A long-planned project by the power company NStar to improve electricity service on the Island is well under way, as work continues on the installation of an undersea cable connecting Falmouth and the Vineyard. The cable is expected to add redundancy to the Vineyard power system and make service more reliable.

The cable — all 29,000 feet of it — will be installed from NStar’s work barge, now moored in the Vineyard Haven Harbor and visible from Beach Road.

“Martha’s Vineyard is traditionally served by four cables,” said Coleman Geary, NStar project manager on the undersea cable job. “And pretty much at any time, we can run on three.”

The newly-installed phone poles on Edgartown-Vineyard Haven Road and Edgartown-West Tisbury Road are also designed to offset the power load. — Mark Lovewell

In 2011, one of the cables — installed in 1975 and dubbed the 75 line — was irreparably damaged and taken out of service. The lack of backup led to problems on the Vineyard power grid over the next two summers, when a second cable failed and the Vineyard was relying on just two lines to provide power.

“We do have faults on them; they get hit by anchors,” Mr. Geary said. “Then we have to run generators.” The cable failed this past July during a heat wave and NStar brought in temporary generators to handle the increased power burden. While temporary units are typically used in summer, NStar relied on 15 this summer, as opposed to the eight used the previous year.

The new cable will “eliminate the need to have those supplemental generators,” Mr. Geary said.

The cable is part of a longterm plan by NStar to improve electricity service. The newly-installed phone poles on Edgartown-Vineyard Haven Road and Edgartown-West Tisbury Road, which have proven controversial for their considerable size, are also designed to offset the power load. The poles were referred to the Martha’s Vineyard Commission for review and approved last November. The undersea cable came before the commission for review in December 2012 and was approved in April 2013.

NStar spokesman Michael Durand said the last time a cable was installed beneath Vineyard Sound was in 1990.

The current installation will permanently replace the failed 75 line.

“The configuration [of this one] is different,” Mr. Durand said, noting that it is a hybrid cable. In addition to three electrical conductors to deliver power, the cable contains two fiber optic cables, each with 48 fibers. The fiber optic cables are owned by Comcast, but because NStar owns the majority of the undersea cable, work is being completed by the electric company.

Mr. Geary called the hybrid cable “enormous,” referring to its length.

Installation begins in the water, with the cable laid across the bottom of the channel and pulled through a long undersea pipe that transmits the cable from land to ocean floor without interfering in the shore zone. Mr. Geary said he is not sure which side of the channel will be installed first — Falmouth or Martha’s Vineyard. Once the cable is pulled through the pipe, it will be buried in the sand

Mr. Geary and his crew began work on the Falmouth side of the Sound in early November, drilling one of the tunnels that houses the underwater pipe that extends 2,000 feet out. Work from Falmouth was completed on New Year’s Eve and activity began on the Vineyard side on Jan. 16.

The Vineyard tunnel is about two-thirds completed, Mr. Geary said. The cable comes ashore on the Vineyard side at a point near West Chop.

“We’ve been slowed down a little bit with the storm for the past couple of days,” he said this week. “With the high winds — for safety reasons, we shut down, and now we’re back up.”

“The plan is to have it all up and running by May,” he added. “It’s going along pretty well.”