Peaks and valleys: How National Grid’s non-wires alternative ensures delivery to Nantucket Island

The fast ferry arrives in Nantucket Harbor.

Promoting the development and implementation of sustainable, innovative and affordable energy solutions is central to what National Grid does every day. So, when customer demand for energy continued to trend upward on the island of Nantucket, they pursued a novel approach to ensure reliable and resilient power.

The island, located off the coast of Massachusetts, has a fairly small, year-round resident population of approximately 11,000. However, during the summer vacation period from the end of May until Labor Day in September, the population swells to more than 50,000. This population fluctuation corresponds directly to an increased need for energy.

Nantucket’s electric power is supplied by two 30-mile submarine cables connected to the Candle Street substation from the mainland. The first was installed in 1996; the second in 2005, to keep pace with the island’s growth. Combined, these cables have a capacity of 71 megawatts (MW). Two small on-island combustion turbine generators (CTG) have a combined capacity of 6 MW of backup power that can be engaged when delivery from the mainland is interrupted.

The site before work began on the new project. Credit: POWER Engineers

Evaluating options

In an N-1 contingency event where one of the transmission cables fails, electrical load on Nantucket Island can exceed the on-island grid’s capacity to serve that load. In addition to the potential risk of cable failure or mainland power interruption, we had recently determined that the existing combustion turbine generators had reached the end of their useful life.

After evaluating the scope of this potential risk to reliability and resiliency, National Grid engineers considered various mitigation options to address the ever-increasing load demand and the need for the electricity infrastructure improvements. The most obvious solution would be to add a third cable from the mainland which would address the growth and demand issues well into the future, but at a significant investment of almost $200 million. Ultimately, a unique solution involving the integration of several additions and improvements proved to be the most optimal.

The solution, referred to as IslandReady, consisted of replacement of the existing CTGs with a larger 15 MW CTG, a battery energy storage system (BESS) rated at 6 MW/48 MWh and the expansion and rebuilding of the existing distribution substation.  Analysis and studies by the Pacific Northwest National Laboratory (PNNL) for the U.S. Department of Energy validated the energy storage business case for the Nantucket application. This alternative was not only more efficient than the current system, but also cost less than one-half of the price of a third cable.

The site showing the proposed Tesla batteries. Credit: POWER Engineers.

Implementing the solution

Next, National Grid collaborated with POWER Engineers as their owner’s engineer to develop specifications and conduct the studies necessary to move forward with designing and supporting the construction of the new 13.2 kV substation, procuring the CTG and BESS and performing detailed design and integration of all the components. The larger CTG has a temperature-dependent capacity that varies between 10 MW and 16 MW to provide system backup and recharge the BESS if the cables fail. A twenty-four hour fuel supply is available to the generator onsite.

For the BESS, they chose Tesla’s 6-MW/48-MWh lithium-ion battery system, made up of 240 Powerpacks, to work in tandem with the CTG. Integrating all the new system components required careful modelling of the electrical network to accurately assess the various potential use cases that are unique to this type of system. Reliability is always an important utility requirement, but it has more significance in isolated areas such as an island.  As a result of the PNNL studies many additional benefits affected by the BESS were identified which further enhanced flexibility, reliability and resilience.

Because of this innovative approach, National Grid’s Nantucket Island customers can rely on this new system to power essential loads in the event of a total power failure. Power demand will follow population growth in the future, but the hybrid BESS/gas CTG will delay the necessity to deploy a third cable for an estimated 15-20 years. They commissioned this new system and substation last fall, which allowed them to retire the aging backup diesel generators. In November 2019, the project received an Energy Storage North America Innovation award for leadership and positive impact on the energy storage industry.

The site as viewed from above. Credit: POWER Engineers.

The team is excited to have developed an energy storage prototype for National Grid in Massachusetts. As currently the largest battery in New England, it will not only provide reliable power to National Grid’s customers but will also provide performance evidence necessary to move energy storage and renewable energy technology forward.

This successful project is further evidence of National Grid’s commitment to transforming its electricity and natural gas networks to support the 21st century digital economy with smarter, cleaner and more resilient energy solutions.

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Andrew Alexiades is a program manager with POWER Engineers for National Grid's New England East-West Solution portfolio of projects. Andrew earned a BEEE degree from Stevens Institute of Technology in New Jersey, U.S. A member of IEEE and Project Management Institute, he has served in numerous project management roles within the energy industry for more than 40 years, including areas of power generation and power delivery both domestically and internationally.

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