CORONA In Transmission Lines

 CORONA

When an alternating current is made to flow across two conductors of the transmission line whose spacing is large compared to their diameters, then air surrounding the conductors (composed of ions) is subjected to di-electric stress, At low values of supply end voltage, nothing really occurs as the stress is too less to ionize the air outside. 

But when the potential difference is made to increase, then the field strength at the surface of conductor increases and then the air surrounding it experiences stress high enough to be dissociated into ions making the atmosphere conducting if the stress increases beyond threshold value of 30 kV (peak) known as the break down voltage of air at normal temperature and pressure. 

This results in electric discharge around the conductors due to the flow of these ions, giving rise to a faint luminescent glow, along with the hissing sound accompanied by the liberation of ozone, which is readily identified due to its characteristic odor. 

This phenomena of electrical discharge occurring in transmission line for high values of voltage is known as the corona effect in power system. If the voltage across the lines is still increased the glow becomes more and more intense along with hissing noise, inducing very high power loss into the system.

FACTORS AFFECTING CORONA

Atmospheric Conditions

It has been physically proven that the voltage gradient for di- electric breakdown of air is directly proportional to the density of air. Hence in a stormy day, due to continuous air flow the number of ions present surrounding the conductor is far more than normal, and hence it is more likely to have electrical discharge in transmission lines on such a day. compared to a day with fairly clear weather.

Condition of Conductors

It has an inverse proportionality relationship with the diameter of the conductors. i.e. with the increase in diameter, the effect of corona in power system reduces considerably. Also the presence of dirt or roughness of the conductor reduces the critical breakdown voltage, making the conductors more prone to corona losses. Hence in most cities and industrial areas having high pollution, this factor is of reasonable importance to counter the ill effects it has on the system.

Spacing between Conductors

As already mentioned, for corona to occur effectively the spacing between the lines should be much higher compared to its diameter, but if the length is increased beyond a certain limit, the di-electric stress on the air reduces and consequently the effect of corona reduces as well. If the spacing is made too large then corona for that region of the transmission line might not occur at all.

METHODS TO REDUCE THE EFFECTS OF CORONA

✓ The use of bundle conductors reduce corona loss Spacing between conductors is selected so that corona is tolerable.

✓  Since the shape of conductors affect corona loss, cylindrical shape conductors have uniform field that reduces corona loss than any other shape.

✓ The voltage stress and electric field gradient should be minimized which can be accomplished by following good high voltage design practices.

✓Using conductors with large radii reduce corona loss.

✓Void free solid conductors and insulators should be used.

✓The terminals on high voltage equipment are designed with smooth round diameter rounded shapes like balls

✓ Addition of corona rings to insulators of high voltage transmission lines.