Rolling blackouts and temporary power outages experienced across the Island last weekend were caused by an undersea cable failure, according to Eversource, with the utility planning to bring 15 backup generators to Island as it conducts repairs.

The cable experienced a "fault" on Friday around 5:00 p.m., Eversource confirmed, with the extraction of the cable and its repairs expected to take two months or longer, just as the Island reaches its peak energy usage rates at the height of summer.

In a letter to Island public safety officials and provided to the Gazette, Eversource community relations and economic development director Ronit Goldstein explained the power outages, which were mainly focused in Edgartown but felt in other Island towns as well.

“One of the submarine cables serving Martha’s Vineyard faulted on Friday, July 16, at 5:14 p.m., and we will be making necessary repairs over the coming weeks,” Ms. Goldstein wrote. “Through a combination of a backup generator already on Martha’s Vineyard coming online and our remote system operators using distribution automation technology like smart switches to reroute power from other circuits, all approximately 2,250 customers who briefly lost power were restored in fewer than five minutes.”

New England’s largest utility, Eversource provides power to the Island through four undersea cables that run between a switching station on Cape Cod and Martha’s Vineyard.

The company operates a substation off the Edgartown-Vineyard Haven Road in Vineyard Haven.

The four undersea cables are supplemented by five diesel-powered backup generators that kick-in when the Island experiences ballooning energy demands during the summer. The utility recently cancelled a plan to replace the generators with an expansive battery-powered storage facility in Oak Bluffs, citing increased cost estimations and an increase in expected on-Island energy demand.

The utility instead hopes to install a fifth undersea cable to meet the Island's expected energy demand through 2030, according to project updates filed on the utility's website.

Meanwhile, the utility plans to bring temporary backup generators to the Island while it conducts repairs to the undersea cable. The letter did not elaborate on what caused the failure.

“To help ensure reliability for residents, businesses and visitors while we make the necessary repairs to the submarine cable, we are taking the extra precaution of bringing 15 extra backup generators to Martha’s Vineyard,” Ms. Goldstein wrote. “These extra generators will be able to provide 28 MW of back-up generation to the Island if needed, and will be strategically placed at the Tisbury landfill, near our Vineyard Haven substation and near the Oak Bluffs DPW facility.”

In a follow-up email with the Gazette, Eversource spokesman William Hinkle said in a statement that cable repairs, which involve intricate logistics, divers and a barge, will take an expected eight to 10 weeks. The utility had not yet identified where the fault on the cable occurred.

"We are currently assessing the location of the fault, which could be along the cable or at an existing splice. There are many moving pieces as we continue to work toward making the necessary repairs as quickly as safely possible, which we believe could be complete in 8-10 weeks weather permitting," Mr. Hinkle wrote. "Next steps include bringing in a diver to further inspect the cable and pinpoint the fault location, and eventually a barge to pull the cable up to make repairs. The logistics for both of these are still being finalized."

The utlity is the sole energy provider for the Island, which experiences a dramatic population increase during the peak summer months. Mr. Hinkle said in the statement Tuesday that Eversource would be able to meet current summer demand despite the cable failure through a combination of shifting loads on active circuits and the back-up generators, as well as existing permanent energy generation on the Island. 

"We are confident there is enough capacity to ensure reliability for our customers on the island – including on the hotter days we can expect to see as the summer continues," Mr. Hinkle said. "This will require constant planning and refining, and precisely how that load is balanced using these options will be dependent on conditions like the weather and specific spots of demand at any given time."

Edgartown town administrator James Hagerty read the letter at a meeting of the Edgartown select board on Monday afternoon. He added that an issue involving a loss of AT&T cell phone service over the weekend was still being investigated.

“We’re still reaching out from AT&T to find out what the situation with that was, because we lost cell phone service concurrent with power service, which could be a major issue in the event of an emergency,” Mr. Hagerty said.