Exelon Corp. and Southwire Co. recently completed a two-year field trial of a new, intelligent transmission conductor. Fiber-optic communication fibers were stranded into the conventional ACSR conductor to provide a dynamic rating tool. The fiber-optic transmission conductor (FOTC) increases the capacity and reliability of a transmission line by providing accurate profiles of the conductor temperature.
The Intelligence Quotient
Optical fibers installed in the conductor communicate with an instrument at one end of the line to report the temperature. The combination of the optical fiber and the instrument combine to result in an intelligent system. FOTC is a special application of distributed temperature sensing (DTS) whose first applications were in the aerospace industry followed by applications in underground cable systems, where localized hot spots could be identified.
DTS determines temperatures by watching the optical fiber change color as temperature changes. A pulse of light sent down the fiber is partially reflected from every part of the long fiber back to the source. The reflection time determines the location of the temperature. Depending on the desired resolution and based on the number of readings per foot, DTS has a range of up to 30 miles (48 km).
Rating the Conductor
Early transmission lines were not heavily loaded because a single static rating was used, which was based on conservative operating parameters where 25°C (77°F) was the assumed ambient temperature, 2 ft/sec was the assumed wind speed flowing across the line and 75°C (167°F) was the maximum allowable conductor temperature. If actual weather conditions are used in the rating process, much higher ratings can be realized. For example, if the wind speed was higher than 2 ft/sec and the ambient temperature were lower than 25°C, the conductor rating would increase substantially, encouraging a search for techniques where real-time weather conditions could be measured. In this respect, seasonal ratings emerged as the first form of dynamic ratings to take advantage of favorable weather conditions to increase line capacity.
In the 1980s, dynamic line ratings took a big step forward with the development of computer models that calculated conductor ratings based on real-time weather data, with ratings that could be adjusted as frequently as every 15 minutes.
The next development was a line-powered device that could provide a temperature measurement at a single point on an energized conductor. More recent developments involved tension monitors and optical sag monitors that interfaced with computer programs to provide real-time ratings. All rating methods have some uncertainty, requiring some safety margin to be applied to prevent clearance violations or conductor damage due to overheating. FOTC, on the other hand, represents a technology that can measure temperature at all points along a line section to determine both steady-state and short-term ratings. Because uncertainty is small, the line capacity can be maximized based on existing weather conditions.
In the FOTC system, the instrument is located in a secure building near the substation. The data collected show the temperature of the fiber as it exits the instrument and is routed through overhead and underground sections to the transmission tower. At about 1000 ft (305 m), the fiber enters the line and the elevated temperature of the line is measured. It is clear from the data that the temperature is not uniform along the span. The four sharp temperature dips are caused by the suspension fittings, which absorb significant heat from the conductor. The live computer display is even more interesting, showing that the refresh rate is fast enough to monitor the effect of wind gusts, which cause the temperature to fall in those sections. The span that crosses a highway displayed temperature excursions that were due to exposure to the cooling effects of the wake of passing trucks and trailers. Sections of line that were sheltered by trees typically ran hotter than line sections in unsheltered areas.
In the two years of testing, Exelon has recorded line temperature, line current and weather parameters, which can be used to better understand the thermal behavior of a line and to test the validity of other rating methods.
Designing the Line
Because optical fibers are fragile and not weather-tolerant, the fibers were placed inside protective stainless steel sleeves, called buffer tubes. The tubes were filled with a protective gel to prevent water intrusion and four such tubes carefully placed in suitable interstices of the stranded ACSR conductor. To fit in the available space, the buffer tubes are only 0.033 inches in diameter with each tube containing a single fiber. The fibers are rated for a maximum temperature of 300°C (572°F), which is considerably higher than the maximum temperature rating of the conductor. With four fibers stranded into the conductor, it was possible to measure the radial temperature gradient and to ensure redundancy in the event of a fiber failure. Other than the four buffer tubes, FOTC is identical to stranded ACSR and was produced on the same assembly line as standard conductors. A 5000-ft (1524-m) length of conductor was manufactured for the test program. To ensure that the FOTC would not compromise the reliability of the transmission line in which it was installed, samples of the conductor were tested at NEETRAC (Forest Park, Georgia, U.S.). The tests demonstrated that its electrical and mechanical characteristics were essentially identical to the standard 1113-kcmil 45/7 ACSR Blue Jay conductor. In tension tests, the conductor was loaded above its rated breaking strength (RBS) of 29,800 lb (13,517 kg) with no breakage of the fibers before the conductor itself finally broke.
Additional laboratory testing was undertaken at the Oak Ridge National Laboratory (Oak Ridge, Tennessee, U.S.), where a large-scale climate simulator was available. The simulator provides an environment of controlled temperature, solar radiation, wind and rain. Two samples of FOTC were mounted in a load frame to maintain a tension of 25% of the conductor's RBS, and thermocouples were placed near the core and near the conductor surface to provide accurate temperature measurements. The optic fibers were spliced in a series arrangement and connected to the DTS instrument. Low-voltage ac current was used to produce the desired conductor temperature, while the climate simulator was adjusted to vary the wind speed, wind direction, solar conditions and rainfall. The evaluation of the test results showed remarkably good agreement between the DTS and the thermocouple temperature measurements.
Designing the Line
After several sites were investigated, the Itasca Substation near Chicago's O'Hare International Airport was selected for the demonstration project. Two problems emerged with respect to finding dead-end fittings and a technique for terminating the fibers from the energized conductor. The problems were resolved by using two polymer insulators to hold the line tension and a fiber-optic instrument bushing to take the fibers from conductor to ground at the tower. Standard aerial cable was used to connect the DTS equipment to the demonstration line. An additional problem involved the necessity to find a suitable connector that would not crush the optical fibers, as would be the case if conventional compression connectors were used. A special cast resin end fitting, normally used for laboratory testing of overhead conductors, was specified because of its high-strength characteristics. These laboratory fittings are stronger than standard fittings but are difficult to install in the field, where a mold must be placed around the conductor and the resin poured into the mold. The buffer tubes are not harmed because their only contact is with liquid resin. After curing, the resin will hold the conductor at its rated breaking strength. The mold is predrilled for yoke plates that attach to standard overhead line hardware. While the buffer tubes carry some of the line tension, testing showed that the strain on the tubes was well within strength limits for both the tubes and the fibers.
Construction
The construction phase, scheduled as soon as the engineering issues had been resolved, was complicated by tight outage schedules and the large number of special skills required. The scheduled outage date in February represented another complication, because the installation of cast resin fittings and the splicing of optical fibers require warm and dry conditions. Tents and heaters on an elevated work platform were provided to ensure that construction could proceed without concern for the weather.
The first construction step was the termination of the FOTC cable. The four buffer tubes were routed into a section of armored hydraulic hose, which protected the tubes from mechanical damage and provided another layer of weatherproofing for the optical fibers. One end of the hydraulic hose was inserted into the resin mold before the resin was poured. The steel core strands were cut at the top of the mold, and a J-shape was formed to ensure that the strands would not slip under extreme loads. After the resin was poured and cured, all of the aluminum strands were carefully formed back into the conductor and terminated with a compression connector. The hydraulic hose, with the four stainless buffer tubes, was connected to a weatherproof fitting on an optical splice enclosure. Inside the enclosure, the buffer tubes were removed and the exposed FOTC fibers were spliced to the fibers at the energized end of the optic bushing. At the bushing's ground end, a second splice box was used to connect the fibers to standard communication aerial cable. All of the optical connections were made with standard communications equipment and techniques.
The demonstration line replaced 4.5 spans of one phase of an existing 138-kV line. A standard compression splice was used to join the Bluejay/FOTC to the standard Bluejay ACSR, which was already in service. Although the optical fibers were probably crushed when crimping the compression fitting, there was no concern because the DTS works from only one end of the fiber.
Conclusion
After almost two years in service, the FOTC performance has been flawless for power delivery and for reliable temperature measurements. An outage caused by lightning affecting the DTS instrument was almost incidental to the performance of the conductor. A reboot was all that was needed to get back in operation. Line temperature over all weather conditions has been recorded and continues to be recorded.
FOTC offers reliability, accuracy and the ability to locate and measure hot spots. The technology permits more coverage than is possible with sag and tension monitors. The data collected can be input to weather-based programs to provide ratings for nearby transmission lines. FOTC can locate damaged conductor, either by locating a fiber break or by identifying a hot spot where strands are damaged or broken. The demonstration line has performed without the need for any maintenance or repair. If access is a concern, the electronics package can be located on the ground in a sheltered environment.
The remaining requirements for FOTC's universal use includes developing splices, end fittings and special repair techniques for the optical fibers.
The incremental cost for FOTC conductor over standard conductor should make it competitive with alternate dynamic rating methods when accuracy, low maintenance, coverage and high reliability are factored into the cost-benefit equation. It is possible to offset the extra cost for the fiber-optic terminations by adding extra fibers for communications.
Exelon (Chicago, Illinois, U.S.) and Southwire (Carrolton, Georgia, U.S.) formed an alliance partnership to design, manufacture and deploy FOTC conductor. A patent has been filed for the conductor design.
Shantanu (Shan) Nandi received an MSEE degree from Iowa State University in 1971 and joined ComEd after graduation. Having held various positions in engineering and planning, he was appointed as the commercial consultant for high-voltage wires and cables in 1992. Nandi is a senior member of IEEE and chairman of Standards for the Insulated Conductor Committee. He is adjunct professor at Oakton Community College in Des Plaines, Illinois, U.S.
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James Crane manages the R&D program for Exelon Energy Delivery, which provides electricity to more than 5 million customers in the Chicago and Philadelphia metro areas. He serves on various research advisory councils, including EPRI, Power System Electric Research Center and the National Electric Energy Research and Application Center. Crane holds a BSEE degree from Purdue University.
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Paul L. Springer III is program manager for mechanical systems at the Georgia Institute of Technology's NEETRAC. Springer received a BSME degree from Georgia Tech in 1977. He is responsible for a mechanical testing laboratory and a vibration laboratory. Springer is registered as a professional engineer in the state of Georgia.
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