaining 20 substations scheduled for later. Basically, the existing multi-drop radial system would have the statistical MUXs added to it, creating two side-buy-side systems that ran over the same communication path. This first project revealed one problem that was encountered with implementing DCB's statistical MUX.
SCADA systems require their data sent in a way that is different from the method most standard protocols use. Instead of being broken down into packets, SCADA systems expect the data to be contiguous. Packeted data can generate communication errors that cause the system to not respond to polls. Unless the data can be properly transmitted and received, the system just will not work. LaTulipe encountered this problem with the first sub-station installation.
" DCB did some programming changes on the chips in the MUX to make sure that the polls and responses were delivered intact. In the second phase of the project other problems were encountered with radio-keying error correction and data integrity," LaTulipe said. " We had to make sure that the MUX's were set up to function in a half-duplex mode to the radio modem properly. DCB also did some follow-up programming on the host end to disable error correction in the MUXs. Also, programming was done to make sure the request-to-send (RTS) on the remote end followed through to the host end, so the host would not think the remote was through talking before it really was."
Without DCB's help, LaTulipe doubts that the system would have worked and enabled Mecklenburg to save over $70,000 a year. " DCB provided us with a way to share the same communications path for two separate systems," says LaTulipe. "The beauty of it is that it will work with any system. It doesn't matter what two systems you have, because it isn't protocol dependent. Also, you can mix speeds on it. Our SCADA system is 9600 baud and the Load Management is 300 baud. Therefore, we are mixing two different protocols and two different baud rates, and DCB's statistical MUXs do not care. It is a very off-the-shelf application that gives you great flexibility.
LaTulipe is a hundred percent happy with the resulting system. " There's not anything left to do on the system, really," LaTulipe said. "We have the latest master station and our communications have been upgraded to 9600 baud. So we're good for at least the next ten years."