When an alternating potential difference between the two conductors of a transmission line is raised beyond a certain limit, a point is reached when a pale violet glow appears on the conductor surface, together with a hissing noise. This phenomenon is known as Corona. Actually, air particles around the conductor surface get ionised and as result corona occurs.
Observations during Corona or Effect of Corona
1. A pale violet glow appears around the conductor.
2. Release of ozone gas (O3)
3. There is a hissing noise.
4. There is radio-interference.
5. Vibration may occur in the conductors.
6. Formation of tuffs and buds i.e. overall conductor radius increases.
7. Corona causes Power loss in the line.
8. Corona induces harmonic current as a result of which charging current increases.
Critical Disruptive Voltage
It is the minimum potential difference required between the conductors to start ionization of air particles around the conductor surface.
Critical Visual Voltage
It is the minimum voltage at which the visual glow of corona occurs.
Corona Power Loss
During Corona, heat is generated due to the collision of the free electrons with conductor surface. The power dissipated in the system due to corona discharge is known as corona loss.
Factors affecting Corona loss or Corona
1. Effect of supply frequency
Corona loss increases with the increase in supply frequency. The corona loss for HVDC lines is less than that for EHV AC transmission lines.
2. Effect of system voltage
Corona loss occurs due to the high electric field around the surface of the conductor. Therefore, higher the system voltage (potential difference between conductors), higher is the electric field and hence, higher is the power loss due to the corona.
3. Effect of conductivity of air
Higher is the conductivity of air, higher is the number of ions and higher is the power loss due to the corona. During rain and thunderstorms, ion contents increases and therefore, the atmosphere becomes more conducting and as the result, corona loss increases.
4. Effect of Density of air
Corona loss increases with the decrease in the density of air. Corona loss is more in hilly regions than that of a similar line in plains due to the reduced value of air density factor at higher altitudes.
5. Effect of conductor radius
With the increase in conductor radius, electric field intensity at the surface of conductor decreases resulting in reduced corona loss.
6. Effect of Conductor Surface
Corona loss is more for stranded conductor than for a solid conductor due to high potential gradient at the surface of a stranded conductor. Corona loss is more for conductors having rough surfaces.
7. Effect of Atmospheric Conditions
Corona loss is more in rainy and bad atmospheric conditions such as fog, sleet and snowstorms.
8. Bundling of Conductors
The effective diameter of the bundled conductor is much larger than that of the equivalent single conductor as a result of which corona loss is reduced.
9. Effect of Distance Between Conductors
Corona loss is inversely proportional to square root of the distance between the conductors. Hence if the distance between conductors is increased corona loss reduces.
10. Polarity of Conductors
For the conductor of positive polarity, corona loss is more than for a conductor of negative polarity.
Methods to reduce the effects of corona
1. The uses of bundled conductors reduce corona loss.
2. Spacing between conductors is selected so that corona is tolerable.
3. Since the shape of conductors affects corona loss, cylindrical shape conductors have a uniform field that reduces corona loss than any other shape.
4. The voltage stress and electric field gradient should be minimized which can be accomplished by following good high voltage design practices.
5. Using conductors with large radii reduce corona loss.
6. Void free solid conductors and insulators should be used.
7. The terminals on high voltage equipment are designed with a smooth round diameter rounded shapes like balls.
8. Addition of corona rings to insulators of high voltage transmission lines
What is the Practical Importance of Corona?
Corona acts as a safety value for the conductor surface and decreases the impact of lightning surges on
the surface of the conductor. It reduces the magnitude of high voltage steep-fronted waves due to lighting or
switching by partially dissipating them as corona loss.