Types of Insulator and their Applications
Different types of Insulators are:
1) Pin type insulator.
2) Suspension type or Disc type insulator.
3) Strain insulator.
4) Shackle insulator.
5) Stay insulator.
Importance of line insulators are
The line conductors are carried over steel towers. The steel towers are constructed on the ground. So the line conductors are to be supported and insulated from steel towers. Line insulators provide insulation and support to line conductors. These also provide safety in power handling.
1) Pin type Insulator
It consists of one or more pieces of porcelain permanently connected together. And this piece is mounted rigidly on the pole cross arm with a pin fixed inside this insulator. Hence, the name is Pin Insulator. (Refer Fig.1.)
Sketch of one place pin insulator
The conductor is placed in the groove at the top of the insulator and it is tied to the insulator by means of binding wire. There is friction between conductor and groove of the insulator and some damage to conductor may occur, so an aluminum tape (binding tape) is wound on the part of the conductor to be placed in the groove and then it is wound by a binding wire. This insulator is used on L. V. line on intermediate pole.
2) Suspension type or Disc type insulator.
The pin insulator becomes bulky and uneconomical if it is to be used beyond 33 kV. Suspension insulator is used above 33 kV. As the name suggests, these are suspended from cross-arm and conductor is supported to them at the bottom portion. Refer Fig.3. The suspension insulator consists of a number of porcelain discs connected in series by
means of mechanical links.
Advantages of Suspension Insulator
(i) Each disc is designed for 11 kV, by connected number of discs in series (a string of insulators), the system can be designed for any desired voltages level.
(ii) if anyone disc fails it can be replaced, there is no need to replace the complete string of discs/ insulators
(iii) Suspension arrangement gives flexibility to the line. The string can swing freely in any
(iv) The distance between earth wire (fixed at the top of pole) and conductor (supported at the direction, thus mechanical stresses are reduced. the bottom of the string insulator) is more compared to the pin insulator. Hence, the line conductors are less affected by lightening
(v) if there is an increase in demand for electrical energy, the number of transmission line conductors has to be increased which is a complicated process. So instead of that, the system voltage is Increased so that increased power can be sent through the same conductors. The conductors now, have to be insulated for higher voltage. in the case of string insulators, this can be easily done by increasing the number of discs.
(vi) Beyond 33 kV suspension insulators are economical than pin insulators.
Types of suspension insulators
There are three types of suspension insulators.
(i) Hewiett suspension insulators.
(ii) Cemented cap type suspension insulator
(iii) Core and link-type suspension insulator.
3) Strain insulator
This is suitable for H.V. starting and end poles.
4) Shackle insulator (Spool insulator)
This is used for low voltages distribution work (below 11 kV). Shackle insulators can be used in a vertical and horizontal position. Fig.5. shows a shackle insulator fixed to a pole. This insulator is used on starting pole and end pole. The conductor is fixed in the groove by means of binding wire.
5) Stay insulator (Guy Insulator/Egg. Insulator.
Stay wire (or guy wire) is used to provide support to a pole erected at corner, sharp turn of line subjected to pull/ tension. There is greater tension or pull on pole at such locations; hence a stay wire is connected to a pole, to provide mechanical strength and stability in the opposite direction. As seen from Fig.6. the clearance between the line conductor and stay wire is less.
In case of an accident, the line conductor may touch stay wire and it can be a great hazard to human and animal life. So stay insulator is inserted in the stay line for safety. The stay wire is inserted in the holes of stay insulators such that even if the insulator is damaged the stay wires remain connected together and support is provided.
For L.T. line the suitable insulators are pin type, shackle type insulators.
Shackle insulator is used on the L.T. line and at terminal points. For H.T. line the suitable insulators are pin type, suspension type, strain (disc) type. Stay insulator is used in stay wire.