Cobb Electric Membership Corp. (Cobb EMC; Kennesaw, Georgia, U.S.) has reduced outage restoration time from more than an hour to just five minutes by employing a distribution automation system with wireless monitoring technology in areas served by manual switches.
Cobb EMC has more than 450 gang-operated disconnect switches with fault-detection capability. In the past, an outage would de-energize the entire feeder, requiring at least an hour to field a crew, locate the outage and restore power to everyone except those on the affected section. With the new wireless monitors located at intervals along the feeder, signals are transmitted within a few minutes of the fault occurrence, allowing the control center to determine the location of the fault and restore power without mobilizing a crew.
Cobb EMC uses Numerex's Cellemetry network, a two-way communications option that employs mass-produced cellular modems and the existing cellular network. This approach adds only a fraction of the normal communications hardware costs to the remote terminal units (RTUs).
Locating the Fault
The challenge in locating a fault has always been pinpointing the location so it can be quickly bypassed by the distribution system. Cobb EMC has installed automated disconnect switches in some locations, but economics dictated manual switches for others. These switches make it possible to redistribute power and to quickly restore service to the majority of customers affected by the outage.
The fault indicators are arranged 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; however, it still took almost half an hour to assemble the crew. During off hours, this time could extend to almost an hour.
After assembly, the crew would drive down the length of the affected feeder to visually inspect the detectors that would be flashing up to the point of the fault. This technique made it possible for the crew to isolate the affected section and report back so the automation system could bypass that section.
An Economical Solution
Because this procedure left customers without power for up to an hour, company engineers considered a variety of remote monitoring systems to rectify the problem. Most of these systems proved to be expensive. For example, a 900-MHz two-way wireless system adds costs of about US$1000 for each monitor. In addition, the company would be faced with either the high cost of constructing its own base communications system or incur high monthly charges in using an existing network. Any solution that involved the installation of a telephone line would have required a minimum monthly charge of $40 for each monitoring point. At this price, the cost of monitoring the entire network would have been too high.
A team organized to develop a low-cost monitoring system focused on the Fisher Pierce system, which took a unique approach to low-cost monitoring. Fisher Pierce, a division of Danaher Corp. (Weymouth, Massachusetts, U.S.), specializes in the development of electronic sensing and control equipment. By using the control system of the everyday analog cellular network and sending small data packets, the system never intrudes on the voice channel, resulting in a monthly cost per monitoring point of only $5. Fisher Pierce's SmartLink monitoring package, which is specially designed for feeder automation applications, only costs $150 to allow two-way communications. To be convinced that a solution as inexpensive as this one would really work, company engineers set up a trial device for field-testing; the solution met all specifications. Cobb EMC has now installed 13 devices and has experienced no serious problems.
Mimicking a Roaming Cellular Phone
The Cellemetry network mimics a roaming cellular telephone, which requires the phone to register on the local cellular network when it is beyond its designated call area. This process, called roaming registration, is transparent to the user. During the roaming setup sequence, the cellular system verifies that the phone attempting to make a call is valid. While verification proceeds, the system transmits a short message over the cellular control channel, typically taking less than 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 data channel technology has several major advantages over wireline, conventional cellular and satellite data services, particularly for applications with small data requirements. First, the costs are considerably lower than the alternatives, while the reliability is high and the reach of the service covers almost all of 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 communicate with a cellular transceiver at a distance far beyond that of cellular voice capability, making it possible to easily cover remote facilities.
Integration with the Control Center
Cobb 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 from a step-down transformer with battery backup. A fully solar-powered version is anticipated for the future.
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, which dials into a PC located in Cobb's control center. Every indicator up to the point of the fault sends an alarm. The PC, which runs a Fisher Pierce application, flashes red to provide a visual alert, while a text message indicates the location of the fault. This entire process can be completed in less than one minute. Within a few more minutes, the control center can restore power through its switching system and dispatch a crew to fix the fault. Under these circumstances, by eliminating the time previously required to visually inspect the line in searching for the fault location, the cost of the monitors is more than fully covered.
Since the concept was validated, Cobb has used the technology in conjunction with its other automated equipment. The SmartLink is used for fault protection at major taps or underground risers that don't have automated switches, providing the performance of an automated switch at a much lower cost.
In the deregulated utility market, where the emphasis is to provide better customer service than potential competitors while keeping costs low, these monitors restore service quickly while reducing operating costs. The goal for ensuring reliable service is important to Cobb, whose distribution system covers more than 7000 miles of line, serving more than 150,000 members, 92% of which are residential services. Cobb is the largest of Georgia's 42 EMCs, the second largest retail supplier in the state and the second largest of the nation's 900 EMCs.
Corbin Clift is manager of special projects for Cobb Electric Membership Corp. Clift joined Cobb in 1974 as a power engineer and, in 1985, was promoted to director of engineering design, which included the mapping department. In 1990, the position of manager of special projects was created to develop, design and direct projects that would maintain a reliable and economical distribution system. Clift's responsibilities include directing the operations of the SCADA system, the distribution automation system and the mobile map project, Cobb Mobile Maps System.