Facilities & Equipment

Northern Pass is a proposed 192-mile transmission project that will carry 1,090* megawatts of clean hydroelectric power from Canada to New Hampshire and New England. The project will consist of two sections: direct current (DC) and alternating current (AC). First the power will be transmitted over a 158-mile DC transmission line, 60 miles of which will be underground, from the Canadian border to a converter terminal in Franklin, NH. There it will be converted to AC and travel along an AC transmission line for 34 miles to a substation in Deerfield, NH, where, it will interconnect to the New England power grid and serve customers in New Hampshire and all other New England states.

 DC Transmission Line

  • +/-320 kV line, approximately 158 miles long, including 60 miles located underground, extending from the Québec/New Hampshire border to the proposed Franklin, NH converter terminal
  • Portions of existing Eversource AC lines along the transmission corridor will be relocated within the existing Eversource right-of-way to support the construction of the Northern Pass DC line

AC Transmission Line 

  • 345 kV line, approximately 34 miles long, extending from the proposed Franklin converter terminal to the Deerfield Substation
  • Portions of existing Eversource AC lines along the transmission corridor will be relocated within the existing Eversource right-of-way to support the construction of the Northern Pass AC line 

Converter Terminal 

  • Located in Franklin, NH, using 21 acres of a 118 acre parcel and allowing for a substantial vegetative buffer
  • A facility that contains the equipment necessary to convert power from direct current (DC) to alternating current (AC)
  • Includes buildings with conversion equipment and controls, an open-air substation with filter banks, and other equipment similar to a conventional substation  

Substation Upgrades

The organization that oversees our regional electric grid, ISO New England, has reviewed a request from Northern Pass to connect to the grid at the Deerfield substation in Deerfield, NH. This review, conducted as part of what is called an I.3.9 request, has determined that the two 345 kV lines between the Deerfield substation and the Scobie Pond substation in Londonderry, NH will need to be upgraded, along with minor upgrades at each substation. This upgrade involves replacing a total of 10 structures along the existing transmission lines to allow the existing lines to carry a greater level of power.   

As noted above, Northern Pass must also make minor upgrades to the substations at Deerfield and Scobie Pond. This will include the construction of an additional 345 kV bay position at the Deerfield Substation, and additional capacitor banks to provide voltage support for the Project, both of which will be done in an area adjacent to the existing substation yard. A 345 kV capacitor bank will also be constructed at the Scobie Pond Substation in an area adjacent to the existing substation yard, and 345 kV breakers will be installed in the existing substation bus to provide voltage support for the Project. 

Structures

  • By changing to the new 1,090 MW technology, the Project was able to reduce structure heights along the DC portion of the line. Structure heights were also reduced along the AC portion of the line between Franklin and Concord by relocating 17 miles of existing 115 kV lines and changing the design of the proposed 345 kV line.  
  • To address potential visual impacts to sensitive public scenic and historic resources, the Project will use the more expensive monopole design for more than 140 structures.
  • Taller structures are used along the route when it’s necessary to maintain the clearances and minimum heights required by the National Electrical Safety Code. A majority of the Eversource transmission line structures that exist along the proposed Northern Pass route today are 45 to 75 feet tall, and operate at a voltage of 115 kV AC or 34.5 kV. The Northern Pass will construct lines that operate at voltages of +/-320 kV DC and 345 kV AC. These higher voltages require greater height and separation for safety. Here are some other reasons that can influence structure height:
    • To span rivers and roadways
    • To cross other lines and structures
    • To account for changes in terrain

 The height of Northern Pass transmission structures is as follows:

  • In the new transmission corridor in the North Country from the Canadian border to the location where the line meets the existing transmission corridor in Dummer, NH, the most common structure height will be 80 to 85 feet. On average, this section has the lowest structure heights of the Project. 
  • In the existing transmission corridor between Dummer and where the line transitions to underground in Bethlehem, the most common structure height is 85 feet to 90 feet. From Bridgewater, where the underground line transitions to an overhead line in the existing transmission corridor, to Franklin, the most common structure height will be 80 to 85 feet.
  • In the existing transmission corridor from Franklin to Deerfield, a majority of the structures (55%) are 100 feet or less, and the most common structure height is 105 feet to 110 feet.  

Photo Simulations

A series of photo simulations have been prepared to show what the Northern Pass transmission corridor will look like from various scenic resource locations along the proposed route. 

Here are examples of photo simulations reflecting design changes.

  • Lancaster
  • Concord