Factors Affecting Corona in Overhead Transmission Lines
Author: Engr. Aneel Kumar
Introduction
The corona effect in overhead transmission lines is a phenomenon that occurs when the electric field intensity around conductors exceeds a critical value, causing ionization of the surrounding air. This ionization produces bluish light, hissing sound, power loss, and ozone gas. While corona may seem undesirable, it also has a few advantages such as reducing overvoltages by absorbing surges.
Corona directly impacts power system efficiency, transmission losses, equipment life, and design cost. Therefore, engineers must understand the factors affecting corona in detail to ensure efficient and reliable design of high-voltage transmission systems.
1. Conductor Size (Diameter)
The diameter of the conductor plays a crucial role in corona formation. A larger conductor size results in lower surface electric field intensity, which increases the corona inception voltage and decreases the chances of corona discharge. This is why bundled conductors are extensively used in EHV (Extra High Voltage) and UHV (Ultra High Voltage) systems.
For example, a 400 kV line with a single conductor may produce heavy corona, but the same line using bundled conductors will experience significantly less corona discharge. This reduces corona loss, audible noise, and radio interference.
2. Spacing Between Conductors
The spacing between conductors directly influences the electric field intensity. If conductors are placed close to each other, the field intensity increases and corona is more likely. By increasing conductor spacing, the disruptive critical voltage can be raised, thus reducing corona.
However, excessive spacing is not desirable because it increases tower size, cost, and right-of-way requirements. Therefore, an optimal balance between mechanical design and electrical performance must be achieved.
3. System Voltage
Corona effect is directly proportional to system voltage. At low voltages (below 30 kV), corona is negligible. However, at 220 kV, 400 kV, and higher transmission lines, corona becomes a major consideration in design. Engineers design lines such that the operating voltage is kept well below the corona inception voltage under fair weather conditions.
If the operating voltage is too close to the critical disruptive voltage, corona becomes continuous, leading to severe power losses and interference.
4. Atmospheric Conditions
The atmospheric conditions significantly affect corona performance. In rain, fog, snow, and dust, the conductor surface becomes irregular due to moisture or particles, which reduces the corona inception voltage. This results in a higher intensity of corona discharge.
In fair weather, corona is less severe and often negligible for properly designed systems. Thus, engineers design lines considering worst-case weather scenarios.
5. Conductor Surface Condition
The surface condition of conductors is another critical factor. Smooth, polished conductors maintain uniform electric field distribution, while rough or stranded conductors intensify the electric field at certain points, leading to early corona onset.
Corrosion, dust deposition, and aging of conductors worsen this effect. This is why transmission companies prefer conductors with protective coatings and periodic maintenance to keep surfaces clean.
6. Line Frequency
The frequency of the supply system also impacts corona. Corona loss increases with frequency because ionized particles oscillate more rapidly. Since most power systems operate at 50 Hz or 60 Hz, this factor remains constant; however, HVDC systems are immune to corona loss due to frequency effects.
7. Air Density and Pressure
The air density factor (δ) depends on altitude, temperature, and pressure. At higher altitudes, the air is thinner and offers less insulation strength, reducing the corona inception voltage. This makes corona more prominent in hilly or mountainous regions compared to coastal or plain regions.
Engineers apply correction factors while designing transmission lines for different altitudes to account for this variation.
Comparative Table: Factors vs. Effect on Corona
| Factor | Influence on Corona |
|---|---|
| Conductor Size | Larger diameter reduces corona effect by lowering surface electric field intensity. |
| Conductor Spacing | Increased spacing raises corona inception voltage and reduces corona discharge. |
| System Voltage | Higher operating voltage increases corona intensity and losses. |
| Atmospheric Conditions | Rain, fog, snow, and dust lower inception voltage, increasing corona. |
| Conductor Surface Condition | Rough, dirty, or corroded surfaces intensify electric fields and promote corona. |
| Line Frequency | Corona loss increases with frequency; HVDC is immune to this effect. |
| Air Density | Low air density at high altitudes reduces inception voltage, increasing corona. |
Advantages of Corona
- Reduces the severity of voltage surges and transients.
- Acts as a natural regulator by limiting voltage peaks.
- Provides a visible and audible warning of over-voltage conditions.
Disadvantages of Corona
- Causes corona loss, reducing transmission efficiency.
- Produces radio interference and audible noise.
- Generates ozone and nitrous oxides, which deteriorate insulation.
- Leads to power quality issues in nearby communication systems.
- Reduce transmission line efficiency.
- Leads to audible hissing and visual glow - undesirable in populated areas.
Conclusion
The corona effect is an inevitable phenomenon in high-voltage transmission lines, but its severity can be minimized through careful design, choice of conductor, optimal spacing, weather considerations, and maintenance. While it has certain advantages such as surge absorption, its disadvantages like power loss and interference outweigh the benefits in most cases.
By understanding the factors affecting corona, engineers can design transmission systems that balance reliability, efficiency, and cost-effectiveness, ensuring long-term stability of the power grid.