It is obvious that if the transmission line is not properly insulated from the support of the tower or pole, then the current will flow through the tower in the direction of the ground and thus become dangerous. Of course, transmission lines are always supported by insulators placed on the poles. Insulators used on towers must have properties such as high mechanical strength, high resistance, and high relative dielectric constant.
The insulator material used in transmission lines is porcelain, but depending on the requirements, talc or glass types can also be used. There are different types of insulators in transmission lines, such as pin insulators, suspension, strain stay and shackle. Insulators such as pin, strain and shackle are suitable for medium and high voltage systems, while shackle and stay are suitable for low voltage applications.
Insulators used to isolate wires from physical supports, such as pins on poles or towers, are called pin insulators. This type of insulator is used in 33kV distribution systems. As the name implies, it is arranged on the pins connected to the conductors. These insulators are made of glass, otherwise they are porcelain.
These insulators are still used in 33 kV distribution systems. Depending on the applied voltage, these insulators are available in different parts, such as 1-part, 2-part or 3-part types. One part type is used in 11 kV distribution systems where the entire insulator is a ceramic/glass shaped part.
If the leakage path of this insulator is on the surface, the length of the surface area needs to be increased in the vertical direction to increase the leakage path.
The internal construction of a pin insulator consists of two main parts, the ceramic and the galvanized steel bolt. The bolt is attached to the base by a glue joint. There are various techniques available to protect the insulator towards the bolt.
The insulator must be designed correctly to overcome the electrical and mechanical stresses on the insulator. The electrical stress on the insulator depends mainly on the line voltage, therefore, the right insulator must be used according to the line voltage. Excess voltage can damage the insulator through breakdown or flashover.
The puncture of an insulator can be occurred due to the electrical discharge from the conductor to pin throughout the insulator. Enough thickness of insulator material must be used to evade a puncture. When such type of puncture occurs then the insulator will be damaged permanently.
The flashover of an insulator can be occurred because of the electrical discharge by designing an arc among the pin of an insulator & line conductor.
It is defined as the ratio of puncture potency to flash overvoltage. It requires high safety factor value so that a flash-over occurs once before the pin type insulator gets punctured. For this kind of insulator, the safety factor value is approximately 10.
