The Corona is the ionization of air surrounding the power conductor. The phenomenon of violet glow, hissing noise, and the production of ozone gas in an overhead transmission line is known as the Corona.
When an alternating potential difference is applied across two conductors that exceed a certain value, called critical disruptive voltage*, the conductors are surrounded by a faint violet glow called the Corona.
Corona Effect in Electrical Transmission Lines: When an alternating potential difference is applied across two conductors that exceed a certain value, called critical disruptive voltage, the conductors are surrounded by a faint violet glow called the Corona Effect.
[*Critical disruptive voltage: It is the minimum phase-neutral voltage at which corona occurs]
When the corona effect occurs in the overhead transmission line there will be a hissing sound, production of ozone, power loss, and radio interference.
Corona Formation in 5 points
♦ Under normal conditions, the air around the conductors contains some ionized particles (i.e., free electrons and +ve ions) and neutral molecules.
♦ When p.d. is applied between the conductors, a potential gradient is set up in the air. Under the influence of a potential gradient, the existing free electrons acquire greater velocities. The greater the applied voltage, the greater the potential gradient, and the more the velocity of free electrons.
♦ When the potential gradient at the conductor surface reaches about 30 kV per cm (max. value), the velocity acquired by the free electrons is sufficient to strike a neutral molecule with enough force to dislodge one or more electrons from it.
♦ This produces another ion and one or more free electrons, which in turn are accelerated until they collide with other neutral molecules, thus producing other ions.
♦ Thus, the process of ionization is cumulative. The result of this ionization is that either corona is formed or a spark takes place between the conductors.
Factors Affecting Corona
The following are the factors upon which corona depends:
♦ Atmosphere: As corona is formed due to the ionization of air surrounding the conductors, therefore, it is affected by the physical state of the atmosphere. In stormy weather, the number of ions is more than normal, and as such corona occurs at much less voltage as compared with fair weather.
♦ Conductor size: The corona effect depends upon the shape and conditions of the conductors. The rough and irregular surface will give rise to more corona because unevenness of the surface decreases the value of breakdown voltage.
♦ Spacing between conductors: If the spacing between the conductors is made very large as compared to their diameters, there may not be any corona effect. It is because a larger distance between conductors reduces the electro-static stresses at the conductor surface, thus avoiding corona formation.
♦ Line voltage: The line voltage greatly affects corona. If it is low, there is no change in the condition of air surrounding the conductors and hence no corona is formed. However, if the line voltage has a greater value, then a corona can be formed.
Methods of Reducing Corona Effect
♦ By increasing conductor size: By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced.
♦ By increasing conductor spacing: By increasing the spacing between conductors, the voltage at which the corona occurs is raised, and hence corona effects can be eliminated. However, spacing cannot be increased too much otherwise the cost of supporting structure (e.g., bigger cross arms and supports) may increase to a considerable extent.
Advantages and Disadvantages of Corona
Advantages of Corona:
» It reduces the magnitude of high voltage steep fronted waves due to lighting or switching by partially dissipating as a corona loss. In this way, it acts as a safety valve to some extent.
» Due to corona formation, the virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stresses between the conductors.
» Corona reduces the effects of transients produced by surges.
Disadvantages of Corona Effect
» 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.
» The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring communication lines.
