Utilities plan new power, TV cable to Martha’s Vineyard

Engineers will drill below the beach and sea floor to install a new power cable.
Photo courtesy of NSTAR

Engineers will drill below the beach and sea floor to install a new power cable.

NSTAR, the public power utility that supplies electricity to Martha’s Vineyard, and Comcast, the cable television provider, plan to install another undersea cable from Falmouth to Martha’s Vineyard, to meet the rapidly increasing demand for power and to replace current cables that have repeatedly failed.

The hybrid cable will carry fiber-optic strands for Comcast to use to distribute television and communications services, as a backup to the lines it already uses.

The project is before the Martha’s Vineyard Commission (MVC), triggered by a provision of MVC rules that requires review of any project in Island waters. The commission has scheduled a public hearing on Thursday, Dec. 6, at 7:15 pm.

According to the MVC staff report, no part of the cable will be visible. The project plan will use existing infrastructure at the landing point on West Chop, including manholes already in place. Plans also show a new manhole and approximately 15 new utility poles.

Power past

In a detailed presentation to the MVC’s land use planning committee on November 26, NSTAR executives and engineers outlined the plan to install the new power cable and the reasons it is needed.

There are now four submarine cables from the mainland to the Island. According to NSTAR, a cable installed in 1995, which comes ashore in Oak Bluffs, failed completely in September 2011, and cannot be repaired. That cable never operated at its designed capacity because of a defect in the cable’s manufacture, according to NSTAR.

A cable installed in 1996, to replace the defective cable installed the year before, is operating now, but it has failed and been repaired four times.

An older cable, installed in 1986, comes ashore in Tisbury. That cable has failed and been repaired six times.

A cable installed in 1990 is now operating. It has never failed. If it did, the Island would be left with much less electricity capacity than is required at peak use times.

Power present

The failure of undersea cables, combined with increasing demand for power, has forced NSTAR to bring powerful diesel generators to the Island, to supplement the power transported by electric cable. Each year, the generators have seen increasing use. There are now five diesel generators capable of producing 2.5 megawatts (MW) each. Three are located at the NSTAR facility in Oak Bluffs and two more are located at the Martha’s Vineyard Airport.

The total installed capacity from the four undersea cables is 55 MW. In July 2011, electricity use on Martha’s Vineyard peaked at 54.7 MW, and in July 2012, use peaked at 55.4 MW. NSTAR predicts that by 2022, the peak load will reach 73 MW.

Power future

The new cable would increase the capacity for electric power transmission coming to Martha’s Vineyard by 45 percent. The 4.5-mile cable would leave the mainland near Mill Road in East Falmouth and come ashore near Squantum Avenue, in the Mink Meadows area of West Chop.

Where the cable goes from land to sea, engineers plan directional underground drilling to install a conduit. Drilling equipment located on land will bore a small tunnel under the ground, then under the seabed, to avoid disruption of fragile barrier beaches and sea grass beds.

In the middle of Vineyard Sound, special underwater trenching equipment will bury the cable three to six feet below the sea floor.

The hybrid cable itself is about 5.5 inches in diameter. The route across Vineyard Sound is not a straight line. Engineers mapped a route that avoids some of the more environmentally sensitive underwater regions.

The Massachusetts Environmental Protection Agency issued a comprehensive permit for the project in August. The project will be scheduled to avoid disruption to shore bird populations.

NSTAR and Comcast still need approval from the Tisbury conservation commission and the MVC.