Urban Towers in U. S. Prove Less Can be More
These days, the art of supporting overhead lines is as much about what is not to be seen as what is actually there. In this respect, Salt River Project’ s 230 kV Shrader line which runs through the suburbs of Phoenix, Arizona is a model of elegant understatement.
The 12 km( 7.7 mile) single circuit portion in particular sets a high standard for unobtrusive urban line design, with its low visual impact 1272kCM Bittern ACSS single conductor in place of the more usual two 954kCM ACSR Rail conductor bundle used by SRP at this voltage. The practical advantage is that the softer annealed aluminum strands operate at higher temperature and achieve twice the thermal and current rating of a fully hardened conductor.
At the same time, a non-drooping configuration of high cantilever strength silicone line post insulators( having 8.9 cm core rods), with 15 ° upswing and no more than 50 % deflection at maximum load, allow 245 m( 800 ft) spans and over a narrower line corridor compared to the alternative of daviet cross-arms and suspension strings.
Finally, a 69 kV under-build on the pre-cambered reduced diameter steel poles features phases inverted from those on the upper portion. This not only reduced net electromagnetic field but also saved structural and foundation costs due to the narrower phase spacing that could be achieved( i. e. 4 m versus the more normal 5.3 m).
106
Photos: INMR ©