PSE's Power Lines
Puget Sound Energy has more than 20,000 miles of transmission and distribution power lines in our 6,000 square mile
service area, primarily in the Puget Sound region of Western Washington.
Transmission and distribution lines are both vital parts of our electric system, but they play very different roles.
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Transmission lines are high-voltage lines that carry electricity from generation plants to substations or from substation to
substation. Transformers at the substation “step down” the electricity’s transmission voltage (55 – 230 kilovolts*) to our
primary distribution voltage (12.5 kV).
A distribution line is a medium-voltage (12.5 kV) line that carries power from a substation to customers. Roughly half
of PSE’s distribution lines are underground. Distribution voltage is stepped down to service voltage through smaller
transformers located along distribution lines.
* A kilovolt (kV) is equal to 1,000 volts of electric energy.
Generation
Transmission via
230/500 kV lines
Transmission via
115 kV lines
Distribution via
12.5 kV lines
Electric Transmission Lines
Above ground versus underground lines
Nearly all of PSE’s transmission lines are located above ground. While we are able to build underground transmission
lines, overhead transmission lines are the first option for their combination of reliability and affordability – both of which are
important to our customers. Per state regulated tariffs, if an overhead route is viable, the additional cost to underground
must be paid for by the group requesting it. This is to help keep costs low for all of our customers. The construction costs for
overhead power lines compared to underground lines can be seen in this table:
Distribution
Overhead
Underground
$590,000 to $830,000 per mile
$2.3 million to $3.2 million per mile
115 kV Transmission $825,000 to $2.1 million per mile $4.7 million to $9.5 million per mile
230 kV Transmission $3 million to $4 million per mile
$20 million to $28 million per mile
In addition to cost, there are other factors to consider. For example:
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Putting power lines underground can have bigger environmental and neighborhood
impacts. Undergrounding transmission lines requires extensive vegetation removal,
trenching and installation of large (as much as 20 feet x 30 feet) access vaults every
quarter mile and can be very disruptive to neighborhoods and the environment.
Underground lines typically take longer to repair, and repairs are more difficult. When
an overhead line fails, our crews can often repair it within hours. Repair of underground
transmission lines can take days and even weeks, depending on the repairs that need
to be made.
Ultimately, the local community must weigh the benefits of undergrounding against these
significantly increased costs. Should the community decide to invest in an underground
solution, PSE will work with the community in this effort.
PSE’s power distribution system:
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2,597 miles of transmission lines (55 kV and above)
20,428 miles of distribution lines (under 55 kV)
○○ 10,357 miles of overhead lines
○○ 10,071 miles of underground lines
428 transmission and distribution substations
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230kV transmission lines
Line maintenance
Overhead transmission line maintenance includes visual line inspections, pole treatment, and vegetation management.
Further investigation can include infrared (IR) scanning and radio frequency scanning. Underground transmission lines are
difficult to maintain due to their sensitivity to design and operating conditions. The cables are sensitive to changes in soil
cover and aboveground changes. The main maintenance activity is patrolling the cable route to verify there have been no
changes in soil depth, cover type, vegetation changes, or other activity that would impact the ability of the cable to dissipate
heat effectively. Visual inspection and IR inspection of the cable ends and vaults is also important to maintain the cable.
Outages and restoration
Transmission outages are less common than outages on the distribution system; usually less than 0.5% of customer outages
are transmission outages. The lower failure rate is due to differences in design, construction, and operation. When there are
transmission outages they typically affect thousands of customers.
If an outage occurs, PSE crews focus first on restoring power to transmission lines that provide power to substations serving
large numbers of customers in a broad geographic area. Priority for power restoration also goes to essential services such
as hospitals, water/waste water systems, energy and transportation.
Once the high-voltage transmission system and substations are back online, PSE crews focus on repairing damage
to distribution and service lines – the lines that directly serve homes and businesses. During an outage, one part of a
neighborhood may have its lights on while another remains without power. Because our electric system works in sections or
circuits, PSE crews re-route power around the damaged areas, thereby isolating the section that requires repairs.
Electric Distribution Lines
Roughly half of our distribution lines are underground.
PSE typically undergrounds distribution lines in all new
developments. This is because it is cost-effective to install
buried cables at the time a neighborhood or building site is
constructed and the costs are shared by the developer.
Burying distribution lines in an older, established
neighborhood could carry a price tag as much as 15 times
the cost of installing above-ground lines. This is because
it requires excavating asphalt and concrete and, in some
cases, working in difficult terrain, particularly in the Puget
Sound region where there are many wetlands. If a business,
neighborhood or community asks PSE to underground
an existing distribution system with overhead lines, PSE’s
regulated tariffs require that business, neighborhood or
community to help pay the costs.
Line maintenance
Overhead distribution line maintenance includes visual line
inspections, pole treatment, and vegetation management;
further investigation can include infrared (IR) scanning
and radio frequency scanning. Underground lines are
more difficult to maintain and require diagnostic testing to
verify proper operation; though vaults can still be visually
inspected and IR scanned similar to overhead components.
Annual outage restoration costs and preventive
maintenance is slightly higher for underground lines due to
the difficulty in troubleshooting and maintaining.
Outages and restoration
12.5 kV distribution lines
While underground distribution cables are better able to withstand wind storms, falling trees and other above-ground
hazards, they may at times fail due to corrosion, fatigue and other stresses below ground. Additionally, since underground
cables are often fed from an overhead power line, an outage on the overhead line can result in an outage to the downstream
underground cable.
Placing distribution lines underground may result in fewer outages caused by storms; however, when outages do occur
they are typically of much longer duration. PSE workers can visually inspect overhead power lines to determine the location
of damage, and due to easy access, can typically make repairs in a few hours. Locating damage to underground cables,
however, is not as easy; we must first locate a fault by testing the cable with specialized fault-finding equipment, which can
take several hours. Once located, we must dig up the area around the fault to repair the cable.
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