Types of Electrical Insulator used in Overhead Transmission Lines

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Types of Electrical insulator

Insulator provides necessary insulation between line conductors or supports and prevent leakage current to flow from conductors to earth.


Among several types of insulators the most comonly used insulators are Pin insulator, suspenson insulator , Strain type insulator , Shackle insulator , Stay or Guy insulator.   

1) Pin type insulator :- 

(a) Pin type insulator are used for supporting line conductors. It consists of single or multiple shells arranged in parallel and mounted on cross-arm of the pole or tower.
  
(b) One piece of insulators can be used upto 11kV, two piece upto 25KV  and 4 to 6 pieces upto 44 to 66 KV 

(c) The designing of sheds or petticoats should be such that even when the outer surface is wet due to rain, the inner dry surface provides sufficient leakage resistance.

(d) On the top of the insulator the conductor bound into a groove which is cemented on to a galvanized steel pin.  

(e) Cost of pin type insulator increases rapidly as the working voltage is increased. Therefore , this type of insulator is not economical beyond 33 KV. 

2) Suspension type insulator :-

(a) For higher voltage (above 33 KV ), it is practical to use suspension type insulator.
 
(b) It consist of a number of porcelain discs connected in series by metal links in the form of a string. 

(c) The conductor is suspended at the bottom end of this string while the other end of this string is secured to the cross-arm of the tower. 

(d) Each unit of disc called Disc insulator  and designed for 11KV. Depending on the working voltage, the desired number of discs can be connected in series. 
 

(e) Suspension arrangement provides greater flexibility to the line which minimise the mechanical stresses.

(g)  Suspension type insulator are generally used in steel towers. 

3) Strain type insulator :-

(a) It is basically an assembly of suspension insulator used in horizontal position, also known as tension insulator.  

(b) The main function of this insulator is to reduce excessive tension of the line .    

(c) Strain type insulator are used when the line is subjected to greater tension such as dead ends, river crossings, sharp curves and on direction change.

(d) Upto 11KV  shackle insulator can be used, but for high voltages strain type insulators should be used.

4) Shackle insulators :-

(a)  In early days , the shackle insulators were used as strain insulators. But now a days they are frequently used for low voltage distribution lines.
 (b)  Shackle insulators is also known as spool insulator and can be used either in a horizontal position or in a vertical position. 
 
 (c) They can be directly fixed to the pole with a bolt or to the cross arm. The conductor in the groove is fixed with a soft binding wire. 

5) Stay/Guy Insulator :-

(a) The insulator  used in the stay wire is called as the stay insulator or egg insulator. It is made of porcelain and is designed such that in case of breakage of the insulator the guy wire will not fall to the ground.

(b) Stay insulator is a type of low voltage insulator designed to fasten and counter weight the dead end pole by connecting with stay wire. 


Properties Required for a Good Insulator :
1) High mechanical strength 
2) High electrical resistance 
3) High relative permittivity 
4) It should be Non-porous
5) High ratio of puncture strength to flashover. 



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