Question

Which type of insulator is used in transmission line?

Answer 1

Insulators are used in electrical transmission &distribution system to support the line conductor& provide clearance fromground & structure.

Insulators are generally classified in following types:


1.Pin Type Insulator:This is the first developed insulators and being used for overhead lines for voltage grade up to 33 kV. The live conductor is place on the top of the insulator and the bottom of the insulator inconnected to earth. The insulator has to withstand the potential stress betweenconductor and earth. When insulator is wet, its outer surface becomes almost conducting. Hence the flash over distance of insulator is decreased. Theelectrical insulator is designed such that the decrease of flash over distance is minimum when the insulator is wet. That is why the upper most petticoat of a pin insulator has umbrella type designed so that it can protect the restlower part of the insulator from rain. The upper surface of top most petticoat is inclined as less as possible to maintain maximum flash over voltage during raining.

2. Post Insulators:Post insulator is suitable for higher voltage. It has higher numbers of petticoats and has greater height. This type of insulator can be mounted onsupporting structure horizontally as well as vertically. The insulator is made of one piece of porcelain and it has fixing clamp arrangement are in both top and bottom end. For highervoltage application Two or more insulators can be fixed together to meet the requirement.


3. Suspension Insulator:Using post insulator in higher voltage is not economical and suspension type insulator is evolved. Disc insulatorsare connected together in series to make a string which is suspension type insulators. As per the voltagegrade the number of discinsulators are increased or decreased so that is suitable for any voltage level. When suspension insulators are used a conductor is always hanging / suspended below the metallic tower level and it is always protected fromlightning. On the other hand in order to maintain minimum clearance between conductor and ground/ equipment the tower height uses to be higher. The amplitude of free swing of conductors is larger in suspension insulator system, hence,more spacing between conductors should be provided.

4. Shackle insulators:It is usually used inlow voltage distribution network. It can be used both in horizontal and vertical position. The conductor in the groove of shackle insulator is fixed with the help of soft binding wire.


5. Stay Insulators:For low voltage lines, the staysare to be insulated from ground at a height. The insulator used in the stay wireis called as thestay insulator and is usually of porcelain and is so designed that in case of breakage of the insulator the wire will not fall to the ground.

6. String Insulators:When suspension string is used to sustain extraordinary tensile load of conductor it is referred as string insulator. When there is a dead end or there is a sharp corner intransmission line, the line has to sustain a great tensile load of conductor or strain. A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties. In string Insulator, each porcelain disc is designed for11 kV. Thus for132 kV overhead line around 12 disc will be assembled.