Download III. Types of underground transmission cables

International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
High Voltage Underground Electric
Transmission System
Hnin War, Thet Tin

Abstract— This paper contains information about electric
transmission lines which are installed underground, rather than
overhead on poles or towers. Underground lines have different
technical requirements than overhead lines and have different
physical, environmental, and construction needs; underground
lines generally cost more than overhead lines. And, this paper
describes types of underground electric transmission lines, the
impacts of siting, how they are built, how they differ overhead
lines and cable layout and system design.
Index
Terms—Cross-linked
Polyethylene
(XLPE),
International Electrotechnical Commission (IEC), Right-of-way
(ROW), Underground (UG)
I. INTRODUCTION
Much new distribution is being placed underground,
especially for taps in suburban residential areas. Whether
urban, suburban, or even rural, all parts of a distribution
circuit can be underground, including the main feeder.
Underground cables carry the load current without
overheating and also without producing excessive voltage.
Underground line installation will be coordinated with the
installation master plan to avoid conflict with construction of
future facilities. Lines will normally be installed adjacent to
roadways in urban, housing, or industrial plant areas, but may
be routed as required to meet the project objectives. A careful
study will be made of all underground utilities in order to
ensure a minimum of interference between electrical lines and
other underground utilities, whether existing, being
constructed, or proposed as a definite future construction
project. Electrical lines will be at least six feet from any steam
or hot water lines, except at crossings where a one-foot
separation from such lines is adequate[1].
II. OVERHEAD VS. UNDERGROUND
Electric energy utilizes overhead and underground means
to deliver power. Overhead networks comprise relatively
low-cost insulators and conductors mounted on poles made of
various materials like wood, steel or concrete. Other overhead
equipments are installed on some of these poles which make it
more cost effective to repair and maintain. On the other hand,
Manuscript received Oct 15, 2011.
Hnin War, Department of Electrical Power Engineering, Mandalay
Technological University, (e-mail: [email protected]). Mandalay
City, Myanmar, +95401669483.
Thet Tin, Department of Electrical Power Engineering, Mandalay
Technological University, Myanmar, +952010693).
this direct exposure has a down side of being highly
susceptible to malfunctioning due to environmental and
manmade breakdowns.
Undergrounding consists of maintenance holes which are
commonly referred to as manholes which are tubes used to
connect underground utilities to the surface. Manholes are
widely used in sewer systems, electrical and communication
systems. These manholes are situated at regular intervals
along the utility path, to allow easy access to maintenance
workers. Rubber and other insulated conductors (cables) are
installed and sliced inside these underground cavities.
Therefore, fault detection and repairs can be a very costly
exercise.
The biggest advantage of underground installation is that it
is less exposed and susceptible to external factors than
overhead. However, somehow or other, this advantage can be
offset by the time and effort spent in locating and repairing
faults should they take place. The nature of the installation
design and the complexity will be determined by the
obligatory standards used for the installation [2].
III. TYPES OF UNDERGROUND TRANSMISSION CABLES
There are two main types of underground transmission lines
currently in use. One type is constructed in a pipe with fluid or
gas pumped or circulated through and around the cable in
order to manage heat and insulate the cables. The other type is
a solid dielectric cable which requires no fluids or gas and is a
more recent technological advancement. The common types
of underground cable construction include high-pressure,
fluid-filled pipe (HPFF), high-pressure, gas-filled pipe
(HPGF), self-contained fluid-filled (SCFF) and solid cable,
cross-linked polyethylene (XLPE)[3].
A. High-Pressure, Fluid-Filled Pipe-Type Cable
A high-pressure, fluid-filled (HPFF) pipe-type of
underground transmission line consists of a steel pipe that
contains three high-voltage conductors. Each conductor is
made of copper or aluminum; insulated with high-quality,
oil-impregnated kraft paper insulation; and covered with
metal shielding (usually lead) and skid wires (for protection
during construction). The conductor and its wrappings are
often referred to as “cables.” The cables are surrounded by
dielectric oil that is maintained at 200 pounds per square inch
(psi). This fluid acts as an insulator and does not conduct
electricity. The steel pipe protects the conductors from
mechanical damage and water infiltration and prevents fluid
leaks. The pipe is protected from the chemical and electrical
environment of the soil by means of a coating and cathodic
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
protection. The pressurized dielectric fluid prevents electrical
discharges in the conductors’ insulation. An electrical
discharge can cause the line to fail. The fluid also transfers
heat away from the conductors. The fluid is usually static and
removes heat by conduction. In some situations the fluid is
pumped through the pipe and cooled through the use of a heat
exchanger.
B. High-Pressure, Gas-Filled Pipe-Type Cable
The high-pressure, gas-filled (HPGF) pipe-type of
underground transmission line is a variation of the HPFF
pipe-type described above. Instead of a dielectric fluid,
pressurized nitrogen gas insulates the conductors. Nitrogen
gas is less effective than dielectric fluids at suppressing
electrical discharges and cooling. To compensate for this, the
conductors’ insulation is about 20 percent thicker than the
insulation in fluid-filled pipes. Thicker insulation and a
warmer pipe reduce the amount of current the line can safely
and efficiently carry.
C. Self-Contained, Fluid-Filled Pipe-Type Cable
The self-contained, fluid-filled (SCFF) pipe-type of
underground transmission line is often the choice for
underwater transmission lines. The conductors are hollow and
filled with an insulating fluid that is pressurized to 25 to 50
psi. In addition, the three cables are independent of each
other. They are not placed together in a pipe. Each cable
consists of the fluid-filled conductor insulated with
high-quality kraft paper and protected by a lead-bronze or
aluminum sheath and a plastic jacket. The fluid reduces the
chance of electrical discharge and line failure. The sheath
helps pressurize the conductor’s fluid and the plastic jacket
keeps the water out.
D. Cross-Linked Polyethylene
The cross-linked polyethylene (XLPE) underground
transmission line is often called “solid dielectric”. The XLPE
type is becoming the national standard for underground
electric transmission lines. This type of line relies on
high-quality manufacturing controls to eliminate any
contaminants or voids in the insulation that could lead to
electrical discharges and breakdown of the line from electrical
stress. The solid dielectric material replaces the pressurized
liquid or gas of other types of cable. This type of construction
has three independent cables. They are not housed together in
a pipe, but are set in concrete ducts or buried side-by-side
directly in specially prepared soil. Each cable consists of a
copper or aluminum conductor, a semi-conducting shield,
cross-linked polyethylene insulation, and an outer covering
consisting of another semi-conducting shield, a metallic
sheath, and a plastic jacket. The insulation is about twice as
thick as the oil insulation used in other types of cable.
IV. INSTALLATION METHODS
Cable installation is one of the costliest items in an
underground cable system project. The underground cable
shall be installed with proper methods and equipment to
minimize cable damage during installation which may cause
higher installation and maintenance cost. Although some
utilities have their own installation crews, many utilities rely
on contractors to install cables. The three main installations
can be classified into eight installation methods are shown in
below [4].
A. Direct Burial
A direct burial installation is defined as installing cable
without pulling into a pipe or duct, but by pulling or laying
cables in an open trench and having the earth in direct contact
with the cable jacket or sheath after backfilling. To avoid the
excavation by a stranger, it is recommended that utility apply
this method only in the specific areas, e.g., in substation.
B. Semi-Direct Burial
Semi-direct burial installation is similar to direct burial but
the cables are pulled into the conduits. Conduits provide
mechanical protection to cables. To avoid the excavation by a
stranger, some utilities apply this method in only the specific
areas, e.g. in substation and foot path.
C. Concrete Troughs
Concrete troughs installation is similar to direct burial but
the cables are pulled into the concrete troughs. Concrete
troughs provide mechanical protection to cables.
D. Concrete Encased
The cables are laid in conduits which are arranged in
rectangular formation and covered by reinforced concrete to
be named as double protection. A group of conduits can also
be called “Duct Bank”. Conduit spacing shall be 25 cm.
E. Trenches
Trenches are commonly used in substations. The trenches
are generally accessible from the surface for the ease of
extension or maintenance. Cables are laid in reinforced
concrete trench and backfilled with sand. The trench can be
easily accessed by opening the top cover. Some utilities apply
this method only in the specific areas, e.g., in substation.
F. Horizontal Directional Drilling (HDD)
HDD process begins with boring a small, horizontal hole
(pilot hole) under the obstacle (e.g., a highway or river) that
needs to be crossed. This is done with a continuous string of
steel drill rod. When the bore head and rod emerge on the
opposite side of the crossing, a special cutter called a back
reamer will be attached and pulled back through the pilot
hole. The reamer bores through the pilot hole, enlarging it so
that a pipe can be pulled through. The pipe is usually pulled
through from the side of the crossing opposite to the drill rig.
Utilities usually apply this method for crossing obstacles such
as a river or a road.
G. Pipe Jacking
Pipe Jacking is a method of constructing a pipeline under
the ground. The technique involves pushing a pipe through
the ground with the thrust provided by hydraulic jacks. The
excavation of soil takes place at the same time the pipe moves
into the ground. This provides a structurally flexible and
watertight finished conduit. This method is commonly
adopted for underground cables. Nowadays, it is the main
method used for laying pipe to preserve the natural
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
environment and maintain pleasant living conditions. Some
utilities use reinforced concrete pipes with internal diameter
from 1.0- 1.8 m as a jacking pipe with small conduits arranged
inside. This is the preferred choice for constructing the main
line because it helps to minimize environmental effects.
H. Tunnels
Burial installation has the advantage of relatively low
construction and installation cost due to the limited civil
works. However, direct burial is not realistic in case of a high
number of circuits (20 or more) or circuits with high
transmission capacities or in countries with high average
temperatures and dry soil, the installation of direct burial is
not realistic any more. With burial installation, big conductor
cross-section would be required because of higher
temperature compared with installation in tunnel. That means
more expensive cable. In addition, cables installed in tunnels
can be inspected and monitored easier. For all these reasons,
more and more utilities opt to install cables in tunnel. In some
cases cables are buried on a long section and installed in a
tunnel, for example at the entrance of substation where there
are many joints or splicing.
V. ACCESSORIES
The following structures are called “accessories,” because
they are a part of all types of underground electric
transmission lines. They are also responsible for 90 percent of
all underground line failures [5].
A. Splices
Splices join separate pieces of conductor. Splices are
needed because there is a limit to the amount of cable that can
be put onto a spool for shipping and there is a limit to the
amount of tension a cable can withstand as it is pulled through
a pipe. The length that works best also depends on the number
of bends and dips in the line. XLPE type lines need a splice
every 900 to 2000 feet. Pipe-type lines need a splice at least
every 3,500 feet. Pipe-type lines require a concrete work vault
to hold each splice. While the connecting piece is only about
7.5 inches long, the materials needed to reduce the electrical
stress results in a joint about 63 inches long (over 5 feet). This
means that the vault must be at least 135 inches long (over 11
feet). Typically, the hole dug for a vault is over 15 to 20 feet
long and about twice as wide as the trench for the pipe. There
are typically one or two “chimneys” or vents to the surface
that are closed with steel covers. The strength of the concrete
must withstand the load of overhead traffic. Vaults are
generally not located under sidewalks or within 125 feet of
street intersections. About two-thirds of XLPE splices are in
permanent concrete vaults and one-third are constructed in
temporary vaults. Temporary vaults are not lined with
concrete but filled in after construction. When XLPE is
installed in concrete ducts, all splices are constructed in
permanent, concrete vaults.
B. Terminations
Underground lines terminate (connect to overhead lines or
to substations) by means of “risers,” which are fastened to
above ground structures. Three spread arms (the
“spreaderhead”) carry the underground line aboveground and
separate the three conductors so that they meet electric code
requirements for the spacing of overhead conductors. If
spreaderheads are placed underground, they are called
“trifurcators” and may be placed in a concrete vault. Porcelain
insulators or housings, contain the actual connections
between the in-earth and in-air portions of the line. These
housings are often called “potheads.” Terminations keep
moisture out of the underground system. Lightning arrestors
are placed close to the terminations to protect the
underground cable from over-voltage damage that can be
caused by nearby lightning strikes. The insulating material is
very sensitive to large voltage changes and cannot be
repaired. Typically a completely new cable is pulled into
placed a lightning damaged cable.
C. Pressurizing Source
For HPFF systems, a pressurizing plant maintains fluid
pressure in the pipe. The pressurizing plant is located at one
end of the line, usually within a substation fence. It includes a
reservoir that holds reserve fluid. An HPGF system does not
use a pressurizing plant, but rather a regulator and nitrogen
cylinder.
These are located in a “gas-cabinet” that contains
high-pressure and low-pressure alarms and a regulator.
Sometimes, a longer SCFF line that varies in elevation has
equipment approximately every mile at splice points to
maintain equal pressure throughout its length. The XLPE
system does not require any pressurization.
VI. SIZING IMPACTS
The impacts of underground transmission lines differ from
those of overhead transmission lines during construction and
afterwards. Underground lines generally cause greater soil
disturbance due to trenching requirements, while overhead
lines disturb the soil primarily at the location of the
transmission poles. Trenching an underground line through
farmlands, forests, wetlands, and other natural areas causes
significant land disturbances.The ROW for underground
transmission lines must be kept clear of trees and bushes,
while small trees and bushes are allowed within the ROW
under overhead lines. Post-construction issues such as
aesthetics, concerns regarding electric and magnetic fields
(EMF), and property values are usually less of an issue for
underground lines. Underground lines are not visible after
construction and have less impact on property values and
aesthetics.
A. Construction Impacts in Suburban and Urban Area
The construction impacts of underground lines are
temporary and, for the most part, reversible. They include
dirt, dust, noise, and traffic disruption. Increased particles in
the air can cause health problems for people who live or work
nearby. Particularly sensitive persons include the very young,
the very old, and those with health problems, such as asthma.
If the ROW is in a residential area, construction hours and the
amount of equipment running simultaneously may need to be
limited to reduce noise levels. In commercial or industrial
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
areas, special measures may be needed to keep access to
businesses open or to control traffic during rush hours.
B. Construction Impacts in Farmland and Natural Areas
Most underground transmission is constructed in urban
areas. In non-urban areas, soil compaction, erosion, and
mixing are serious problems, in addition to dust and noise.
During construction, special methods are needed to avoid
mixing the topsoil and lower soil horizons and to minimize
erosion.
The special soils often placed around an underground line
may slightly change the responsiveness of surface soils to
farming practices. Trees or large shrubs would not be allowed
within the ROW due to potential problems with roots. Some
herbaceous vegetation and agricultural crops may be allowed
to return to the ROW.
VII. CABLE LAYOUT AND SYSTEM DESIGN
The dimensioning of a high voltage cable system is always
based on the specifications and demands of the project at
hand. The required detail for calculation are the type of cable
insulation, nominal and maximum operating voltage,
short-circuit current with statement of the effect time,
transmission capacity or nominal current, operating mode:
permanent operation or partial load operation (load factors)
and ambient conditions: type of installation, ambient
temperature ( including external effects ) and special thermal
resistance of the ground.
The calculation of the admissible load currents (ampacity)
and the cable temperatures is performed in accordance with
the IEC publication 60287.
1) Electrical Field
In initial approximation, the main insulation of a high
voltage XLPE cable can be regarded as a homogenous
cylinder. Its field distribution or voltage gradient is therefore
represented by a homogenoius radial field. The value of the
voltage gradient at a point x within the insulation can
therefore be calculated as:
Ex 
U0
r
rx ln  a
 ri



(kV/mm)
where, U0 = operating voltage (kV)
rx = radius at position x (mm)
ra = external radius above the insulation (mm)
ri = radius of the internal field delimiter (mm)
The electrical field strength is highest at the inner
semiconductor and lowest above the insulation (below the
external semiconductor, rx = ra ).
2) Capacity, Charging Current
The operating capacity depends on the type of insulation
and its geometry. The following formula applies for all radial
field cables:
Cb 
5.56ε r
(µF/km)
D
ln  
d
where, εr = relative permittivity (XLPE=2, 4)
D = diameter over main insulation (mm)
d = diameter over inner semiconductor (mm)
Single-core high voltage XLPE cables represent an
extended capacitance with a homogenous radial field
distribution. Thus a capacitive charging current to earth
results in the following formula:
I C  U 0ωCb (A/km)
where, U0 = operating voltage (kV)
ω = angular frequency (1/s)
Cb = operating capacity (µF/km)
3) Inductance, Inductive Reactance
The operating inductance in general depends on the
relation between the conductor axis spacing and the external
conductor diameter. Practically, two cases have to be
considered:
Laying formation: trefoil
The operating inductance for all three phases calculates as:
 a 
 (H/km)
L m  2  104 ln 
 0.779rL 
where, a = phase axis distance (mm)
rL = diameter of conductor over inner semiconducting
layar (mm)
Laying formation: flat
The mean operating inductance for the three phases
calculates as
 a 
 (H/km)
L m  2  104 ln 
0.779r
L 

where, a' = phase axis distance (mm)
rL = diameter of conductor over inner semiconducting
layer (mm)
The inductive reactance of the cable system calculates for
both cases as:
X = ωL (Ω/km)
where, ω = angular frequency (1/s)
4) Current-dependent losses
The current-dependent losses consist of ohmic conductor
losses, losses through skin effect, losses through proximity
effect and losses in the metal sheath.
(i) Ohmic conductor losses
The ohmic losses depend on material and temperature. For
the calculation of the ohmic losses I2 R, the conductor
resistance stated for 20°C (R0) must be converted to the
operating temperature θ of the cable:
R = R0 (1+α (θ-20°C)) (Ω/km)
where, α = 0.0393 for copper and 0.0403 for aluminium
The conductor cross-section and admissible DC resistance at
20°C (R0) correspond to the standards series pursuant to IEC
60228.
(ii) Losses through skin effect
The losses caused by the skin effect, meaning the
displacement of the current against the conductor surface, rise
approximately quadratic with the frequency. This effect can
be reduced with suitable conductor constructions, e.g.
segmented conductors.
(iii) Losses through proximity effect
The proximity effect detects the additional losses caused by
magnet fields of parallel conductors through eddy currents
and current displacement effects in the conductor and cable
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
sheath. In practice, their influence is of less importance,
because three-conductor cables are only installed up to
medium cross-sections and single-conductor cables with large
cross-sections with sufficient axis space. The resistance
increase through proximity effects relating to the conductor
resistance is therefore mainly below 10%.
(iv) Losses in the metal sheath
High voltage cables are equipped with metal sheaths or
screens that must be earthed adequately. Sheath losses occur
through circulating currents in the system, eddy currents in the
cable sheath (only applicable for tubular types) and resulting
sheath currents caused by induced sheath voltage ( in
unbalanced earthing methods.
5) Calculation of the induced voltage
The induced voltage Ui within a cable system depends on
the mutual inductance between core and sheath, the conductor
current and finally on the cable length:
Ui = XM I L (V)
where ,XM = mutual inductance between core and sheath
(Ω/km)
I = conductor current per phase (A)
L = cable length
Two cases must be considered for the determination of the
maximum occurring voltage and for the dimensioning of the
surge arresters:
I = IN Normal operating current (A)
I = IC Three-pole short-circuit current (A)
The mutual inductance between core and sheath calculates
from the following formula:
XM = ω LM (Ω/km)
where, ω = angular frequency (1/s)
LM = the mutual inductivity between core and sheath
(H/km). The mutual inductivity between core and sheath L
calculates as follows:
For installation in trefoil formation:
 2a 
 (H/km)
L M  2  107 ln 
 dM 
For installation in flat formation:
 2  3 2a 
 (H/km)
L M  2  10 7 ln 

d
M


6) Short-Circuit current capacity
The short-circuit currents can be converted with the
following formula:
I k,x
1

I k,1s
tc
tangential (belts etc.) forces. The amplitude of a dynamic
force in general is calculated applying the following formula:
2  10 7  I s
Fs 
(kN/m)
a
2
where, a = phase axis distance (mm)
Is  κ 2Ic
where, Is = impulse short-circuit current (kA)
κ = surge factor (usually defined as 1.8)
Ic = short-circuit current (kA)
VIII. CONCLUSION
The electrical transfer from one place to another with
standard regulation is one of the major problems in the field of
electric power engineering. On the other hand, the choice of
size is also important in order to enough the amount of current
that flows on the line due to the transfer of power. To get
standard regulation, it has become mandatory for most
modern cities worldwide to use underground power cables.
This demonstrates that undergrounding has become feasible
economically and technologically. Using underground cables
for transmission system, the many advantages are obtained
such as minimal visual impact and lightning problems, high
level of personal and public safety, no fallen lines, not
affected by ice, snow, rain, wind, dust, smoke or fog, land use
minimized, high reliability and availability.
ACKNOWLEDGMENT
The author would acknowledge to Dr. Khin Thuzar Soe,
Associate Professor and Head, Department of Electrical
Power Engineering, Mandalay Technological University for
her kindly permission and suggestion throughout the
preparation of paper.
Especially, the author would like to express deeply
gratitude to her supervisor, U Thet Tin, Lecturer, Department
of Electrical Power Engineering, Mandalay Technological
University for his valuable advice, encouragement help
throughout the period of study.
The author is deeply grateful to her parents who specially
offered strong moral and physical support, care and kindness.
REFERENCES
[1]
[2]
where, Ik,x = short-circuit current during x seconds (kA)
tc = duration of short-circuit (s)
Ik,1s = short-circuit current during one second (kA)
7) Dynamics forces
Single-core cables have to be fixed in their position at
certain intervals. The calculation of dynamic forces for cable
systems is important for the determination of the fixing
interval and the layout of the fixing devices. It has to be
distinguished between radial (e.g. clamps, spacers) and
[3]
[4]
[5]
Brown, R., “Undergrounding Assessment Phase 1 Final Report
Literature Review and Analysis of Electric Distribution Overhead to
Underground Conversion”: Raleigh, 2007.
Pansini, A.J. and Smalling, K.D., ‘Undergrounding Electric Lines’.
Second ed, Lilburn: The Fairmont Press, Inc.1993.
“Underground Transmission Systems Reference Book,1992 Edition”.
“Power Cable Handbook”, Harmonisation of Power Distribution
Systems in the Lower Mekong Subregion, 2011 Edition.
“Educational and Project Materials supplied by Power Delivery
Consultants, Inc. of Ballston Lake. New York”.
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Volume 1, Issue 1, July 2012
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