Undersea Electric Cable To Nantucket Back Online

National Grid announced Monday that the undersea cable which provides electricity to Nantucket and had been broken down since April 30th has been restored. The 36-megawatt cable had been out of service for nearly two weeks, and utility company National Grid stated the problem was identified as equipment at the Candle Street substation on Nantucket.

The cable is one of two undersea transmission lines that run nearly 30 miles beneath Nantucket Sounds to substations on Cape Cod. The island relies on both for power, although only one is sufficient to cover Nantucket's electricity needs during the offseason.

Had it happened during the summer - when the island's peak electricity usage approaches 60-megawatts - it could have been a different story.

The precarious situation, however, prompted the island's electric utility provider National Grid and Eversource, another utility company that supplies power to the cables, to take the unusual step of transporting 11 large generators to the island over the weekend.

But by Monday, National Grid spokesman Bob Kievra released a statement announcing that the cable was back online.

“National Grid restored service Sunday night to an undersea cable that transmits electricity to Nantucket," said Kievra, National Grid's manager for corporate affairs in Massachusetts. "The cable went offline April 30 and National Grid’s remaining infrastructure supplied sufficient capacity for the island with no customer impact. The repairs, conducted by National Grid employees and contractors, involved maintenance replacement items on equipment at the Candle Street substation on Nantucket.

"Out of an abundance of caution, we had brought additional assets to the island last week/over the weekend, part of our overall response efforts to the situation," Kievra added in reference to the generators that were transported to Nantucket. 

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.

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 April 30th.

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.