COLLEGE OF SCIENCE
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EPE3262_ Transmission and Distribution
 The line insulators, and their mechanical properties;
 The surges; causes; effects and protections
 the corona and corona losses; interference between
communication lines.
 the sag and tension of a given profile in electrical power line
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Insulators
What is an insulator and It’s purpose in an overhead power lines?
•The 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 i.e., line conductors must be properly insulated from
supports. This is achieved by securing line conductors to supports with
the help of insulators.
•The insulators provide necessary insulation between line conductors
and supports and thus prevent any leakage current from conductors to
earth.
•Used to provide necessary clearance between conductors and metal
works , the conductors are attached to insulators.
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The insulators should have the following desirable properties :
(i) High mechanical strength in order to withstand conductor load,
wind load etc.
(ii) High electrical resistance of insulator material in order to avoid
leakage currents to earth.
(iii) High relative permittivity of insulator material in order that
dielectric strength is high.
(iv) The insulator material should be non-porous (not allowing liquid
or air to pass through it), free from impurities and cracks otherwise
the permittivity will be lowered.
(v) High ratio of puncture strength to flashover.
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•
The most commonly used material for insulators of overhead line
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
• The most suitable materials for insulators are:
Toughened glass and
Porcelain
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Toughened glass:
Can withstand great tension.
Glass is cheaper than porcelain in
simpler shapes.
Used very rarely.
For lines below 25kV with short
spans.
Porcelain:
Mechanically stronger than glass
Less affected by changes in
temperature
Most commonly used material for
manufacturing of insulators.
used for high-voltage power
transmission
Toughened glass
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Porcelain
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Types of Insulators
•The successful operation of an overhead line depends to a
considerable extent upon the proper selection of insulators.
•There are several types of insulators The most commonly
used are pin type, suspension type, strain insulator and
shackle insulator.
 Pin type insulators:
o Consists of non- conducting material such as glass, porcelain
, wood, polymer and plastic.
o Small , simple in construction and cheap.
o For higher voltages they tend to be more heavy and costly.
The cost of pin type insulator increases rapidly as the working
voltage is increased.
o Pin insulator are used for transmission and distribution of
electric power at voltage up to 33kv
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 Suspension type insulators:
•Used for lines above 33kV
•Also known as disc or string insulators.
•Consists of porcelain discs mounted one above the other.
•The conductor is suspended below the point of support by an insulator
string.
•They consist of a number of porcelain discs connected in series by metal
links in the form of a string.
•The conductor is suspended at the bottom end of this string while the other
end of the string is secured to the cross-arm of the tower.
•This type of insulator is not economical beyond 33kv.
•Each unit or disc is designed for low voltage, say 11 kV. The number of
discs in series would obviously depend upon the working voltage.
For instance, if the working voltage is 66 kV, then six discs in series will be
provided on the string.
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Suspension type insulators
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Advantages
(i) Suspension type insulators are cheaper than pin type insulators for
voltages beyond 33 kV.
(ii) Each unit or disc of suspension type insulator is designed for low voltage,
usually 11 kV. Depending upon the working voltage, the desired number of
discs can be connected in series.
(iii) If any one disc is damaged, the whole string does not become useless
because the damaged disc can be replaced by the sound one.
(iv) 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.
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Cont….Advantages…….
(v) 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. Therefore, the
additional insulation required for the raised voltage can be easily
obtained in the suspension arrangement by adding the desired
number of discs.
(vi) 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.
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 Strain Insulators:
•Similar to suspension type insulators.
•When there is a dead end of the line or there is corner or sharp curve, the line is
subjected to greater tension.
•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, as at long river spans, two or more strings are used in
parallel.
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 Shackle Insulator :
•Now a days, shackle insulators are frequently
used for low voltage distribution lines.
•Such insulators can be used either in a
horizontal position or in a vertical position.
•They can be directly fixed to the pole with a bolt
or to the cross arm.
•They can be directly fixed to
the pole with a bolt or to the
cross arm.
Fig. Shackle Insulator
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 The surges; causes; effects and protections
 What is surges in power system?
• Electrical devices are designed to handle specific
voltages. Unfortunately, in cases where devices
are subject to higher voltages than they are
designed for, damage can occur.
• A power surge is:
 an abnormally high voltage lasting for a short
period of time
 a transient wave of voltage, current or power in
an electrical circuit.
• This is usually a sub-cycle overvoltage lasting less than a half-cycle of the
regular voltage waveform, and can either be additive or subtractive with
positive or negative polarity. https://www.sunpower-uk.com/glossary/what-is-a-power-surge/
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• This is usually an oversupply of the voltage from
a source such as mains supply or generator and
lasting only for a few microseconds. Typical
surges may last for 50 microseconds and even
though it’s only a very short duration, they surge
can reach 6000 Volts and 3000Amps by the
time they arrive at the powered equipment
 Causes of power surges
• Surges are produced by various events such as equipment faults, lightning
discharges, capacitor bank building and load switching. The ability of the
equipment to withstand the transients greatly impact on its reliability.
• There are different types of power surges based on the sources and voltages
involved. A surge can range from five to ten volts when a hair dryer is turned
on or several thousand volts when a lightning strikes a transformer
https://www.sunpower-uk.com/glossary/what-is-a-power-surge/
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 Effects of power surges
• The power surges, which are brief overvoltage disturbances or spikes may
reach amplitudes of as high as tens of thousands of volts, and can
degrade, damage, or destroy electronics equipment within an industrial or
manufacturing facility, or within a home or commercial building or it can
damage the lines and equipment’s connected to the system
• Surges are common causes of failures in electronics and computer
equipment.
• Low level surges won’t cause instant damage such as blowing fuses or
melting electronic components, instead it causes gradual degradation of
the components until they completely fail. The larger power surges cause
instantaneous damage, blowing up fuses, semiconductors, resistor and
capacitors
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 How to protect equipment from electrical power surges?
The surges can be suppressed before reaching the equipment. There are
different methods that can be used depending on magnitude of expected
surges and the powered equipment. The most common method is the use of
surge protective devices (SPDs).
The SPDs limit the transient overvoltage from reaching the equipment, hence
preventing damage to the equipment. They divert the surge to an alternative
route such as the ground and hence drive the excess voltage away from the
powered equipment.
These components vary in their:
The common devices used are: •Effectiveness
•Reliability
•Metal Oxide Varistor (MOV)
•Availability
•Silicon Avalanche diode (SAD)
•Energy capability
•Gas Discharge Tubes (GDT)
•Cost
https://www.sunpower-uk.com/glossary/what-is-a-power-surge/
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 the corona and corona losses;
interference between communication
lines.
 the sag and tension of a given profile in
electrical power line
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 Corona in an overhead transmission line
https://www.electricaltechnology.org/2018/02/corona-effect-discharge-transmissionlines-power-system.html
https://muniracademy.com/corona-effect/
• Electric power transmission practically deals with the bulk transfer of
electrical energy, from generating stations situated many kilometers away
from the main consumption centers or the cities
• A power system transmission lines are separated by certain spacing when
potential difference is applied across the power conductors in transmission
lines then air medium present between the phases of the power conductors
acts as insulator medium however the air surrounding the conductor subjects
to electro static stresses .
• When the potential increases still further then the atoms present around
the conductor starts ionize
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……………….Corona in an overhead transmission line
• Corona was electrical discharge emerge around overhead line conductor,
due to air flow where would disturb radio waves and creating lost power.
• When a normal ac voltage is applied across two conductors with enough
spacing between them, there is no change in the atmospheric conditions
surrounding the conductors. But if the voltage exceeds a particular limiting
value, then the air surrounding the conductors will gets ionized and
luminous glow (weak purple color) will rise with hissing sound. This
phenomena is called corona.
• Corona is the phenomenon in which hissing sound, violet glow and
production of ozone takes place in an overhead transmission line. It is not
necessary that the violet glow appears, but the hissing sound is always
present in Corona. The effect which is the cause of corona is known as
Corona Effect.
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Further increase in the potential difference in the transmission lines then a
faint luminous glow of violet color appears together along with hissing noise.
This phenomenon is called corona
The corona discharge
around a high voltage coil.
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Large corona discharge (white) around
conductors energized by a 1.5 million volt
transformer in a laboratory.
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• Corona is a type of localised emission resulting from transient gaseous
ionisation in an insulation system when the voltage stress i.e., voltage
gradient exceeds a critical value,is also known a partial discharge .
• Corona can also occur within voids in insulators as well as at the
conductor/insulator interference
• The corona effect in power system has a predominant role in reducing the
efficiency of EHV (extra high voltage lines).
• During wet and humid conditions, water drops collect on the conductors
and increase corona activity. under these conditions , a crackling or
humming sound may be heard in the immediate vicinity of the line.
• Then the ions produced in this process repel with each other and attracts
towards the conductor at high velocity which intern produces other ions by
collision. The ionized air surrounding the conductor acts as a virtual
conductor and increases the effective diameter of the power conductor.
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The phenomenon of corona is affected by the physical state of the
atmosphere as well as by the conditions of the line
(i) Atmosphere. As corona is formed due to ionization of air surrounding
the conductors, therefore, it is affected by the physical state of
atmosphere. In the stormy weather, the number of ions is more than
normal and as such corona occurs at much less voltage as compared
with fair weather.
(ii) Conductor size. The corona effect depends upon the shape and
conditions of the conductors. The rough and irregular surface will give
rise to more corona because unevenness of the surface decreases the
value of breakdown voltage. Thus a stranded conductor has irregular
surface and hence gives rise to more corona that a solid conductor.
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…………Factor Affecting Corona
(iii) Spacing between conductors. If the spacing between the conductors
is made very large as compared to their diameters, there may not be any
corona effect. It is because larger distance between conductors reduces
the electro-static stresses at the conductor surface, thus avoiding corona
formation.
(iv) Line voltage. The line voltage greatly affects corona. If it is low,
there is no change in the condition of air surrounding the conductors and
hence no corona is formed. However, if the line voltage has such a
value that electrostatic stresses developed at the conductor surface
make the air around the conductor conducting, then corona is formed.
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The phenomenon of corona plays an important role in the design of an
overhead transmission line.
Therefore, it is profitable to consider the following terms much used in the
analysis of corona effects:
(i) Critical disruptive voltage. It is the minimum phase-neutral voltage at
which corona occurs.
(ii) Visual critical voltage. It is the minimum phase-neutral voltage at which
corona glow appears all along the line conductors.
(iii) Power loss due to corona. Formation of corona is always accompanied
by energy loss which is dissipated in the form of light, heat, sound and
chemical action.
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Consider two conductors of radii r cm and spaced d cm apart. If V is the
phase-neutral potential, then potential gradient at the conductor surface is
given by:
In order that corona is formed, the value of g must be made equal to the
breakdown strength of air.
The breakdown strength of air at 76cm pressure and temperature of 25ºC
is 30 kV/cm (max) or 21·2 kV/cm (r.m.s.). In order that corona is formed,
the value of g must be made equal to the breakdown strength of air.
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If Vc is the phase-neutral potential required under these conditions, then,
The above expression for disruptive voltage is under standard conditions
i.e., at 76 cm of Hg and 25ºC.
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If these conditions vary, the air density also changes, then it will affect
corona, thus altering the value of go. The value of go is directly proportional
to air density.
Thus the breakdown strength of air at a barometric pressure of b cm of
mercury and temperature of tºC becomes δ go where
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Correction must also be made for the surface condition of the conductor.
This is accounted for by multiplying the above expression by irregularity
factor mo.
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(ii) Visual critical voltage. It is the minimum phase-neutral voltage at which
corona glow appears all along the line conductors. It has been seen that in
case of parallel conductors, the corona glow does not begin at the disruptive
voltage Vc but at a higher voltage Vv, called visual critical voltage. The
phase-neutral effective value of visual critical voltage is given by the
following empirical formula :
where mv is another irregularity factor having a value of 1·0 for polished
conductors and 0·72 to 0·82 for rough conductors.
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(iii) Power loss due to corona. Formation of corona is always accompanied
by energy loss which is dissipated in the form of light, heat, sound and
chemical action. When disruptive voltage is exceeded, the power loss due
to corona is given by :
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Advantages and disadvantages of Corona
 Advantages
(i) Due to corona formation, the air surrounding the conductor becomes
conducting and hence virtual diameter of the conductor is increased. The
increased diameter reduces the electrostatic stresses between the
conductors.
(ii) Due to lightning and switching, surges are produced, thus Corona
reduces the effects of transients produced by surges.
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Disadvantages
(i) Corona is accompanied by a loss of energy. This affects the transmission
efficiency of the line.
(ii) Ozone is produced by corona and may cause corrosion of the conductor
due to chemical action.
(iii) The current drawn by the line due to corona is non-sinusoidal and hence
non-sinusoidal voltage drop occurs in the line. This may cause inductive
interference with neighbouring communication lines.
Radio Interference:
Corona produces radiations which create disturbance in the radio waves, this
is known as Radio Interference. The signal affected by it can be television,
radio or communication signal. It affects the area of few kilometers
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Methods of Reducing Effect of Corona
careful design should be made to avoid corona on the sub-stations or busbars rated for 33 kV and higher voltages otherwise highly ionised air may
cause flash-over in the insulators or between the phases, causing
considerable damage to the equipment.
The corona effects can be reduced by the following methods :
1. By increasing the spacing between the conductors: Corona Discharge
Effect can be reduced by increasing the clearance spacing between the
phases of the transmission lines .
However increase in the phases results in heavier metal supports. Cost and
Space requirement increases.
2. By increasing the diameter of the conductor: Diameter of the conductor
can be increased to reduce the corona discharge effect. By using hollow
conductors corona discharge effect can be improved.
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……………Methods for reducing corona:
3. By using Bundled Conductors:
By using Bundled Conductors also corona effect can be reduced this is because
bundled conductors will have much higher effective diameter compared to the
normal conductors.
4. By Using Corona Rings or Grading Rings:
This now does not have much significance as Corona Rings or Grading Rings
are present on the surge arresters to equally distribute the potential along the
Surge Arresters or Lightning Arresters which are present near the Substation
and in the Transmission lines.
Bundled
Conductors
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1. Determine the critical descriptive voltage and corona loss for a 3-phase line
operating at 110kV which has a conductor of 1.25 cm diameter arranged in a
3.05metre delta. Assume air density factor of 1.07 and the dielectric strength of air
to be 21kV/cm.
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2. A 3-phase line has conductors 2 cm in diameter spaced equilaterally 1
m apart. If the dielectric strength of air is 30 kV (max) per cm, find the
disruptive critical voltage for the line. Take air density factor δ = 0·952 and
irregularity factor mo = 0·9.
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3. A 3-phase, 220 kV, 50
Hz transmission line
consists of 1·5 cm radius
conductor spaced 2 metres
apart in equilateral
triangular formation. If the
temperature is 40ºC and
atmospheric pressure is 76
cm, calculate the corona
loss per km of the line.
Take mo = 0·85.
Solution
the corona loss is given by :
Review exercises found in the book of
Principles of power systems, by Mehta
pages 184-186.
Available in UR/CST library
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 Sag an Overhead Lines
• Erecting an overhead line, it is very
important that conductors are under
safe tension.
• If the conductors are too much stretched
between supports in a bid to save
conductor material, the stress in the
conductor may reach unsafe value and
in certain cases the conductor may
break due to excessive tension.
In order to permit safe tension in the conductors, they are not fully stretched
but are allowed to have a dip or sag.
• The difference in level between points of supports and the lowest point on
the conductor is called sag.
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A conductor suspended between two equilevel supports A and B.
The conductor is not fully stretched but is
allowed to have a dip. The lowest point
on the conductor is O and the sag is S.
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• Conductor sag and tension is an important consideration in the
mechanical design of overhead lines.
• The conductor sag should be kept to a minimum in order to reduce the
conductor material required and to avoid extra pole height for sufficient
clearance above ground level
• It is also desirable that tension in the conductor should be low to avoid
the mechanical failure of conductor and to permit the use of less strong
supports
• The tension on the conductor depends on the following factors:
 Weight of the conductor
 Wind effects
 Ice loading
 Temperature variation
clearance above ground level
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(i) When supports are at equal levels.
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 Consider a conductor between two equilevel supports A and B with O as
the lowest point.
 It can be proved that lowest point will be at the mid-span. Refer fig.below in
which ‘O’ is the lowest point of the conductor spacing.
• L=length of span in meters
• W=weight of the conductor per unit length
• T=tension on the conductor
• Consider any point ‘P’ on the conductor
whose coordinate are ‘x’ and ‘y’.
• There are two external forces acting on the
portion ‘OP
1. Tension T
2. The weight wx which acts at a distance of
x/2 from the point O or P, as OP=x acting
downward
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Equating the moments of above two forces about point O,
we get,
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We can see this condition at
(ii) When supports are at unequal levels.
hilly areas.
• Conductor suspended between two supports A and B which are at different
levels.
The lowest point on the conductor is O.
Let
l = Span length
h = Difference in levels between two supports
x1 = Distance of support at lower level (i.e., A)
from O
x2 = Distance of support at higher level (i.e. B)
from O
T = Tension in the conductor
If w is the weight per unit length of the conductor,
then,
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If w is the weight of conductor
per unit length of the
conductor, then,
𝑤𝑥 2
2𝑇
From 𝑦 =
by replacing y=S1 or S2, and X= X1
or X2 , we get
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Effect of wind and ice loading.
• The above formulae for sag are true only in still air and at normal temperature
when the conductor is acted by its weight only.
• In actual practice, a conductor may have ice coating and simultaneously
subjected to wind pressure
The weight of ice acts vertically
downwards i.e., in the same
direction as the weight of
conductor. The force due to the
wind is assumed to act
horizontally i.e., at right angle to
the projected surface of the
conductor.
Hence, the total force on the conductor is the vector sum of horizontal and
vertical forces as shown in this
fig.
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Example: A 132 kV transmission line has the following data : Wt. of conductor
= 680 kg/km ; Length of span = 260 m Ultimate strength = 3100 kg ; Safety
factor = 2 Calculate the height above ground at which the conductor should be
supported. Ground clearance required is 10 metres.
clearance above ground level
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Solve example found in
the book of Principle of
power
system
by
Mehta page 190-198
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 Some Mechanical Principles
Some important points in the mechanical design of
overhead transmission lines.
(i) Tower height
Tower height depends on the length of the span.
With long spans, relatively few towers are required
but they must be tall and costly. It is not usually
possible to determine the tower height and span
length on the basis of direct construction costs
because the lightning hazards increase greatly as
the height of the conductor is increased.
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(ii) Conductor clearance to ground level (h1):
The conductor clearance to ground at the time of greatest
sag should not be less than some specified distance
(usually between 6 and 12 m), depending on the voltage,
on the nature of the country and on the local laws.
The greatest sag may occur on the hottest day of summer
on account of the expansion of the wire or it may occur in
winter owing to the formation of a heavy coating of ice on
the wires. Special provisions must be made for melting ice
from the power lines.
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(iii) Sag and tension :
When laying overhead transmission lines, it is necessary to allow a
reasonable factor of safety in respect of the tension to which the conductor
is subjected. The tension is governed by the effects of wind, ice loading
and temperature variations. The relationship between tension and sag is
dependent on the loading conditions and temperature variations. For
example, the tension increases when the temperature decreases and there
is a corresponding decrease in the sag. Icing-up of the line and wind
loading will cause stretching of the conductor by an amount dependent on
the line tension
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(iv) Stringing charts : For use in
the field work of stringing
conductors, temperature-sag
temperature tension charts
plotted for the given conductor
loading conditions.
the
and
are
and
These charts are very helpful
while stringing overhead lines.
Sag
Tension
(v) Conductor spacing :
Spacing of conductors should be able to provide safety against flash-over
when the wires are swinging in the wind. The proper spacing is a function of
span length, voltage and weather conditions. The use of horizontal spacing
eliminates the danger caused by unequal ice loading. Small wires or wires of
light material are subjected to more swinging by the wind than heavy
conductors. Therefore, light wires should be given greater spacing.
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(vi) Conductor vibration : Wind exerts pressure on the exposed surface of the
conductor. If the wind velocity is small, the swinging of conductors is harmless
provided the clearance is sufficiently large so that conductors do not approach
within the sparking distance of each other. A completely different type of
vibration, called dancing, is caused by the action of fairly strong wind on a wire
covered with ice, when the ice coating happens to take a form which makes a
good air-foil section. Then the whole span may sail up like a kite until it
reaches the limit of its slack, stops with a jerk and falls or sails back. The
harmful effects of these vibrations occur at the clamps or supports where the
conductor suffers fatigue and breaks eventually. In order to protect the
conductors, dampers are used.
Review exercises found in the book of Principles of power systems, by Mehta pages 200-201.
Available in UR/CST library
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End of lecturer 3
Any question(s) ??????
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𝑤𝑥 2
𝑦=
2𝑇
GMR=0.7788r
(ε0 = 8.85 x 10-12 F/m)