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While the unit reads slowly, it has a high comprehension.
More than 100 rural electric cooperatives (four affiliated with Minnkota Power Cooperative, Grand Forks, North Dakota, U.S.) and at least three investor-owned utilities have ordered or are operating the latest in slow-speed, ow-cost communication systems for automatic meter reading. Known as the Turtle, the digital system can communicate accurately over long distances via power line carrier, making it especially cost effective for spread-out rural-type distribution systems.
Minnkota-associated cooperatives who have been installing and testing the Turtle system include Beltrami Electric Cooperative, Bemidji, Minnesota, U.S.; Clearwater-Polk Electric Cooperative, Bagley, Minnesota; Cass County Electric Cooperative, Kindred, North Dakota; and Roseau Electric Cooperative, Roseau, Minnesota.
Turtle's technology resulted from a 1990 National Rural Electric Cooperative Association (NRECA) US$ 187,000 contract with Hunt Technologies, Inc., Pequot Lakes, Minnesota. NRECA directed Hunt to research what is known as ultra narrow bandwidth communication (UNB) on distribution circuits. The goal: develop a low-cost method of communication that doesn't require speed for applications within the industry.
The system consists of a Turtle meter reader transmitter in each meter that sends its signal through the power line. The signal is monitored by one channel per meter on the Turtle receiver's circuit cards in the substation. The receiver, in turn, converts the signals to RS-232 for sending to a central office by SCADA or telephone (Fig. 1).
UNB Makes PLC Attractive
As explained in the NRECA report and an IEEE tutorial paper by Turtle's inventor, Paul C. Hunt, bandwidth of a communications signal is the range of frequencies it occupies. Wide bandwidth signals require more energy to overcome noise than do narrow bandwidth signals. For comparison, to achieve similar range, a TV transmitter (6000 kHz bandwidth) requires 200,000 W; a music broadcast transmitter (60 kHz bandwidth) requires 2000 W; and a voice-only transmitter (3 kHz bandwidth) requires 100 W. However, a UNB transmitter (0.00001 kHz bandwidth) requires only 0.003 W. Thus, Morse code transmissions can get through when voice cannot, and conversely, the faster a signal sends information, the more bandwidth it requires.
UNB's disadvantage is slow speed. It can only be used in applications where the data can be tolerated to arrive slowly, on the order...