Underground service and a truly self-healing distribution system improves customer service within the automation zone.
The growth of Murfreesboro, Tennessee, U.S., presented the Murfreesboro Electric Department (MED) with a unique opportunity. In 2005, the city constructed Medical Center Parkway, a 4-mile (6.4-km) gateway from the interstate to the town center. In addition to a new hospital with more than 200 beds, many new businesses were attracted to this location. A new Embassy Suites Hotel with a convention center was planned, and the area attracted a 100-acre (40.5-hectare) mall along with many other medical offices, shopping venues and restaurants.
To enhance the community and area around Medical Center Parkway, city leaders determined the area should be served by underground facilities. The city installed duct banks for MED to use to supply the area with electricity. The utility had never installed underground feeders that served more than 400 A. To provide service to this area, multiple feeder circuits capable of 800 A would be needed. The major questions were what routes and cable size could be used to serve this new load and what protection equipment should be used.
MED decided to use parallel runs of Okonite's Okoguard ethylene-propylene-rubber 500-kcmil copper 15-kV primary cable for the main feeders, each of which would supply approximately 800 A of capacity. In 2006, three feeders and two substations were available to supply Medical Center Parkway. The growth in this area dictated feeds from several different substations. Four circuits now feed this area, with plans for three more circuits and a new substation.
Underground distribution protection is different than overhead distribution protection. An outage on overhead lines generally can be located by sight. With underground lines, equipment that isolates the problem to a smaller area is used and sections are tested one at a time. Fault detectors can help locate the fault, but unless sectionalizing devices are in the line, underground outages can take hours to locate. Automation helps the fault location process by sectionalizing the line automatically, greatly reducing outage time and not impacting reliability index numbers.
MED then turned to the question of protection. The utility found that below-grade equipment at the 800-A to 900-A range was not readily available; only a few vendors were able to provide the required switching equipment. MED decided on S&C Electric Co.'s Vista equipment, which is submersible and rated for 900 A. Vaults manufactured by CDR Systems Corp. were large enough to accommodate Vista equipment with up to six switches or fused sections. When the lid tops are open, one can treat the equipment as open air, eliminating the need to provide forced air for confined spaces. MED then decided to look at the possibility of distribution automation for the Medical Center Parkway.
The decision to automate distribution is both hard and easy. The quantitative justification is difficult, but the qualitative improvements to customers and the community are significant. The challenge is cost justification. Since one automated, padmounted switch can cost more than US$100,000, the challenge is not trivial. Lost revenue from a 30-minute outage of a 500-A primary circuit is about $100 to the utility. Clearly, this is not the determining factor.
However, customer losses can be significant. The cumulative effect of a 30-minute outage that impacts 150 commercial customers, each of whom loses only $100 in revenue during the event, creates a community cost of $15,000. The actual lost revenue cost is most likely much higher than this figure, which makes cost justification for automation easier. Payback for that $100,000 equipment is probably close to four to five events. Payback for an overhead switch can be one to two events.
The decision was made to begin automating the area around the hospital. Once MED decided to install distribution automation, an initial plan was made for five Vista switchgear locations along the Medical Center Parkway. The CDR vaults were installed at critical locations and a Vista 642 (six bays, four switches, two fault-interrupting bays) and Vista 633 (six bays, three switches, three fault-interrupting bays) were ordered.
MED also made plans to install five overhead S&C Scada-Mate switches and one padmounted switch with motor operators in the surrounding area. Five breakers at two different substations were equipped with S&C Universal Interface Modules, which allowed the breakers to communicate with the other distribution automation switches and work together to create automation zones of protection on the distribution system. All this equipment included the S&C 5800 Series Automatic Switch Control with IntelliTEAM II Automatic Restoration System software. The communications medium was Landis+Gyr's spread-spectrum UtiliNet radios that hopped communications from one radio to another. Each switch location communicates with its neighbors to form a mesh network, so each switch knows what the next switch in the zone is doing.
As new customers have been added to the electric system in the Medical Center Parkway area, MED has added to the automation system. Some of the locations along Medical Center Parkway were preplanned, but an automation system almost always has a natural growth progression. The more area that is covered, the easier it is to see where the next piece of automated switchgear could be placed.
Communications is a key part of distribution automation technology. MED has gone through one complete change to newer, faster S&C SpeedNet radios. These new radios help speed up communications, which is important to the speed of response of the distribution automation equipment. The utility is making plans to replace some of the radios with Ethernet fiber ports that will make communications almost instantaneous. All the radios report back to a head-end radio at MED's Primary substation, connected to MED's main office with fiber. In turn, this connection supplies information to a proxy server that collects all the current switch data and reports it to an EFACEC Advanced Control Systems supervisory control and data acquisition (SCADA) system.
MED operations staff can observe, record and operate all the automatic switches from the operations center in the main office through the EFACEC Advanced Control Systems SCADA system. Engineers or operations staff also can access the equipment remotely using the configuration and diagnostic software. This is a valuable engineering tool for maintenance and troubleshooting.
To date, there have been five automation events where portions of the distribution feeder had to be reconfigured to keep the power on for customers. Two of the events occurred when two separate tornadoes swept through the city. All of the events to date have been weather related on the overhead distribution system. The largest automation event occurred in August 2010 when a violent lightning storm came through the city.
MED's Primary and Blackman substations are the two current substations that feed into the Medical Center Parkway area. Primary substation has three breakers that feed into the automation area and Blackman has one breaker that feeds into the area. The August 2010 storm caused an event that tripped the whole yard at Primary substation, including Tennessee Valley Authority's 161-kV switchyard. MED lost three feeds into the automation area, the worst-case scenario. The one circuit feeder that was left picked up almost 60% of the circuits that were out in less than two minutes. The outage to the substation was more than 45 minutes long.
The process worked as planned. As another section energized, the system kept up with how much new load was being added. When the Blackman feeder got to around 760 A, the automatic switching stopped because no more capacity was left. The automation logic looks at available spare capacity when making restoration decisions, so the alternate feeder will not overload. Without automation, this event would have left the new Middle Tennessee Medical Center hospital in the dark. Since the automation was in place, the hospital was out of power for less than a minute. The lesson learned is that with automation, feeder capacity is very important. Excess capacity is necessary for feeders that supply the automation zone.
A month before the August event occurred, another lightning storm caused the same type of event where the Primary substation and Tennessee Valley Authority switchyard were tripped. All the automation switches opened on the primary circuits, but the backfeed operation failed because of a bad battery on the motor operator at the open point between Primary substation and Blackman substation. Battery maintenance is critical to keeping the automation system ready to operate at a moment's notice. Equipment continues to evolve where battery tests are now performed automatically, which would have prevented this failure.
Real-time data in the automation zone is of utmost importance. If automation teams are not ready to switch, then automatic operations will not take place. When crews are working on sections of line in the automation zone, automation must be shut down, but care must be taken to get the automation teams back to ready mode after work is complete. Without SCADA information or network connection or both, keeping up with the status of the automation system can be difficult. Personnel radio verification at each physical location is quite time consuming. SCADA alarms flag when problems are present (automation teams are not in ready mode).
A network connection with a head-end radio is a much smoother way to look at each switch. The only real barriers on the network were radio settings issues and information technology firewall issues allowing the correct Internet protocol addresses to punch through.
MED has three 161/46-kV substations that feed the 46-kV transmission system around Murfreesboro, with five 46/12.47-kV distribution substations to supply customers. During one of the tornadoes, a section of the 46-kV transmission line tripped. Before the utility could re-energize the line, a crew was dispatched to the field to check for trouble. The crew was pulled away from other work and customers were without power for about 45 minutes while the line was checked, even though there was no actual problem with the line. The line could have been re-energized in less than a minute with automation.
MED has decided to use its automation expertise to automate the 46-kV loop around Murfreesboro. By automating this loop, MED will get a lot of bang for the money spent. Since motor operators, which work in conjunction with the EFACEC ACS SCADA system, have been installed previously on key transmission switches, the only real issues to automate the transmission loop are the installation of controls and communications. Due to the importance of the transmission system, MED will install a fiber loop to all these control points and will use the latest S&C controls and software. With automation installed, the tornado outage could have been limited to a few seconds instead of 45 minutes.
One big lesson learned has been how to get the line crew to accept distribution automation technology. The line crew at MED has been included in the installation of and training on all the automated equipment. Ongoing training is an essential part of automation acceptance. The line crew needs to know what to expect if an automatic operation takes place. They need to know what process has to occur to return a circuit to normal after an automated event. They need to be aware of what controls have to be disabled so they are safe as line work is done in an automation zone.
The line crew at MED has come a long way to accept what distribution automation can do for the electric utility. The complete success of an automation project depends on the understanding all operating personnel have of the project.
The scale and experience with automation continues to grow at MED. The more the utility uses its distribution automation system, the more the system is refined. The capabilities of the distribution automation equipment, software and communications continue to improve.
Mark Kimbell (firstname.lastname@example.org) joined Murfreesboro Electric Department in 1999 and was appointed chief engineer in 2000. Prior to MED, he worked for the Gallatin Department of Electricity for 12 years. Kimbell has a BSEE degree from Tennessee Technological University and a MBA from Middle Tennessee State University. He is a member of the IEEE, and serves on the R&D Committee of Tennessee Valley Public Power Association and Murfreesboro's Board of Electrical Examiners.
CDR Systems www.cdrsystems.com
EFACEC Advanced Control Systems www.efacec-acs.com
S&C Electric Co. www.sandc.com
Tennessee Valley Authority www.tva.com