Overhead insulator are the insulator which are used in overhead transmission line to prevent unwanted flow of electric current. The overhead line conductors are supported on the electric poles or towers. In order to prevent the flow of current to earth through poles, the line conductors must be properly insulated from supports. This is achieved by placing insulators between line conductors and supports. These insulators are used to hold the overhead bare conductor. Insulator has a high resistive path which opposes the flow of current. Thus these insulators play important role in the working of overhead transmission lines.
The overhead insulator are placed in open environment, therefore insulator must have following characteristics:
1.High Mechanical Strength
The overhead insulator should have high mechanical strength in order to withstand the conductor load and load due to wind pressure.
2.High Dielectric Strength
The insulator should have high dielectric strength to withstand the high voltage without damaging. It reduces the size of insulator for same electric potential.
3.High Insulation Resistance
The overhead insulator should have high insulation resistance because high insulation resistance prevents the leakage current.
The insulator should be non porous because pores permit water and dust to accumulate, which reduces the dielectric strength of insulator.
TYPES OF INSULATOR
The overhead insulator has following types:
1.Pin Type Insulator
Pin type insulators are placed on the cross arm of electric poles. These insulators have groove on the upper side to hold the overhead line conductor. Pin type insulator can be used up to 33kV but above 33kV, these insulators are not economical and becomes bulky. For the lower voltages, Single or one piece type of insulator is used. This type of insulator may have one, two or maximum three petticoats or rain sheds. These rain sheds or petticoats are designed in such a way that when these insulators are wet, even then a sufficient dry space is provided by the inner sheds.
2. Suspension Insulator
Suspension insulators are consist of porcelain disc which are mounted one above each. These porcelain discs has metal cap at its top and metal pin under it. The size of string of porcelain discs depends upon the voltage of the transmission line. The line conductor is held below the point of support. These type of insulator are cheaper than pin type insulator. If one string get damage due to any reason, it can be replaced rather than changing the whole unit. The suspension insulators has flexible strings, thus it is freely swing in any direction. But due to this, larger spacing between conductors is required, hence larger cross arms are required.
3. Strain Insulator
Strain insulators are basically special type of mechanical strong suspension insulator. These insulator are used to take the tension of overhead line conductor at line terminal. The overhead line has greater strains at the dead ends, on sharp turns, at river crossing or at the corners of streets. These insulators are employed to take up theses strains, hence these insulators are called, Strain Insulator. The strings of strain insulators are placed in horizontal direction. In case, when tension or strain of overhead line is very high, the two or three strings can be used in parallel. For low voltage line up to 11kV, Strain insulator can be used but for high voltage line strain insulator with the assembly of suspension insulators is used.
4. Shackle Type Insulator
Shackle type insulators are used in low voltage distribution network. These insulators are also called Spool insulator. Shackle insulator can be used in both horizontal and vertical direction. The line conductor is passed through the gap left between the clamp and the insulator and is tied along the groove with the help of soft binding wires of same material as that of the line conductor. Now days, these insulators are less used because usage of underground cable for distribution cable has increased.
5. Stay Type Insulators
Stay insulators are used to prevent the stay wire getting energize from the broken live wire. These insulators are also called Egg Insulators. Stay insulators basically provides insulation between stay clamps and electric poles. These insulators are mounted at the height of minimum 3 meters from ground. The stay insulator has two holes at right angle to each other. Through these holes, two ends of the stay wire are looped in such a way that in case, The stay insulator breaks, the stay wire will not fall on the earth. In this arrangement, the stay insulator remains in under compressive stress position, thus the compressive strength of the insulator is very high.
INSULATING MATERIALS USED
The insulating materials used for overhead insulators are:
- Fibre etc.
Porcelain and Glass are the mostly used insulating material.
The most commonly used insulating material is Porcelain whereas other insulating materials like Glass, Rubber etc are used for some special applications. Porcelain is produced by firing a mixture of Kaolin, Feldpare and Quartz at a controlled temperature. The porcelain has following properties:
1. It has very high mechanical strength.
2. Porcelain is not affected by dirt particles.
3. It has high dielectric strength- 60kV/cm
4. It has very high compressive strength- 70000kg/cm2
5. It has tensile strength- 500kg/cm2
Glass is the second most used insulating material after porcelain. It is also cheaper than porcelain. The glass insulators are produced by mixing the different type of materials like lime and quartz powder at high temperature and then cooling it down in the mold. The glass insulator has following properties:
1. Glass has very high dielectric strength- 140kV/cm
2. It has very high compressive strength- 10000kg/cm2
3. It has tensile strength- 35000kg/cm2
4. Glass insulator has low coefficient of thermal expansion
5. Glass has very long life.