Line and Substation Upgrades

Northern Pass will connect to the regional grid at an existing substation in Deerfield, NH.  In order to determine how the project could interconnect at that location, ISO-NE had studied Northern Pass’ original 1200 MW design and concluded that certain electrical upgrades to the New England regional transmission grid are required to allow for Northern Pass to connect without adversely impacting grid reliability.  Per the study, under certain conditions, without any system modifications, power flowing into Deerfield substation over Northern Pass and into the regional grid will cause two transmission lines to exceed their present rating.  The first line is located between Deerfield Substation and Scobie Pond substation in Londonderry, NH. The second line is between Scobie Pond substation and a substation located in Buxton, Maine.   

To address the potential for line overloads, additional terminal equipment (a circuit breaker and connecting bus work) will be required to loop the Scobie Pond to Buxton, ME line into and out of Deerfield substation.   The line currently runs past Deerfield substation without an electrical connection.  In addition, ten existing transmission structures between Deerfield and Scobie Pond substations would need to be raised in order to achieve the required ratings.  The increase in height for all but one structure is approximately 5 feet.  One structure will be increased approximately 10 feet.   The structures are located in the towns of Deerfield (5), Raymond (3) and Chester (2).  

Finally, to maintain system voltage levels, additional equipment will be installed at both Deerfield and Scobie Pond substations. 

ISO-NE is currently studying the project's new 1000 MW design (with a potential to deliver up to 1090 MW), so the identified upgrades described above are subject to change.  The project does not anticipate that the results will be materially different for the lower capacity project.