What is the corona effect in a power system?

Corona occurs around the conductors which has no insulation around the conducting material.So it is seen mainly in over head line conductors.When voltage applied to the conductor exceeds 30 kv then the air particles will ionise and provides a path in flowing the power instead of through the conductor.This occurs why only beyond 30kv ?

Because the air dielectric strength is 30kv/cm

The air particles will break and provides a path for power flow when voltage exceeds beyond 30kv/cm

This effect we can observe in places where we can have moisture content more in air because moisture content decreases the dielectric strength of air and thus air breaks down at lower voltage levels.Corona effect converts electrical energy into light and sound energy.

We can see a bluish or violet colour around conductor with a hissing sound.

Corona effect is the result of ionization of air in overhead transmission line.

Air is not pure insulator, it contains good amount of free electrons. Air’s dielectric strength is 30 kV/cm, it means that if we keep two conductors parallel to each other at a distance of 1 cm and apply more than 30 kV voltage then air will ionize resulting in break down and both conductors will be shorted.

Electric field is maximum at the surface of the conductor in overhead transmission line, zero at the center of conductor and outside it is perpendicular to the surface.

All the excess charges resides on the surface of the conductor and these are the main cause of corona, they ionizes the air around the conductor which causes hissing noise and luminous glow in the transmission line; it also causes the formation of ozone. But to see corona, transmission line voltage should be high enough to ionize the air.

There are two important terms used in corona effect:

Disruptive voltage: It is the voltage at which air break down happens.

Visual Critical Voltage (Vc): It is the voltage at which corona begins.

The loss due to this corona effect is known as corona loss which, according to peek’s formula is,

This formula shows the dependency of corona loss on various parameters.