The Mt. Pleasant Tech Interconnection involved the design of six 345 kV circuits and one 138 kV circuit. A new circuit was strung onto the vacant side of an existing double circuit 345 kV line. The existing structures consisted of elliptical tubular steel poles and lattice tower structures. A new 1.5-mile corridor of two double circuit 345 kV lines was constructed to connect the existing line with a new substation. A small portion of the existing 345 kV transmission line had a sister circuit which vacated its line position and was constructed on a new single circuit 345 kV 1-mile corridor. There was also a separate double circuit 345-138 kV line that was uprated and moved into new positions at the substation.
Due to the age of the existing 345 kV transmission line and that the arms on the vacant side had been installed without load there was a concern that the structures were damaged from vibration over time. The need for a steel pole inspection plan was identified and put into place by ECI to determine if any structural remediation was necessary. The existing lattice towers were also modeled and analyzed using PLS-TOWER. Several of the lattice tower structures were found to be inadequate for the new loading. Instead of replacing the towers ECI developed a tower remediation plan to replace/reinforce members as necessary and put certain restrictions on construction sequencing/methods to ensure that the towers were not overloaded during construction.
There was a significant amount of other local infrastructure upgrades going on at the same time as the transmission line work. The design had to take into account multiple future scenarios for various road projects as well as new underground utilities. ECI also had to work closely with the FAA to ensure that all appropriate permits were obtained for the work as there were multiple active airports within the vicinity of the project.