Online Electrical Engineering
Electrical Power Transmission System and Network
Short Transmission Line
Medium Transmission Line
Long Transmission Line
ABCD Parameters of Transmission Line
Electrical Transmission Tower Types and Design
Performance of Transmission Line
Methods of Transmission Tower Erection
Basic Concept of Transmission Tower Foundation
Design of Foundations of Transmission Towers in different Soils
Insulation Coordination in Power System
Types of Electrical Insulator | Overhead Insulator
Electrical Insulator | Insulating Material | Porcelain Glass Polymer Insulator
Electrical Insulator Testing | Cause of Insulator failure
Test for Thickness of Insulation of Power Cable
Electrical Power Substation Engineering and Layout
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Electrical Isolator or Electrical Isolation Switch
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ZnO Gapless Lightning Arrester or Surge Arrester
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Electrical Power Cable
Conductor Resistance Test of Electrical Power Cables
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Annealing Test for Wires and Conductors
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Tariff of electricity in India
High Voltage Transmission Lines
Power System Stability
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Remote Controlling of Circuit Breaker by Microcontroller
Electrical Insulator | Insulating Material | Porcelain Glass Polymer Insulator
Under Electrical Transmission
Insulating MaterialThe main cause of failure of overhead line insulator, is flash over, occurs in between line and earth during abnormal over voltage in the system. During this flash over, the huge heat produced by arcing, causes puncher in insulator body. Viewing this phenomenon the materials used for electrical insulator, has to posses some specific properties.
Properties of Insulating MaterialThe materials generally used for insulating purpose is called insulating material. For successful utilization, this material should have some specific properties as listed below- 1. It must be mechanically strong enough to carry tension and weight of conductors. 2. It must have very high dielectric strength to withstand the voltage stresses in High Voltage system. 3. It must possesses high Insulation Resistance to prevent leakage current to the earth. 4. The insulating material must be free from unwanted impurities. 5. It should not be porous. 6. There must not be any entrance on the surface of electrical insulator so that the moisture or gases can enter in it. 7. There physical as well as electrical properties must be less effected by changing temperature.
Porcelain InsulatorPorcelain in most commonly used material for over head insulator in present days. The porcelain is aluminium silicate. The aluminium silicate is mixed with plastic kaolin, feldspar and quartz to obtain final hard and glazed porcelain insulator material. The surface of the insulator should be glazed enough so that water should not be traced on it. Porcelain also should be free from porosity since porosity is the main cause of deterioration of its dielectric property. It must also be free from any impurity and air bubble inside the material which may affect the insulator properties.
Properties of Porcelain Insulator
|Dielectric Straingth||60 KV / cm|
|Compressive Strength||70,000 Kg / cm2|
|Tensile Strength||500 Kg / cm2|
Glass InsulatorNow days glass insulator has become popular in transmission and distribution system. Annealed tough glass is used for insulating purpose. Glass insulator has numbers of advantages over conventional porcelain insulator<
Advantages of Glass Insulator1. It has very high dielectric strength compared to porcelain. 2. Its resistivity is also very high. 3. It has low coefficient of thermal expansion. 4. It has higher tensile strength compared to porcelain insulator. 5. As it is transparent in nature the is not heated up in sunlight as porcelain. 6. The impurities and air bubble can be easily detected inside the glass insulator body because of its transparency. 7. Glass has very long service life as because mechanical and electrical properties of glass do not be affected by ageing. 8. After all, glass is cheaper than porcelain.
Disadvantages of Glass Insulator1. Moisture can easily condensed on glass surface and hence air dust will be deposited on the wed glass surface which will provide path to the leakage current of the system. 2. For higher voltage glass can not be cast in irregular shapes since due to irregular cooling internal cooling internal strains are caused.
Properties of Glass Insulator
|Dielectric Straingth||140 KV / cm|
|Compressive Strength||10,000 Kg / cm2|
|Tensile Strength||35,000 Kg / cm2|
Polymer InsulatorIn a polymer insulator has two parts, one is glass fiber reinforced epoxy resin rod shaped core and other is silicone rubber or EPDM (Ethylene Propylene Diene Monomer) made weather sheds. Rod shaped core is covered by weather sheds. Weather sheds protect the insulator core from outside environment. As it is made of two parts, core and weather sheds, polymer insulator is also called composite insulator. The rod shaped core is fixed with Hop dip galvanized cast steel made end fittings in both sides.
Advantages of Polymer Insulator1. It is very light weight compared to porcelain and glass insulator.
2. As the composite insulator is flexible the chance of breakage becomes minimum. 3. Because of lighter in weight and smaller in size, this insulator has lower installation cost. 4. It has higher tensile strength compared to porcelain insulator. 5. Its performance is better particularly in polluted areas. 6. Due to lighter weight polymer insulator imposes less load to the supporting structure. 7. Less cleaning is required due to hydrophobic nature of the insulator.
Disadvantages of Polymer Insulator1. Moisture may enter in the core if there is any unwanted gap between core and weather sheds. This may cause electrical failure of the insulator. 2. Over crimping in end fittings may result to cracks in the core which leads to mechanical failure of polymer insulator.
In addition to these, some other disadvantages might be experienced. Let us give a practical example where many difficulties are faced in maintaining a distribution network in Victoria Australia due to polymeric insulator.
There are many Cockatoos, Galahs & Parrots in that area of Australia, which love to chew on polymeric strain insulators. Here, the 22KV network has many of polymeric strain insulators installed and now after a few years of installing polymeric strain insulators, the authority is now replacing many of them back with Glass disc insulators.
Another disadvantage is that they have had post type polymeric insulators melt and bend in bushfire areas. They have a concrete pole and a steel cross arm that survives a bushfire, however the polymers in some cases fail. This would not be the case with glass or porcelain insulators.
They have also had polymeric insulators fail in areas close to the ocean coastline where there are high salt levels in the air.
- Subject to bird attack by Parrots, Cockatoos & Galahs.
- Not resilient to bushfire temperatures.
- Not recommended for location near surf beaches due to salt spray.
The information is contributed by Robert Lancaster of Australian Electricity Supply Industry
Types of InsulatorThere are mainly three types of insulator likewise
1. Pin Insulator 2. Suspension Insulator 3. Stray Insulator
In addition to that there are other two types of electrical insulator available mainly for low voltage application, e.i. stay insulator and shackle insulator.