CL&P is building two new underground 115- kV electric transmission lines that will run under city streets from Stamford, through Darien to Norwalk, Connecticut. The project is needed to provide additional electric capacity and reliable electric supply in southwest Fairfield County, the fastest growth area in the state, and to assure the area's electric grid conforms to national and regional reliability criteria.
The project comprises 8.7 mi of solid-dielectric cable installed beneath city streets from the Norwalk Substation in Norwalk to the Glenbook Substation in Stamford. The estimated cost of the project is $223 million. Regulatory review of the project began in March 2004 with construction underway in October 2006. The cables are on schedule to be energized later this year. Most of the work on public roadways is being done during night-time hours. Off-roadway work and tunneling is largely done during daylight hours. Currently construction work is being done in the Town of Darien and in Norwalk. Restoration paving of disturbed roads is underway in Stamford.
Transmission cables are installed in a duct bank placed in a 4-ft by 6-ft deep trench and backfilled with concrete. Splice vaults are installed in sets of three about every 1800 ft along the duct bank. Each vault requires an excavation 14 ft deep, 14 ft wide and 36 ft long. At each set of vaults, the duct bank is split into segments providing a unique entrance and exit route for each circuit through its corresponding splice vault.
Equipment upgrades were necessary at both substations to make them compatible with the Glenbrook cables. At the Norwalk substation, where power is reduced from 345 kV to 115 kV, new cable terminations and towers were installed. The Glenbrook Substation in Stamford received new circuit breakers and switches.
The same technology used to bore a pair of tunnels under the English Channel is being used on a smaller scale by CL&P to tunnel paths through rock for the new 115-kV transmission line. CL&P's Glenbrook Cables project team drilled a pair of five-foot diameter tunnels about 220 ft under railroad lines using a small boring unit (SBU). A technician operates the SBU from a compartment inside the unit, which rides on rails as it bores into the rock. See photo. Each tunnel took between 10 and 15 days to complete.