Properties of Insulators

Introduction

It has been mentioned earlier that the overhead line conductors are bare without any insulating covering over them. While the metal structure in the form of towers is used to support such live conductors. To avoid the flow of current to the earth from live conductors through supports, there must be a safe clearance between live conductors and the supports. Thus the live conductors must be perfectly insulated from the supports. Hence the insulators are used between live conductors and the supports. The main function of the insulators is to provide perfect insulation between the live conductors and the supports and to prevent any leakage current from the live conductors to earth through the supports.

In highly polluted area, dust and other pollutants settle on the insulator's surface. These particles provide conductive path for the flow of leakage current and subsequent flashover on insulator surface. To avoid deterioration of insulator surface, proper and adequate creepage distance is required. To enhance creepage, greater number of insulators are provided on the transmission lines in heavily polluted areas.

The insulators are fitted with arcing horns. The arcing horns are used to pass away the impulse voltage in the transmission line which causes insulator failure due to flashover arc on the surface of the insulator.

1. Properties of Insulators

To provide necessary insulation, the insulators must have the following properties,

1. The insulators along with providing the insulation, also provide support to the conductors. Hence to withstand the conductor load alongwith wind pressure etc, the insulators must be mechanically very strong.

2. The main function of the insulators is to resist any leakage current. Thus the insulators must have very high insulation resistance.

3. The insulators must be free from internal impurities such as holes, cracks, laminations etc. This reduces the permittivity of the insulators.

4. The dielectric strength of the insulators must be very high.

5. To have high dielectric strength, the relative permittivity of the insulating material should be very high.

6. The insulating material should be nonporous.

7. The insulators should not be affected by the changes in the temperature.

8. Due to very high voltage of line conductors, there is possibility of flash over between line conductors and earth. The flash over means production of arc due to which extreme heat gets produced. This is the main cause of insulator failure. 

Hence the insulators must have high ratio of puncture strength to flash over voltage.

Review Question

1. Why the insulators are used with overhead lines ? Discuss the desirable properties of insulators.