Underground vs. Overhead Transmission

TOTL Citizen Advisory Group
 April 15, 2015
Transmission and Distribution
Transmission System

Transmission lines
connect power
generation plants to
substations, other
power generating
plants, and other
utilities at high
voltages.
Distribution System

Distributes electricity
to each customer's
residence, business, or
industrial plant at
lower voltages.
Transmission and Distribution
Transmission
Distribution
Underground and Overhead
Transmission


Undergrounding
transmission lines is less
common
Underground cables
have different technical
requirements than
overhead and have
different impacts
Distribution

Undergrounding
distributions lines is
more common
Impacts





Design
Construction
Repair and Maintenance
Cost
Aesthetics
Underground vs. Overhead Transmission

Design Issues
 Specialized
engineering skills required
 Extensive study required to determine site-specific
subsurface obstructions or obstacles
 Longer timeframe for design
 Need to provide larger budget contingency
 Flood plain and wetland issues require special
consideration
 Environmental impacts
Underground vs. Overhead Transmission

Construction Concerns
 Space
for large vaults (8’ x 10’ X 20’)
 Longer construction time frame
 Dewatering in wet areas during construction
 Significantly more impacts to surrounding properties
 Open

trenches
min. 5’ wide x 5’ deep
 Specialized
backfill
Underground vs. Overhead Transmission

Operational Concerns
 Difficult
to identify outage location
 Requires specialized work force
 Long lead time for delivery of materials
 Need
to warehouse specialized spare materials
 Increased
maintenance
 Shorter life span
 Dewatering and cleaning of equipment in vaults
Underground vs. Overhead Transmission

Cost
Typical underground costs are 8 to 10 times the cost of
overhead construction
 Typical life of underground is approximately one-half the
life of overhead construction
 Depending on route may have significantly more
unanticipated problems with associated costs
 4-Cable system required to increase reliability which adds
cost
 Specialized workforce increases cost
 Wetland mitigation may be substantially more depending
on route
 Warehousing of spare materials and equipment

Underground Transmission

Generally used:
 in
densely populated and urban settings
 where sufficient right-of-way is not available
 to reduce visual impacts
 riser
poles at each end of the underground cable are large
and support additional equipment that create visual impacts

Reliability
 May
have fewer outages than overhead
 When outages occur they will be more difficult to
locate and may take significantly more time to repair