The one and only work an insulator is to keep a live conductor electrically separated from the earth. The main material used for making is a solid insulator is either porcelain or glass. Although porcelain is more widely used for the purpose, nowadays, pyrex glass is also used for the same purpose. Pyrex glass insulators are comparatively cheaper. Although pyrex glass insulators are not used for more than 11KV system.
There are two types of insulators normally used in an LT line. These are pin insulators and shackle insulators. Pin insulators are used at a tangent and a small-angle pole. On the other hand, shackle insulators are used at a large angle pole and a terminal pole.
LT Pin Insulator
LT pin insulator is made by a single piece of porcelain insulator. The porcelain part of the insulator is called the shell of the insulator.
There are two types of pin insulators. One is suitable for the conductor of 10mm diameter an another is suitable for the conductor of 19mm diameter.
Shackle insulators are also available in two types. The diameter of the first one is 75mm with 90mm height. The diameter of the second one is 100mm with 115mm height. The first type of shackle insulators is used for the conductor of 16 to 48 square mm cross-section. The second type of insulators is used for the conductors of the above cross-sections.
Each of the pin insulators has a threaded GI stud projected from its bottom. This threaded stud is used to fix the pin insulator on the cross arm with the help of nuts.
HT Pin Insulator
HT pin insulators are of different sizes and different shapes. Single shell HT pin insulators are used up to 11KV system. Multi shell HT pin insulators are used for higher voltage class systems. Each shell has its own rainshade. The shells are fixed together with the help of a special type of cement. Rainshade of the shells protects the rainwater to fall on the pin from the top of the insulator. Therefore a rainshade keeps dry the surface on its bottom. Also, the rainshade increases the creepage distance of the insulator.
To fit the pin in the insulator there is a hole provided below the bottommost shell of the insulator. The hole is provided with a threaded metal thimble. The metallic pin is inserted in this thimble. The metal and design of the pin are so chosen that at any abnormal tension on the conductor the metal pin becomes bend first. Therefore the tension cannot affect the porcelain portion of the insulator. Therefore design and material of the pin are important factors for a pin insulator. According to the tension withstand capacity, there are mainly two types of pins available in the market for the purpose. These are the pin of 1000 pounds and the pin of 2000 pounds. The general factor of safety of the pin of a pin insulator is taken as 2.5. The type and size of the selected pin depend on the weight of conductors, angle of diversion, and span length of the overhead line, etc. The average height of the conductor from the cross-arm for 11KV system is 175 mm. On the other hand, the average height of the conductor from the cross arm for 33KV system is 275 mm. During fixing a pin insulator on the cross arm we must use spring washer in addition to normal flat washer and nuts. A spring washer reduces the chance of losing the nuts during any vibration of the conductor.
Pin insulators could not be used for more than a 33kv system. This is because in that case the size of the insulator will be quite large and the weight of the insulator will be quite heavy. Therefore, fitting and fixing such an insulator on the cross arm with the help of a pin will be difficult and impractical.
Structurally a post insulator is similar to a pin insulator. A post insulator is a single shell insulator. Normally on the top of a post insulator, there is a clamp provided for fixing the conductor with it. A special type of fitting is also provided at the bottom of the insulator for fixing it with the supporting structure. A specific number of post insulators can be fitted one above other to support a higher voltage bus. Such assembly of post insulators is called insulators stack.
Disc insulators are used in all overhead lines above 33KV. Although disc insulators are also often used in 11KV and 33KV overhead lines. There are certain advantages of using disc insulators.
Advantages of Using Disc Insulator
- Disc insulators can be used for any level of voltages by connecting disc insulators one by one in series. This assembly is called the insulator string.
- If one disc insulator is damaged in disc insulator string, we can just replace that damage insulator without replacing the entire string.
- Since the string is flexible, the mechanical tresses initiated in the conductors do not affect directly the cross arm.
- The string is suspended from the cross arm and the conductor is fitted at the bottom of the string. So, the position of the conductor is below the cross arm. This position of the line conductors sometimes safes the line from direct lightning.
The Disadvantage of Using Disc Insulator
The main disadvantage of using disc insulators is that the height of supporting towers and the length of cross arms are more than those that are required for pin and post insulators.
There is a metal cap fitted on the top of each disc insulator. One ball pin is provided at the bottom of the disc. The porcelain shell is provided in between the top metal cap and button ball pin. The top metal cap and bottom ball pin are attached to the porcelain shell with a special type of cement.
Types of Disc Insulators
There are two types of disc insulators available in the market. These are ball socket type and clevis type. The diameter of both types of disc insulators is 255 mm or 10 inches. One disc insulator is made for 11KV.
According to the tension withstand capacity, disc insulators are categorized into types. These are suspension insulators and strain insulators. Both types of insulators are constructionally similar. But the tension withstanding capacity of a strain insulator is more than that of a suspension insulator. Suspension insulators are used at tangent towers.
On the other hand, strain insulators are used at angle towers, terminal towers, and river crossing towers, etc.
It is previously told that a disc insulator is normally manufactured for 11KV voltage. For higher voltage applications, a number of such disc insulators are fitted one after another. Such assembly of disc insulators is called the insulator string.
How many disc insulators are used in a string depends on the system voltage. But it is mandatory to provide one extra disc to a strain insulator string than a normal suspension insulator string. To save an insulator string from flashes of lightning and other kinds of transient overvoltages, it is provided with arcing horns at the bottom and the top of the string. The gap between the tip of the bottom and top horns is always less than the length of the insulator string. Because of that during an abnormal transient overvoltage condition, the flashover takes place between the top and bottom arcing horn tips. Therefore the insulators remain saved.