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.