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Wireless Monitor Helps Cut Service
Downtime from 60 to 5 Minutes
A Special WDN Contribution from NumereX, Aug. 20, 2002
Enhancing a distribution automation system with a wireless monitoring technology helps an
electric utility reduce the time required to restore partial service after an outage from 60 to 5
minutes in areas served by manual switches. Cobb Electric Membership Corporation in
Kennesaw, Georgia, has over 350 gang operated disconnect switches with fault detection
capability. In the past, an outage meant the entire feeder line went down and it took an hour or so
to field a crew, locate the outage, and restore power to everyone except those on the affected
section. Now, the new wireless monitors, located at intervals along the feeder, send a signal
within a few minutes of the fault, allowing the control center to easily determine its location and
restore power without sending out a crew. Numerex's Cellemetry network, the two way
communications option used by Cobb, is very economical, utilizing mass produced cellular
modems and the existing cellular network. This combination provides two way communications
service for only a few dollars per month while adding only a fraction of the normal
communications hardware costs to the RTU.
Challenge of locating the fault
Formed in 1938, Cobb Electric Membership Corporation (EMC) started as an electric utility with
489 residential members and 14 commercial accounts. With over 150,000 members and growing,
Cobb EMC is the largest of Georgia's 42 EMCs, the second largest retail supplier of electricity in
Georgia, and the second largest of the nation's approximately 900 EMCs. As a not for profit,
customer-owned corporation, Cobb EMC is dedicated to providing its member/owners with the
best service at the lowest possible price. The challenge has always been pinpointing the location
of outages so they can be quickly bypassed by the distribution system. The company has
installed automated disconnect switches in some locations but economics dictated manual
switches for others. These switches make it possible to redistribute power and quickly restore
service to the great majority of customers affected by an outage.
The utility has fault indicators arrayed strategically throughout most feeder lines with visual
indicators that flash from the time of the fault until they are reset. In the past, as soon as the
outage occurred, the manager in the control center would assign a field crew to locate the outage.
It could take 20 or 25 minutes to scramble the crew during the day or perhaps 45 minutes during
off hours. The crew then drove down the length of the affected feeder line for the purpose of
visually inspecting the fault detectors. Normally, every detector would trip and provide a visual
signal up to the point of the fault, while detectors located after the fault would not trip. This made
it possible for the crew, typically within 20 to 30 minutes, to isolate the affected section and report
back so the automation system could bypass it. Because Cobb EMC is located in a congested
suburb of Atlanta, slow traffic sometimes substantially delayed the visual line survey.
Finding an economical solution
Of course, this approach leaves the company’s customers without power for an hour or so. Cobb
EMC engineers considered a variety of remote monitoring systems but each of them was quite
expensive. A 900 MHz two-way wireless system adds costs in the neighborhood of $700 to
$1200 to each monitor's cost and the company would also have been faced either with the very
high cost of constructing its own base system or the hefty monthly charges involved in using an
existing network. Any solution that involved installation of a telephone line would have required a
minimum monthly line charge of $40 or so for each monitoring point. At these prices, the cost of
monitoring the entire network would have been too high to make it economical. The engineers
continued their search for a less expensive yet still reliable monitoring method.
Corbitt Clift, Manager of Special Projects for Cobb EMC, led the team assigned to develop a low
cost monitoring solution. “I was interested when I heard about the Fisher Pierce system because
it was a totally unique approach to low-cost monitoring,” Clift said. Fisher Pierce, a division of
Danaher Corporation located in Weymouth, Massachusetts specializes in the development of
electronic sensing and control equipment for the electric utility industry. “Basically, they use the
control system of the every-day analog cellular network. Because they send very small data
packets, they never have to intrude on the voice channel. The result is that the cost per
monitoring point is very low, only about $5 per month. Their SmartLink monitoring package, which
is specially designed for feeder automation applications, only costs about $150 to allow two-way
communications. I had to be convinced that a solution that was this inexpensive would really work
so I asked them to set up a trial device and put it on in the field for a few months. It passed with
flying colors. We now have 13 devices out in the field and have never had any serious problems.”
Mimicking a roaming cellular phone
Numerex's Cellemetry network works by mimicking a roaming cellular telephone. A person
turning on a cellular phone outside the local service area causes the phone to register on the
local cellular network before obtaining service. This process is called roaming registration and is
transparent to the user. During the roaming setup sequence, the cellular system verifies that the
phone attempting to make calls is valid. During the process, the system transmits a short
message over the cellular control channel typically taking less then 30 seconds and never tying
up the voice channel of the cellular system. Like a roaming registration, SmartLink messages are
sent to a Cellemetry Gateway that provides immediate transmission of data to the Cellemetry
Data Service customer.
This unique data channel technology has several major advantages over the alternatives of
wireline, conventional cellular and satellite data services, particularly for applications whose data
size requirements are small. First, the costs are considerably lower than the alternatives yet the
reliability and reach of the service virtually covers North America. Additionally, the small size of
the data packets provides a robustness that extends the range of the service considerably
beyond conventional cellular networks. For example, these data transmissions can penetrate
multiple building walls without difficulty and can typically communicate with a cellular transceiver
at a distance far beyond that of cellular voice capability, making it possible to easily cover remote
facilities.
Integrating with control center
Cobb EMC uses several versions of the SmartLink monitor, which is available in different models
for feeder fault monitoring, capacitor or sectionalizing switch control and status or end of feeder
voltage monitoring. The device is powered from the distribution line via a step down transformer.
Battery backup is provided and a fully solar powered version will be available soon. When a fault
occurs, the device’s transmitter reports the faulted phase through the cellular system to a base
station operated by Cellemetry Data Service. The alarm is immediately routed to Fisher Pierce’s
network that dials into a PC located in Cobb’s control center. Every indicator up to the point of the
fault sends an alarm and the PC, which runs an application developed by Fisher Pierce, flashes
red to provide a visual alert while a text message indicates the location of the fault. This entire
process can be completed within 35 seconds to one minute of fault occurance. Within a few
minutes, the Cobb EMC control center can restore power through their switching system and
dispatch a crew to fix the fault. Eliminating the time previously required to visually inspect the line
more than pays for the cost of the monitors.
Clift said, after validating the concept, Cobb EMC has begun to utilize it extensively in conjunction
with its other automation equipment. “We use the SmartLink for fault protection at major taps or
underground risers that don’t have automated switches. Basically they allow us to provide the
performance of an automated switch at a much lower cost. In the new deregulated utility market,
the bottom line is providing better customer service than competitors while keeping costs low.
These monitors help us restore power much more quickly than we were able to do in the past
while actually reducing our operating costs. So it shouldn’t come as any surprise that we are
planning to add a considerable number of these units in the near future.”
Cobb EMC's distribution system covers approximately 415 square miles. The physical plant
consists of over 7,300 miles of line located within Bartow, Cherokee, Cobb, Fulton, and Paulding
Counties. In 1999, Cobb EMC sold 2.8 billion kilowatt hours of electricity with revenue exceeding
$228 million. Cobb EMC's customer base is 92 percent residential and 8 percent
commercial/industrial. Based on total kilowatt hour sales, revenue is approximately 30 percent
commercial/industrial and 70 percent residential. Cobb EMC's total utility plant is valued at over
$371 million, with assets of over $447 million.
For more information about Cellemetry Data Service technology, contact NumereX 1600
Parkwood Circle Ste. 200 Atlanta, GA. 30339 or visit the NumereX Web site at www.nmrx.com.
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