1. DIFFERENT TYPES OF
INSULATORS AND
INSULATING MATERIALS
USED IN OVER HEAD AND
UNDERGROUND LINES
Assignment No. 1
By: Puneet Chaudhary
Roll no. :324/17
2. WHAT ARE INSULATORS ?
• A material or an object that does not
easily allow heat, electricity, light, or
sound to pass through it.
• An electrical insulator provides very
high resistance so that practically no
current can flow through it.
3. PROPERTIES OF INSULATING
MATERIALS ?
• Resistivity of a good insulator is very
high.
• Dielectric strength of a material is the
ability to withstand electric stresses
without breaking down.
• Relative permittivity (or dielectric
constant) is the ratio of the electric flux
density produced in the material to that
produced in vacuum.
• Electrical dissipation factor (dielectric
loss) is the ratio of the power lost in the
material to the total power transmitted
through it.
6. FOLLOWING ARE HOW BOTH
TRANSMISSION LINES ARE SEEN OR
INSTALLED
OVERHEAD TRANSMISSION UNDERGROUND TRANSMISSION
7. DIFFERENT TYPES OF MATERIALS
USED IN MAKING ELECTRICAL
INSULATORS IN OVERHEAD:
Wood and Glass Insulators :
Telegraph lines came before power
utility lines and glass worked effectively
for telegraph lines as the voltage was
low. Before the age of high voltage
power (1870s onward), electricity was
used primarily in telegraph and
telephone technology. At that time
some people including a young Thomas
Edison built systems using glass bottles
mounted on top of nails stuck into
trees and poles.
8. Polymer Insulators
Polymer insulators are composed of a fibre
glass rod covered by polymer weather
sheds. Polymer weather sheds are generally
made from silicon rubber. Few other
materials may also be used for weather
sheds, such as polytetrafluorethylene (PTFE
or Teflon), EPM, EPDM etc. Polymer
insulator are sometimes also called
as composite insulators or silicon rubber
insulators. They are almost 90% lighter than
porcelain insulators and still offer almost
equal or better strength.
Long Rod Insulators
A long rod insulator is basically a porcelain
rod with an outside weather shed and
metal end fittings. The main advantage of
long rod design is the elimination of metal
parts between the units, thereby increasing
the insulator's strength. Long rod
insulators can be used at suspension
locations as well as tension locations.
9. Ceramics :
High voltages created new problems for insulator design. Glass would crack
apart with high temperatures and failed to withstand certain high temperatures,
so the solution lay in Czech pottery. Master potters were hired to create
porcelain products for the new electrical revolution of the 1880s. Simply
changing to existing porcelain products was only a temporary solution, because
the needs for even higher voltages came about in the 1890s. Chemists and
material engineers helped design higher performance porcelain insulators with
special coatings and designs.
10. ALL THE ABOVE TYPES ARE USED IN
OVERHEAD TRANSMISSION LINES
11. INSULATIONS IN UNDERGROUND
LINES
In general, the insulating materials used in cables should have the following
properties :
• High insulation resistance to avoid leakage current.
• High dielectric strength to avoid electrical breakdown of the cable.
• High mechanical strength to withstand the mechanical handling of cables.
• Non-hygroscopic i.e., it should not absorb moisture from air or soil. The moisture tends
to decrease the insulation resistance and hastens the breakdown of the cable. In case
the insulating material is hygroscopic, it must be enclosed in a waterproof covering like
lead sheath.
• Non-inflammable.
• Low cost so as to make the underground system a viable proposition.
• Unaffected by acids and alkalies to avoid any chemical action
12. TYPES OF INSULATION USED IN CABLES
IN UNDERGROUND TRANSMISSION:
Rubber
Vulcanized India rubber has greater
mechanical strength, durability and
wears resistant property than pure
rubber. Its main drawback is that
sulfur reacts very quickly with copper
and for this reason, cables using VIR
insulation have a tinned copper
conductor. The VIR insulation is
generally used for low and moderate
voltage cables.
13. Impregnated Paper
It consists of chemically pulped paper
made from wood chippings and
impregnated with some compound such
as paraffinic or naphthenic material. It is
because it has the advantages of low cost,
low capacitance, high dielectric strength,
and high insulation resistance. For smaller
installations, where the lengths are small
and joints are required at a number of
places, VIR cables will be cheaper and
durable than paper insulated cables.
14. Varnished Cambric
The cambric is lapped on to the
conductor in the form of a tape and its
surfaces are coated with petroleum jelly
compound to allow for the sliding of one
turn over another as the cable is bent.
Polyvinyl chloride (PVC)
Polyvinyl chloride has high insulation
resistance, good dielectric strength and
mechanical toughness over a wide range
of temperatures. As the mechanical
properties (i.e., elasticity etc.) of PVC are
not so good as those of rubber,
therefore, PVC insulated cables are
generally used for low and medium
domestic lights and power installations.
15. FOLLOWING SHOWN IS DIFFERENCE IN
SIZE OF CABLES USED :
Thicker cables are used in
underground transmission where
another one is used in overhead
transmission due to thickness in
underground it is much costlier than
the another.
