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Chapter – 4
Insulators
Introduction
•Overhead line conductors should be
supported on the poles or towers in such a
way that currents from conductors do not
flow to earth through supports. This is
achieved by securing line conductors to
supports with the help of insulators.
•Insulators provide necessary insulation
between line conductors and supports and
thus prevent any leakage current from
conductors to earth.
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Desirable Properties of Insulators
1)High mechanical strength to withstand
conductor load, wind load etc.,
2)High electrical resistance to avoid leakage
currents to earth.
3)High relative permittivity so that the
dielectric strength is high.
4)Insulator material should be non-porous, free
from impurities and cracks otherwise the
permittivity will be lowered.
5)High ratio of puncture strength to flashover.
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•Most commonly used material is
porcelain but glass, steatite and special
composition materials are also used to a
limited extent.
•Porcelain is produced by firing at a high
temperature a mixture of kaolin, feldspar
and quartz. It is stronger mechanically
than glass, gives less trouble from
leakage and is less effected by changes
of temperature.
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Types of Insulators
•Successful operation of an overhead line
depends to a considerable extent upon
the proper selection of insulators.
•There are several types of insulators but
the most commonly used are pin type,
suspension type, strain insulator and
shackle insulator.
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1)Pin type insulators
Part section of a pin type insulator is shown
Pin type insulators...
• Pin type insulator is secured to the
cross-arm on the pole.
• There is a groove on the upper end of
the insulator for housing the
conductor.
• Conductor passes through this groove
and is bound by the annealed wire of
the same material as the conductor.
Pin type insulators...
These insulators are used
for the transmission and
distribution of electric
power up to 33kV.
Beyond 33kV, pin type
insulators become too
bulky & uneconomical.
Causes of insulator failure
• Insulators are required to withstand
both mechanical & electrical stresses.
• Electrical stress is primarily due to line
voltage and may cause the breakdown
of the insulator.
• Electrical breakdown of the insulator
can occur either by flash-over or
puncture.
Causes of insulator failure...
In flashover, an arc
occurs
between
the line conductor
and insulator pin
(i.e., earth) and
discharge
jumps
across the air gaps,
following shortest
distance (a+b+c).
Causes of insulator failure...
• 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, discharge occurs
from conductor to pin through the
body of the insulator in which case, the
insulator is permanently destroyed due
to excessive heat.
Causes of insulator failure...
• In practice, sufficient thickness of
porcelain is provided in the insulator
to avoid puncture by the line voltage.
• Ratio of puncture strength to flashover
voltage is known as safety factor i.e.,
Puncture strength
Safety factor of insulator 
Flash - over voltage
 10 for pin type insulators
2) Suspension type insulators
Usual practice to use suspension type insulators
for high voltages (>33kV).
Suspension type insulators...
• Number of porcelain discs are connected in
series by metal links in the form of a string.
• Conductor is suspended at the bottom end
of this string while the other end of the
string is secured to the cross-arm of tower.
• Each disc is designed for low voltage, say
11kV. Number of discs in series will depend
upon the working voltage – 6 discs for 66kV.
Suspension type insulators...
Advantages:
1) Cheaper than pin type insulators for
voltages beyond 33 kV.
2) Each disc of suspension type insulator is
designed for low voltage, usually 11 kV.
Desired number of discs can be connected
in series depending upon the working
voltage.
Suspension type insulators...
3) Whole string does not become useless
because only the damaged disc can be
replaced by the good one.
4) 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.
Suspension type insulators...
5) Additional insulation required for the raised
voltage can be easily obtained in the
suspension arrangement by adding the
desired number of discs.
6) 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.
3) Strain insulators
• When there is a dead end of the line or there
is corner or sharp curve, the line is subjected
to greater tension which can be relieved by
using strain insulators.
• For low voltage lines (<11kV), shackle
insulators are used as strain insulators.
• Strain insulator for HV transmission lines
consists of an assembly of suspension
insulators.
Strain insulators...
• The discs of strain
insulators are used
in the vertical plane.
• When the tension in
lines is exceedingly
high, as at long river
spans, two or more
strings are used in
parallel.
4) Shackle insulators
• Shackle insulators were used as strain
insulators earlier but now, they are
frequently used for LV distribution lines.
• These insulators can be used either in a
horizontal position or in a vertical position.
• These insulators can be directly fixed to the
pole with a bolt or to the cross arm.
Shackle insulators...
• The fig. shows a
shackle insulator
fixed to the
pole.
• The conductor
in the groove is
fixed with a soft
binding wire.
Potential Distribution over
Suspension Insulator String
• A string of suspension
insulators consists of a
number of porcelain discs
connected in series through
metallic links.
• Fig. shows 3-disc string of
suspension insulators.
Potential Distribution over
Suspension Insulator String...
Porcelain portion of each disc is in
between two metal links and
hence forms a capacitor C as
shown in Fig. This is known as
mutual capacitance or selfcapacitance. Charging current I is
same through all discs and hence
voltage across each unit is same
i.e., V/3 as shown.
Potential Distribution over
Suspension Insulator String...
• Capacitance
also
exists
between metal fitting of each
disc and tower or earth in
actual practice. This is known
as shunt capacitance C1.
• Due to this shunt capacitance,
different current flows through
all the discs of the string
leading to different voltages.
Observations regarding the potential
distribution
1)Voltage impressed on a string of suspension
insulators does not distribute itself uniformly
across the individual discs due to the presence
of shunt capacitance.
2)Disc nearest to the conductor has maximum
voltage across it. As we move towards the
cross-arm, the voltage across each disc goes
on decreasing.
Observations regarding the potential
distribution...
3) Unit nearest to the conductor is under
maximum electrical stress and is likely to be
punctured. Thus, the potential across each
unit need to be equalized.
4) If the voltage impressed across the string
were DC, then voltage across each unit
would be the same. This is because the
insulator capacitances are ineffective for DC.
String Efficiency
• Voltage applied across the string of
suspension insulators is not uniformly
distributed across various units or discs.
• The disc nearest to the conductor has much
higher potential than the other discs.
• This unequal potential distribution is
undesirable and is usually expressed in
terms of string efficiency.
String Efficiency...
• The ratio of voltage across the whole string
to the product of number of discs and the
voltage across the disc nearest to the
conductor is known as string efficiency i.e.,
Voltage across the string
n  Voltage across disc nearest to conductor
n = number of discs in the string
String efficiency 
where
• The greater the string efficiency, the more
uniform is the voltage distribution.
Mathematical expression
• Fig. shows the equivalent
circuit for a 3-disc string.
• Let the self capacitance of
each disc be C.
• Further, let the shunt
capacitance C1 be some
fraction
K
of
self
capacitance i.e., C1 = KC.
Mathematical expression...
• Starting from the cross-arm or tower, the
voltage across each unit is V1, V2 and V3
respectively as shown.
• Applying Kirchhoff’s current law to node A,
we get,
I2  I1  i1 or V2C  V1C  V1C1
or

V2C  V1C  V1 KC
V2  V1 1  K 

1
Mathematical expression...
• Applying Kirchhoff’s current law to node B,
we get,
I3  I2  i2 or V3C  V2C  V1  V2  C1
or
V3C  V2C  V1  V2   KC
or
V3  V2  V1  V2  K  V1K  V2 1  K 
2

 V1 K  1  K  




V3  V1 1  3K  K
2

V2  V1 1  K 

2 
Mathematical expression...
• Voltage between conductor and earth (i.e.,
tower) is
V  V1  V2  V3

 V1  V1 1  K   V1 1  3K  K

 V1 3  4K  K 2


2

V  V1 1  K  3  K   3  & hence,
Voltage across top unit, V1 
V
1  K  3  K 
Mathematical expression...
• Voltage across second unit from top,
V2 = V1 (1 + K)
• Voltage across third unit from top,
V3 = V1 (1 + 3K + K2)
% String efficiency
Voltage across the string

 100
n  Voltage across disc nearest to conductor
1  K  3  K 

V

 100 
3  V3
3 1  3K  K 2


Methods of improving String
Efficiency
The potential distribution in a string of
suspension insulators is not uniform
The maximum voltage appears across the
insulator nearest to the line conductor
 Decreases progressively as the cross arm is
approached
 The insulation of the insulator nearest to the
line conductor is stressed to the highest)
 This causes its break down or flash over
 The breakdown of other units will take
place in succession
 This necessitates to equalize the potential
across the various units of the string i.e. to
improve the string efficiency
Methods to improve the string efficiency:
1) By using longer cross-arms:
 The value of string efficiency depends upon the value of K i.e., ratio of shunt
capacitance to mutual capacitance
 The lesser the value of K, the greater is the string efficiency and more
uniform is the voltage distribution
 The value of K can be decreased by reducing the shunt capacitance
In order to reduce shunt capacitance longer
cross-arms should be used.
Limitations of cost and strength of tower do
not allow the use of very long cross-arms
 In practice, K = 0·1 is the limit that can be
achieved by this method.
2) By grading the insulators:
 Insulators of different dimensions are so
chosen that each has a different
capacitance
 The insulators are capacitance graded
 The string is assembled in such a way that
the top unit has the minimum capacitance,
Increasing progressively as the bottom unit
(i.e., nearest to conductor) is reached
As voltage is inversely proportional to
capacitance, this method tends to equalize
the potential distribution across the units in
the string
This method has the disadvantage that a
large number of different-sized insulators
are required
3) By using a guard ring
A guard ring which is a metal ring electrically
connected to the conductor and surrounding the
bottom insulator as shown
The idea is to cancel exactly the pin to tower
(shunt capacitance) charging currents
The guard ring introduces capacitance between
metal fittings and the line conductor
 The guard ring is designed in such a way that shunt
capacitance currents i1, i2 etc. are equal to metal
fitting line capacitance currents i1’, i2’ etc.
Consequently the same charging current I flows
through each unit of the string
There will be uniform potential distribution across
the units.
Testing of Insulators
Three categories of tests:
1) Flash-over Tests:
a) 50 per cent dry impulse flash-over test.
b) Dry flash-over and dry one-minute test.
c) Wet flash-over and one-minute rain test.
a) 50 per cent dry impulse flash-over test
 The test is made on a clean insulator
 The impulse generator delivers at least 20 positive 1/50
microsecond impulse wave
 50% of the positive impulses applied cause flash-over of
the insulator
 Next, about 20 negative 1/50 impulse is applied
 50% of the positive impulses applied cause
flash-over of the insulator
In each case the insulator must not be damaged
b) Dry flash-over and dry one-minute test

A voltage of power frequency is applied to a clean
insulator
The voltage is gradually increased up to the
specified value
This voltage is maintained for one minute.
 The voltage is then increased gradually until flashover occurs.
 The insulator is then flashed-over at least
four more times
 The voltage is raised gradually to reach
flash-over in about 10 seconds
 The insulator must not be damaged
c) Wet flash-over and one-minute rain test
 The insulator is sprayed with water at an angle
of 45 degrees
 The temp. of the water is within 10°C of the
ambient temperature in the neighborhood
 The insulator must withstand the test-voltage
specified for one minute
The voltage is then gradually raised until flashover occurs
 The insulator is then flashed at least four more
times
The time taken to reach the flashover voltage
being in about 10 seconds.
2) Sample Tests:
a) Temperature-cycle test
b) Mechanical test
c) Electro-mechanical test
d) Puncture test
e) Porosity test
a) Temperature-cycle test
 The insulator is subjected three times to the
following temperature cycle
 Immersed for T minutes in a water-bath at
70° C
 The main water bath is at a lower
temperature
 Next, taken out, and immersed as quickly in
a main water bath and left in this bath for T
minutes.
The insulator must withstand this
series of tests without damage to the
porcelain or glaze.
b)
Mechanical Tests
Applied to pin insulators and line post insulators.
 The test is a bending test, in which a load of three
times the specified maximum working load (twice
for a post insulator) is applied for one minute.
There must be no damage to the insulator,
In the case of the post insulator, the load is then
raised to three times
 There must be no damage to the insulator or its
pin (or pins)
c) Electro-mechanical test

Applied to suspension or tension units only
 the insulator is mechanically stressed to a tension of
2.5 times the specified maximum working load
 this being maintained for one minute.
Simultaneously, 75 per cent. of the dry spark-over
voltage is applied.
 Simultaneously 75% of the dry spark-over voltage is
applied
 The insulator should not be damaged
d) Puncture test
The insulator is completely immersed in insulating
oil at room temperature
 The voltage raised as rapidly as is consistent with
correct measurement
 The insulator must not be punctured