Electrical Insulator

What is an Electrical Insulator?

An electrical insulator (also referred to as an insulator) is used in an electrical system to prevent unwanted flow of current to the earth from its supporting points. The insulator plays a vital role in the electrical system. An electrical insulator is a very high resistive path through which practically no current can flow.

In transmission and distribution systems, the overhead conductors are generally supported by supporting towers or poles. The towers and poles both are properly grounded. So there must be an insulator between tower or pole body and current-carrying conductors to prevent the flow of current from conductor to earth through the grounded supporting towers or poles.

Insulating Material

The main cause of failure of the overhead line insulator is flashover, which occurs between line and earth during abnormal overvoltage in the system. During this flashover, the huge heat produced by arcing causes puncher in the insulator body. Viewing this phenomenon the materials used for electrical insulator has to possess some specific properties.

Properties of Insulating Material

The materials generally used for the insulating purpose is called insulating material. For successful utilization, this material should have some specific properties as listed below-

It must be mechanically strong enough to carry the tension and weight of conductors.It must have a very high dielectric strength to withstand the voltage stresses in High Voltage transmission systems. It must possess high Insulation Resistance to prevent leakage current to the earth.The insulating material must be free from unwanted impurities.It should not be porous.There must not be any entrance on the surface of the electrical insulator so that the moisture or gases can enter it.There physical as well as electrical properties must be less effected by changing temperature.

Porcelain Insulator

Porcelain is the most commonly used material for overhead insulators in the present day. The porcelain is aluminum silicate. The aluminum 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

Property        Value(Approximate)
Dielectric Strength     60 kV / cm
Compressive Strength 70,000 Kg / cm²
Tensile Strength 500 Kg / cm²

Glass Insulator

Nowadays glass insulators have become popular in transmission and distribution systems. Annealed tough glass is used for the insulating purpose. Glass insulator has numbers of advantages over conventional porcelain insulator

Advantages of Glass Insulator

It has a very high dielectric strength compared to porcelain.Its resistivity is also very high.It has a low coefficient of thermal expansion.It has a higher tensile strength compared to porcelain insulator.As it is transparent in nature the is not heated up in sunlight as porcelain.The impurities and air bubbles can be easily detected inside the glass insulator body because of its transparency.Glass has a very long service life because the mechanical and electrical properties of glass do not be affected by aging.After all, glass is cheaper than porcelain.

Disadvantages of Glass Insulator

Moisture can easily be condensed on the glass surface and hence air dust will be deposited on the wed glass surface which will provide a path to the leakage current of the system.For higher Voltage glass can not be cast in irregular shapes since due to irregular cooling internal strains are caused.

Properties of Glass Insulator

Property    Value(Approximate)
Dielectric Strength 140 kV / cm
Compressive Strength 10,000 Kg / cm²
Tensile Strength 35,000 Kg / cm²

Polymer Insulator

A polymer insulator has two parts, one is glass fiber reinforced epoxy resin rod-shaped core and the other is silicone rubber or EPDM (Ethylene Propylene Diene Monomer) made weather sheds. The rod-shaped core is covered by weather sheds. Weather sheds protect the insulator core from the outside environment. As it is made of two parts, core and weather sheds, polymer insulator is also called a composite insulator. The rod-shaped core is fixed with Hop dip galvanized cast steel made end fittings on both sides.

Advantages of Polymer Insulator

It is very lightweight compared to porcelain and glass insulator.As the composite insulator is flexible the chance of breakage becomes minimum.Because of lighter in weight and smaller in size, this insulator has lower installation costs.It has a higher tensile strength compared to a porcelain insulator.Its performance is better, particularly in polluted areas.Due to lighter weight polymer insulator imposes less load to the supporting structure.Less cleaning is required due to the hydrophobic nature of the insulator.

Disadvantages of Polymer Insulator

Moisture may enter in the core if there is any unwanted gap between core and weather sheds. This may cause the electrical failure of the insulator.Over crimping in end fittings may result in cracks in the core which leads to mechanical failure of polymer insulator.

In addition to this, 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 insulators.

There are many Cockatoos, Galahs, and Parrots in that area of Australia, which loves to chew on polymeric strain insulators. Here, the 22 kV network has many 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 bush fire areas. They have a concrete pole and a steel cross arm that survives a bush fire, 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, and Galahs.Not resilient to bushfire temperatures.Not recommended for a location near surf beaches due to salt spray.

The information is contributed by Robert Lancaster of Australian Electricity Supply Industry

Types of Insulators

There are mainly three types of insulator likewise

Pin InsulatorSuspension InsulatorStray Insulator

In addition to that, there are other two types of electrical insulators available mainly for low voltage applications, i.e. stay insulator and shackle insulator. Read more about the various types of insulators used in transmission lines.

Corona

This is electrical Corona 

A corona discharge is an electrical discharge brought on by the ionization of a fluid such as air surrounding a conductor that is electrically charged. Spontaneous corona discharges occur naturally in high-voltage systems unless care is taken to limit the electric field strength. A corona will occur when the strength of the electric field (potential gradient) around a conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same property as a gas discharge lamp.

In many high voltage applications, corona is an unwanted side effect. Corona discharge from high voltage electric power transmission lines constitutes an economically significant waste of energy for utilities. In high voltage equipment like cathode ray tube televisions, radio transmitters, X-ray machines, and particle accelerators, the current leakage caused by coronas can constitute an unwanted load on the circuit. In the air, coronas generate gases such as ozone (O3) and nitric oxide (NO), and in turn, nitrogen dioxide (NO2), and thus nitric acid (HNO3) if water vapor is present. These gases are corrosive and can degrade and embrittles nearby materials, and are also toxic to humans and the environment.

Corona discharges can often be suppressed by improved insulation, corona ring, and making high voltage electrodes in smooth rounded shapes. However, controlled corona discharges are used in a variety of processes such as air filtration, photocopiers, and ozone generators.