Converter Sites And Cable Routing
As proposed, Green Line will include two new HVDC converter stations, one in Houlton, Maine and one in Boston or Salem, Massachusetts. HVDC converter stations convert between AC and DC power, thus enabling direct current power to flow through the new DC transmission line 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 be compatible with the character of the surrounding built or natural environment.
SUBMARINE CABLE ROUTING
The cable will be laid beneath the Gulf of Maine 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. 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.