PSEG Long Island is on track to start work in less than a month on a closely watched $30 million project that will bring a needed third power line to Shelter Island.

PSEG vice president John O’Connell said he expected approval soon of an agreement with the Shelter Island Heights Property Owners Corp. and signoffs on permits needed to begin work. “Things are coming together,” he said.

PSEG will use a contractor to drill three 12-inch diameter tunnels over six-tenths of a mile between Greenport and Shelter Island, up to 120 feet under the seabed.

The water at its deepest there is 90 feet. The full cable from a Southold substation through Greenport, under the bay and on Shelter Island, will be 3.1 miles long.

Preparations for work come as LIPA trustees will vote next month on a new policy for reviewing and publicizing major powerline projects and giving impacted communities the chance to pay for undergrounding. Shelter Island rejected a previous proposal to build a new substation on the island, a plan that would have obviated the need for the $30 million cable. PSEG spokesman Jeff Weir said the final plan comports with the new LIPA policy. “We looked at a number of options and we believe the one we chose meets all the criteria” for bringing cost-effective power to Shelter Island.

No long-term facilities other than the power line and a new pole will be placed on Shelter Island, where the cable will emerge on Chase Avenue, O’Connell said.

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Stella Lagudis, general manager of the Shelter Island Heights Property Owners Corp., said talks were progressing but wouldn’t discuss specifics. “If we don’t sign, they’re not starting,” she said, adding it would be “the goal not to hold it up.”

Greenport Village has signed off on the plan, which will bring it $1.3 million in cash and new electric infrastructure upgrades. Greenport has its own municipal electric system.

The undersea drilling, which failed in a 2013 attempt to the west before PSEG’s tenure, will involve less distance and fewer angles than before, when a drill bit broke inside the nearly complete tunnel, O’Connell said.

That tunnel was 36 inches around and nearly a mile long. “One really big tunnel brings a lot of challenges,” O’Connell said. “This is more certain and less risky.”

Most of the drilling beneath a narrow portion of Peconic Bay will happen 20 to 120 feet below the seabed, and only one 13,000-volt cable will be put in place. The other two will be available “should there be any problems,” O’Connell said.

Two other working 13,000-volt cables already feed Shelter Island, one from Sag Harbor and the other from Southold. A third line that supplied power to the village was lost during superstorm Sandy.

New cable will run underground from a Southold substation for around 2.2 miles down Route 25 to Fifth Street in Greenport Village.

Drilling equipment will be on Fifth Street in Greenport near a public beach. The work is expected to be completed by May 1.