Presentationon

“Disadvantages of corona, radio interference, inductive interference

between power and communication lines”

Introduction

Disadvantages of corona.

Radio interference.

Inductive interference between power and communication lines

It has been observed that, if the spacing between two conductors is less than their diameters, & if applied potential between them gradually increased, a violet glow occurs round the conductor, along with a hissing sound, at a certain value of the potential. This phenomenon is known as corona.

The conductors have air gap between them, so air acts as a dielectric medium. The dielectric strength of air at normal temperature and pressure is 30kV/cm.

The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as corona.

For corona effect to occur effectively, two factors here are of prime importance as mentioned below:-

1) Alternating electrical potential difference must be supplied across the line.

2) The spacing of the conductors, must be large enough compared to the line diameter.

This breakdown accompanied by following phenomenon :

Hissing noise is produced.

A faint violet glow appears which is visible in the dark.

Production of ozone gas.

Radio interference occurs.

There is a loss of power.

Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased. The increased diameter reduces the electro static stress between the conductors.

Corona reduces the effect of transient produced by surges.

Effects of transients produced by lightning surges and other causes are reduced. Charge induced on the line by surges will be partially dissipated as a corona loss. In this way it acts as a safety valve.

Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.

Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.

Harmonics are produced in the line.

During the formation of corona, ozone gas is formed. This gas reacts with the material of the conductors and causes corrosion.

Electromagnetic interference, also called radio frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction.

High voltage transmission lines and substations can cause radio interference.

Since radio noise is associated with corona, it mainly depends on the potential gradients at the conductors.

It is also influenced by all those factors which influence corona, such as air density, humidity, wind contaminates, imperfections and precipitation.

The radio noise in bad weather is about 10-25 db higher than the radio noise during fair weather.

The radio interference is caused by electro-magnetic waves in the frequency range of broad cast frequencies.

Radio interference : 0.5 – 1.6 MHz By using bundled conductors, surface voltage stress, corona

and radio interference can be reduced.

Corona produces inductive interference between transmission lines and wireless lines, signal.

Radio interference due to corona is of the importance for lines above 200 kV.

The electromagnetic and the electrostatic effects mainly depend on what is the distance between power and communication circuits and the length of the route over which they are parallel.

The unacceptable disturbance which is produced in the telephone communication because of power lines is called Telephone Interference.

This noise is lesser in bed weather.

Radio interference field produces by transmission line is inversely proportional to radio frequencies.

This shows that higher frequency band service like T.V., frequency modulated broadcasting, microwave relay and radars have less chance of affecting their services.

In Extra High Voltage lines the radio noise is a very important consideration is designing a line.

There are various factors influencing the telephone interference. These factors are as follows:

1) Because of harmonics in power circuit, their frequency range and magnitudes.

2) Electromagnetic coupling between power and telephone conductor.

3) Due to unbalance in power circuits and in telephone circuits.

4) Type of return telephone circuit i.e. either metallic or ground return.

5) Screening effects.