A Third Undersea Cable To Power Nantucket May Be Needed Much Sooner Than Originally Thought

Nantucket will need a new extension cord to the mainland to keep the lights on - and it may become necessary much sooner than the town and its utility provider National Grid expected.

The island's peak demand for electricity is growing at five times the Massachusetts statewide average, leading the town and National Grid to conclude a third undersea cable will be necessary by 2033, just eight years from now, a project that could cost more than $200 million. A previous estimate had anticipated the third cable wouldn't be needed until 2044.

In an April 3rd letter to the Massachusetts Grid Modernization Advisory Council (GMAC), Nantucket's town counsel Devan C. Braun wrote that "Nantucket’s electricity demand continues to grow at an unprecedented rate and already exceeds the capacity of each undersea delivery cable from Cape Cod...A failure of one of the two undersea cables during the peak season would leave the island without sufficient capacity to power essential services, posing a significant risk of prolonged outages and widespread disruption."

The letter was released by the town on Tuesday.

"The island’s peak load of 55 MW has already exceeded the capacity of one undersea cable, a 6 MW Tesla Battery Energy Storage System (BESS), and a 10 MW turbine generator installed in 2019," Braun wrote. "As such, discussions are occurring with National Grid about a third undersea cable to maintain appropriate levels of reliability in future years. A third cable, if deemed necessary, would likely follow the same cost structure, placing a significant financial burden on island ratepayers."

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 island’s first 28-mile undersea electric cable to the mainland was installed beneath Nantucket Sound back in 1996 when the island’s peak electricity usage was just 20 megawatts. The $27 million 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.

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.

The potential failure of one of the two existing undersea cables became a real-world situation last May when the original cable that runs to Harwich failed and was offline for nearly two weeks. The situation prompted 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 for backup power.

"To mitigate this potential catastrophe, the Town requests clarity and transparency on the State’s and/or EDCs’ (electric distribution companies) plans for deploying additional roll-on generators and/or other contingency measures during this process," the town wrote to the state advisory council.

The letter also called on National Grid to outline a detailed timeline for the
planning, permitting, and installation of the third undersea cable, as well as determine what other local distribution infrastructure upgrades the company is planning.

"The accelerated timeline (for the third cable) highlights the rapid growth in electricity demand and the urgency of addressing these challenges," the town wrote. "Recent failures also underscore the vulnerability of current infrastructure, particularly during peak periods, making greater urgency and transparency essential."

The upgrades believed to be necessary, including the $200 million third undersea cable, have the potential to raise Nantucket's already expensive electricity prices, which are significantly higher than the national average. Island ratepayers are already subject to a 15 percent cable facility surcharge on their bills to finance the costs of the existing two cables. To minimize the financial impact, the town is urging National Grid and the state to explore federal and state grant programs for grid resiliency.

While the installation of renewable energy systems - such as solar panels and land-based wind turbines - has been considered one way the island could forestall a third undersea cable, the letter to the state advisory board describes a significant hurdle the town has encountered on that front.

"Nantucket also faces a natural 'reverse capacity limit' which restricts the amount of local renewable energy that can be developed on the island without risking backfeeding events across the undersea cables to the mainland," Braun wrote. "This limit remains undefined, creating significant uncertainty and hindering the efficient
advancement of local solar projects and key non-wire alternatives."

While the town recently completed a new 232-panel, 104.4-kilowatt (DC-kW) rooftop solar array at the Surfside Wastewater Treatment Facility, and the number of private solar installations has continued to grow, the power backfeeding issue has created concern about the future capacity of renewable energy projects. The town's letter requests the exploration of "3VO" (overvoltage) protection at the Candle Street substation "to unlock Nantucket’s renewable energy potential and ensure grid reliability as electrification continues to accelerate."