The Dallas/Fort Worth (DFW) International Airport began experiencing significant failures in its power distribution infrastructure in 1989. There were earlier operational problems involving separable connectors as early as 1982, but in 1989 seven cable failures raised concern about possibly more serious systemic problems. A study was launched in November of 1989 to understand the nature of the problems and failures and to develop conceptual plans for improving the overall distribution system infrastructure and replacing existing cabling.
The original DFW electrical system was placed into service in 1970 and was an underground system with above-grade structures for only substations and pad-mounted equipment. The initial system consisted of eleven radial 25 kV feeders (from two 138 to 25 kV substations), approximately 57.5 circuit-miles, installed in a duct/manhole system consisting of approximately 350 manholes and 29 miles of duct bank. A separable connector system was used down the manholes to allow feeder sectionalizing. The early problems with the separable connectors and the later cabling failures provided growing evidence for better isolation and sectionalizing of the problems. A plan was developed to address:
The desire to use as much of the existing facilities as possible, to keep disruptions to a minimum, to
maintain reliability during the reconfiguration, and to keep costs down clearly pointed to a loop feed system
as the best redesign choice. This would basically convert a dual radial pair of feeders into a two feeder
loop, with one feeder back-standing the other. This required that all tapped laterals be
incorporated into the loop or fused off the main feeders, and that, ultimately, loop feed switching capability
be added at all switch gear locations.
In the event of a fault on the new loop feed configuration, the customers with automatic transfer service would still automatically transfer to their alternate source. The manual transfer customers, however, must now wait until the fault could be located using fault indicators installed at the switch gear sites. To take advantage of the loop feed system's increased sectionalizing flexibility, the fault indicator information must be readily retrievable by operating personnel, even from the many locations that were not readily accessible such as the terminal area vaults. With this information available, power to all the other affected customers could be restored with no more than three switch operations. This would take much less time than all themanual switch gear operations and cable transfers required with the original dual radial system.