Eversource Energy Greenwich Substation and Line Project H-1 February 2015 H. Underground Transmission System Design In addition to analyzing potential routes, the Company considered several different design technologies for the proposed underground transmission supply lines, settling on two underground cable technologies: HPFF pipe type cable and cross-linked polyethylene (“XLPE”) cable. The load at the Greenwich Substation does not require the larger conductors that are available with XLPE cable technology. The Company concluded that two supply lines would be required to ensure a reliable power source. Based on the Company’s analysis, use of two HPFF cable circuits were determined to be the most appropriate for the Project for the following reasons: • The HPFF cable can be provided in longer lengths, so fewer vaults and cable splices will be required along the route, resulting in a more cost-effective Project. Also, fewer vaults result in less accessories such as cable splices, which improves reliability since accessories have a higher rate of failure; • A HPFF cable splice vault is smaller than an XLPE cable splice vault, and unlike XLPE cables, the splices for both HPFF circuits can be housed within the same splice vault. This results in less excavation than a comparable XLPE cable system, and therefore quicker construction and less impact to the community along the route; • HPFF cable systems have the ability to circulate the dielectric fluid to smooth out hot spots along the cable route. This provides a great advantage over XLPE cable systems when running parallel to existing heat sources, such as the existing distribution circuits along Railroad Avenue or segments of the route requiring deeper installation, such as the HDD crossings; • The three (3) power cables for each circuit are installed in a single 8-inch pipe versus three (3) individual 8-inch PVC conduits in a concrete duct bank, therefore the HPFF cable system is easier to route and install and should result in a shorter construction duration; and, • HPFF cable systems can be upgraded with forced-cooling equipment to expand the load carrying capacity in the future.
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H. Underground Transmission System Design. Underground Transmission System Design In addition to analyzing potential ... cable would consist of a3500-kcmil segmental copper conductor
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Eversource Energy
Greenwich Substation and Line Project H-1 February 2015
H. Underground Transmission System Design
In addition to analyzing potential routes, the Company considered several different
design technologies for the proposed underground transmission supply lines, settling on
two underground cable technologies: HPFF pipe type cable and cross-linked
polyethylene (“XLPE”) cable. The load at the Greenwich Substation does not require the
larger conductors that are available with XLPE cable technology. The Company
concluded that two supply lines would be required to ensure a reliable power source.
Based on the Company’s analysis, use of two HPFF cable circuits were determined to
be the most appropriate for the Project for the following reasons:
• The HPFF cable can be provided in longer lengths, so fewer vaults and cable
splices will be required along the route, resulting in a more cost-effective Project.
Also, fewer vaults result in less accessories such as cable splices, which improves
reliability since accessories have a higher rate of failure;
• A HPFF cable splice vault is smaller than an XLPE cable splice vault, and unlike
XLPE cables, the splices for both HPFF circuits can be housed within the same
splice vault. This results in less excavation than a comparable XLPE cable
system, and therefore quicker construction and less impact to the community
along the route;
• HPFF cable systems have the ability to circulate the dielectric fluid to smooth out
hot spots along the cable route. This provides a great advantage over XLPE
cable systems when running parallel to existing heat sources, such as the existing
distribution circuits along Railroad Avenue or segments of the route requiring
deeper installation, such as the HDD crossings;
• The three (3) power cables for each circuit are installed in a single 8-inch pipe
versus three (3) individual 8-inch PVC conduits in a concrete duct bank, therefore
the HPFF cable system is easier to route and install and should result in a shorter
construction duration; and,
• HPFF cable systems can be upgraded with forced-cooling equipment to expand
the load carrying capacity in the future.
Eversource Energy
Greenwich Substation and Line Project H-2 February 2015
A 115-kV HPFF underground transmission line system is comprised of the following
termination structures and foundations, and a cathodic protection system. The Project’s
HPFF underground 115-kV line system would consist of two (2) 8-inch steel pipes in a
common trench, in which the two HPFF lines would be installed, along with a 6-inch fluid
return pipe for fluid circulation, and four (4) fiber optic cables (2 for communications and
2 for dynamic temperature sensing).
The electrical cable carbon steel pipes would be installed in a trench encased in low-
strength concrete slurry, also known as fluidize thermal backfill (“FTB”) and capped by a
protective layer of high-strength concrete. Figure H-1 illustrates a typical trench cross
section.
Eversource Energy
Greenwich Substation and Line Project H-3 February 2015
Figure H-1 Typical High Pressure Fluid Filled (HPFF) Trench Cross Section with Two Line Pipes, Fluid Return Pipe and Communications and Duct Temperature
Sensors Ducts
Eversource Energy
Greenwich Substation and Line Project H-4 February 2015
H.1 Lines
The 115-kV HPFF transmission system would consist of three (3) cables per line. Each
cable would consist of a 3500-kcmil segmental copper conductor insulated to 115 kV
with paper insulation and would be approximately three (3) inches in diameter. Figure
H-2 illustrates the cross section of a typical 3500-kcmil segmental copper conductor