Monday, October 6, 2014

Corona Loss in Transmission Lines; Hissing sound in transmission lines

Corona Effect

Corona phenomenon is the ionization of the surrounding air near power conductor. Free electrons are always present in free space because of radioactivity and cosmic rays. As the potential between the conductors is increased, the gradient around the surface of the conductor increase. The free electrons will move with certain velocity depending upon the field strength.These free electrons will collide with molecules of air and in-case they have high velocity they will dislodge the electrons from molecules and which will leads to increase in no. of electrons. This will form a electron avalanche. 

In case the ratio of spacing between conductors to the radius of conductor is less than 15, flashover will occur between conductors before ionization.

Corona loss occurs with hissing sound which is very clearly listened in HT lines. There sound is even very clearer during rainy seasons.

Corona loss is given by formula as below:-

Corona loss= 241X10-5 f+25 (r/d)1/2    (Vp-Vo) 2

d= air density correction factor

Corona loss directly proportional to (Vp-Vo) 2
Vo= rgd (ln d/r)
Where g= Dielectric strength and is equal to 30 KV/cm peak at NTP
So v is directly proportional to diameter
also corona loss is  proportional to (r/d)1/2
So by combining both effects we see that corona loss reduces as conductor diameter increases

Corona Effect in Transmission Line

When an alternating electric 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 beyond some threshold value of around 30 kV known as the critical disruptive voltage, then the field strength increases and then the air surrounding it experiences stress high enough to be dissociated into ions making the atmosphere conducting. 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 which must be accounted for.

Factors Affecting Corona Effect in Power System.

As mentioned earlier, the line voltage of the conductor is the main determining factor for corona in transmission lines, at low values of voltage (lesser than critical disruptive voltage) the stress on the air is too less to dissociate them, and hence no electrical discharge occurs. Since with increasing voltage corona effect in a transmission line occurs due to the ionization of atmospheric air surrounding the cables, it is mainly affected by the conditions of the cable as well as the physical state of the atmosphere. Let us look into these criterion now with greater details :

Atmospheric Conditions for Corona in Transmission Lines

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 its more likely to have electrical discharge in transmission lines on such a day, compared to a day with fairly clear weather. The system has to designed taking those extreme situations into consideration.

Condition of Cables for Corona in Transmission Line.

This particular phenomena depends highly on the conductors and its physical condition. 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.