With Cold Shrink, termination takes less than one hour to complete. In 1989, when Texas Utilities Electric (TU Electric), Fort Worth, Texas, U.S., began replacing all the XLPE cable at Dallas/Fort Worth International Airport (DFW) with PILC cable, it was like performing open-heart surgery on a person running a marathon. Critical electrical-dependent airport operations had to remain up and running the entire time.
TU Electric had selected PILC cable because of its reliability, but the utility's engineers had to develop new standards for 25-kV terminations, since TU Electric's experience with PILC in downtown Dallas and Fort Worth was based on 15 kV. Common 25-kV potheads were too big for the switchgear specified for the project. Searching for an effective alternative, TU Electric first developed a hand-taped stress cone that worked well but was labor-intensive. By the summer of 1993, TU Electric was working with 3M to develop a live-front Cold ShrinkTM termination that could effectively seal the oil and be installed in less than one hour (versus six hours for the hand-taped stress cone). Although introduced late in the project, the Cold Shrink PILC termination was the answer for the airport job, where time was of the essence.
Has Four 138-kV Feeds DFW encompasses 18,247 acres, includes four terminal buildings and 117 boarding gates, employs more than 37,000 people and boasts of having the world's largest airport hotel, a 1400-room Hyatt Regency. It is supported by four 138-kV lines and two 138-/25-kV substations, one on the northwest side, the other on the southwest. Both substations receive power from two underground 138-kV circuits with automatic sectionalizing equipment. Radiating from each of these substations is an underground 25-kV distribution system encased in a network of concrete conduits and manholes. The system is designed to provide continuous service if one substation goes down. Two 25-kV feeders lie along each side of the International Parkway, providing a two-source feed from each substation to all critical loads in the central terminal area.
The cable replacement project at DFW began officially with the formation of the DFW Airport Task Force in November 1989. Eighty-six cable, connection and equipment failures had occurred at the airport since 1973, blamed in part on early XLPE technology. The testing of cable samples revealed treeing caused by water migration. It was clear that all the XLPE needed to be replaced because it was the same 1971 vintage.
The task force had two primary objectives: Restore the system integrity.
Incorporate better components, techniques and design concepts whenever possible.
To these ends, PILC cable was selected because of its proven reliability (service life can exceed 70 years) and because it would work particularly well in the naturally wet and corrosive environment at DFW.
One problem, however, soon became apparent. The preferred switch-gear for the project was not designed to accept PILC cable (TU Electric wanted to be able to use the same switchgear on non-PILC cable elsewhere). The hand-taped stress cone proved to be an effective solution, but weeks were spent teaching cable splicers how to make the termination. Because of the time it took to install each stress cone, an elaborate schedule of clearances and cutovers was implemented so that service to each DFW customer could remain uninterrupted.
Live-Front Termination For dead-front terminations, crewmen were already using a Cold Shrink T-body that 3M had recently adapted for use on 25-kV PILC cable. TU Electric engineers asked if the same technology could be applied to a live-front termination. 3M saw no reason why not and soon developed the prototype for what is now known as the 5800-Pb series Cold Shrink Quick Term II Termination for both 15- and 25-kV cable.
The termination consists of a high-dielectric-constant stress-control tube insulated with a molded silicone skirt that minimizes leakage currents and skirt-to-skirt conductive paths. The skirt design also features an extended base that, when combined with mastic, ensures a seal at the cable jacket and where the neutral wires emerge. Cable oil is stopped using a specially designed thin-wall EPDM Cold Shrink tube covered with four wraps of a special white restricting tape. The installation method is shown in Figs. 1-5.
Working with 3M engineers, TU Electric crewmen installed five of the live-front Cold Shrink terminations on fused switchgear serving the roadway lighting system at DFW. At one location, cable splicers who had never used the termination were able to install three in two hours (Fig. 6). TU Electric estimates that an experienced splicer could install one termination in about 30 minutes (Fig. 7). Although no oil leaks were experienced with the Cold Shrink termination, laboratory tests indicated that actual oil pressures could greatly exceed the design of the termination (see sidebar). The Cold Shrink terminations were modified to withstand these high pressures and have not leaked in more than two years of service, while leaks have occurred on many of the hand-taped stress cones.
Changed Feeder Configuration To improve sectionalizing throughout, the DFW project changed the circuits from a dual-radial to a loop-feed configuration. This activity involved the installation of a fiber-optic SCADA system that will provide remote monitoring and fault indication as well as remote control of switchgear and other equipment.
Before the project began, the DFW airport system consisted of XLPE Al cable as follows: 29 circuit-miles of 1000 kcmil, 7.5 circuit-miles of No. 4/0, and 21 circuit-miles of No. 1/0. By the end of 1994, all had been removed and replaced with approximately 37 circuit-miles of new 500-kcmil PILC Cu and 16.1 circuit-miles of new No. 2/0 PILC Cu cable.
In the vault areas, the radial feeder taps from the manholes into the vaults have been replaced with feeder loops into and out of the vaults. The fuse-only switchgear in the vaults has been replaced with dead-front SF6 gas-insulated switchgear that offers a combination of fusing, switching, motor-operated switching and circuit- grounding capabilities. Forty-three existing fuse-only, one-position switches have been replaced with 32 SF6 five-position switches. Two of these switches per switchgear have supervisory controlled motor operators. Two switchgears also have been installed in a new American Airlines vault in the 2E terminal area and additional SF6 switches have been installed in vaults in the 2W terminal area.
Also, 68 S&C Electric padmount switchgear units have been replaced with new S&C padmount switchgear that features motor operators to accommodate the electronics equipment required for supervisory control. Forty-eight new S&C padmount switchgear locations have been established to accommodate the change to a loop-feed configuration and to establish three cross ties between the feeders on the east spine and those on the west spine. Five switchgear locations were eliminated.
Conclusion Although only a small part of the overall DFW project, installation of Cold Shrink terminations on live- front 25-kV switchgear offered TU Electric a good opportunity to learn something new about Cold Shrink products as well as PILC cable terminations. TDW
Gary D. Hendley is a senior engineer with TU Electric, Fort Worth, TX, which he joined in 1979. He has the BSEE degree from Texas AM University. He has worked in HVAC design, facilities inspection and customer electrical problems and testing and is presently involved in distribution design and standards with TU Electric's Major Distribution Design Dept. Mr. Hendley is a registered professional engineer in Texas.