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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)