Converter Sites And Cable Routing
As proposed, Green Line will include two new HVDC converter stations, one in Wiscasset, Maine and one in Boston. NEITC has identified sites for the converter stations near to the existing Maine Yankee 345 kV substation in Wiscasset and the existing K Street 345 kV substation in South Boston. HVDC converter stations convert between AC and DC power, thus enabling direct current power to flow through the new submarine cable and alternating current power to synchronize with the existing AC grid.
Converter stations are the most visible element of the Green Line project. The converter station layout, which has a foot print of 10 to 20 acres, consists of four sections: an open-air high voltage AC switchyard section, a valve hall, a control building, and a spare parts building. The open air section includes the following electric components: AC circuit breakers and bus-work, AC harmonic filter circuits, surge arrestors on the AC side, converter transformers, and air core smoothing reactor. The valve hall houses the thyristor valves. The control building contains control and protection systems, communication facilities, auxiliary supply batteries, valve cooling skid, air conditioning system, and offices. Spare parts and miscellaneous storage will be housed in the fourth section. Generally, they are similar in appearance to a medium scale industrial building with an adjacent electric switchyard and can be designed to reflect the character of the surrounding built or natural environment.
It is expected that the converter stations will connect to the AC grid via an underground 345 kV line. The converter stations will be sited in close proximity to the waterfront in order to minimize the terrestrial reach of the DC cable.
WISCASSET
On the Wiscasset end of the line there is ample industrial zoned property with sufficient setbacks and natural screening to allow the building to sit on the property with little disruption to the surrounding landscape. NEITC is still evaluating the cable route from the shoreline and coast to the open ocean, but it is expected that the cable will be placed beneath the Sheepscot River. The Sheepscot River is deep enough to accommodate a cable-laying vessel up to the town of Wiscasset. NEITC intends to work closely with local stakeholders, such as fishermen, wormers, lobstermen, environmentalists, boaters, town officials, and others to determine a route for the cable that will best minimize environmental impacts.
BOSTON
On the Boston end, laying cable beneath or in the vicinity of Boston Harbor will require extensive environmental studies, careful planning and close coordination with all regulatory agencies, as well as close coordination with regulations under the new Massachusetts Oceans Act of 2008. The Green Line team’s demonstrated experience with laying cable beneath New York Harbor on the Neptune project will prove very helpful in approaching the installation of a cable in Boston Harbor. It will also require review of previous projects that laid electric cable beneath Boston Harbor. On land, the converter station’s building design is flexible enough to blend in with surrounding urban landscape in a way that reflects either the historic architecture of Boston’s past or the modern architecture of its future. NEITC will work with local designers and planners to help determine the most suitable appearance of the building.
CABLE ROUTING
The cable will be laid beneath the Gulf of Maine between AC/DC converter stations located in Wiscasset, Maine and Boston, Massachusetts, a distance of approximately 140 miles. Preliminary investigations of possible siting routes show that water depths in the Gulf range from zero feet in shoreline areas to 900 feet in an area known as Murray Basin, which is 36 nautical miles east of Rockport, Massachusetts. South of Wiscasset water depths are generally 480 feet or less within 12 nautical miles of the shoreline. The cable length and water depths are well within the design and construction parameters of existing submarine cables installed and operating around the world.
Existing survey data provided on NOAA charts show that most of the ocean bottom in the potential route is comprised of sand and mud. There are also areas with rocky or hard bottom conditions. Navigational constraints are found in the major port areas such as Portland, Portsmouth, and Boston, including vessel separation areas and approach corridors.