Construction of HV Transmission Lines etc.pptx

Construction of High Voltage Lines,Types
of Towers,Types of Insulators and Their
Electrical and mechanical
Charecteristics

What is a Transmission Line
 A transmission line is a high voltage power line that carries the electrical
energy from generating stations to the electric substations.
 The transmission lines are segmented into two sub-sections namely :
 primary transmission lines
 and secondary transmission lines.
 The primary transmission lines carry the electrical power from generating
stations to the receiving stations, while the secondary transmission lines carry
the electricity from receiving stations to the electrical distribution
substations who further distribute power to the end users.
 The transmission line operates at very high voltages such as 33 kV, 66 kV, 132
kV, 220 kV, 440 kV, 765 kV, and so on.
 The steel towers are usually used to support the transmission lines.

History of transmission Lines
 The first transmission of electrical impulses over an extended distance was demonstrated on July 14, 1729,
by the physicist Stephen Gray.[The demonstration used damp hemp cords suspended by silk threads
 However the first practical use of overhead lines was in the context of telegraphy. By 1837 experimental
commercial telegraph systems ran as far as 20 km. Electric power transmission was accomplished in 1882
with the first high-voltage transmission between Munich and Miesbach (60 km). 1891 saw the construction
of the first three-phase alternating current overhead line on the occasion of the International Electricity
Exhibition in Frankfurt, between Lauffen and Frankfurt.
 In 1912 the first 110 kV-overhead power line entered service followed by the first 220 kV-overhead power
line in 1923. In the 1920s RWE AG built the first overhead line for this voltage and in 1926 built
a Rhine crossing with the pylons of two masts 138 meters high.
 In 1953, the first 345 kV line was built by The L.E. Myers Co. and put into service by the Ohio Valley
Electric Corporation in the United States.[ In Germany in 1957 the first 380 kV overhead power line was
commissioned (between the transformer station and Rommerskirchen). In the same year the overhead line
traversing of the Strait of Messina went into service in Italy, whose pylons served the Elbe crossing 1. This
was used as the model for the building of the Elbe crossing 2 in the second half of the 1970s which saw the
construction of the highest overhead line pylons of the world.
 Earlier, in 1952, the first 380 kV line was put into service in Sweden, in 1000 km (625 miles) between the
more populated areas in the south and the largest hydroelectric power stations in the north. Starting from
1967 in Russia, and also in the US and Canada, overhead lines for voltage of 765 kV were built. In 1982
overhead power lines were built in Soviet Union between Elektrostal and the power station at Ekibastuz,
this was a three-phase alternating current line at 1150 kV (Powerline Ekibastuz-Kokshetau). In 1999, in
Japan the first powerline designed for 1000 kV with 2 circuits were built, the Kita-Iwaki Powerline. In 2003
the building of the highest overhead line commenced in China, the Yangtze River Crossing

Classification of transmission lines
By operating Voltage
 Overhead power transmission lines are classified in the electrical power industry by the range of voltages:
 Low voltage (LV), less than 1000 volts, used for connection between a residential or small commercial
customer and the utility.
 Medium voltage (MV; distribution), between 1000 volts (1 kV) and 69 kV, used for distribution in urban and
rural areas.
 High voltage (HV; subtransmission less than 100 kV; subtransmission or transmission at voltages such as 115 kV
and 138 kV), used for sub-transmission and transmission of bulk quantities of electric power and connection to
very large consumers.
 Extra high voltage (EHV; transmission) – from 345 kV, up to about 800 kV used for long distance, very high
power transmission.
 Ultra high voltage (UHV), often associated with ≥ ±800 kVDC and ≥ 1000 kVAC
By length of the line
 The overhead transmission line is generally categorized into three classes, depending on the length of the
line:
 Lines shorter than 50 km are generally referred to as short transmission lines.
 Lines between 50 km and 150 km are generally referred to as medium transmission lines.
 Lines longer than 150 km are considered long transmission lines.
 This categorization is mainly done for the ease of performance analysis of transmission lines by power
engineers.

Components in the Transmission Lines
 Support Structures
 Insulators
 Conductors
 Ground Wires
 Dampers

Construction of Power Transmission Lines
There are several steps needed prior to transmission line poles being installed. The process
can vary depending on the size of the line, soil conditions, terrain and other variables.
 1. Soil surveys and property staking: Before acquisition begins, field survey and soil
information must be obtained to finalize design. A soil boring is drilled at structure
locations to determine the mechanical properties of the soil. Soil borings typically take
place about a year before the start of construction. Right-of-way (ROW) agents request
access to the property and coordinate between the soil boring contractor and the property
owner. The soil boring locations are staked and existing underground utilities are located
prior to borings. Once final pole locations are determined, they are staked in the field
with tree clearing limits, right-of-way boundaries and other property features mapped as
needed. Typically, a 150-foot wide easement will be required for 345 kV lines.
 2. Construction access and tree clearing: Construction access routes to the ROW are
identified and obtained during the ROW acquisition process. The access is typically 25 to
30 feet wide and is needed so large equipment including a drill rig, concrete trucks and a
crane can be delivered to the site. Tree clearing and other vegetation removal will take
place on the identified access route and the area within the easement. Matting is
sometimes put down in wet or soft areas to prevent compaction, minimize soil disturbance
and improve site safety.

Construction of Power Transmission Lines
 3. Mobilizing equipment and delivering material: A crane, drill rig, concrete truck,
boom trucks, trailers, structures, steel casing and rebar cages are some of the
equipment and materials that will be moved into the site for construction.
 4. Foundation construction: Construction crews will begin drilling for structure
foundations. Two types of foundations are typically used for transmission line
projects, either drilled pier foundations or direct embed foundations. Reinforced
concrete drilled pier foundations typically range from 6 to 9 feet in diameter and are
drilled 20 to 40 feet deep. Once drilling is complete, reinforcing steel and anchor
bolts are placed in the hole and concrete is poured. Drilled pier foundations typically
take one to two days to complete unless rock is encountered. Direct embed
foundations typically range from 3 to 5 feet in diameter and are 15 to 30 feet deep.
Once the hole is drilled, the pole base section is placed in the hole and then
backfilled with rock, soil or concrete. Direct embedded foundations typically take 2
to 4 hours to complete.
 5. Installing the structure: High voltage transmission structures are usually steel
poles. The poles are assembled at the foundation site and set in place with the use of
cranes and other heavy equipment. A pole can be assembled and set in place in one
day.

Construction of Power Transmission
Lines
 6. Stringing conductor: After all structures are erected in an area, the next
step is to install conductor (wire). Conductor is pulled from one structure to
the next through a pulley system temporarily placed on the structures. After
a section of conductor is pulled through a series of structures, the conductor
is attached to insulators, which are attached to the structure and the pulleys
are removed. Trucks, heavy equipment and sometimes helicopters are used in
this process. Other equipment including bird diverters, spacers and galloping
devices are also installed.
 7. Land restoration: Following construction, the ROW is cleaned up and
restored. This work may include tile and fence repair, rut removal,
decompaction, tilling, seeding and possible wetland restoration.

What is a Transmission Tower?
 A transmission tower supports an overhead power line. The other names of
transmission towers are power transmission towers, power towers, and electricity
pylons. The transmission towers carry high-voltage transmission line to transport
power from the generating station to electrical substations. The electrical
substations transport power to the end users through distribution lines.
 Transmission towers have to carry the heavy transmission conductors at a
sufficient safe height from the ground. In addition to that, all towers have to
sustain all kinds of natural calamities. Therefore, the transmission tower design is
an important engineering job where civil, mechanical, and electrical engineering
concepts are equally applicable.
 Transmission towers have to carry the heavy transmission conductors at a
sufficient safe height from the ground depending on the voltage
(132kV/220kv/400kv/765kv). Thus, the transmission towers maintain the minimum
ground clearance according to the system voltage.
A transmission tower supports every high voltage transmission line. A transmission tower (also known as a power transmission tower, power tower, or electricity pylon) is a tall structu

Parts of Transmission Tower
An EHT transmission tower consists of the following parts:
 The peak of the tower(the portion above the top cross arm)
 The cross arm (Cross arms of the transmission tower hold the transmission
conductor)
 Cage of transmission tower(portion between tower body and the peak is
known as a cage of transmission tower)
 Transmission Tower Body(The portion from the bottom cross arms up to the
ground level)
 Leg of transmission tower
 Stub/Anchor Bolt and Base plate assembly of the transmission tower.

Peak Of Transmission Tower
The top portion of the transmission tower is called the peak of the transmission tower.
It is situated above the top cross arm, and it carries an earth shield wire connected to the tip of the tower’s peak.

Cross arm of Transmission Tower
 Cross arms hold the transmission conductors. The size of the cross arms is different for the transmission of
various voltages.
 The dimension of the cross arm depends on the following parameters.
 Configuration of tower
 Level of Transmission Voltage
 Minimum forming angle for stress distribution.

Cage of Transmission Tower
 The shape of the cage may be a square or triangle, depending on the height
of the tower.

Body of the Tower
 The spacing between the lowest cross arm of the tower and the ground is
called the transmission tower body.
The transmission tower body provides the height of the tower required for ground clearance.

The leg and base plate of the Transmission Tower
 The transmission tower stands on its leg. There may be one, two, or more
legs according to the requirement of the tower structure.
 Stub/Anchor Bolt and Base plate assembly:This part holds the entire structure
of the tower.

Types of Transmission Towers
In terms of their technical background, the following is a list of transmission tower types.
Suspension Towers
High voltage suspension towers are designed to withstand only the weight of the conductor when they are positioned in
a straight line. Conductors on suspension towers can be held in place by I strings, V strings, or a combination of both.
Suspension transmission towers run on straight-line routes where there is less than a 5-degree deviation in angle.
Transposition Towers
They are most common in three-phase line systems and are often used to support and maintain long transmission lines
where the weight in the centre of the span puts significant stress on the structure. A transposition tower is supported by
a body, a cage, and a peak. Conductors and lines are positioned at a distance that prevents contact between the cross-
arms of the towers.
Tension/Angle Towers
They support the directional change of transmission lines in turning points where the angle of deviation is more than 5
degrees. The tower is reinforced with anchors to counteract pressure exerted against the angle. The transmission line
that connects two angle towers is called a section. The length of the section depends on geographic location,
easements, possible placement possibilities, and the ultimate destination.
Special Towers
Custom towers are built when conditions dictate a significant change in angle or when a substantial amount of
additional support is needed based on environmental factors. Among the factors that affect transmission tower
construction are heavy wind, porous soil, freezing rain, and others. In addition, these towers are used in areas involving
long-span river crossing, valley crossing, power line crossings above existing lines, power line crossings below existing
lines (Gantry type structures), tapping existing lines, unique termination towers and the like.

Transmission Tower design
There are various designs for transmission structures. Two common types are:
 Lattice Steel Towers (LST):It consists of a steel framework of individual structural components that
are bolted or welded together
 Tubular Steel Poles (TSP):It consists of hollow steel poles fabricated either as one piece or as
several pieces fitted together.
The basic specifications to be considered before the design of the tower is mentioned below.
 Voltage
 The number of circuits.
 Type of conductors.
 Type of insulators.
 The possible future addition of new circuits.
 Tracing of the transmission line.
 Selection of tower sites.
 Selection of rigid points.
 Selection of conductor configuration.
 Selection of height for each tower.

TOWER HEIGHT
 The tower design philosophy mainly considers the factors of the tower height, base width, top
damper width, and cross arm’s length.Typical tower height is in the range of 15 to 55 m (49 to 180 ft).
 The height of the tower is determined by- H=H1+H2+H3+H4
 where,
H1 =Minimum permissible ground clearance;
H2 =Maximum sag;
H3 =Vertical spacing between conductors;
H4 =Vertical spacing between earth wire and top conductor.
 Minimum permissible ground clearance: Normally minimum ground clearance in an Electrical power
transmission line varies depending on the voltage level.
 Maximum Sag:Sag Template is a tool with the help of which the position of towers on the profile is
decided so that they conform to the limitations of vertical and wind loads on any particular tower, as
per I.E. Rules.
 Vertical spacing between conductors:The vertical space between power conductors plays an
important role in spacing between the cross arms.
 Vertical clearance between earth wire and top conductor:The main factors which affect determining
the location of the earth wire on the transmission tower are the minimum difference in
 Suspension insulator length
 The drop of earth wire to Suspension clamps
 Angle of shield

INSULATORS
 Insulators must support the conductors above the structure and where the
conductor hangs below the structure.
 They must withstand both the normal operating voltage and surges due to
switching and lightning.
 Insulators must withstand environmental factors such as fog, pollution, or salt
spray.

Types of Insulators
 There are 5 types of insulators used in transmission lines as overhead insulation:
 Pin Insulator
 Suspension Insulator
 Strain Insulator
 Stay Insulator
 Shackle Insulator
 Pin, Suspension, and Strain insulators are used in medium to high voltage systems.
While Stay and Shackle Insulators are mainly used in low voltage applications.

Insulators
 Insulators are broadly classified as either pin-type, which support the conductor
above the structure, or suspension type, where the conductor hangs below the
structure.
 The invention of the strain insulator was a critical factor in allowing higher
voltages to be used. They must also be strong enough mechanically to support
the full weight of the span of conductor, as well as loads due to ice
accumulation, and wind.
 Suspension insulators are made of multiple units, with the number of unit
insulator disks increasing at higher voltages. The number of disks is chosen based
on line voltage, lightning withstand requirement, altitude, and environmental
factors such as fog, pollution, or salt spray. In cases where these conditions are
suboptimal, longer insulators must be used. Longer insulators with longer
creepage distance for leakage current, are required in these cases.

Insulators
 Porcelain insulators may have a semi-conductive glaze finish, so that a small current (a few
milliamperes) passes through the insulator. This warms the surface slightly and reduces the
effect of fog and dirt accumulation. The semiconducting glaze also ensures a more even
distribution of voltage along the length of the chain of insulator units.
 Insulators are usually made of wet-process porcelain or toughened glass, with increasing
use of glass-reinforced polymer insulators. However, with rising voltage levels, polymer
insulators (silicone rubber based) are seeing increasing usage.
 Polymer insulators by nature have hydrophobic characteristics providing for improved wet
performance. Also, studies have shown that the specific creepage distance required in
polymer insulators is much lower than that required in porcelain or glass. Additionally, the
mass of polymer insulators (especially in higher voltages) is approximately 50% to 30% less
than that of a comparative porcelain or glass string. Better pollution and wet performance
is leading to the increased use of such insulators.
 Insulators for very high voltages, exceeding 200 kV, may have grading rings installed at
their terminals. This improves the electric field distribution around the insulator and makes
it more resistant to flash-over during voltage surges.

Pin Insulators
 The pin type insulator is shown in the figure. The pin type insulator is fitted to the cross-arm
on the pole. The pin type insulator has a groove on the upper end of the insulator for housing
the line conductor. The line conductor passes through this groove and is bounded by the
annealed wire made up of the same material as the line conductor.
 Pin insulators are the earliest developed overhead insulator, but are still commonly used in
power networks up to 33 kV system. Pin type insulator can be one part, two parts or three
parts type, depending upon application voltage.
 In a 11 kV system we generally use one part type insulator where whole pin insulator is one
piece of properly shaped porcelain or glass.
 As the leakage path of insulator is through its surface, it is desirable to increase the vertical
length of the insulator surface area for lengthening leakage path. One, two or more rain
sheds or petticoats on the insulator body are provided to obtain long leakage path.
 In addition to that rain shed or petticoats on an insulator serve another purpose. These rain
sheds or petticoats are designed in such a way that while raining the outer surface of the
rain shed becomes wet but the inner surface remains dry and non-conductive. So there will
be discontinuations of conducting path through the damp pin insulator surface.
 In higher voltage systems – like 33KV and 66KV – manufacturing of one part porcelain pin
insulator becomes more difficult. The higher the voltage, the thicker the insulator must be to
provide sufficient insulation. A very thick single piece porcelain insulator is not practical to
manufacture.
 In this case, multiple part pin insulator are used, where some properly designed porcelain
shells are fixed together by Portland cement to form one complete insulator unit.Generally
use two parts pin insulators is used for for 33KV, and three parts pin insulator for 66KV
systems.

Post Insulators
 Post insulators are similar to Pin insulators, but post
insulators are more suitable for higher voltage
applications.
 Post insulators have a higher number of petticoats
and a greated height compared to pin insulators. We
can mount this type of insulator on supporting
structure horizontally as well as vertically. The
insulator is made of one piece of porcelain and it has
clamp arrangement in both top and bottom end for
fixing.

Main Differences between Pin and Post Insulators
The main differences between pin insulator and post insulator are:
 1. Pin insulator is generally used up to 33KV system. Post insulator
suitable for lower voltage and also for higher voltage
 2. Pin insulator is single stag. Post insulator can be single stag as well
as multiple stags
 3.Conductor is fixed on the top of the Pin insulator by
binding.Conductor is fixed on the top of the Post insulator with help
of connector clamp
 4.Two Pin insulators cannot be fixed together for higher voltage
application.Two or more post insulators can be fixed together one
above other for higher voltage application
 5.Metallic fixing arrangement provided only on bottom end of the Pin
insulator. Metallic fixing arrangement provided on both top and
bottom ends of the Post insulator

Suspension Insulators
 In higher voltage, beyond 33KV, it becomes uneconomical to use pin insulator because size, weight of the
insulator become more. Handling and replacing bigger size single unit insulator are quite difficult task. For
overcoming these difficulties, suspension insulator was developed.
 The suspension type insulators are generally used with the steel towers. These insulators consist of a number
of porcelain discs connected in series by metal links in the form of a string. Each insulator of a suspension
string is called disc insulator because of their disc like shape. The line conductor is suspended at the bottom
end of this string and the other end of the string is fixed to the cross-arm of the steel tower.
 It is a usual practice to use suspension type insulators when the operating voltage is high (> 33 kV). In the
suspension type insulator, each insulator disc is designed for low voltage (say 11 kV). Therefore, the number
of discs in a string would depend upon the working voltage. For example, if the operating voltage is 132 kV,
then 12 discs in series will be provided on the string.
Advantages of Suspension Insulator:
 Each suspension disc is designed for normal voltage rating 11KV (Higher voltage rating 15KV), so by using
different numbers of discs, a suspension string can be made suitable for any voltage level.
 If any one of the disc insulators in a suspension string is damaged, it can be replaced much easily.
 Mechanical stresses on the suspension insulator is less since the line hanged on a flexible suspension string.
 As the current carrying conductors are suspended from supporting structure by suspension string, the height
of the conductor position is always less than the total height of the supporting structure. Therefore, the
conductors may be safe from lightening.

Suspension Insulator
Disadvantages of Suspension Insulator
 Suspension insulator string is costlier than pin and post
type insulator.
 Suspension string requires more height of supporting
structure than that for pin or post insulator to maintain
same ground clearance of current conductor.
 The amplitude of free swing of conductors is larger in
suspension insulator system, hence, more spacing
between conductors should be provided.

Strain Insulator
 Strain Insulators
 The strain insulators are used when there is a dead end of the transmission
line or there is a corner or sharp curve and the line is subjected to the
greater tension. The strain insulator is shown in the figure.
 The strain insulator consists of an assembly of suspension insulators. The
insulation discs of the strain insulators are used in the vertical plane. In the
places, where the tension in the transmission line is exceedingly high, then
two or more strings of discs are used in parallel.

Design Consideration of Insulators
 The live conductor attached to the top of the pin insulator which is at the live
potential. We fix the bottom of the insulator to supporting structure of earth
potential. The insulator has to withstand the potential stresses between conductor
and earth. The shortest distance between conductor and earth, surrounding the
insulator body, along which electrical discharge may take place through the air, is
known as flashover distance.
 When the insulator is wet, its outer surface becomes almost conducting. Hence
the flashover distance of insulator is decreased. The design of an electrical
insulator should be such that the decrease of flashover distance is minimum when
the insulator is wet. That is why the uppermost petticoat of a pin insulator has
umbrella type designed so that it can protect, the rest lower part of the insulator
from the rain. The upper surface of the topmost petticoat is inclined as less as
possible to maintain maximum flashover voltage during raining.
 The rain sheds are made in such a way that they should not disturb the voltage
distribution. They are so designed that their subsurface is at a right angle to the
electromagnetic lines of force.

Properties of HV Insulators
The basic performance of high voltage insulator includes electrical, mechanical and thermal performance. Also, it
has environmental and ageing resistance properties.
 ① Electrical performance: the destructive discharge along the insulation surface is called flashover, which is the
main electrical performance of high-voltage insulator. For different voltage levels, the basic performance includes
electrical, mechanical and thermal performance. Besides, it has environmental and ageing resistance properties.
 The withstand voltage requirements of are different, and the indicators include power frequency dry and wet
withstand voltage, lightning impulse withstand voltage, lightning impulse cut-off withstand voltage, operation
impulse withstand voltage, etc. To avoid a breakdown in operation, the breakdown voltage of high-voltage insulator
is higher than the flashover voltage. In the factory test, the porcelain insulator which can be broken down generally
goes through the spark test, that is to say, frequent sparks occur on the surface of the high-voltage insulator for a
certain period to see whether it is broken down. Its basic properties include electrical, mechanical and thermal
properties. Also, it has environmental and ageing resistance properties.
 Corona test, radio interference test, partial discharge test and dielectric loss test are also required. In high altitude
area, the electrical strength of insulator decreases due to the decrease of air density, so its withstand voltage should
be increased when converted to standard atmospheric conditions. The flashover voltage of polluted high-voltage
insulator when it is damp is much lower than its dry and wet flashover voltage. Therefore, in a polluted area, it is
necessary to strengthen insulation or adopt pollution resistant insulator, and its creepage distance (ratio of creepage
distance to rated voltage) should be higher than that of the normal type. Compared with AC high-voltage insulator,
DC insulator has poor electric field distribution, adsorption of pollution particles and electrolysis, low flashover
voltage, and generally requires special structure design and larger creepage distance.
 ② mechanical performance of high-voltage insulator: the model of the high-voltage insulator is often affected by the
gravity and tension of conductor, wind force, icing weight, self-weight of an insulator, the vibration of the conductor,
the mechanical force of equipment operation, short-circuit electric force, earthquake and other mechanical forces.
Relevant standards have strict requirements for mechanical properties.
 ③ Thermal performance of high-voltage insulator: outdoor high-voltage insulator is required to be able to withstand
sudden temperature change. For example, porcelain insulator is required to pass several cold and hot cycles without
cracking. Due to the current passing through the insulating sleeve, the temperature rise and allowable short-time
current value of its parts and insulating parts shall meet the requirements of relevant standards.

Properties of Insulators
 In overhead transmission systems, insulators provide necessary insulation between each
phase and grounded cross arm to prevent the leakage of current to earth as well as to
support line conductors. This demands that insulators should possess high electrical insulation
resistance, a high mechanical strength-to-weight ratio , high thermal conductivity, easy
molding and lower maintenance cost . Because of the nature of the duties, the selection of
insulators depends both on the economic consideration and assurance of long-term
performance .
 Historically, traditional porcelain and glass insulators have been used extensively as outdoor
high-voltage insulation, and their required material properties have been known for a long
time . Decades of in-service experience illustrates that these materials possess excellent
resistance to electrical and environmental stresses and, at the same time, are cost effective
. However, from a utility perspective, their labor-intensive installation and maintenance
complexities due to their bulkiness and poor performance in highly polluted areas are the
associated concerns and challenges associated with these materials .
 Polymeric insulators, with their advent in 1960 , were presented as a potentially attractive
alternative to ceramics for use by electric utilities and equipment manufacturers. A
polymeric material, by virtue of its chemical formulation, offers greater ease in creating
complex designs, as well as fabrication. The substantial advantages which a polymeric
insulator offers include light weight , a compact line design and mitigation against
consecutive failures. Furthermore, its low surface energy means that the polymeric insulator
surface’s hydrophobicity remains intact even under wet conditions . The light weight of a
polymeric insulator has an impact on the economic design of towers, thus making the up-
gradation of transmission voltage possible without any significant dimensional changes to
supporting towers .

 The commonly used polymeric materials for high-voltage outdoor insulators are silicone
rubber (SiR), ethylene propylene diene monomer (EPDM), epoxy resins and ethylene vinyl
acetate (EVA) These materials are used to manufacture sheds which offer required creepage
distance, while the mechanical strength is provided by the fiberglass rod. Some other
polymeric materials such as polytetrafluoroethylene (PTFE), polyolefin elastomers (POE) and
high-density polyethylene (HDPE) find their applications at a low voltage level both in
outdoor and indoor environments . The polymeric materials (SiR, EPDM, and epoxy resins) are
classified as thermoset elastomers, while the remainder are thermoplastic polymers. The
thermoset elastomers have been extensively studied for manufacturing sheds of the first
generation of polymeric insulators .
 In the early 1960s, outdoor insulators manufactured from epoxy resins were used for the first
time in the UK. However, surface damage and poor performance at cold temperatures were
the main constraints leading to their failure in outdoor environments .Later, polymeric
insulators were used elsewhere, and the statistics of their worldwide use shows the extensive
use of SiR and EPDM. The popular use of SiR as a shed material is due to its characteristics
offering the ability to recover surface hydrophobicity in a harsh environment, excellent
breakdown strength and high volume resistivity. However, it is less resistant to tracking, weak
mechanically, and is also expensive. On the other side, EPDM exhibits better resistance to
tracking/erosion and is relatively stronger mechanically. However, it has low surface and
volume resistivity as compared to SiR. Considering the overall performance, polymeric
insulators made of SiR were reported to have a relatively superior standing.

 The major concern associated with polymeric insulators is their life expectancy. Being organic
in nature, SiR, EPDM, and epoxy in their pure forms are bound to age during their extended
exposure to the outdoor environment. This results in the partial degradation of their
dielectric, thermal and mechanical properties .Under electrical and environment stresses in
real working environments, polymeric sheds experience degradation which is weakly bonded
at the molecular level. Electrical stresses, which include corona discharges and dry band
arcing, along the highly stressed surface regions cause surface tracking/erosion and finally
cause the puncturing of the sheds .The environmental stresses which contribute to the aging
of polymeric insulators are dry sunlight heating in arid areas, ultraviolet (UV) radiation,
moisture, and acid rain, etc. . These environmental parameters may lead to a thermal impact
due to heating and radiation, the corrosion of metal-end-fittings and the flow of leakage
current on the degraded surface under moist conditions. It may cause flashovers and
ultimately failure of the sheds, leading to permanent failure of the insulators.
 Having understood the failure mechanisms of the first generation of polymeric insulators
under prevailing electrical and environmental stresses, researches are going on to modify
these polymeric insulator materials through the incorporation of micro and nano fillers.
 The polymeric insulators of the second generation are fabricated using different polymeric
materials loaded with different fillers. Researchers have studied and analyzed the thermal,
dielectric, and mechanical performance of SiR, EPDM, and epoxy filled with micro, nano, and
micro/nano hybrid fillers to achieve enhanced electrical, thermal and mechanical properties

Insulator Cross-sectional View
The following figure shows a suspension type (for above 33 kV).

Insulator Chain
Number of separate discs are joined with each other by using metal links
to form a string. The insulator string is suspended from the cross arm of
the support.

Composite Insulator
1. Sheds to prevent bridging by ice and snow.
2. Fibreglass reinforced resin rod.
3. Rubber weather shed.
4. Forced steel end fitting.

Construction of HV Transmission Lines etc.pptx

Recommended

Recommended

More Related Content

Similar to Construction of HV Transmission Lines etc.pptx

Similar to Construction of HV Transmission Lines etc.pptx (20)

More from Satish Pydimarla

More from Satish Pydimarla (20)

Recently uploaded

Recently uploaded (20)

Construction of HV Transmission Lines etc.pptx