Overhead line Insulators
The Construction of an overhead transmission (or Distribution) lines are secured to the supporting structures by means of insulating fixtures in order that there is no current leakage to the Earth through the Supports.
Thus the Insulators play an important part in the successful operation of Overhead transmission (or distribution) lines.
The Insulators used must have high Mechanical strength to withstand the load because of Conductor weight, hight insulation resistance to avoid leakage of currents to earth and high ratio of rapture strength to the flash - over Voltage. The material used should not be porous and there should be no effect of change in Temperature.
Simultaneously it must be free from internal impurities and cracks and should be imperious to the fluids and gases in the atmosphere.
The material most commonly used for Insulators is porcelain, but glass, steatite and special composition materials, are also used to a limited extent. Porcelain is extensively used material and is mechanically stronger than glass. In tension this material usually weak and does not withstand tensile stresses more than 500kg/cm².
The dielectric strength and compressive strength of an mechanically sound porcelain Insulators are about 40kv/cm of its thikness and 70,000 kg/cm² respectively.
Glass is cheaper than porcelain in the simpler shapes and if properly toughened and annealed gives high resistivity and dielectric strength (140 kv per cm of thikness of the material).
Glass is quite homogeneous material and can withstand high compressive stresses as compared to porcelain. Glass Insulators, however can be used up to 25kv under Atmospheric conditions and we'll up to 50kv in dry atmosphere.
Steatite has a much higher tensile and bending stresses than porcelain and can advantageously br used at tension tower of when the transmission line take a sharp turn . The special artificial material is used in Insulators for low voltage and have an important advantage that these can be easily moulded into any shape without any internal stress.
Types of Insulators
These are many types of Insulators.
1) Pin type insulators
2) Suspension type Insulators
3) Strain Insulators
4) Shackle Insulators
5) Egg or stay Insulators
Various types of Insulators are briefly described Below.
1. Pin Type Insulators :- The pin type insulator is designed to be mounted on a pin which in turn is installed on the cross arms of the pole.
For lower Voltages generally one piece type of Insulators is used. For high voltage transmission lines larger, stronger pin type insulator are used. The high voltage pin type insulator differ in construction from low voltage type in that they consist or two or three pieces of porcelain cemented together. These prices form what we call petticoat. These are available for use up to 50,000 volts.
The main Advantages of pin type insulator is that it is cheaper. In many cases one pin Insulator can do the work of two suspension Insulators.
Secondly the pin Insulator Requires a shorter pole to give the same Conductor clearance above the ground since the pin Type Insulator raises the conductor above the cross arm while the suspension Insulators suspends it below the cross arm.
However, its beyond operating Voltage of 80,000 volts become uneconomical . The modern practice is not to use pin type Insulators beyond 33,000 volts. Insulators of this type are used intermediate poles on straight runs. These do not take tantion.
2. Suspension Type Insulators :- With the increase in Operating voltage, the insulation Required increase. Transmission line use extremely high voltages, 220kv, for example. At these Voltage the pin type Insulators become bulky , cumbersome and costly.
Besides, the pin which must Hold it would have to be inordinately Long the large. In order to meet the problem of Insulators for These high voltage, the suspension Insulator was developed.
The suspension Insulator hangs from the cross arm, as opposed to the pin Insulator which sits on the top of it. The line Conductor is attached to its lower end. Because there in no pin problem, we can put any distance between the suspension Insulator and the Conductor just by adding more Insulators to the "string".
The entire unit of suspension Insulators is called a string consists of depends upon the working Voltage, the weather conditions, the type of transmission construction, and the size of Insulator used. It is noteworthy that in a string of suspension Insulators one more Insulators can be replaced without replacing the whole string.
3. Strain Insulators :- Sometimes a line is to withstand great stain, for instance at a dead end or at a corner or on sharp turns. In such a circumstances for LT (low tantion) lines shackle Insulators are used but for HT ( high tension ) transmission lines strain Insulators consisting of an assembly of suspension Type Insulators are used.
Because of its peculiarly important job, a strain Insulators must have considerable
Strength as well as the necessary Electrical properties.
The discs of strain Insulators are Insulators are employed in a vertical plane where at as suspension Insulators are used in horizontal plane.
4). Shackle Insulators :- The Shackle or spool type Insulator, which is easily identified by its shape, is usually used on it lines. Both the low voltage Conductors and the house services wires are attached to the Shackle Insulator.
5). Egg or stay Insulators:- Such Insulators are of egg shape and used in guy cables, where it is necessary to insulate the lower part of the guy cable from the pole for the safety of the people on the ground.
These are provided at a height of about 3m from the ground level.