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INSULATORS
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Introduction The overhead line conductors should be supported on the poles or towers in
such a way that current from the conductors do not flow to earth through
supports.
This is achieved by securing line conductors to supports with the help of
insulators.
An insulator, also called a dielectric, is a material that resists the flow of
electric current. An insulating material has atoms with tightly bonded valance
electrons.
Properties of Insulators High mechanical strength in order to withstand conductor and wind loads
etc.
High electrical resistance to avoid the leakage current.
High relative permittivity.
High dielectric strength to withstand overvoltages.
The insulator material should be free from impurities and cracks, non porous.
High ratio of puncture strength to flashover.
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Insulator Failure
The electrical failure of insulators occur either by puncture or by flashover.
The arc passes through the body of the insulator causes puncture and
becomes useless.
Flashoveris caused by the arc discharge between the conductor and earth
through air surrounding the insulator
These are generally caused by line
surges or due to formation of wet
conducting layer over the insulator surface.
For satisfactory operation the flashover
should occur before the puncture.
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Pin Type Insulator This type of insulator is supported on a forged steel pin or bolt securing thecross arm of the supporting structure. Placed in the cross arm of the pole.
Used for transmission and distribution upto 33 kV.
Groove is provided for housing of the conductor.
Cost of this type of insulators increases rapidly as working voltage increases.
(not economical beyond 33 kV)
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Suspension Type InsulatorThe cost of a pin-type insulator increases very rapidly as the working voltage is
increased, for high voltages pin type is therefore uneconomical, and there is the further
disadvantage that replacements are expensive.
For these reasons, high-voltage lines are insulated by means ofsuspension insulatorsin which the combination of individual insulator dics, which are connected to metal links toform a flexible chain or a string. These are suspended to the cross arm of the support.
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Suspension type InsulatorsUsed for
high voltages (>33kV)
Consists of number of porcelain
discs connected in series by metal links
in the form of string.
Conductor is suspended at thebottom
end of the string and other end is fixed
in the cross arm of the tower.
Each disc is designed to insulate up to
11 kV. Therefore the number of disc in a string
depends upon the voltage level of
transmission. (typically for 66 kV, 6
disc)
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Advantages of Suspension Insulators
Cheaper than pin type beyond 33 kV
Each disc is designed for 11 kV. Hence depending upon the working voltage,
desired number of disc can be connected in series. Any damage disc can be replaced. Whole string of discs will not be useless if
one disc damages.
Arrangement provides greater flexibility to the line.
Insulator is free to swing in any direction and can take up the position of
minimum mechanical stress. Generally used with steel towers. Since conductor runs below earthed cross
arm, it can provide protection from lightning.
http://www.utilex.com/over_power_trans.htm -
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http://www.myinsulators.com/acw/bookref/insulator/gonentrans-fig9.19.jpghttp://www.chinainsulators.com/products/cap_pin_suspension_insulator_ANSI.htm -
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Strain Type Insulators
These insulators are designed for handling stresses at an angle positions where there is achange in the direction of the line or at terminations of the line. A typical strain
insulator is a piece of glass or porcelain that is shaped to accommodate two cables or acable shoe and the supporting hardware on the support structure (hook eye, or eyelet on asteel pole/tower).
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Strain Insulators used in dead end of the
line where the line is subjected to greater
tension.
For HV X-mission line, it consists of assembly of suspension insulators as
shown.
For high tension in the line two or
more strings of strain insulators can
be used in parallel
For low voltage (< 11 kV), Shackle
Insulator used as strain insulator
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Shackle Insulators
Can also be used as strain insulator.
Frequently used for low voltage
(400 V) distribution line.
Can be used both horizontal and
vertical position.
Directly fixed to the pole with bolt
or can be fixed in the X-arm.
Used where we need to run the line
with certain bending with some angle. (where
turning of the roadetc. occurs)
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http://www.chinainsulators.com/products/Shackle_spool_insulator_BS2.htm -
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Insulators have got certain materials according to differentapplications in Electrical field of Engineering. Some of them arementioned here:
PORCELAIN INSULATORS
GLASS INSULATORS
RUBBER INSULATORS
PLASTIC INSULATORS
WOOD INSULATORS
COMPOSITE INSULATORS
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PORCELAIN INSULATORS
The strength and durability of porcelain is ideal for the power industry. They are
glazed in brown color.Porcelain pin type insulators in the United States began to appear in volume around the
turn of the century.
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GLASS INSULATORS
North American threaded glass pin type insulators are the most commonly collected typeof insulator.
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RUBBER INSULATORS
Rubber was invented in the mid 1800's and has since been used as an electrical insulator.More recently rubber insulators have been used in place of glass to protect from damage.
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PLASTIC INSULATORS
In recent years plastic is available to replace glass and porcelain for most insulator types.These range from communication pin types to high tension insulators
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WOOD INSULATORS
Although uncommon, there are a number of Insulator styles made from wood. The bestknown and most desirable wood insulators were made from the dense wood lignumvitae for the San Francisco trolley system.
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COMPOSITE INSULATORS
There are a class of insulators made from a number of materials that are a mixture ofmultiple components. These are frequently made from an insulating filler material(Fiberglass, asbestos, mica, etc.) mixed with a binder material (resin, tar, shellac, etc.) toform a moldable material that will stand up to outdoor use. Almost all varieties ofinsulators have been molded out of composition materials, but all the pin types aresomewhat uncommon
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Causes of Insulation Failure
- Insulators required to withstand both mechanical andelectrical
stress.
- High electrical stress may cause breakdown ofinsulators.
-- Flashover**
(due to arc formation in the surface between conductorand pin)
-- Puncture** (discharge occurs from conductor to pin via body of
insulator)