Short Answer:

Corona discharge in transmission lines is a phenomenon where the air around a high-voltage conductor becomes ionized, causing a faint glow, hissing sound, and sometimes a loss of power. It usually occurs when the voltage level exceeds a certain limit, known as the critical disruptive voltage, and is more common in humid or rainy conditions.

This discharge results in energy loss, electromagnetic interference, and damage to nearby insulation. Engineers try to reduce corona discharge by using smooth and larger-diameter conductors or bundled conductors to lower the electric field intensity at the conductor surface.

Detailed Explanation:

Corona Discharge in Transmission Lines

Corona discharge is a form of electrical discharge caused by the ionization of air surrounding a high-voltage conductor. It occurs when the electric field strength around the conductor becomes strong enough to break down the air molecules, turning them into ions. This usually happens when the conductor is operated at very high voltage, particularly in extra high voltage (EHV) and ultra-high voltage (UHV) transmission systems.

Although corona appears as a faint bluish glow and makes a hissing noise, its effects are much more serious, including power losses, signal interference, and equipment damage. Understanding this phenomenon is crucial for designing efficient and reliable transmission systems.

Conditions Causing Corona Discharge

1. High Voltage
   When the line voltage exceeds a certain limit, the air insulation between conductors and the ground begins to break down, causing ionization.
2. Sharp Conductor Edges
   Corona is more likely to happen at sharp edges or pointed conductors because they create stronger electric fields.
3. Small Diameter Conductors
   Conductors with smaller diameters have higher surface electric field intensity, making them more prone to corona discharge.
4. Atmospheric Conditions
   High humidity, rain, dust, and pollution reduce air’s dielectric strength, increasing the chances of corona.
5. Spacing Between Conductors
   If conductors are placed too close to each other, the electric field between them becomes stronger, leading to corona.

Effects of Corona Discharge

1. Power Loss
   Corona leads to continuous energy loss in the form of heat, light, sound, and chemical reactions. These are known as corona losses, which lower the efficiency of power transmission.
2. Radio and TV Interference
   Corona produces electromagnetic noise, which can interfere with communication signals, especially near long EHV lines.
3. Audible Noise
   The ionized air produces a hissing or crackling sound, especially during wet weather or at night when ambient noise is low.
4. Ozone and Nitric Acid Formation
   Corona discharge leads to the formation of ozone (O₃) and nitric acid, which are corrosive and harmful to both equipment and the environment.
5. Insulation Damage
   The discharge stresses insulation systems, reducing their life span and increasing the risk of faults or breakdowns.

Methods to Reduce Corona Discharge

1. Use of Larger Diameter Conductors
   Larger conductors reduce the electric field intensity on the surface, thus lowering the chance of ionization.
2. Bundled Conductors
   Using two or more conductors per phase spreads out the electric field, reducing corona formation.
3. Smooth Surface Conductors
   Conductors with polished or smooth surfaces help to minimize electric field concentration.
4. Increasing Conductor Spacing
   Wider spacing between phase conductors reduces the field strength between them, reducing the corona effect.
5. Operating Below Critical Disruptive Voltage
   Keeping the operating voltage below the threshold where ionization begins helps prevent corona discharge.

Real-Life Examples

• In overhead lines, especially at voltages above 220 kV, corona becomes a serious concern. That’s why bundled conductors are commonly used in such lines.
• During foggy or rainy nights, visible corona glow and hissing sound are often observed near substation terminals or transmission towers.
• Corona is rarely a problem in underground cables, as they are shielded and insulated from atmospheric effects.

Conclusion

Corona discharge in transmission lines is an unwanted but natural result of high electric field strength around conductors, especially in high-voltage systems. It causes energy losses, noise, and damage to equipment. Although it cannot be fully eliminated, it can be effectively minimized by using design techniques such as bundled conductors, larger conductor diameters, and proper spacing. Managing corona discharge is essential to ensure efficient, safe, and interference-free power transmission.