This largest transmission line upgrade project in National Grid’s history consisted of double circuit horizontally configured 115 kV transmission lines that were originally constructed in the 1920’s with 4/0 hard drawn copper conductors.
The A127/B128 Lines run from Millbury substation in Massachusetts to the Harriman Hydro Station in Vermont. The west section of the lines runs from Leicester, Massachusetts to Harriman Station, a distance of 67 miles consisting of 509 lattice structures.
Since their construction, the electrical codes have been revised and the lines did not comply with current clearance requirements. To improve clearances 477 kcm 26/7 ACCR conductor was chosen. This is a proprietary product manufactured by the 3M Company and has a maximum operating temperature of 464°F. In order to maintain clearances it was necessary to install this new conductor at 7,000 lbs at NESC, Heavy, which is considerably higher than the original design loadings and required analysis and reinforcement of all the towers.
Much of the line was in heavily wooded areas consisting of rough terrain including swamps, lakes and rock outcrops. Over 27 percent of tower foundations were rock anchor foundations in various stages of deterioration requiring selective improvement. The remaining foundations were steel grillages.
Acting as an extension of the National Grid Transmission Line Engineering staff, TRC performed analysis and design for the tower reinforcements utilizing PLS-CADD and PLS-TOWER; developed specifications for procurement of the replacement poles; performed site inspections and developed reinforcement plans for tower foundations; reviewed pole vendor steel pole designs; developed foundation design packages for the new steel poles; and developed wire stringing packages for the new cables and related hardware changes.
TRC was able to work with and supplement National Grid engineering staff to implement this critical project. TRC was able to respond quickly to changes in the project scope and modify design products appropriately. Interface was required with all levels of the National Grid organization including project management, construction services, procurement, permitting, community outreach, and scheduling