High transmission lines also known as high tension lines carries high voltages in terms of KVS. Air act as a dielectric medium between the conductors of transmission lines. Due to high voltages, electrostatic field is produce around the conductor. When voltage gradient at the surface of any conductor exceeds a particular value called as critical disruptive voltage (1), high electrical field intensity is produce which is greater than the dielectric strength of air (medium) surrounding the transmission line. This causes the electrical breakdown and ionization of air around the nearby line.
The ionized air let the charging current flow through it to due to which hissing sound or faint violet glow arc is produce around the conductor. Electrical discharge due to the ionization of air give rise to a phenomenon known as Corona Discharge or Corona Effect. Di-electric strength of air is around 30kV/cm. Corona discharge causes power loss known as corona loss.
(1) : The maximum allowable voltage of transmission line at which corona loss doesn’t occur is called critical disruptive voltage.
How can we reduce reduce Corona loss in transmission Lines ?
Corona loss cause power losses ↗ and they are undesirable. These losses can be minimized but cannot be eliminated completely. There are various factors which affect the breakdown voltage of air and needs to be control in order to reduce corona loss in transmission line. Corona loss depends on the following factors:
Spacing between Conductors
If spacing between the conductors is increased, critical disruptive voltage at which corona occurs also increases. In that way, corona loss reduces. It is to be noted that the spacing between the conductors should not be increase too much since the cost of supporting towers increases in that case. Hence proper distance should be maintain between the conductors.
Line Voltage :
Voltage of transmission line affects corona. In case of low line voltages, electric field intensity produce will be less due to which corona loss is absent. However, the problem arises when the line voltages has such a high value that the develop electric intensity at the conductor surface makes the surrounding air to get ionized. In that case, air becomes conducting due to which hissing noise or violet glow is produce across the conductor. This shows the formation of corona.
Radius of Conductors :
Corona loss can be decrease by increasing the radius of conductor. For that purpose, bundled conductors are used in high voltage transmission lines. Bundles conductors ( more than one conductor per phase) reduces the voltage gradient at the conductor surface which results in less corona loss.
Corona discharge can be control by using corona control rings. These are circular conducting rings which reduces the electric field gradient below the corona threshold value due to which corona loss is prevented.
Shape of Conductors :
Corona loss can be decreased by using conductors having smooth surface. If there are irregularities on the conductor surface, such that there are scratches on the conductor or sharp edges are present. In that case, value of critical disruptive voltage decreases due to which corona loss increases.
Environmental Factors :
Weather is an important factor which affects the corona loss. During rainy weather, moisture increases the conductivity of air due to which breakdown voltage of air decreases. Once breakdown voltage of air decreases, corona loss increases.
Corona loss is inversely proportional to the air density ( i.e. corona loss increases with the decrease of air density). In hilly areas, density of air is low due to low pressure. Hence transmission line passing through hilly areas experience high corona losses since air density is low in that region.
Advantages of Corona :
When lightning or switching occurs, high voltage surges may flow across the transmission line. Corona discharge reduces the steepness of overvoltage surges due to which it is not harmful for transmission line equipment’s ( transformers, Circuit Breakers etc. )
Disadvantages of Corona :
Corona causes power losses due to which efficiency of transmission line is reduced.
Corona discharge is responsible for causing electromagnetic interference in radio and TV signals.
This effect also produces ozone which causes corrosion of conductors.
Due to Corona, non- sinusoidal voltage waves occurs which causes inductive interference in the nearby communication line.
Corona discharge is the result of an electric field strength which varies with the distance from the wire. Field strength is higher near the conductor and ionization of air occurs there. Corona loss occurs when electric field strength is greater than the dielectric strength of air. Moreover, corona discharge rapidly took place at sharp points, corners and edges of conductor. Hence it is necessary to use conductors having a smooth surface in order to prevent corona loss.
That’s all. Hope this will helps you.
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