One Of The Two Undersea Cables That Provide Electricity To Nantucket Is Broken Down

Jason Graziadei •

Screen Shot 2024 05 03 at 11 59 35 PM

One of the two undersea cables that provide electricity to Nantucket has been down since Tuesday, April 30th, and National Grid crews have yet to resolve the issue.

Given the timing of the outage outside of the peak season, the single 38-megawatt undersea cable remaining in service and the island’s backup generator/battery storage unit system are sufficient to cover Nantucket’s electricity needs. Had it happened during the summer, however, it could have been a different story.

After several days of inquiries by the Current, a National Grid spokesman on Saturday confirmed the undersea cable was down.

“National Grid is currently investigating an equipment issue involving one of two transmission cables servicing Nantucket,” Bob Kievra, National Grid’s manager of corporate affairs for Massachusetts, to the Current in an email. “National Grid’s remaining transmission infrastructure has sufficient capacity to service Nantucket and no customer impact is expected while repairs are underway to the out-of-service cable.”

The failure of the undersea cable leaves the island in a precarious situation, one that it has prepared for over the years as Nantucket’s energy demand has increased substantially, but a scenario that is still uncharted waters.

As Nantucket town manager Libby Gibson stated in 2019: “We face a serious risk if one of those cables goes down. If one goes down it is not enough to keep us going.”

The trouble began on the afternoon of Tuesday, April 30th, when there was a roughly 30-second power outage after National Grid crews ran the island’s backup generator on Bunker Road to discharge power from the unit during a scheduled operation. The older of the two undersea electric cables went after this process was completed, but it's unclear if it was related.

On Wednesday, crews from the electrical contracting firm KOBO traveled to the island to work with the local National Grid team to diagnose and fix the problem, but have so far been unsuccessful. While the one remaining cable can meet the island’s electricity demand in May, there is concern about the upcoming surge of demand around Memorial Day Weekend.

“National Grid expects repairs will be completed later this month, prior to Memorial Day,” Kievra stated. “A battery storage system on Nantucket is always ready to support the island’s energy needs but our existing cable currently can satisfy all of Nantucket’s energy demand.”

Nantucket's local National Grid leaders declined to comment.

The island’s first 28-mile extension cord to the mainland was installed beneath Nantucket Sound back in 1996 when the island’s peak electricity usage was just 20 megawatts. The undersea cable installed by National Grid that year ran from Nantucket to Harwich on Cape Cod with a capacity of 36 megawatts. It brought to an end the era in which the island generated its own electricity at the former downtown Nantucket Electric Company power plant along the waterfront that featured six massive diesel generators.

As Nantucket’s electricity usage continued to grow, a second undersea cable was installed in 2006 to meet the demand. This one connected the island’s substation on Candle Street to Kalmus Beach in Hyannis. The $41 million, 38-megawatt cable was paid for through a National Grid surcharge on Nantucket electric bills.

While the two undersea cables are owned and operated by National Grid, it’s another utility company, Eversource, that supplies power to the cables.

Sources told the Current that it is the original undersea cable - the one installed 28 years ago - that went offline on Tuesday.

In 2019, National Grid installed a bank of new Tesla battery storage units on the island that were intended to replace Nantucket’s backup power source: two aging 6-megawatt diesel generators. The new 6-megawatt / 48-megawatt-hour lithium-ion Tesla system was paired with a 15-megawatt generator and power control house on National Grid’s three-acre property east of Nantucket Memorial Airport. At the time, the installation was the largest of its kind in the Northeast. During the current breakdown, the battery system has not yet been needed given the island's lower peak usage for May. 

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National Grid's Tesla battery storage system on Bunker Road. Photo by Jason Graziadei

The town of Nantucket’s Energy Office has spearheaded a “Beat the Peak” initiative to reduce peak energy usage on the island during the late afternoons and evenings (approximately 5 p.m. to 9 p.m.) to forestall the need for a third undersea electric cable. That project could cost local ratepayers as much as $175 million and would “be disruptive to the island economy, precious land, and sea life,” according to the Energy Office.

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