345kV–Rating Study & Tower Raising Project

345kV–Rating Study & Tower Raising Project, Illinois

345kV – L11212 & L8012 (50) Mile Lattice Tower Upgrade

The purpose of this project was to upgrade an existing (50) mile 345kV lattice tower line with double bundled conductor, through a combination of nips, tucks, floating dead-ends, and tower raising.

Project Scope:
Work entailed modeling the existing line in PLS-CADD, using LiDAR survey. The desired line rating was provided by our Client, and clearance violation reports were completed under the new line rating. A best solution matrix was developed, demonstrating which combination of nips, tucks, floating dead-ends, and tower raises were needed to be most cost-effective, minimizing outage timeframes. Design criteria was also developed by the Client. The basis of the matrix was determined by compiling design criteria for each combination of fixes, which were then considered and assigned a value based on line reliability, schedule, and worker safety. In determining a final solution, the following were also considered: access to tower site; constructability; minimizing distance traveled to construction sites, land owner issues, and permitting issues for construction equipment traveling on local roads.

The design criteria, regarding nips and tucks, required anti-cascading, mid-span poles due to excessive longitudinal imbalances on the towers, for a large quantity of nips and tucks.  Design criteria for nips and tucks without any anti-cascading mid-span poles, was (1) nip or tuck floating dead-end every (2) miles.

Overall basis for the alternatives was to minimize the number of fixes in order to meet the new clearance requirement.

Once the solution was finalized, tower extensions were designed for lifting the towers. A structural evaluation of the tower being lifted was also performed to ensure lifting the tower from the bridge, with the inclusion of the wires, was satisfactory and safe. A criteria guide for lifting the tower during construction was developed for construction to review, stating the maximum wind speeds allowed during a tower lift. The rigging devices for lifting the tower were also designed, fabricated, and tested.

The process for tower lifting was included in the design criteria, and listed which order and conditions were needed to be followed prior to lifting the towers. The first step needed was to lower the static wires prior to lifting, to minimize the load on the towers. Next, the installation of the spreader beam, which is attached to the tower bridge, was connected along with temporary stiffeners designed to spread the load of the tower over several joints. The tower was then unbolted from the stubs and lifted (1) foot and held (5) minutes to observe a solid and safe hoist. After the hold period, the tower was lifted to the elevation (2) feet above where the extension would rise to. The extension is then constructed and the tower lowered and bolted. The static wires were re-installed on the peaks, and the crews then begin clean-up of the site. On average, the construction averaged (4) hours per tower site.