Conductor Materials

• The conductor is one of the most important items in a transmission and

distribution systems of electric power, the cost of the conductor material
accounts for a major part of the total cost. So proper choice of conductor
material and size of the conductor is of utmost importance.
• The conductor material should have the following characteristics:
1. High electrical conductivity i.e. low specific resistance.
2. High tensile strength in order to with stand the mechanical stresses.
3. Low specific gravity in order to give low weight per unit volume.
4. Low cost in order to be used over long distances .
5. Easy availability.
6. Should not be brittle.
• No single conductor material meets all of the above requirements. Hence
a compromise will have to be made between the cost and the desired
electrical and mechanical properties in the selection of a conductor
material for a given situation.
• The most commonly used materials are copper, aluminum and their
alloys.
 Stranded Conductors
• All conductors are preferably stranded in order to increase flexibility as
well as the mechanical strength of the conductor.
• Usually a central wire is surrounded by successive layers of wires
containing 6, 12, 18, 24,……wires.
• If there are n layers of strands of equal diameter in a circular strand
formation with one central strand, the following general formulae are
applicable:
Total number of conductors in a strand of n layers = 1+ 3n(1+n)
Overall diameter of stranded conductor with n layers, D = (1+2n)d
Where d is the diameter of each strand. Thus a 7 strand conductor has a
central strand with 6 outers; the 19 strand conductor has central strand
with 6 strands in the first layer and 12 strands in next layer.
 Stranded Conductors Contd….
• The conductor size is specified by its equivalent copper cross sectional
area and the number of strands with the diameter of each strand. For
example a conductor made up of 19 strands each of diameter 2.9 mm
having equivalent copper area of 130 mm2 is designated as 130 mm2,
19/2.9 mm conductor.
• The equivalent cross-section of a stranded conductor is the area of cross
section of a solid conductor of the same material and length as the
stranded conductor and having the same resistance at the same
temperature. For convenience the conductors are identified by their code
names assigned by the manufacturers.
 Types of Conductors
 Hard-Drawn Copper Conductors
1. Copper for overhead lines is hard- drawn to give a relatively high
tensile strength.
2. It has high electrical conductivity.
3. It does not corrode in normal atmosphere.
4. It has higher current density.
5. It is quite homogeneous, durable and of high scrap value.
6. These conductors are most suitable for distribution work where
spans are short and for service connections to buildings.
 Cadmium Copper Conductor
1. The tensile strength of copper is increased by approximately 50 % by
adding about 0.7 % to 1.0% cadmium to it. The conductivity is
however reduced by 15 to 17 % .
2. This alloy posses the advantage of easy jointing, more resistance to
atmospheric corrosion, better resistance to wear.
 Types of Conductors Contd….
3. Use of these conductors will be economical for a line with long spans
and small cross section i.e. where the cost of conductor material is
comparatively small in comparison to that of supports.
4. The smaller diameters renders these conductors unsuitable to be
used on high voltage lines where corona losses are serious.
 ALL ALUMINIUM CONDUCTOR – AAC
1. AAC conductor is also known as aluminum stranded conductor. AAC
conductor is manufactured from electrolytically refined aluminum,
having purity of minimum 99.5% of aluminium (with minimum
Conductivity of 61%). This is cheaper in cost and lighter in weight
than copper conductors. The tensile strength is less as compared to
copper conductors.
2. Has higher diameter than that of copper and being light its handling,
transportation and erection becomes economical. Corona effect is
reduced due to higher conductor diameter.
3. AAC conductor is used mainly in urban areas because the spacing is
short and the supports are close. It also corrosion resistant and is
thus used very extensively in coastal areas.
 Types of Conductors Contd….
 ALUMINIUM CONDUCTOR STEEL REINFORCED – ACSR
1. ACSR conductor consists of a solid or stranded steel core surrounded
by strands of aluminum.
2. ACSR Conductor has high Tensile Strength and light weight.
3. Variable steel core stranding enables desired strength to be achieved
without sacrificing ampacity.
4. The sag is small and the line can be designed with shorter supports or
longer spans for a given sag.
5. Due to the greater diameter of ACSR Conductor a much higher
corona limit can be obtained causing big advantages on high as well
as extra high voltage Overhead lines.
6. The presence of steel in ACSR conductors creates difficulty in making
dead ends. The other trouble is that of corrosion which is due to an
electrochemical action between aluminium and steel core.

Fig. of ACSR conductor

 Types of Conductors Contd….
 ALL ALUMINIUM ALLOY CONDUCTOR – AAAC
1. AAAC conductor is made from aluminum-magnesium-silicon alloy of
high electrical conductivity containing Magnesium (0.6-0.9%) &
Silicon (0.5-0.9%) to give it better mechanical properties after
treatment.
2. Higher Strength to weight ratio compared to ACSR Conductor.
3. Better sag Characterstics.
4. Better Corrosion resistant than ACSR Conductor.

Construction of AAC & AAAC

Conductors
 Insulators
• Insulators are used to separate line conductor from each other and from the
supporting structures. While designing an insulator the following considerations are
made:
1. The insulator should have high permittivity
2. It should posses high mechanical strength
3. It needs to have a high resistance to temperature changes
4. The leakage current to earth should be minimum to keep corona loss and radio
interference within reasonable limits.
5. The insulator material should not be porous and should be impervious to gases in
atmosphere and should be free from impurity and cracks which may lower the
permittivity.
• the electrical failure of insulators occurs either by puncture or flashover. In the case
of a puncture the arc passes through the body of the insulator rendering it useless.
• Flashover is caused by an arc discharge between conductors and earth through the
air surrounding the insulator. It is either due to line surges or due to the formation of
wet conducting layer over the insulator surface. Here the insulator is not damaged as
was in the case of a puncture.
• Sufficient thickness of the material is provided in the insulator to prevent the
puncture under surge conditions. Flashovers are reduced by increasing the resistance
to leakage currents.
• The length of the leakage path is made large by constructing several layers called
rainsheds. They keep the inner surfaces relatively dry in wet weather conditions and
thus provide sufficient leakage resistance to prevent a flashover.
• The insulator is provided with semi-conducting glaze over its exposed surface to
reduce surface deposition of dirt, smoke ,etc on the surface which may produce
flashover at the operating voltage.
• For satisfactory operation the flashover should occur before puncture. The ratio of
puncture voltage to flashover voltage called factor of safety is kept as high as
possible. The flashover voltage is reduced considerably by moisture and surface
deposits.
• For satisfactory operation , the rainsheds should have the shapes like those of
equipotential surfaces and the insulator body should be constructed along the
lines of electrostatic field around the pin.
• Insulators are of three types :-
1. Pin type.
2. Suspension type.
3. Strain type.
 PIN TYPE INSULATOR

• pin insulator is supported on a forged steel pin or bolt which is secured to the cross
arm of the supporting structures.
•The conductor is tied to the insulator on the top groove on straight line positions and
side groove in angle positions by annealed binding wire of the same material as
conductor.
•A lead thimble is cemented in to the insulator body to receive the pin.
•Single piece type pin insulators are used for lower voltages , but for higher voltages
two or more pieces are cemented together to provide sufficient thickness of porcelain
and adequate leakage path or creepage path.
•The increased weight , size and cost of pin type insulator puts a limit to its use above
66 KV and therefore the suspension insulators are used for high voltage work.
 SUSPENSION INSULATOR
• A suspension insulator consists of a number of separate insulator units connected
with each other by metal links to form a flexible chain or a string.
• The insulator is suspended from the cross arm of the support. The conductor is
attached to the lowest unit.
• Suspension insulators offer the following advantage:-
1. Each unit is designed to operate at 11 KV voltage, so that a string can be attached
by connecting several units to suit service voltage and weather conditions.
2. In the case the line is to operate on a higher voltage in future to cope with the
increased load , additional units would be introduced to the same string . In the
case of damage to one unit only the damaged insulator but not the whole string is
to be replaced.
3. The string is free to swing in any direction and therefore greater flexibility is
provided. The tension in successive spans are balanced hence lines can be
designed for longer spans and higher mechanical loading.
4. There is decreased liability to lightning disturbances if the string is suspended
from a metallic supporting structure , which works as a lightning shield for
conductor.
The disadvantage being that since the string is hung from support the tower
height is to be increased and greater spacing between the conductors is to be
provided to allow for swinging.
 There are two types of suspension insulators:-
1) Cap and pin type
2) Hewlett or interlink type

Fig a Fig b Fig c

•Hewlett or interlink type is not so common in use and suffers from the
disadvantage that porcelain between links are highly stressed and hence its
puncture strength is lesser as compared to other types.
•Fig a, b & c shows the most common type Cap and pin insulator . These
units are joined together either by ball and socket(fig a) or clevis-pin
connections(fig b & c)
 STRAIN INSULATORS
• They are designed to handle mechanical stresses at angle positions where
there is a change of direction of the line or at terminations of the line.
• Suspension insulator string arranged in horizontal position serve as strain
insulators for high voltage lines which are having longer spans and greater
mechanical loadings.
• For low voltage lines shackle & pin insulators serve the purpose.
• In case single string is not sufficient to take the load two or more strings in
parallel may be employed for higher conductor tensions.
 Stay 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
 Shackle 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 increased use of underground cable
for distribution purpose.
• The tapered hole of the 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.
 V- Strings
• A single string insulator follows the conductor and sways like pendulum in
a strong wind.
• V strings are used to prevent conductor movement at towers. They find
increased application in high voltage transmission systems:
• The advantage of them are as follows:-
1. The insulator swing is reduced and lesser spacing can be provided
between the conductors.
2. Right of way width is reduced.
3. Reduction in phase spacing reduces line reactance and therefore the
system power limits are higher.
4. The size of the tower and cross arm decrease. This results in saving in
cost .
• They are suitable for single circuit lines.
 INSULATOR MATERIALS
• Overhead Line Insulators Are Produced From Toughened Glass Or Porcelain.
• Pre-stresses or toughened glass have their surface layers in state of high
compression due to which their resistance to withstand mechanical and thermal
stresses is greater.
• Advantages of toughened glass over porcelain insulators:-
1. Toughened glass have greater puncture strength.
2. They posses greater mechanical strength and thus less breakage in transport and
installation.
3. They have high thermal shock resistance and therefore damage from power
flashover is reduced.
4. Toughened glass have longer life.
• Disadvantage being that moisture readily condenses over its surface.
• Now-a-days a polymer insulator are being used more. It is a combination of
fibre glass and epoxy polymer instead of porcelain.
• It has high mechanical strength , high thermal shock resistance to reduce
damage due to flashover , lesser hardware corrosion and better performance in
polluted atmosphere.