The Connecticut Light & Power (CL&P) Stamford Reliability Cable Project, for which construction began early this past Spring, is moving steadily toward its scheduled completion date of year end 2014. According to CL&P’s Anuj Mathur, Project Manager — Transmission Projects, duct bank installation is approximately 95% complete and cable installation is about 75% complete.
The approximate 1.5-mi underground transmission line links two existing CL&P substations: the Glenbrook Substation and the South End Substation located near the Stamford Transportation Center, the transportation hub for rail and bus traffic into and out of the city. The project in this heavily trafficked area has required close coordination with City of Stamford officials to help identify a cable route that would best minimize the impact of installing a 115-kV transmission line under city streets. The route selected also had to avoid conflict with the Stamford Urban Transitway Phase II, the city’s roadway improvement project now underway in the same area.
By connecting two substations that are currently fed from separate sources, the Stamford Reliability Cable Project addresses both the growing demand for electricity in a city that is experiencing substantial economic growth and the improvement of the reliability of the power supply in Southwest Connecticut. “The installation will improve the reliability of the transmission system that serves the Stamford and Greenwich areas and ensure compliance with mandatory national and regional reliability standards,” said Mathur. “By providing an additional source of electric supply into the South End Substation, we can better distribute power flows that presently serve Stamford and surrounding area, especially under contingency conditions,” he said.
A major challenge to this installation was how to cross the railroad tracks of the Metro- North Railroad and not disturb rail traffic. Trenchless excavation, also known as pipe jacking or pipe ramming, a tunneling method for installing underground pipes with minimum surface disruption, is being used. The trenchless excavation required a 200-ft jacking operation from a jacking pit to the receiving pit. From the jacking pit approximately 21 ft deep, a pipe was “pushed” to a receiving pit approximately 13 ft deep. Mathur said, “It’s a very complicated task with respect to safety compliance and managing groundwater intrusion. A well point system at the jacking pit and receiving pit was used to handle ground water. The trenchless excavation was just completed and was accomplished with no interruption to Metro-North train service.” According to Mathur, the operation, completed ahead of schedule, went very smoothly.
The 115-kV cable being installed has cross-linked polyethylene (XLPE) insulation with a 3500-kcmil copper conductor. Each cable used for each of the three phases of the line is approximately 4 1/2 inches in diameter. The cable is pulled into a duct bank with a “two over two” configuration for a total of four ducts. One duct is used for each of the three phases and the fourth duct is a spare. The ducts are made of polyvinyl chloride (PVC) and are encased in concrete before backfilling with soil. Mathur said, “With the trenchless excavation just completed, we are now continuing the duct bank installation from the jacking pit to where the duct bank has already been installed on the street. In addition to the cable pulling, there is also cable splicing to be done, high voltage testing, and completion of terminations within the substations.” Splicing will occur within three precast concrete splice boxes or vaults approximately 24 ft by 9 ft by 9 ft spaced about 1800 to 2000 ft apart. “Ongoing work includes installing the racks to support the cables and the splices and, of course, the splicing work as well,” said Mathur.
Much of the work had to be done during nighttime hours, as preferred by City of Stamford traffic officials. But, Mathur said, “Working at night has not been a problem and has actually helped us expedite the project since there is much less traffic to contend with.” He added, “From concept to construction we have had fantastic communication and cooperation from City of Stamford officials who have supported this project from the start. We also have worked very closely with the Connecticut Department of Transportation and Metro-North Railroad. These close working relationships are helping us bring this project to its successful conclusion right on schedule.”
Major contractors on this project include:
- General Cable — cable manufacturer and cable splicing.
- New River Electrical Corp. — cable pulling.
- Bond Brothers, Inc. — supplier and installer of duct bank.
- Matrix S.M. Electric Co., Inc. — substation upgrades within Glenbrook and South End substations.