Comparison of Overhead
and
Underground
Presentation 1.3
Instructor: Frank Frentzas
Underground System Design
TADP 547
Overhead vs. Underground
Electric Power Lines create backbone of bulk power network
(69 kV lines and above are typically considered transmission).
Historically, transmission lines has been overhead in rural and
underground in urban areas.
Overhead lines predominantly used at 345 kV and above.
Overhead lines have more current carrying capacity than
underground cables, and multiple cables/phase are typically
required to match overhead line capacity.
However, overhead lines are getting harder to install due to
public opposition for environmental and aesthetic reasons.
Overhead vs. Underground (cont.)
Some European countries now prohibit (or limit) new overhead
line installations.
For every kilometer of overhead line installed a percentage of
underground circuits must be installed to offset the overhead.
Some European countries now mandated replacement of
existing overhead transmission lines with underground circuits
at 230 kV and 400 kV levels.
Cost to install an overhead line is less than that of an under-
ground line. However, overhead lines are less reliable.
Reliability measured by the time a line is available to carry load.
Key Factors
Key factors to consider when choosing transmission line design:
– Feasibility
– Cost
– Reliability
– Environmental impact
Feasibility
Feasibility is first key factor:
Is it technically feasible to construct an overhead or under-
ground line? Overhead circuits require more right-of-way.
Overhead lines may require a bigger foot print. However, under-
ground lines may have a greater environmental impact during
construction.
Overhead construction impact is generally located at the poles
or towers, whereas underground is one continuous route which
can impact the feasibility of the installation.
Other underground utilities can impact installation or proposed
/specified route. For example …..
Feasibility - Congestion
Congestion of underground facilities
Feasibility – Submarine Cables
Submarine cable across large waterways
Feasibility (cont.)
Voltage level of circuit? Overhead is currently the only reliable
option at 500 kV and above.
(Some underground lines do exist at 500 kV, but they are new
and their long-term reliability has yet to be established.)
Length of the line can also determine which option to choose.
A long underground line will have to be divided in sections and
additional inductors installed to compensate for line
capacitance.
Typically, underground lines are installed when overhead
cannot be constructed or permitted.
Cost
Cost plays an important role when designing a line since most
utilities pass the cost on to customers.
Unless there is clear evidence that an overhead line cannot be
constructed, Utility Commissions will not allow utilities to recover
the cost of installing an underground line.
It can take several months to a year to construct a mile of under-
ground cable whereas an overhead line can be constructed in a
few months.
Cost to build and install an underground line is greater than an
overhead line due to the higher material, infrastructure, and
labor costs. (The ratio of underground to overhead cost is 2 - 4
times at lower voltages, and 8 - 14 at higher voltage levels.)
Cost (cont.)
Cost of operating high voltage High Pressure Fluid Filled (HPFF)
and Low Pressure Fluid Filled (LPFF) underground lines are
approximately two times higher than operating overhead lines.
Higher cost is primarily due to maintenance costs of pressurizing
and cathodic systems, and higher line losses.
However solid dielectric lines require less maintenance than
those above.
Reliability
Line reliability refers to the number of times a transmission line
is interrupted and the amount of time it is available to carry load/
power.
Underground cables are less susceptible to weather related
outages. However, they generally require longer repair times
which can result in a significant difference in overall system
reliability indicators.
Weather related interruptions and outages are the main cause
of overhead line faults.
Overhead line faults are easy to locate and can be repaired
within a day, unless multiple structures are damaged during a
storm.
Reliability – Line Damage
Overhead line damaged during an ice storm
Reliability – Line Damage (cont.)
Overhead lines damaged during an ice storm
Reliability – Line Damage (cont.)
Overhead line damaged during a tornado
Reliability – Line Damage (cont.)
Overhead line damaged during a tornado
Reliability – Fault Location
Underground faults are hard to locate, and repairs can take from
several days to several weeks.
Underground lines are not effected by weather like overhead
lines. However, they are exposed to mechanical damage by
contractors or other utilities digging in the same area.
Dig-ins are the number one cause of failures for underground
cables followed by accessory failures due to workmanship
during installation.
Damage to pipe coating on HPFF pipe type systems can lead to
corrosion and eventually a leak.
Fault Location – Cable Damage
138 kV Underground cables damaged due to a dig-in
138 kV cables
in concrete
encased duct
bank
Fault Location – Cable Damage (cont.)
138 kV Underground cables damaged due to a dig-in
Fault Location – Cable Termination
138 kV Cable termination failure due to workmanship
Fault Location – Pipe Leak
Pipe Leak due to coating damage
Fluid
Leak
Environmental
Overhead lines span long distances - in some cases over 1,000
feet from tower to tower.
Ground area between the towers untouched and can be returned
to it’s original state after construction.
Overhead lines have minimal impact to environment, although in
some locations trees and vegetation may need to be managed to
avoid contact with line conductors.
Tree contacts with line conductors can cause outages, brush
fires and personal injuries.
Low growth vegetation typically planted near or around overhead
lines.
Environmental (cont.)
Underground facilities present some environmental impacts
which include:
– Continuous trenching during construction and installation of
splice bays or manholes - which are typically 20 - 30 feet
long and 6 - 10 feet wide.
– Alterations to soil and surface water drainage patterns -
mostly due to thermal backfills used around conduits or
cables.
– Heat dissipation from energized cables to surrounding soil –
which can effect vegetation.
Environmental (cont.)
– Continuous trenching requires the removed soil to be hauled
away (if not used for backfill) and properly disposed off.
– Contaminate soil and/or waterways when fluid filled
transmission lines experience a leak.
– Spill prevention and containment plans required lines
containing a large volume of fluid (typically over 1,500
gallons).
– Visual impact to environment and landscape is eliminated
when installing underground lines since only termination sites
are visible.
Summary - Underground Line Advantages
Have smaller land requirement or foot print.
Do not effect real estate/property values.
Can be installed when overhead lines are not feasible or
permitted.
Lower maintenance costs when XLPE cables used.
More reliable since they are not effected by weather (storms, ice,
tornados, etc.).
Less exposure to vandalism.
Minimal visual impact to environment and landscape.
Summary - Underground Line Advantages (cont.)
Have lower EMF values.
No Radio or TV interference (RI/TVI) due to corona discharge.
Contact with live conductors eliminated - which can cause
injuries and/or trees or brush fires.
Summary - Underground Line Disadvantages
Higher initial cost. However, differential costs decreasing with
more and more underground installations.
Underground line installation could result to higher electric rates.
Longer repair times (7 to 10 days vs. 1 day for overhead) and
costs for typical repairs – excluding storms.
Longer to identify underground faults than overhead faults.
Continuous trench required during installation can impact
environment especially in sensitive areas.
Soil thermal and drainage conditions modified which can effect
overall drainage and vegetation.
Summary - Underground Line Disadvantages (cont.)
Length of AC cable circuit limited (or requires compensation).
Presence of splice bays or manholes every 1,500 - 2,500 feet
can impact surroundings.
Multiple cables-per-phase required to match capacity of over
head circuit.
All repairs must be done with lines de-energized.