Eversource has substantially completed a major electric system reliability upgrade for Martha's Vineyard, placing two upgraded submarine transmission cables into service and eliminating the need for five diesel generators that had supported peak electricity demand on the island for decades.

The project includes one new submarine cable and one replacement cable connecting Martha's Vineyard to the Massachusetts mainland, along with upgrades to substations, underground infrastructure and distribution equipment. Collectively, the improvements add approximately 37 MW of capacity while increasing system reliability and redundancy.

The upgraded system was tested during the July 3-4 holiday weekend heatwave, when Martha's Vineyard reached a record peak electric demand of 66.5 MW. According to Eversource, the new cables remained in service throughout the event, allowing the island to meet its highest-ever electric demand without relying on local diesel generation.

Prior to the project, portions of the island's electric system relied on submarine cable infrastructure dating to the 1980s, while diesel generators were used to supplement electricity supply during periods of high demand.

The project included:

• Installation of more than 11 miles of new submarine cable beneath Vineyard Sound.
• Construction of a new underground duct bank and manhole system in Falmouth.
• Upgrades at the Falmouth Substation.
• Distribution system improvements in Oak Bluffs, West Tisbury and other parts of Martha's Vineyard.
• A dedicated circuit to improve electric reliability for Falmouth Hospital.

A key outcome of the project is the retirement of five diesel generators that had provided seasonal generation support for the island. Two of the generators dated to the 1940s. Eversource said retiring the units will reduce emissions and noise while supporting the island's transition to a cleaner electric system.

The reliability upgrades are the result of nearly a decade of planning, permitting, environmental review and coordination with local communities, state and federal agencies, fisheries representatives and other stakeholders.

Eversource said the investment is part of its broader grid modernization strategy to improve system resilience while supporting future electrification, economic growth and Massachusetts' clean energy goals.