Building the Transmission Lines

Constructing the transmission lines from Stoughton to Boston involved several steps including:

• Building the underground network that housed the transmission line
• Installing the cables that carry the electricity

Building the underground network

A network of underground pipes was installed along the 18 mile transmission route from Stoughton to the NSTAR substations in South Boston and Hyde Park in order to hold the cables that will transport electricity along the transmission lines. These eight-inch steel pipes are coated in a corrosion-protective material and buried underground. In order to set this network, construction crews excavated a five foot wide trench in the roadway, about 4.5 feet deep, to install pipe type cable. 

The crews then lowered sections of steel pipe into the trench. After the pipes were lowered into the trench, sections of pipe are welded together and tested to insure integrity of the weld. The trench was then backfilled with concrete and the roadway was repaved.  Then the crew moved farther along the route to excavate the next section of trench.

Roughly every 3,000 feet along the route, construction crews also installed manholes. Manholes are enclosures that are used to reach the underground network in order to install and splice the transmission cables or perform maintenance on the transmission system when it is complete. 

The installation of manholes involved digging a hole 10 feet wide, 20 feet long and 10-12 feet deep about every 3,000 feet along the transmission path. Once the hole was excavated, a prefabricated manhole was delivered to the site and lowered by a crane into the newly excavated hole. Once the manhole was in place, the hole iwas backfilled and the pavement restored.

Installing transmission cables 

Once the underground pipes are in place, crews brought reels of transmission cable to the manholes. Using these access points they pulled sections of cable through the pipes.

Once the cable is installed, splicers climbed into the manholes and spliced or joined these cables together.  Work at each location was performed 24 hours a day, for a period of about 7 days.

In order to complete this operation, the splicers needed to control the climate in the manholes.  To achieve this controlled environment, when crews removed the manhole cover to begin the splicing operation, they parked a splicing trailer over the manhole where the work was to be performed.  Through a trap door in the bottom of the van, the crews created a sleeve running between the manhole and the van to prevent moisture from entering the manhole.  Splicing lines together is a complex, continuous 24-hour operation that takes about one week per cable to complete.  Therefore, the splicing trailer and the crews at the manhole remained on-site until the splicing was complete.

Installing station equipment

While road crews and linemen worked to build the underground transmission network, construction crews at the station locations installed electrical equipment that will help deliver the electricity flowing through these lines.  There are two basic groups of equipment that needed to be installed:

• Transforming equipment
• Switching equipment

Switching equipment

Since the new transmission lines access power flowing through an existing overhead transmission line that runs between Holbrook and Walpole through the Town of Stoughton, switching equipment was installed below these overhead wires at a new switching station  constructed by NSTAR along Route 138, in Stoughton.  This station  allowed the new underground lines to access electricity traveling from the overhead electrical network as well as provide circuit switching capability and circuit protection.

Transforming equipment

Electricity traveling through the underground transmission lines reaches NSTAR substations in South Boston and Hyde Park.  Both of these stations currently house transformers that reduce voltage from the transmission lines to a level usable by area residents and businesses.