Underground and Overhead Transmission Lines
This fact sheet summarizes several of the major differences between Factors that can influence the selection of
underground and overhead transmission lines. The information was drawn underground cables
from a number of sources, including major studies completed in different
Four main factors normally influence the decision on whether underground
parts of the world by parties interested in determining the viability of
cables are considered as an alternative to meet the required electricity
underground transmission cables to meet specific electricity needs. This
need for a project. These factors are:
information does not present a position for or against their use in the
context of the Heartland or any other transmission project. • Technical feasibility;
• Reliability of service;
Underground feasibility study being conducted by the • Environment; and
Alberta Electric System Operator • Overall cost.
The Alberta Electric System Operator (AESO’s) is currently conducting a In some circumstances, location specific constraints may also play a role in
study to assess the technical feasibility and cost of using high voltage the decision to use underground cables.
500 kV Alternating Current (AC) underground cables for the Heartland
Transmission Project. In particular the AESO’s study will look at the Technical feasibility
feasibility of deploying up to an 8 – 10 kilometre section of AC underground
Technical feasibility refers to factors regarding the maturity of the
cables as one component of a larger AC overhead project.
technology, and whether it can be integrated into an existing transmission
system and operated dependably. Technical feasibility for 500 kV
The AESO’s study will focus on AC underground. Work completed during the
underground cables is difficult to determine as there are currently very few
AESO’s project alternative analysis determined that Direct Current (DC)
other underground 500 kV cables in service in the world and only one with
underground was not an acceptable alternative for the Heartland Project.
an underground line length greater than five kilometres that we were aware
Factors effecting this decision included:
of at time of printing. The AESO’s study will include an examination of the
• reduced operational reliability (risk of long outage durations); technical feasibility of deploying up to an 8 – 10 kilometre section
• future reduced flexibility (higher incremental cost to increase of 500 kV AC underground cables as one component of a larger AC
capacity) ; and overhead project.
• higher cost (higher initial installation and ongoing power losses
associated with the conversion of AC to DC). Reliability of service
Reliability of service refers to the amount of time a transmission line
For more information regarding the AESO’s summary report of its analysis
is available to transport electricity. Underground cables are much less
of the various alternatives, titled “Alberta’s Industrial Heartland Bulk
susceptible to weather-related outages. However, longer repair times
Transmission Development”, visit their web site at www.AESO.ca. The
for underground cables result in a significant difference in overall
AESO’s study of the feasibility of AC underground cables is expected to be
completed by late summer.
Reliability of service for 500 kV underground cables is difficult to determine
as there are currently very few other underground 500 kV cables in service
in the world. However, information published by the European Commission
Alternating Current (AC): an electric current that reverses its for 400 kV overhead lines shows outage statistics of 0.126 hours per
direction at regularly recurring intervals. In North America, the kilometre per year for overhead lines compared to 6.4 hours per kilometre
standard for alternating current is 60 complete cycles each per year for comparable underground cables. Using these statistics,
second. Such electricity is said to have a frequency of 60 hertz. assuming a 100-kilometre 400 kV transmission line, the average time
where the overhead lines would be out of service is 12.6 hours per year
Direct Current (DC): An electric current flowing in one direction compared to almost 27 days per year for underground cables. These outage
only and substantially constant in value. The current supplied statistics take into account both scheduled maintenance outages and
from a battery is direct current. unplanned outages.
Information available as of May 1, 2009
Environment one with an underground line length greater than five kilometres that
Underground cables eliminate the potential for bird contacts (which we were aware of at time of printing to use as a comparison. However,
could cause injury or death to the birds) that overhead lines present. available information related to 400 kV underground facilities indicate
Underground cables also largely eliminate the visual impact of the that costs are normally between four and 20 times higher than equivalent
transmission facilities on the landscape. Underground facilities present overhead options. For example, if the cost of an underground line is 10
some environmental impacts, including underground vaults, localized times more expensive, then the cost of a 50 kilometre 500 kV underground
heating of the soil and alterations in ground and surface water drainage line would be approximately $1.5 billion in comparison to $150 million
patterns. The construction process for underground installation requires for an overhead line. Using the same assumptions, if the line were to be
soil stripping, continuous trenching and underground vaults every several underground for 10 kilometres and then overhead for the remaining 40
hundred metres. The underground vaults can be 10 metres long by four kilometres the cost of the transmission line itself would increase from $150
metres wide. After construction is complete, the heat produced by energized million for an entirely above-ground line to $420 million.
cables dissipates into the surrounding soil and may result in localized
impacts on vegetation. Drainage may need to be altered temporarily or As to the cost of operating high voltage underground cable, available
permanently depending on the construction of underground cable trenches. information related to 400 kV installations indicates that life cycle
operating costs are 1.5 to 2.5 times higher than overhead lines. This is due
Overhead lines span across long distances with approximately 350 metres to the higher maintenance costs and higher line losses.
between tower locations. This leaves the ground and subsurface virtually
untouched, with the exception of tower locations, after construction is In Alberta, for every $100 million added to a transmission project’s cost,
complete. Overhead lines have minimal impacts on most environmental the average monthly bill for each resident in Alberta would increase by
features including natural heritage (plants and animals), riparian (rivers, approximately 13 cents. An underground 50 kilometre line would add
streams), cultural heritage (archaeological) and paleontological features, approximately $2 per month to every residential electricity bill in Alberta.
but do have some environmental impacts. Depending on the location, some A ten kilometre section of underground would add approximately $0.35 per
tree and vegetation management may be required in the area surrounding month to every residential bill in Alberta.
the transmission line. Tree and vegetation management includes the use of
mechanical, chemical and other techniques to encourage the growth of low Electric and magnetic fields
vegetation to reduce the risk of a fire or outage caused by branches or tree Electric and magnetic fields are everywhere there is electricity.
limbs contacting transmission lines.
Electric fields can be shielded by such things as insulation, trees,
Overall cost building walls, and vehicles. Underground cables and their accompanying
The cost of any technical solution is a consideration for Albertans because insulation and shielding effectively stop electric fields. As overhead power
the construction and operation of transmission facilities is paid for by all lines are bare wires, electric fields can be measured near these lines.
Alberta ratepayers through their electricity bills.
Magnetic fields cannot be shielded by insulation, cement or earth and
The cost to build, install and operate underground cables is greater will be measurable at both underground and overhead installations.
than overhead cables. This is due to the higher cost of materials (more The ability to tightly space underground cables may allow for greater
underground cables are required to carry the same amount of power as one cancellation of magnetic fields by the cables themselves when compared
overhead line), the more labour and time intensive construction process to overhead lines. However, given their closer proximity to ground level, the
and the higher cost of maintaining underground facilities and higher line measured magnetic field above underground cables is typically similar or
losses associated with underground cables operated at certain load levels. stronger than at ground level directly below overhead lines. Field levels for
underground cables decrease more quickly compared to overhead lines as
For the Heartland Project, preliminary estimates suggest that a 500 kV you move further away from them.
AC overhead line could cost approximately $3.0 million per kilometre
to construct, not including the cost of substations. Costs for 500 kV
underground cables are more difficult to estimate as there are currently For information on the proposed Heartland Transmission Project:
very few other underground 500 kV cables in service in the world and only Call Toll-Free: 1-888-441-7192
Information available as of May 1, 2009