What is an insulator? It is a material which cannot conduct electrical current. It have high resistivity and low conductivity. Atom of the ins have tightly bound electron and therefore cannot move throughout of material. Due to the electrons are static and not freely moving ,current cannot flow through it easily. In an overhead line the conductor are supported by mean of insulating fixture which is called insulator. In overhead line it is mounted on the cross arm and the line conductor are attached with it.
The important properties that an overhead line ins must posses are
- High relative permittivity so as to provide high dielectric strength
- High mechanical strength so as to bear the load due to the weight of line conductor, wind force and ice loading if any
- High ratio of rupturing strength to flash-over voltage.
- Ability to withstand large temperature variation i-e it should not crack with high temperature during summer and low temperature during winter
- High insulating resistance in order prevent leakage of current to earth.
The material which is most commonly used as a insulator are porcelain ,glass ,rubber, air ,plastic and paper .For an over head transmission line porcelain are most commonly used.
Porcelain is produced by firing at a controlled temperature a mixture of kaolin, feldspar and quartz. It is mechanical stronger then glass .It gives less trouble from leakage and is less susceptible to temperature variation and its surface is not affected by dirt deposits .On the other hand It is not so homogenous as glass, owning to the shelf of porcelain is glazed during manufacturing process and its satisfactory performance in service depend to a considerable extend on the preservation of this glaze which is only of the order of 25 Microns in thickness .
The dielectric strength and compressive strength of porcelain are about 6.5/ mm of thickness and 700kg/mm2 respectively .If this insulating material is manufactured at low temperature its mechanical property improve but the material remain porous and on putting it in service it may deteriorated ,on the other hand if this material is manufactured at high temperature porosity is reduced but the material become brittle. The porosity of this insulating material also reduce its dielectric strength due to the bubble left and impurities left in the materials.
Glass is cheaper than porcelain in the simpler shape .Owning to high dielectric strength ,the glass insulator have simpler design and even one piece design can be used .Glass is quite homogenous material and can withstand higher compressive stress as compared to porcelain. Glass also have lower coefficient of thermal expansion which minimize the strain due to temperature change and owning to transparent nature flaws in the material can be readily detected by visual examination.
The main disadvantage of glass is that moisture more readily condenses on its surface and facilitates the accumulation of the and deposits as a result giving high surface leakage. Glass insulator can used up to 25 KV under ordinary atmospheric conditions and suitable up to 50 KV in dry atmosphere .
Factor involved insulator design
The insulator are required to withstand both electrical and mechanical stresses .It must also have high surface leakage so as to avoid any current leakage to earth. The design must be such that the stress developed owning to contraction and expansion in any part of the insulator does not lead to any defect.
In transmission line insulator may be classified as following
- pin type
- suspension type
- Strain type
Pin type insulators
As the name suggests, pin type insulator is design to be mounted on a pin which in turn is installed on the cross-arm of the pole. Insulator is screwed on the pin and electrical conductor is placed in the groove at the top of insulator and is tied down with soft copper or soft aluminum binding wire according to material of the conductor.
For low voltage transmission and distribution line pin type insulator made of glass are generally used. Pin type insulator made of porcelain material are design for voltage up to about 90 KV but are seldom use on line above 60 KV
Suspension type insulator
As the cost of pin type ins increased repeatedly with the working voltage is increased there for this type of insulator is not economic beyond 33 KV.For voltages higher than 33 KV it is usually practiced to used suspension type insulator .
The suspension type ins hang from the cross-arm ,as opposite to the pin insulator sit on the top of it.The line is attached to the lower end of the insulator. These insulators consists of a number of porcelain disc connected in series by metal link in the form of a “ String”. the number of insulator comprising a string depend upon the following factor
- working voltage
- Type of Transmission line construction
- size of insulator used
- weather condition
Following are the advantage of suspension type over pin type
- suspension type are usually cheaper in cost for operating voltage above 50 KV
- The flexibility of the line is greatly improved with suspension type .The conductor is secured by a clamp but the insulator string being simply suspended is free to swing in any direction .This arrangement reduces mechanical stresses
- Each unit is designed for a comparatively low working voltage so depending upon the working voltage the number of unit to be connected in series may be obtained
- In case of damage ,the whole string does not become useless because the damages part can easily be replaced
- When the line insulation is required to be increase ,the traditional insulator can very easily be added to the string.
- As the conductor is hung below the earthed cross-arm the tower function as a lightning rod, this arrangement provide partial protection from lightning.
The disadvantage of suspension insulator is that large spacing between conductors are required then with pin type insulator due to large amplitude of the swing of the conductor
Suspension type insulator are of following types generally
- Hewllet or interlinking type
- cemented cape type core
- Core-and-line type
Strain insulator may be of pin or suspension type. This type is satisfactory Up to about 30KV but for higher voltage the suspension type are generally used
These are used on dead-end tower or corners of Transmission and distribution line or when making very long Spans. Extra heavy suspension unit are made for such service but often standard unit may be used. on ordinary straight line that dead-end Tower a single string is often sufficient but for self server service more string may be connected in parallel .
Shackle Insulator will be mounted in a clamp. This type can be directly fixed to the pole with a bolt or to the cross –arm. In the groove the conductor is fixed with a soft binding wire.
Earlier, this type was used as a strain insulator. These days they are used for low voltage distribution line
It is tested by following testing methods
- mechanical test
- electrical test
- environmental test
- temperature cycle test
- radio interference test
Such type are of egg shape also called strain or guy insulator and are used in guy cable where it is necessary to insulate the lower part of the guy cable from the pole for the safety of people and animal on the ground. This type consists of a porcelain piece pierced with two holes at right angles to each other through which two end of the guy wire all looped. This keep the porcelain between the two loop in compression and the guy wire remain in position even if the insulator break due to any reason .These are provide at a height of about 3m in from the ground level. The size of ins used depends upon the tensile strength of the stay wire.
Insulator failure causes
The following are the cause of failure
- mechanical stress
- dust deposition
- short circuit
- improper vitrification
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