Insulators
The overhead line conductors should be supported on
the poles or towers in such how that currents from conductors don't flow to
earth through supports i.e., line conductors must be properly insulated from
supports. This is often achieved by securing line conductors to supports with
the assistance of insulators. The insulators provide necessary insulation
between line conductors and supports and thus prevent any leakage current from
conductors to earth. Generally, the insulators should have the subsequent
desirable properties:
1. High mechanical strength so as to face up to
conductor load, wind load etc.
2. High electric resistance of insulator material so
as to avoid leakage currents to earth.
3. High relative permittivity of insulator material
so as that dielectric strength is high.
4. The insulator material should be non-porous, free
from impurities and cracks otherwise the permittivity are going to be lowered.
5. High ratio of puncture strength to flashover.
The most commonly used material for insulators of
overhead line is porcelain but glass, steatite and special composition
materials also are wont to a limited extent. Porcelain is produced by firing at
a heat a mix of kaolin, feldspar and quartz. It's stronger mechanically than
glass, gives less trouble from leakage and is a smaller amount effected by
change of temperature.
Types of Insulators
The successful operation of an overhead line depends
to a substantial extent upon the right selection of insulators. There are
several sorts of insulators but the foremost commonly used are pin type,
suspension type, strain insulator and shackle insulator.
1.
Pin type insulators: The part section of a pin type
insulator. Because the name suggests, the pin type insulator is secured to the
cross-arm on the pole. There's 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 an equivalent material because the conductor.
Pin type insulators are used for transmission and
distribution of electrical power at voltage up to 33 KV. Beyond operating
voltage of 33 KV, the pin type insulators become too bulky and hence
uneconomical.
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 thanks to excessive heat. In
practice, sufficient thickness of porcelain is provided within the insulator to
avoid puncture by the road voltage. The ratio of puncture strength to
flash-over voltage is understood as factor of safety i.e.,
Safety factor of insulator= Puncture strength/Flash-over
voltage
It is desirable that the worth of factor of safety
is high in order that flash-over takes place before the insulator gets
punctured. For pin type insulators, the worth of factor of safety is about 10.
2.
Suspension type insulators: The value of pin type insulator
increases rapidly because the working voltage is increased. Therefore, this
sort of insulator isn't economical beyond 33 KV. For top voltage (>33 KV),
it's a usual practice to use suspension type insulators. They contains variety
of porcelain discs connected serial by metal links within the sort of a string.
The conductor is suspended at rock bottom end of this string while the opposite
end of the string is secured to the cross-arm of the tower. Each unit or disc is
meant for low voltage, say 11 KV. The amount of discs serial would obviously depend
on the working voltage. As an example, if the working voltage is 66 KV, then
six discs serial are going to be provided on the string.
Advantages
1. Suspension type insulators are cheaper than pin
type insulators for voltages beyond 33 KV.
2. Each unit or disc of suspension type insulator is
meant for low voltage, usually 11 KV. Depending upon the working voltage, the
specified number of discs are often connected serial.
3. If anybody disc is broken, the entire string
doesn't become useless because the damaged disc are often replaced by the sound
one.
4. The suspension arrangement provides greater
flexibility to the road. The connection at the cross arm is such insulator
string is liberal to swing in any direction and may take up the position where
mechanical stresses are minimum.
5. Just in case of increased demand on the cable,
it's found more satisfactory to supply the greater demand by raising the road
voltage than to provide another set of conductors. The extra insulation
required for the raised voltage are often easily obtained within the suspension
arrangement by adding the specified number of discs.
6. The suspension type insulators are generally used
with steel towers. Because the conductors run below the earthed cross-arm of
the tower, therefore, this arrangement provides partial protection from
lighting.
3.
Strain insulators: When there's a dead end of the road or
there's corner or sharp curve, the road is subjected to greater tension. So as
to relive the road of excessive tension, strain insulators are used. 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 string are used in parallel.
4.
Shackle insulators:
In
early days, the shackle insulators were used as strain insulators. But now a
days, they 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. A shackle insulator fixed to the pole. The conductor in the groove is
fixed with a soft binding wire.
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