Transmission line insulators

www.youtube.com/c/brainamplifier
www.youtube.com/c/brainamplifier www.facebook.com/brainamplifier
0
2017
www.facebook.com/brainamplifier
TRANSMISSION LINE INSULATOR

1
SR.
NO.
CONTENT PG
NO.
1. Introduction 4
2.
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10
2.11
2.12
2.13
2.14
2.15
2.16
2.17
2.18
Definitions
Tracking
Treeing
Erosion
Chalking
Crazing
Cracking
Hydrolysis
Puncture
Specified mechanical load (s.m.l)
Tensile load
Routine test load (r.t.l.)
Cantilever load
Compressive load
Maximum working combined loads
Working cantilever load (w.c. L.)
Maximum design rating (mdr)
Proof-test load
Delamination
5
5
5
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
3.
3.1
Dielectric properties of insulation
Dielectric strength or breakdown voltage
6
6
4.
4.1
4.2
4.3
4.3.1
4.3.2
4.4
4.4.1
4.4.2
4.4.3
4.4.4
4.4.5
4.4.6
4.4.7
4.4.8
4.4.9
Insulating material
Properties of insulating material
Porcelain insulator
Glass insulator
Advantages of glass insulator
Disadvantages of glass insulator
Polymer insulator
Core
Housing
Weathersheds
End fitting
Coupling zone
Interface
Characteristics
Advantages of polymer insulator
Disadvantages of polymer insulator
7
7
7
8
8
8
9
9
9
9
9
9
9
10
10
10
5.
5.1
5.2
Types of insulators based on construction
Type a
Type b
11
11
11

2
6.
6.1
6.1.1
6.1.2
6.1.3
6.1.4
6.1.5
6.1.6
6.2
6.2.1
6.2.2
6.3
6.3.1
6.3.2
6.4
6.4.1
6.4.2
6.4.3
6.4.4
6.5
Types of insulators based on use
Pin insulator
Structure
Petticoats
Causes of insulator failures
Designing consideration of pin insulator
Dimensions for pin insulators
Helically formed pin insulator types
Post insulator
Suspension insulator
Advantages of suspension insulator
Disadvantages of suspension insulator
Strain insulator
Ball & socket type
Tongue & clevis type
Stay/guy strain insulator
Types of guy insulators
Type of insulators for guy insulators
Basic insulator level for guy insulators
Mechanical strength for guy insulators
Shackle insulator or spool insulator
11
11
11
12
12
13
13
13
14
15
15
15
16
16
17
17
17
17
18
18
18
7. Comparison between ceramic and polymer insulator 19
8. Clearance and creapage distance 20
9. Marking for insulator 20
10.
10.1
10.2
10.3
10.4
10.5
10.6
Causes of insulator failure
Cracking of insulator
Defective insulation material
Porosity in the insulation materials
Improper glazing on insulator surface
Flash over across insulator
Mechanical stresses on insulator
20
20
20
20
21
21
21
11.
11.1
11.2
11.2.1
11.2.2
11.2.3
11.3
11.4
11.5
11.6
11.7
11.8
11.8.1
11.8.2
11.8.3
11.8.4
Insulator design
Basic design concepts
Material selection
Core
Weathersheds
End fittings
Insulator design
Pollution consideration
Table-1
Pollution severity levels
Relation between the pollution level and the specific creepage
distance
Application of the "specific creepage distance" concept
Parameters characterizing the profile
Influence of the position of insulators
Influence of the diameter
Determination of the creepage distance
21
21
21
21
22
22
22
23
23
24
24
24
24
24
24
25

3
12.
12.1
12.2
12.3
12.4
12.5
12.6
Insulation consideration
Waveform- lightening impulse
Waveform- switching impulse
Critical flashover voltage (cfo)
Withstand voltage
Basic (lightning) impulse insulation level (bil)
Basic (switching) surge impulse insulation level (bsl)
26
26
26
27
27
27
27
13.
13.1
13.2
Consideration of interference and corona
Generating processes
Surface corona discharges around highly stressed electrodes
27
27
27
14. Consideration of capacitance effects 28
15.
15.1
15.1.1
15.1.2
15.1.3
15.2
15.2.1
15.2.2
15.2.3
15.2.4
15.2.5
15.2.6
15.2.7
15.2.8
15.2.9
15.2.1
Insulator selection
Mechanical parameters
Loading considerations-ice and wind
Table-3: loading parameters
Table-4: steps used in calculating the total load on a conductor
Electrical parameters
Cifo selection
Table-5 recommended bil at various operating voltages
Low frequency ling duration (60hz) selection
Switching surge selection
Table-6 typical switching surge levels
Contamination performance
Calculation of insulator leakage distance example:
Grading ring selection
Minimum corona extinction level
Maximum riv
29
29
29
29
30
30
30
31
32
32
32
32
33
33
33
33
16.
16.1
16.2
16.3
16.4
Insulator characteristics
Service conditions
Physical characteristics
Electrical characteristics
Mechanical characteristics
34
34
34
34
34
17.
17.1
17.2
17.3
17.4
17.4.1
17.4.2
17.4.3
17.4.4
17.5
17.5.1
17.5.2
17.5.3
17.5.4
17.6
17.6.1
17.6.2
Insulator testing
Type tests
Acceptance tests
Routine tests
Flashover test
Power frequency dry flashover test
Power frequency wet flashover test or rain test
Power frequency flashover voltage test
Impulse frequency flashover voltage test
Performance tests
Temperature cycle test
Puncture voltage test
Porosity test
Mechanical strength test
Routine tests
Proof load test
Corrosion test
36
36
36
36
36
36
36
36
37
37
37
37
37
37
37
38
38

4
INTRODUCTION
In the case of a power system joined together by overhead transmission lines, the transmission
conductors are required to carry the current between the source and the receiver and the
insulators are required to insulate the mechanical supports from the current carrying lines.
Traditionally, the line insulators were made out of ceramics and glass. These materials were readily
available and had the advantage of being cheap, having excellent insulating and dielectric
properties and ease of construction. On the other hand, however, these insulators had the
disadvantage of being heavy, easily broken and suffering from disintegration when subjected to
polluted and exposed atmospheric conditions.
Thus, the need was felt to develop new type of insulators and new type of insulating material that,
while carrying all the advantages of the traditional insulators, overcame the disadvantages. In the
nineteen thirties and forties, new organic insulators were developed as a possible replacement for
the conventional organic insulators; however, these insulators suffered from the problems of
vulnerability from exposure to atmosphere and thus were deemed to be unfit for outdoor
applications.
Later in the nineteen fifties, epoxy resin insulators were developed; these were heavy, susceptible
to degradation from exposure to ultraviolet radiations and thus were never fully integrated into the
power systems.
Since the advent of nineteen eighties, silicone rubber has been widely used in manufacturing of line
insulators because of its resistance to weather conditions and the hydrophobic characteristics of
rubber, which are permanent and allow advancement in the capacity of voltage of pollution the
insulator can withstand. As a result of the above stated properties, the use of the composite
polymers has increased tremendously in power system applications.
Generally, a basic insulator comprises of a core that bears the mechanical stress, a housing to
protect the core from the weather and end fitting to transfer the tensile load. A mixture of glass
with epoxy resin is for the formation of matrix inside the core that imparts the mechanical strength
to the structure. The end fittings that are used to transfer the mechanical tension of the line and
the supports are made from aluminium, cast iron, forged steel, etc. The use of rubber housing is to
introduce insulation and isolation and also shield the core from atmospheric elements.
As a result of the hydrophobic properties of the silicone rubber composite, the new generation
insulators can be designed to be more compact and lighter.

5
DEFINITIONS
TRACKING
Tracking is an irreversible deterioration by the formation of paths starting and developing on the surface of
an insulating material. These paths are conductive even under dry condition. Tracking can occur on surface
in contact with air and also on the interfaces between different insulating materials.
TREEING
Treeing is the formation of micro-channels within the material. The micro-channels can be either conducting
or non-conducting and can progress through the bulk of the material until electrical failure occurs.
EROSION
Erosion is an irreversible and non-conducting deterioration of the surface of the insulator that occurs by loss
of material. This can be uniform, localized or tree-shaped.
CHALKING
Chalking is a surface condition where in some particles of the filler become apparent during weathering,
forming a powdery surface.
CRAZING
Crazing is the formation of surface micro- fractures of depths up to 0.1 mm.
CRACKING
Cracking is any surface fracture of a depth greater than 0.1mm.
HYDROLYSIS
Hydrolysis is a chemical process involving the reaction of a material with water in liquid or vapour form. It
can lead to electrical or mechanical degradation.
PUNCTURE
Puncture can be characterized by a disruptive discharge occurring through a solid dielectric (e.g., shed,
housing, or core) causing permanent loss of dielectric strength.
SPECIFIED MECHANICAL LOAD (S.M.L)
The S.M.L. is a load specified by the manufacturing, used for mechanical tests in this specification. It forms
the basis of the selection of composite insulators.
TENSILE LOAD
Tensile load is the load applied in- line with the longitudinal axis of the insulator rod and away from the end
metal fitting.
ROUTINE TEST LOAD (R.T.L.)
The R.T.L. is the load applied to assembled composite insulators during Routine Tests. It is equal to 50% of
the S.M.L.
CANTILEVER LOAD
Cantilever load is a load applied at the conductor position on the insulator, perpendicular to the conductor,
and perpendicular to the rod of the insulator. This load is also called bending.

6
COMPRESSIVE LOAD
Compressive Load is applied in- line with the longitudinal axis of the insulator rod and towards the base
end.
MAXIMUM WORKING COMBINED LOADS
The maximum working combined loads are the simultaneously applied cantilever and compression loads.
They produce a bending moment that should not exceed the bending moment induced by the working
cantilever load rating alone.
WORKING CANTILEVER LOAD (W.C. L.)
Working cantilever load is a load that must not be exceeded in service.
MAXIMUM DESIGN RATING (MDR)
The maximum mechanical load that the insulator is designed to withstand continuously for the life of the
insulator.
PROOF-TEST LOAD
The routine mechanical load that is applied to an insulator at the time of its manufacture.
DELAMINATION
Delamination is the loss of bonding of fibers to matrix.
DIELECTRIC PROPERTIES OF INSULATION
DIELECTRIC STRENGTH OR BREAKDOWN VOLTAGE
The dielectric material has only some electrons in normal operating condition. When the electric
strength is increased beyond a particular value, it results in breakdown. That is, the insulating
properties are damaged and it finally becomes a conductor. The electrical field strength at the time
of breakdown is called breakdown voltage or dielectric strength.
DIELECTRIC
MATERIAL
DIELECTRIC
STRENGTH(KV/MM)
DIELECTRIC
CONSTANT
Air 3 1
Oil 5-20 2-5
Mica 60-230 5-9

7
INSULATING MATERIAL
The main cause of failure of overhead line insulator, is flash over, occurs in between line and earth
during abnormal over voltage in the system. During this flash over, the huge heat produced by
arcing, causes puncher in insulator body. Viewing this phenomenon the materials used for electrical
insulator, has to possess some specific properties.
PROPERTIES OF INSULATING MATERIAL
 It must be mechanically strong enough to carry tension and weight of conductors.
 It must have very high dielectric strength to withstand the voltage stresses in High Voltage
system.
 It must possess high Insulation Resistance to prevent leakage current to the earth.
 The insulating material must be free from unwanted impurities.
 It should not be porous.
 There must not be any entrance on the surface of electrical insulator so that the moisture or
gases can enter in it.
 There physical as well as electrical properties must be less affected by changing temperature.
PORCELAIN INSULATOR
The porcelain is aluminium silicate. The aluminium silicate is mixed with plastic kaolin, feldspar and
quartz to obtain final hard and glazed porcelain insulator material. The surface of the insulator
should be glazed enough so that water should not be traced on it. Porcelain also should be free
from porosity since porosity is the main cause of deterioration of its dielectric property. It must also
be free from any impurity and air bubble inside the material which may affect the insulator
properties.

8
PROPERTY VALUE(APPROXIMATE)
Dielectric
Strength
60 KV / cm
Compressive
Strength
70,000 Kg / cm2
Tensile
Strength
500 Kg / cm2
GLASS INSULATOR
Now a days glass insulator has become popular in transmission and distribution system. Annealed
tough glass is used for insulating purpose. Glass insulator has numbers of advantages over
conventional porcelain insulator.
ADVANTAGES OF GLASS INSULATOR
 It has very high dielectric strength compared to porcelain.
 Its resistivity is also very high.
 It has low coefficient of thermal expansion.
 It has higher tensile strength compared to porcelain insulator.
 As it is transparent in nature, it is not heated up in sunlight as porcelain.
 The impurities and air bubble can be easily detected inside the glass insulator body because
of its transparency.
 Glass has very long service life as because mechanical and electrical properties of glass do
not get affected by ageing.
 After all, glass is cheaper than porcelain.
DISADVANTAGES OF GLASS INSULATOR
 Moisture can easily condense over glass surface and hence air dust will be deposited on the
wet glass surface which will provide path to the leakage current of the system.
 For higher voltage glass cannot be cast in irregular shapes since due to irregular cooling
internal strains are caused.

9
PROPERTY VALUE(APPROXIMATE)
Dielectric
Straingth
140 KV / cm
Compressive
Strength
16,000 Kg / cm2
Tensile
Strength
35,000 Kg / cm2
POLYMER INSULATOR
CORE
The core is the internal insulating part of a composite insulator. It is intended to carry the
mechanical load. It consists mainly of glass fibres positioned in a resin matrix so as to achieve
maximum tensile strength.
HOUSING
The housing is external to the core and protects it from the weather. It may be equipped with
weather sheds. Some designs of composite insulators employ a sheath made of insulating material
between the weathersheds and the core. This sheath is part of the housing.
WEATHERSHEDS
Weathersheds are insulating parts, projecting from the housing or sheath, intended to increase the
leakage distance and to provide an interrupted path for water drainage.
Basic polymer shed materials used are silicone rubber, EPM, EPDM, CE, and polytetrafluorethylene
(PTFE or Teflon). To obtain desired electrical and mechanical properties these basic material are
combined with various fillers, including aluminium trihydrate.
END FITTING
End fitting transmit the mechanical load to the core. They are usually made out of metal.
COUPLING ZONE
The coupling zone is the part of the end fitting that transmits the load to the line, to the tower, or
to another insulator. It does not include the interface between the core and the fitting.
INTERFACE
An interface is the surface between different materials.
Examples of interface in composite insulators are as follows:
 Glass fibre/impregnating resin
 Filler/polymer
 Core/housing
 Housing/weathersheds
 Housing/end fitting
 Core/end fittings

10
CHARACTERISTICS
 whole moulding and whole injection
 crimped end fittings
 artistic looking
 small volume
 high bending and torsion strength
 good anti-explosive performance
 Light weight (65-80% less than ceramic insulator)
 Silicon rubber sheds provide perfect hydrophobic performance
 good resistance to ageing, tracking and erosion
 Stable behaviour at extreme climatic conditions
 Long term surface hydrophobicity
 Suitable for polluted environment, salty atmospheres etc.
 Resistance to breakage and vandalism, practically unbreakable
 Superior anti-tracking properties
 High mechanical strength
 Ease of installation (easier handling with lighter equipment and labour at the job site)
 Resistance to Seismic Shock
 Free of cleaning, economical maintenance and suitable for difficult maintenance area
 No need of zero value check
 easy and economical to transportation and installation
 Excellent anti-pollution performance, suitable for high polluted area.
 Free of cleaning, economical maintenance and suitable for difficult maintenance area.
 No need of zero value check.
 Reduce the purchasing quantities for spare parts.
ADVANTAGES OF POLYMER INSULATOR
 It is very light weight compared to porcelain and glass insulator.
 As the composite insulator is flexible the chance of breakage becomes minimal.
 Because of lighter in weight and smaller in size, this insulator has lower installation cost.
 It has higher tensile strength compared to porcelain insulator.
 Its performance is better particularly in polluted areas.
 Due to lighter weight polymer insulator imposes lesser load to the supporting structure.
 Less cleaning is required due to hydrophobic nature of the insulator.
DISADVANTAGES OF POLYMER INSULATOR
 Moisture may enter in the core if there is any unwanted gap between core and weather sheds.
This may cause electrical failure of the insulator.
 Over crimping in end fittings may result to cracks in the core which leads to mechanical failure of
polymer insulator.
 Subject to bird attack by Parrots, Cockatoos and Galahs.
 Not resilient to bushfire temperatures.

11
 Not recommended for location near surf beaches due to salt spray.
Let us give a practical example where many difficulties were faced in maintaining a distribution
network in Victoria Australia due to polymeric insulator. There were many Cockatoos, Galahs and
Parrots in that area of Australia, who love to chew on polymeric strain insulators. Here, the 22 KV
network has many of polymeric strain insulators installed and now after a few years of installing
polymeric strain insulators, the authority is now replacing many of them back with Glass disc
insulators. Another disadvantage is that they had post type polymeric insulators melt and bend in
bush fire areas. They have a concrete pole and a steel cross arm that survives a bush fire; however
the polymers in some cases fail. This would not be the case with glass or porcelain insulators. They
have also had polymeric insulators fail in areas close to the ocean coastline where there are high
salt levels in the air.
TYPES OF INSULATORS BASED ON CONSTRUCTION
TYPE A
An insulator or an insulator unit in which the length of the shortest puncture path through solid
insulating material is at least equal to half the length of the shortest flashover path through air
outside the insulator.
TYPE B
An insulator or an insulator unit in which the length of the shortest puncture path through solid
insulating material is less than half the length of the shortest flashover path through air outside the
insulator.
 Type A insulator are of solid core type.
TYPES OF INSULATORS BASED ON USE
PIN INSULATOR
Pin Insulator is earliest developed overhead insulator, but still popularly used in power network up
to 33 KV systems. Pin type insulator can be one part, two parts or three parts type, depending upon
application voltage.
In 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.
STRUCTURE
As the name suggests, the pin type insulator is secured to the cross-arm on the pole with a pin.
There is a groove on the upper end of the insulator for housing the conductor. The conductor
passes through this groove and is bound by the annealed wire of the same material as the
conductor.

12
PETTICOATS
In order to obtain lengthy leakage path, one, two or more rain sheds or petticoats are provided on
the insulator body. In addition to that rain shed or petticoats on an insulator serve another purpose.
These rain sheds or petticoats are so designed, that during 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 wet pin insulator surface. In higher voltage like
33KV and 66KV manufacturing of one part porcelain pin insulator becomes difficult. Because in
higher voltage, the thickness of the insulator become more and a quite thick single piece porcelain
insulator cannot manufactured practically. In this case we use multiple part pin insulator, where a
number of properly designed porcelain shells are fixed together by Portland cement to form one
complete insulator unit. For 33KV two parts and for 66KV three parts of pin insulators are used.
CAUSES OF INSULATOR FAILURES
Insulators are required to withstand both mechanical and electrical stresses. The latter type is
primarily due to line voltage and may cause the breakdown of the insulator. The electrical
breakdown of the insulator can occur either by flash-over or puncture. In flashover, an arc occurs
between the line conductor and insulator pin (i.e., earth) and the discharge jumps across the air
gaps, following shortest distance. Figure shows the arcing distance (i.e. a + b + c) for the insulator.

13
In case of flash-over, the insulator will continue to act in its proper capacity unless extreme heat
produced by the arc destroys the insulator. In case of puncture, the discharge occurs from
conductor to pin through the body of the insulator. When such breakdown is involved, the insulator
is permanently destroyed due to excessive heat. In practice, sufficient thickness of porcelain is
provided in the insulator to avoid puncture by the line voltage. The ratio of puncture strength to
flashover voltage is known as safety factor.
DESIGNING CONSIDERATION OF PIN INSULATOR
The live conductor attached to the top of the pin insulator is at a potential and bottom of the
insulator fixed 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 air,
is known as flash over distance.
 When insulator is wet, its outer surface becomes almost conducting. Hence the flash over
distance of insulator is decreased. The design of an electrical insulator should be such that the
decrease of flash over distance is minimum when the insulator is wet. That is why the upper
most petticoat of a pin insulator has umbrella type designed so that it can protect the rest lower
part of the insulator from rain. The upper surface of top most petticoat is inclined as less as
possible to maintain maximum flash over voltage during raining.
 To keep the inner side of the insulator dry, the rain sheds are made in order that these rain sheds
should not disturb the voltage distribution they are so designed that their subsurface at right
angle to the electromagnetic lines of force.
 The pins shall of single piece obtained preferably by the process of forging.
 They shall not be made by joining, welding, shrink fitting or any other process using more than
one piece material. The pins shall be of good finish, free from flaws and other defects.
 The finish of the collar shall be such that sharp angle between the collar and the shank is
avoided. Aluminium ferrous pins, nuts and washers, except those made of stainless steel, shall be
galvanized. The threads of nuts and taped hole when cut after galvanizing shall be well oiled or
greased.
DIMENSIONS FOR PIN INSULATORS
 Pins shall be of small steel head type S 165 P as per IS : 2486 (Part-II) having stalk length of 165
mm and shank length of 150 mm with minimum failing load of 16 KN.
HELICALLY FORMED PIN INSULATOR TIES
 Helically formed ties used for holding the conductor on the pin insulator shall be made of
aluminium alloy or aluminized steel or aluminium clad steel wires and shall conform to the
requirements of IS : 12048. The ties shall be suitable for pin insulator dimensions of and
conductor sizes specified.
 Elastomer pad for insulator shall be used with the ties to avoid abrasion of the conductor coming
into direct contact with the insulator.

14
POST INSULATOR
Post insulator is more or less similar to pin insulator but former is suitable for higher voltage
application. Post insulator has higher numbers of petticoats and has greater height. This type of
insulator can be mounted on supporting structure horizontally as well as vertically. The insulator is
made of one piece of porcelain but has fixing clamp arrangement in both top and bottom end.
PIN INSULATOR POST INSULATOR
It is generally used up to 33KV system It is suitable for lower voltage and also for
higher voltage
It is single stag It can be single stag as well as multiple
stags
Conductor is fixed on the top of the insulator by binding Conductor is fixed on the top of the
insulator with help of connector clamp
Two insulators cannot be fixed together for higher
voltage application
Two or more insulators can be fixed
together one above other for higher
voltage application
Metallic fixing arrangement provided only on bottom end
of the insulator
Metallic fixing arrangement provided on
both top and bottom ends of the insulator

15
SUSPENSION INSULATOR
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. In suspension
insulator numbers of insulators are connected in series to form a string and the line conductor is
carried by the bottom most insulator. Each insulator of a suspension string is called disc insulator
because of their disc like shape.
ADVANTAGES OF SUSPENSION TYPE
 Suspension type insulators are cheaper than pin type insulators for voltages beyond 33 kV.
 If anyone disc is damaged, the whole string does not become useless because the damaged disc
can be replaced.
 The suspension arrangement provides greater flexibility to the line. The connection at the cross
arm is such that insulator string is free to swing in any direction and can take up the position
where mechanical stresses are minimum.
 In case of increased demand on the transmission line, it is found more satisfactory to supply the
greater demand by raising the line voltage than to provide another set of conductors. The
additional insulation required for the raised voltage can be easily obtained in the suspension
arrangement by adding the desired number of discs.
 The suspension type insulators are generally used with steel towers. As the conductors run
below the earthed cross-arm of the tower, therefore, this arrangement provides partial protection
from lightning.
 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. For instance, if the working voltage is 66 kV, then six discs in series will be provided on the
string.
 Mechanical stresses on the suspension insulator are less since the line hanged on a flexible
suspension string.
DISADVANTAGES OF SUSPENSION INSULATOR
 Suspension insulator string 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.

16
STRAIN INSULATOR
When suspension string is used to sustain extraordinary tensile load of conductor it is referred as
string insulator. When there is a dead end or there is a sharp corner in transmission line, the line has
to sustain a great tensile load of conductor or strain. A strain insulator must have considerable
mechanical strength as well as the necessary electrical insulating properties.
For low voltage lines (< 11 kV), shackle insulators are used as strain insulators. However, for high
voltage transmission lines, strain insulator consists of an assembly of suspension insulators. The
discs of strain insulators are used in the vertical plane. When the tension in lines is exceedingly
high, at long river spans, two or more strings are used in parallel.
BALL & SOCKET TYPE
RATED
SYSTEM
VOLTAGE
NUMBER OF DISC INSULATOR USED
IN STRAIN TYPE TENSION
INSULATOR STRING
NUMBER OF DISC INSULATOR USED IN
SUSPENSION INSULATOR STRING
33KV 3 3
66KV 5 4
132KV 9 8
220KV 15 14

17
TONGUE & CLEVIS TYPE
STAY/GUY STRAIN INSULATOR
For low voltage lines, the stays are to be insulated from ground at a height. The insulator used in
the stay wire is called as the stay insulator and is usually of porcelain and is so designed that in
case of breakage of the insulator the guy-wire will not fall to the ground.
 The porcelain insulator shall be sound, free from defects, thoroughly verified and smoothly
glazed.
 The design of the insulator shall be such that the stresses to expansion and contraction in any
part of the insulator shall not lead to its deterioration.
 The glaze, unless otherwise specified, shall be brown in colour.
 The glaze shall cover the entire porcelain surface parts except those areas that serve as supports
during firing.
TYPES OF GUY INSULATORS
 The standard guy strain insulators shall be designated as ‘A’ or ‘C’ as per IS: 5300.
 The recommended type of guy strain insulators for use on guy wires of overhead lines of 11 KV
voltage level is as follows:
Power Line Voltage: 11KV
Designation of Insulators: C
Dry one minute Power Frequency withstand Voltage: 27 KV (rms)
Wet one minute Power Frequency withstand Voltage: 13 KV (rms)
Minimum Failing Load: 88(KN)
TYPE OF INSULATORS FOR GUY INSULATORS
 The standard guy strain insulators shall be of designations ‘A‘or ‘C‘as per IS: 5300.
 The recommended type of guy strain insulators for use on guy wires of overhead lines of
different voltage levels are as follows :
LINE VOLTAGE DESIGNATION OF INSULATOR
415/240Volt A Type
11KV C Type
33KV C Type (2 No’s of Strings in Series).

18
BASIC INSULATOR LEVEL FOR GUY INSULATORS
DESIGNATION OF
INSULATOR
DRY ONE MIN POWER FREQUENCY
WITHSTAND
WET ONE MIN POWER
FREQUENCY WITHSTAND
A Type 18 KV (rms) 8 KV (rms)
C Type 27 KV (rms) 13 KV (rms)
MECHANICAL STRENGTH FOR GUY INSULATORS
DESIGNATION OF INSULATOR MINIMUM FAILING LOAD
A Type 44 KN
C Type 88 KN
SHACKLE INSULATOR OR SPOOL INSULATOR
The shackle insulator or spool insulator is usually used in low voltage distribution network. It can be
used both in horizontal and vertical position. The use of such insulator has decreased recently after
increasing the using of underground cable for distribution purpose. The tapered hole of the spool
insulator distributes the load more evenly and minimizes the possibility of breakage when heavily
loaded. The conductor in the groove of shackle insulator is fixed with the help of soft binding wire.

19
COMPARISON BETWEEN CERAMIC AND POLYMER INSULATOR
FACTOR CERAMIC POLYMER
Weight Heavy, approx. wt. of 400 kv is
about 135 kgs.
90% lighter than porcelain insulators, but
offer an equal to better strength. Approx. wt.
of 400 kv is less than 14 kgs
Packing & transport Risky & expensive Easy & economical
Installation Risky, expensive and more labour
required
Very easy & economical
Handling Difficult Easy
Maintenance cost High Low
Vandalism More susceptible Highly resistant
Breakages & secondary
damage
Highly fragile - 16 to 15% breakages
are reported during transportation,
storage and installation
Composite insulators are flexible and
therefore, highly resistant to breakages.
Mechanical failure Reduction in mechanical strength
and separation due to pings getting
eroded.
Single piece hence no such problem
Resistance to flashovers
& punctures
Low High
Anti-tracking and
erosion resistance
Very low - poor tracking resistance Excellent tracking resistance avoids erosion or
tracking of the housing material.
Dielectric strength Lower than polymer Excellent insulation performance
Contamination &
pollution
Highly affected Not affected and has longer life
Hydrophobicity Non hydrophobic, porcelain surface
forms water film on the surface
making easy path leading to more
flash overs
The hydrophobicity properties of silicon
rubber provide excellent insulating behaviour
and resists wetting by forming beads of water
without the need of washing or greasing even
in humid or polluted climates. Hence low
failure rate combined with low overall
operating and maintenance costs.
Self-cleaning quality No. - dirt, sand, salt & snow are
easily attracted
Yes. Due to hydrophobicity recovery
characteristic
Tensile strength Good Excellent due to crimping technology.
Maintenance Needs maintenance like cleaning,
washing and greasing
No maintenance is required
Design Design flexibility is limited. Requires
larger and heavier towers for
installation and more space.
Polymer insulator design allows for adaption
to suit specific needs such as creepage
distance. Results in space saving and lower
cost
Manufacturing process Porcelain insulators require long
manufacturing process leading to
long delivery time. Manufacturing
process causes pollution & health
risk .
Pollution free, safe, short process time leading
to short delivery periods
Safety Porcelain insulators are susceptible
to explosion & breakages, due to
high fragile properties , stone
throwing etc.
Composite insulators provide very high level
of safety, superior flexibility and strength. Not
susceptible to explosion. No breakages due to
stone throwing etc.

20
CLEARANCE AND CREAPAGE DISTANCE
CREEPAGE DISTANCE FOR INSULATOR
HIGHEST SYSTEM
VOLTAGE
MODERATELY
POLLUTED
ATMOSPHERES
TOTAL
HEAVILY POLLUTED
ATMOSPHERES
KV MM TOTAL (MM) PROTECTED(MM)
3.6 75 130
7.2 130 230
12 230 320
24 430 560
36 580 840 420
72 1160 1700 850
123 1850 2800 1400
145 2250 3400 1700
245 3800 5600 2800
420 6480 9660 4830
MARKING FOR INSULATOR
 Name or trademark of manufacturer.
 Month and year of manufacture.
 Minimum failing load in KN.
 ISI certificate mark, if any.
 Markings on porcelain shall be printed and shall be supplied before firing.
CAUSES OF INSULATOR FAILURE
CRACKING OF INSULATOR
The porcelain insulator mainly consists of three different materials. The main porcelain body, steel
fitting arrangement and cement to fix the steel part with porcelain. Due to changing climate
conditions, these different materials in the insulator expand and contract in different rate. These
unequal expansion and contraction of porcelain, steel and cement are the chief cause of cracking
of insulator.
DEFECTIVE INSULATION MATERIAL
If the insulation material used for insulator is defective anywhere, the insulator may have a high
chance of being puncture from that place.
POROSITY IN THE INSULATION MATERIALS
If the porcelain insulator is manufactured at low temperatures, it will make it porous, and due to
this reason it will absorb moisture from air thus its insulation will decrease and leakage current will
start to flow through the insulator which will lead to insulator failure.

21
IMPROPER GLAZING ON INSULATOR SURFACE
If the surface of porcelain insulator is not properly glazed, moisture can stick over it. This moisture
along with deposited dust on the insulator surface produces a conducting path. As a result the
flash over distance of the insulator is reduced. As the flash over distance is reduced, the chance of
failure of insulator due to flash over becomes more.
FLASH OVER ACROSS INSULATOR
If flash over occurs, the insulator may be over heated which may ultimately results into shuttering
of it.
MECHANICAL STRESSES ON INSULATOR
If an insulator has any weak portion due to manufacturing defect, it may break from that weak
portion when mechanical stress is applied on it by its conductor. These are the main causes of
insulator failure.
INSULATOR DESIGN
BASIC DESIGN CONCEPTS
The basic designs of polymer insulators evolve around three essential components. These are, a
core, a sheath or weathersheds and metal end fittings.
The end fittings are attached to the core in various ways to develop the required mechanical
strength for the intended application.
The core consists of axially aligned glass fibres bonded together by means of an organic resin. The
unprotected core with end fittings by itself is not suitable for outdoor high voltage application, as
moisture, ultraviolet rays, contamination, acid rain, ozone and voltage are conducive to the
degradation of the core material and leading to electrical and mechanical failure. Hence, a
protective sheath or weathersheds made from various polymer materials that have been
compounded for outdoor electrical applications are applied over the core in various ways to protect
the core and to provide maximum electrical insulation between the attachment ends. It is quite
clear that with such a diversity of possible constructions, the performance of polymer insulators
depends on the selection of materials and on the design and construction of the insulators.
MATERIAL SELECTION
CORE
The mechanical strength member of polymer insulator is a fiberglass rod. The rod, normally referred
to as the core of the insulator, consists of between 70 and 75% by weight of axially aligned glass
fibres bonded by an organic resin. The resin system can be either polyester or epoxy and the rod
either cast or protruded.
Today’s core is protruded in various diameters with electrical grade E-type glass fibres and polyester
Resin. Two critical process parameters in the protrusion of fiberglass rod for insulators are pulling
speed and temperature of the forming die. An axial crack develops when the outside of the rod
cures more quickly than the centre of the rod. This occurs when either the die temperature is too
high for the pulling speed or outside of the rod sets, shrinkage during curing of the bulk of the rod
and produces an axial crack in the centre of the rod. Bonding of glass fibres to the polyester resin is

22
affected by process parameters as well. In pulling glass fibre, the fibre is sized or treated chemically
for protection against mechanical damage during handling. Optimum bonding requires
compatibility of the chemical systems; otherwise, the rod may exhibit porosity and wick moisture by
capillary. The rod may still be acceptable for mechanical applications but not for polymer insulators.
WEATHERSHEDS
weathersheds for polymer insulators are manufactured from materials such as bisphenol or
cycloaliphatic epoxy resins, thermoplastic rubber, and ethylene-propylene diene Monomer (EPDM)
and silicone elastomers. These materials are compounded with various types of inorganic fillers
such as silica and hydrated alumina with concentrations ranging from a few percent to 70% by
weight.
Today, the elastomeric materials of EPDM and silicone containing a minimum of 70% by weight of
hydrated alumina that are in use by most of manufacturers are favoured for weathersheds with
silicone rubber clearly showing the best performance over all other types. Failures of some first
generation polymer insulators with epoxy resin weathersheds have been attributed to
depolymerisation by hydrolysis.
Depolymerisation refers to the destruction of the molecular structure of polymer materials.
Hydrolysis is the result of a chemical reaction, which takes place between the ions of water and the
free ends of polymer chemical chain, which causes depolymerisation to occur.
Also, insulators made from epoxy resins contain locked-in mechanical stresses that develop during
curing of the resin. This occurs when mixing or curing of the resin is uneven. Circumferential cracks
between sheds sometimes develop during storage of the insulator because of the locked- in
stresses.
However, more often the cracks develop in service as the stresses are aggravated by low
temperature and line tension. The cracks extend down to the core, thereby exposing the core to
moisture.
Elastomers are the best weathershed material, as they do not contain locked- in mechanical
stresses from the curing process. Also, elastomers are preferred at low temperatures where impact
resistance is important.
Another problem that surfaced early in the experience of first generation designs were the effect of
outdoor weathering on weathersheds. Weathering affects all polymer materials to some extent and
being a natural phenomenon includes the effects of heat, humidity, rain, wind, contaminants in the
atmosphere and ultraviolet rays of the sun. Under such conditions, the weathersheds of polymer
insulators may be permanently changed, physically by roughening and cracking and chemically by
the loss of soluble components and by the reactions of salts, acids and other impurities deposited
on the surface. Surface become hydrophilic and moisture more easily penetrates into the volume of
the weathersheds.
END FITTINGS
The end fittings are cast from either aluminium or malleable iron in sand castings.
Some aluminium end fittings are cast in a permanent mould. The strengths of these fittings are
more consistent than the sand-cast ones. Forged and extruded aluminium end fittings have
strengths that are also quite consistent. However, if swaging is the method of attaching the fittings
to the core, both permanent mould and forged aluminium fittings may develop cracks in service.
INSULATOR DESIGN
Elastomeric weathersheds in first generation insulators are individually moulded and glued to the
core and to each other by an epoxy adhesive. In some designs, silicone gel or silicone caulking
sealant is applied. In other designs, silicone gel or grease is used to fill the air space between the
sheds and the core. Although epoxy glue provides some measure of protection against water entry,
there is some uncertainty in the lifetime of such a seal. Insulators constructed from individual sheds
are known to permit water reaching the core and have failed during hot line high-pressure water
washing.

23
Most of the insulators are moulded from either EPDM or silicone elastomers, in one-piece having an
aerodynamic design, that is fully vulcanized to the core. The insulators are terminated with end
fittings that are swaged onto the core sealing the insulator from moisture reaching the core. Some
designs or end fittings of the insulators are equipped with power arc interceptors. These serve to
protect against loss of seal during flashover. Aluminium end fittings of a unique design are swaged
onto the core there by effectively sealing the sleeved ends from moisture.
POLLUTION CONSIDERATION
TABLE-1
POLLUTION
CATEGORY
ENVIRONMENT DESCRIPTION MINIMUM
SPECIFIC
NOMINAL
LEAKAGE
(MAX. S.D.D.) MM/KV IN/KV
I-light
(0.06 mg/cm2)
 Areas without industrial and with low density
of
 Houses equipped with heating plants.
 Areas with low density of industries or houses
 But subjected to frequent wind and / or
rainfall.
 Agricultural areas.
 Mountainous areas.
 All areas situated 16 km to 20 km from the sea
 And not exposed to wind directly from the
sea.
16 0.63
Ii-medium
(0.20 mg/cm2)
 areas with industries not producing
particularly
 Polluting smoke and / or with average density
of
 Houses equipped with heating plants.
 areas with high density of houses and / or but
 Subjected to frequent winds and / or rainfall.
 areas exposed to winds from the sea but not
too
 Close to the coast (at least several kilometers
 Distance).
20 0.79
Iii- heavy
(0.60 mg/cm2)
 areas with high density of industries and
 Suburbs of large cities with high density of
 Heating plants producing pollution.
 areas close to the sea or in any case exposed
to
 Relatively strong winds from the sea.
25 0.98
IV-very
Heavy
(>0.60
mg/cm2)
 Areas generally of moderate extent, subjected
to
 Conductive dusts and to industrial smoke
 Producing particularly thick conductive
deposits.
 Areas generally of moderate extent, very close
to
 The coast and exposed to sea-spray or to very
 Strong and polluting winds from the sea.
 Desert areas, characterized by no rain for long
 Periods, exposed to strong winds carrying
sand
 And salt, and subjected to regular
condensation.
31 1.22

24
S.D.D. = Salt Deposit Density
POLLUTION SEVERITY LEVELS
For the purposes of standardization, four levels of pollution are qualitatively defined, from light
Pollution to very heavy pollution. Table-1 gives, for each level of pollution, an approximate
Description of some typical corresponding environments. Other extreme environmental conditions
Exist which merit further consideration, e.g. Snow and ice in heavy pollution, heavy rain, and arid
Areas.
RELATION BETWEEN THE POLLUTION LEVEL AND THE SPECIFIC CREEPAGE
DISTANCE
For each level of pollution described in Table-1, the corresponding minimum nominal specific
Creepage distance, in millimetres per kilovolt (phase-to phase) of the highest voltage for insulator is
Also given.
APPLICATION OF THE "SPECIFIC CREEPAGE DISTANCE" CONCEPT
In order to successfully apply the "specific creepage distance" concept, certain dimensional
Parameters characterizing the insulator shall be taken into account.
The important parameters, that have to be taken into account, are:
PARAMETERS CHARACTERIZING THE PROFILE
The profile of an insulator is characterized by the following parameters:
 Minimum distance, c, between sheds
 Ratio s / p between spacing and shed overhang
 Ratio ld / d between creepage distance and clearance
 alternating sheds
 Inclination of sheds
 Parameters characterizing the entire insulator
 Creepage factor C.F.
 Profile factor P.F.
INFLUENCE OF THE POSITION OF INSULATORS
There is normally some change in the pollution performance of insulators designed for use in The
vertical position when they are used in an inclined or horizontal position. Generally the Change is
for an improvement in performance, but in certain cases a reduction may result, due For example to
the cascade effect of heavy rain.
INFLUENCE OF THE DIAMETER
Various laboratory tests appear to indicate that the pollution performance of post insulators
Decreased with increasing average diameter.
 The following values for kd are proposed, k being a factor to increase the creepage distance With
average diameter Dm in millimeters.
 average diameter Dm < 300 mm: kd = 1
 300 ≤Dm ≤500 mm: kd = 1.1
 Dm > 500 mm: kd = 1.2
For a given profile, the average diameter Dm is given by:

25
1. Regular sheds Dm = D 1 + D2 / 2
2. Alternating sheds Dm = D1 + D2 + 2D 1 / 2
Where:
L is the total creepage distance of the insulator.
D(l) is the value of the diameter at a creepage distance l, measured from one electrode.
DETERMINATION OF THE CREEPAGE DISTANCE
The minimum nominal creepage distance of an insulator situated between phase and earth is
Determined, according to the pollution level of the site, by the relation:
Minimum nominal creepage distance = minimum specific creepage distance X highest
System voltage phase-to-phase for the equipment X kd
Where:
KD is the correction factor due to diameter
If insulators are to be used between phases (phase-spacers for instance), the creepage distance
should Be multiplied by v3 (for a three-phase system).

26
INSULATION CONSIDERATION
There are basically three factors to consider when designing the insulators.
 The 50/60-Hz power voltage.
 Surge voltages caused by lightning.
 Surge voltage caused by switching.
Surge voltages provide the most stringent test and the rationale for the standard impulse voltage
wave Form; that is, if the insulator is properly insulated to withstand surges, it can usually
accommodate the Highest expected 50/60 Hz voltage. Insulators are more tolerant of short
duration overvoltage than Sustained values. For the purpose of impulse testing, a standard
waveform is defined, as shown in Figure. The waveform is referred to as T1 X T2, where both values
are conventionally given in Microseconds. The crest value of the waveform V.
The value T1 is the rise time to crest, whereas the Value T2 is the fall time to 0.5 V.
A convenient analytical representation of the pulse waveform is the double-exponential expression
V(t) = V1[exp(-t / t2) – exp(-t / t1)]
Where
T2 = T2 / ln (2) = 1.443T2
T1 = T1 / 5 = 0.2T1
V1 = V exp(T1 / 1.443T2)
WAVEFORM- LIGHTENING IMPULSE
WAVEFORM- SWITCHING IMPULSE

27
For a given well-defined voltage waveform, under specified test conditions, the following
Terminology is defined:
CRITICAL FLASHOVER VOLTAGE (CFO)
The crest (maximum) voltage for which the probability of Flashover is 0.50.
WITHSTAND VOLTAGE
The crest voltage 3s below the CFO
BASIC (LIGHTNING) IMPULSE INSULATION LEVEL (BIL)
The crest voltage for which the probability of Flashover is 0.16, using a 1.2/50 ms test pulse.
BASIC (SWITCHING) SURGE IMPULSE INSULATION LEVEL (BSL)
The crest voltage for which the probability of flashover is 0.16, using a 250/2500 s test pulse.
CONSIDERATION OF INTERFERENCE AND CORONA
GENERATING PROCESSES
Interference with radio and television (ri and tvi) may arise when electrical discharges run on
Insulators and inject high-frequency currents into associated conductors, which radiate
Electromagnetic waves. The types of discharge which generate interference are:
Micro sparks between Water drops or metal fittings, the latter especially in cases of corrosion.
Discharges across dry bands on leaky surfaces.
SURFACE CORONA DISCHARGES AROUND HIGHLY STRESSED ELECTRODES
Surface corona discharges are again relatively slow phenomena, incapable of heavy generation at
Vhf, but principal sources at lower frequencies. Surface discharges may be prevented by
hydrophobic treatment. This not only inhibits dry-band formation but also gives good voltage
Grading, thus removing the over voltages, which cause other types of discharge. It might appear
that the installation of a corona ring would smother capacitive over voltages, while avoiding the
usual Power loss, which follow installation of a corona ring.
Both the above-mentioned types of discharge are, to some extent, weather dependent. Water may
Cause droplet discharges while suppressing contact discharges by virtue of its high conductivity
and Permittivity.

28
Dry bands are not a fair-weather phenomenon, the existence of which depends on the design of the
insulator and the geometry of the insulators of fittings.
Surface corona, if in fair Weather, for which reason it has long been the practice to specify higher
than normal voltages for Corona inception on insulator.
CONSIDERATION OF CAPACITANCE EFFECTS
The distribution of capacitance along an insulator, and their size, govern the electric stresses,
which excite generation of interference, and the coupling of the generator to the radiating
antenna.
A composite insulators like a cylinder of dielectric having a relative permittivity about 6. The field
Intensity falls away rapidly with increasing distance from the live terminal. The generating
discharges occur at or near the live terminal, and the capacitance, which couples the high
frequency Currents into the radiating circuit, i.e. the line and tower, is small.
Composite insulators are thus significantly quieter as interfering sources than string of discs.
In a string of discs, quite large capacitance – of the order of 30 pf – is connected in cascade
through the fittings. The voltage distribution is governed purely by these and by the stray
capacitance to line and ground, in dry conditions. In such a voltage-dividing circuit the partition is
independent of frequency: identical distributions therefore exist for the power- frequency and for
the radio-frequency Voltage. The units at the line end are more prone to surface corona than the
rest.
Because of the high unit capacitance the sources are closely coupled into the line, which presents
load impedance Equal to one-half of the line’s surge impedance. It is common practice to relieve the
line end Overvoltage by means of stress- grading fittings.
Some of the devices which are used to minimize surface corona, in cases like these where ‘quiet’
Insulators are essential. Tests showed those gradients between 16 and 14 kv/cm is sufficient to
break Down air in contact with insulator over gaps of a few centimetres.

29
INSULATOR SELECTION
MECHANICAL PARAMETERS
The process for selecting the strength rating of composite or ceramic insulators is identical. Based
on strength alone, a composite insulator can directly replace insulators made of porcelain or glass.
Composite insulator’s strength rating is as defined in the Table-2.
Historically, when selecting an insulator’s strength rating a two to one safety factor has been
applied because all materials have a time- load characteristic that reduces their residual strength
over time.
Most manufacturer rate composite insulators at a maximum working load equal to 50% of the
Ultimate load rating.
TABLE-2
LOADING CONSIDERATIONS-ICE AND WIND
Establishing the everyday working load of an insulator requires that ice and wind conditions for the area of
application be considered. Ice and wind can add considerable load to the conductor and insulators, resulting
in greater loads. Table-3 lists parameters to use for calculating ice and wind loads for each region.
The following example determines the total loading of ice, wind and conductor weight. This calculation
does not include the weight of the insulators and other items that increase the load on the insulators.
Normally, such items should be added to the final load. A composite insulator’s weight is a small percentage
of the total weight; therefore, omitting it from the calculation will have minimal effect on the outcome.
TABLE-3: LOADING PARAMETERS
LOADING RADIAL ICE
THICKNESS
(IN)
HORIZONTAL
WIND PRESSURE
(LBS./SQ. FT.)
TEMPERATURE
(DEGREE F)
CONSTANT
(LBS./FT.)
Heavy 0.50 4 (40 mph) 0 0.30
Medium 0.25 4 (40 mph) +15 0.20
Light 0 9 (60 mph) +30 0.05
LOADING NON CERAMIC CERAMIC
SUSPENSION POST
Maximum Short Term
Load (one minute)
SML (Specified
Mechanical Load)
UCL (Ultimate
Cantilever Load)
M&E (Mechanical and
Electrical Strength)
Maximum Temporary
Load (one week)
50% to 60% of the
SML
Not Defined Not Defined
Maximum Working
Load (continuous)
RTL (Routine Test
Load) 50% of the
SML
WCL (Working
Cantilever Load)
50% or UCL
TPL (Tension Proof)
50% of the M&E and
WCL 40% of the UCL

30
TABLE-4: STEPS USED IN CALCULATING THE TOTAL LOAD ON A
CONDUCTOR
1. Select ice and wind
Parameters
Determine maximum radial ice thickness and
wind pressure
From the local standard.
2. Calculate vertical load Conductor and ice weight between insulator
span, including
Spacers and other equipment the conductor
supports.
3. Calculate horizontal load Horizontal wind pressure on the conductor,
radial ice thickness,
Spacers and other equipment the conductor
supports.
4. Calculate combined load Vector sum of the horizontal and vertical
loads.
5. Calculate total conductor load Combine load plus the loading constant
from table 3.
ELECTRICAL PARAMETERS
An insulator’s electrical parameters should be selected to maintain an economic balance between system
performance and the cost of over insulation. Increasing an insulator’s electrical parameters will provide
greater system protection at a higher price and, for long power lines the cost can be substantial. Therefore,
insulation requirements for power systems should be considered on an individual basis because of the
uniqueness of power system configurations, grounding techniques, and protection scheme.
CIFO SELECTION
Generally, an acceptable flashover probability is incorporated into the design of a transmission line because
it’s not economically feasible to protect against all occurrences of lightning overvoltages, These
overvoltages result from shielding failures caused by direct strokes to the conductor, from back-flashover
caused by strokes to the tower and shield wire, and from induced voltages caused by strokes to nearby
grounded objects. When a flashover occurs, an insulator is not normally affected and will recover, unlike
transformer insulation that will be damaged. This ability allows an insulator’s flashover level to be selected
by a statistical process that gives it an expected flashover probability. The capability of an insulator to resist
flashover caused by lightning strokes relies on the air gap distance (Dry Arc) between its live and ground
end. Therefore, as the system insulation level increases, so must the insulator’s length.

31
TABLE-5 RECOMMENDED BIL AT VARIOUS OPERATING VOLTAGES
VOLTAGE CLASS
(KV)
BIL
(KV)
REDUCED BIL
(KV)
15 116
23 150
34.5 200 125
46 250
69 350
92 450
115 550 450/350
138 650 550/50
161 750 650/550
196 900
230 1650 900/825/750
287 1300 1175/1650/900
345 1550 1425/1300/1650
500 1800 1675/1550/1300
The equipment used in a power system comprises items having different breakdown or withstand
Voltages and different voltage time characteristics. In order that all items of the system are
adequately Protected there is a need to consider the situation as a whole and not items of plant in
isolation; i.e.
The insulation protection must be coordinated. To assist this process, standard insulation levels are
recommended and these are summarized in Table-5. Reduced basic insulation impulse levels are
used when considering switching surges and are also summarized in Table-5.
Statistical data has shown that the CIFO should be applied at three standard deviations (s) above
the system BIL; this will provide an acceptable flashover probability. Since the standard deviation is
equal to 3% the required CIFO can be computed directly from the system BIL.

32
1. The BIL is usually expressed as a per unit of the peak (crest) value of the normal operating
voltage to earth; e.g.
for a maximum operating voltage of 362 kV.
1 p.u. = v2 X (362 / v3) = 300 kV
so that a BIL of 2.7 p.u. = 816 kV Calculation of using formula:
Using Typical System BIL
CIFO = BIL/ 0.91
LOW FREQUENCY LING DURATION (60HZ) SELECTION
Insulators must perform under two types of electrical stresses, i.e., those caused by temporary
overvoltages and those caused by the steady state nominal system voltage. Typically the
occurrence of an overvoltage is an uncommon event and limited to a relatively short time. This type
of stress can be determined by a system analysis and then be accounted for when selecting the
insulator requirements. For system voltages through 230 kV, the dry flashover value is typically five
times the maximum line to ground power frequency voltage. This margin results in a wet flashover
value of three to five times the line to ground power frequency voltage. Normally, experience with
the area and design or a system study is required to determine the minimum insulation strength.
SWITCHING SURGE SELECTION
A switching overvoltage is caused by a switching operation or a fault on the system that occurs
during line reclosing, fault clearing, and switching of capacitive or inductive circuits. Surge
developed by these operations is generally of short duration, highly damped, and has unpredictable
amplitudes.
Switching surge requirements depend on the transmission- line design and location, i.e.; clearances
between live parts and ground, altitude, and system grounding. For system voltages below 300 kV
the probability of a switching surge flashover is negligible and lightning surges are of more concern.
Above 300 kV, grounding and shield design determine whether a switching surge or lightning surge
has the greater influence. Often, because of improved grounding techniques, tower designs, and
line shielding methods, switching surge levels are more representative of the system requirements.
Table-6 lists typical switching surge overvoltage from the Transmission Line.
TABLE-6 TYPICAL SWITCHING SURGE LEVELS
*Per unit voltage, expressed in maximum phase to ground system voltage.
CONTAMINATION PERFORMANCE
The long term electrical performance of a polymer insulator is contingent upon the weatherability
of the polymer material and the level of electrical stress subjected to the material. If the material
weathers poorly or the electrical stress is too high, the polymer could prematurely degrade and lead
to a shortened service life.
CAUSE OF SWITCHING SURGE MAXIMUM PER UNIT*
OVERVOLTAGE
Energize 200 mile line, no closing resistors 3.5
Fault initiation on unfaulted phase 2.1
Fault clearing 1.7 to 1.9
Energize line and transformer 1.2 to 1.8
First restrike on capacitor bank 3.0

33
The weathering of the material is a natural event caused by many factors, some of which are; wind,
rain, heat, cold, humidity, pollutants from factories and automobiles, and ultraviolet rays from the
sun. A polymer material's ability to endure these conditions for an extended period is dependent
upon its chemical composition.
Hydrophobicity is the characteristic of silicone rubber that sets it apart from other commonly used
insulator materials. Since silicone rubber does not readily wet-out, leakage current remains low and
flashover are prevented. Even when the insulators are heavily contaminated, silicone has the unique
ability to impart hydrophobicity to the contaminated layer due to the migration of low molecular
weight silicone polymer molecules into the contaminate. Contamination flashover performance of
silicone is far superior to other insulation materials, and because the contamination layer is
hydrophobic, power washing of silicone insulators is not required. To determine pollution category,
use Table-1.
CALCULATION OF INSULATOR LEAKAGE DISTANCE EXAMPLE:
Pollution category II-Medium
Required Leakage = (system kV line) X (5% voltage regulation) X (leakage/kV line)
GRADING RING SELECTION
Corona discharges can form on the metallic ends of an insulator when the electric-field intensity
exceeds the dielectric strength of the surrounding air. Several factors that affect the onset of
corona are air pressure, humidity, end fitting material, and voltage level. If the end fitting has sharp
edges or is irregular in shape, corona can result from a concentration of the electric field. Grading
rings reshape the electric field, thus, reducing corona discharge on the insulator, high RIV levels,
and power losses. If corona persists, it may damage the polymer material and lead to insulator
failure.
The following recommendations for corona extinction and RIV are given by proposed ANSI standard
C29.12:
MINIMUM CORONA EXTINCTION LEVEL
115% of the nominal line to ground voltage.
MAXIMUM RIV
160 mV at 115% of nominal line to ground voltage.
The elevation at which insulator is applied is an important consideration when specifying grading
rings. Tests have shown that corona extinction and inception levels decrease with increasing
altitude; approximately a 1% change for every 1600 feet above sea level. However, the effect is not
as significant below 3000 feet since the bulk of corona tests occur near this level. Therefore,
corona extinction levels must be adjusted for applications above 3000 feet. Grading rings are
selected according to the corona extinction level. A variety of ring shapes and sizes are necessary
to meet the requirements of the different voltage levels and insulator configurations. Generally, by
increasing a corona rings diameter or thickness the inception level increases.

34
INSULATOR CHARACTERISTICS
SERVICE CONDITIONS
i) Weather Degree
ii) Pollution Category
iii) Altitude
iv) Insulator Configuration
PHYSICAL CHARACTERISTICS
i) Section Length
ii) Dry Arc Distance
iii) Leakage Distance
ELECTRICAL CHARACTERISTICS
i) Max. System Voltage
ii) System BIL
iii) Insulator CIFO
iv) Min. Dry 60 Hz Flashover
v) Min. Wet 50/60 Hz Flashover
vi) RIV
MECHANICAL CHARACTERISTICS
Specified Mechanical Load (S.M.L.) - for Suspension Insulator
Ultimate Cantilever Load - for Line Post or Station Post Insulators

35
INSULATOR TESTING
TYPE TESTS
Tests carried out to prove conformity with the specification. These are intended to prove the
general qualities and design of a given type of insulator.
ACCEPTANCE TESTS
Tests carried out on samples taken from the lot for the purpose of acceptance of the lot.
ROUTINE TESTS
Tests carried out on each insulator to check requirements which are likely to vary during
production.
According to the British Standard, the electrical insulator must undergo the following tests
1. Flashover tests of insulator
2. Performance tests
3. Routine tests
FLASHOVER TEST
There are mainly four types of flashover test performed on insulators-
POWER FREQUENCY DRY FLASHOVER TEST
1. First the insulator to be tested is mounted in the manner in which it would be used practically.
2. Then terminals of variable power frequency voltage source are connected to the both electrodes
of the insulator.
3. Now the power frequency voltage is applied and gradually increased up to the specified value.
This specified value is below the minimum flashover voltage.
4. This voltage is maintained for one minute and observes that there should not be any flash-over or
puncture occurred.
The insulator must be capable of sustaining the specified minimum voltage for one minute without
flash over.
POWER FREQUENCY WET FLASHOVER TEST OR RAIN TEST
1. In this test also the insulator to be tested is mounted in the manner in which it would be used
practically.
2. Then terminals of variable power frequency voltage source are connected to the both electrodes
of the insulator.
3. After that the insulator is sprayed with water at an angle of 45o
in such a manner that its
precipitation should not be more 5.08 mm per minute. The resistance of the water used for
spraying must be between 9 kΩ 16 11 kΩ per cm3
at normal atmospheric pressure and
temperature. In this way we create artificial raining condition.
4. Now the power frequency voltage is applied and gradually increased up to the specified value.
5. This voltage is maintained for either one minute or 30 second as specified and observe that there
should not be any flash-over or puncher occurred. The insulator must be capable of sustaining
the specified minimum power frequency voltage for specified period without flash over in the
said wet condition.
POWER FREQUENCY FLASHOVER VOLTAGE TEST
1. The insulator is kept in similar manner of previous test.
2. In this test the applied voltage is gradually increased in similar to that of previous tests.
3. But in that case the voltage when the surroundings air breaks down is noted.

36
IMPULSE FREQUENCY FLASHOVER VOLTAGE TEST
The overhead outdoor insulator must be capable of sustaining high voltage surges caused by
lightning etc. So this must be tested against the high voltage surges.
1. The insulator is kept in similar manner of previous test.
2. Then several hundred thousand Hz very high impulse voltage generator is connected to the
insulator.
3. Such a voltage is applied to the insulator and the spark over voltage is noted.
4. The ratio of this noted voltage to the voltage reading collected from power frequency flash over
voltage test is known as impulse ratio of insulator.
This ratio should be approximately 1.4 for pin type insulator and 1.3 for suspension type insulators.
PERFORMANCE TESTS
TEMPERATURE CYCLE TEST
1. The insulator is first heated in water at 70o
C for one hour.
2. Then this insulator immediately cooled in water at 7o
C for another one hour.
3. This cycle is repeated for three times.
4. After completion of these three temperature cycles, the insulator is dried and the glazing of
insulator is thoroughly observed. After this test there should not be any damaged or deterioration
in the glaze of the insulator surface.
PUNCTURE VOLTAGE TEST
1. The insulator is first suspended in insulating oil.
2. Then voltage of 1.3 times of flash over voltage is applied to the insulator.
A good insulator should not puncture under this condition.
POROSITY TEST
1. The insulator is first broken into pieces.
2. Then these broken pieces of insulator are immersed in a 0.5 % alcohol solution of fuchsine dye
under pressure of about 140.7 kg ⁄ cm2
for 24 hours.
3. After that the sample is removed and examined.
The presence of a slight porosity in the material is indicated by a deep penetration of the dye into
it.
MECHANICAL STRENGTH TEST
The insulator is applied by 2½ times the maximum working strength for about one minute. The
insulator must be capable of sustaining this much mechanical stress for one minute without any
damage in it.
ROUTINE TESTS
Each of the insulators must undergo the following routine test before they are recommended for
using at site.

37
PROOF LOAD TEST
In proof load test of insulator, a load of 20% in excess of specified maximum working load is applied
for about one minute to each of the insulator.
CORROSION TEST
1. The insulator with its galvanized or steel fittings is suspended into a copper sulphate solution for
one minute.
2. Then the insulator is removed from the solution and wiped, cleaned.
3. Again it is suspended into the copper sulphate solution for one minute.
4. The process is repeated for four times.
Then it should be examined and there should not be any disposition of metal on it.

38

Transmission line insulators

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (11)

Similar to Transmission line insulators

Similar to Transmission line insulators (20)

Recently uploaded

Recently uploaded (20)

Transmission line insulators