Corona in Electrical System: Causes, Effects, Corona Loss & Prevention
Corona in electrical system is an important phenomenon associated mainly with high-voltage transmission lines and high-voltage electrical equipment. When the electric field around an energized conductor becomes sufficiently strong, it can ionize the surrounding air and produce a localized electrical discharge known as corona discharge or corona effect.
Corona may appear as a faint bluish or violet glow and can produce a characteristic hissing or crackling sound. It can also cause power loss, electromagnetic interference, ozone formation and other effects that need to be considered while designing and operating high-voltage systems.
In this article, we will understand what corona is, how corona occurs, factors affecting corona, corona loss, advantages and disadvantages of corona, and methods used to reduce the corona effect in electrical systems.
What is Corona in Electrical System?
Corona is a localized electrical discharge that occurs around a high-voltage conductor when the electric field at or near its surface becomes strong enough to ionize the surrounding air.
Normally, air acts as an electrical insulator. However, when the electric field becomes sufficiently high, free electrons can gain enough energy to produce further ionization. This creates charged particles around the conductor and results in a partial discharge.
Unlike a complete electrical breakdown or flashover, corona is generally confined to a region around the conductor where the electric field is strongest.
Corona is particularly important in high-voltage AC and DC transmission systems, where conductor voltage and electric-field stress can be very high.
How Does Corona Effect Occur?
The formation of corona can be understood in a few simple steps:
- A high voltage is applied to the conductor.
- The electric field develops around the conductor.
- As voltage increases, the electric field near the conductor surface also increases.
- When the field reaches the required ionization level, electrons in the surrounding air become accelerated.
- These electrons collide with air molecules and produce additional ions and electrons.
- A localized discharge develops around the conductor.
- Light, sound, electromagnetic interference and energy loss may occur.
The voltage at which corona begins is generally referred to as the corona inception voltage or, in traditional transmission-line analysis, the critical disruptive voltage.
Why Does Corona Occur in Transmission Lines?
Corona is more likely in high-voltage transmission systems because increasing voltage increases the electric-field stress around conductors.
1. High Operating Voltage
Higher transmission voltage produces greater electric-field stress around the conductor. Therefore, corona becomes an important design consideration in EHV and UHV transmission systems.
2. Small Conductor Diameter
A smaller conductor has a stronger electric field near its surface for a given operating condition. Increasing the effective conductor diameter can therefore help reduce corona.
3. Rough or Irregular Conductor Surface
A smooth conductor distributes the electric field more uniformly. Scratches, sharp points, damaged strands, dirt and other surface irregularities can locally increase electric-field intensity and encourage corona formation.
4. Reduced Air Density
Air density decreases with altitude. Lower air density reduces the voltage required for ionization, which can make corona more likely at high-altitude locations.
5. Rain, Fog and Moisture
Weather conditions can significantly influence corona. Water droplets and other surface conditions can create regions of enhanced electric-field stress. Corona activity and associated audible noise can become more significant during wet weather.
6. Conductor Spacing and Configuration
The arrangement and spacing of conductors influence the electric-field distribution around a transmission line. Proper conductor spacing and suitable line geometry are therefore important in high-voltage design.
Corona Loss in Transmission Lines
One of the most important disadvantages of corona is power loss.
During corona discharge, part of the electrical energy is dissipated in the surrounding air. This energy does not contribute to useful power delivered to the load and is therefore considered a transmission-line loss.
Corona power loss becomes particularly important when designing high-voltage and extra-high-voltage transmission lines.
An empirical expression commonly associated with corona-loss calculations is based on Peek-type relationships. A simplified form used in engineering analysis relates corona loss to parameters such as frequency, air-density factor and the difference between operating voltage and disruptive critical voltage.
A key point is that corona loss is not simply a fixed percentage of transmission power. It depends strongly on the electrical and environmental conditions of the line.
Effects of Corona in Electrical Systems
Corona can have several effects on electrical systems.
1. Power Loss
Corona consumes a portion of the transmitted electrical energy. In high-voltage transmission systems, this can affect overall transmission efficiency.
2. Audible Noise
Corona can produce a characteristic hissing, buzzing or crackling sound. The sound is generally more noticeable under unfavorable weather conditions such as rain.
This audible noise is one of the factors considered when planning high-voltage transmission lines near populated areas.
3. Radio and Electromagnetic Interference
Corona produces short-duration current pulses and electromagnetic radiation. These disturbances can interfere with nearby radio and communication systems.
Corona-generated electromagnetic interference is therefore an important consideration in the design of high-voltage overhead transmission lines.
4. Ozone Formation
Corona discharge can produce ozone and other reactive species because of electrical activity in the surrounding air.
Ozone can participate in chemical reactions and may contribute to deterioration of nearby materials under certain conditions.
5. Insulation and Equipment Stress
Persistent corona around high-voltage equipment can indicate excessive electric-field stress. If corona is associated with defective insulation, sharp edges, poor connections or damaged components, it can become a warning sign of a larger insulation problem.
For this reason, corona detection is useful in high-voltage equipment condition monitoring.
Advantages of Corona Effect
Although corona is usually discussed as an undesirable phenomenon, it can have some beneficial effects.
1. Reduction of Sudden Voltage Surges
Corona can dissipate a portion of the energy associated with steep-fronted high-voltage surges. In this sense, it can act to some extent as a natural energy-dissipation mechanism.
2. Reduction of Electric-Field Stress
Ionization around the conductor changes the local electric-field conditions. This can partially redistribute electric-field stress around the conductor.
However, these effects should not be interpreted as a reason to intentionally operate a transmission line with excessive corona. Transmission-line designs generally aim to keep corona within acceptable limits.
Disadvantages of Corona
The major disadvantages of corona include:
- Power loss in transmission lines
- Reduced transmission efficiency
- Audible noise
- Radio and electromagnetic interference
- Ozone and reactive chemical formation
- Possible deterioration of materials
- Additional environmental and maintenance considerations
- Indication of excessive electric-field stress in equipment
Therefore, controlling corona is an important part of high-voltage electrical-system design.
How to Reduce Corona Effect in Transmission Lines?
Engineers use several methods to minimize corona.
1. Increase Conductor Diameter
A larger conductor diameter reduces the electric-field intensity at the conductor surface. This increases the voltage at which corona begins.
This is one reason why suitable large-diameter conductors are used for high-voltage transmission applications.
2. Use Bundled Conductors
In EHV and UHV transmission systems, several subconductors can be arranged together as a bundle.
A bundled conductor system increases the effective electrical radius of the phase conductor and helps reduce surface electric-field stress. It can therefore improve corona performance.
3. Use Smooth Conductors
Conductor surfaces should be maintained as smooth as practical. Sharp points, damaged strands and other irregularities can increase local electric-field concentration.
Proper conductor handling during installation is therefore important.
4. Increase Conductor Spacing
Appropriate phase-to-phase and phase-to-ground spacing can improve the electric-field distribution and help control corona.
However, increasing spacing also affects tower dimensions, right-of-way requirements and project cost, so an engineering balance is required.
5. Use Corona Rings and Grading Rings
Corona rings and grading rings are commonly used around high-voltage equipment and insulation systems to distribute electric-field stress more uniformly.
They are particularly useful around terminals, bushings, insulator assemblies and other areas where sharp changes in geometry could produce high local electric-field concentrations.
6. Proper Equipment Design
High-voltage equipment should avoid unnecessary sharp edges and abrupt changes in conductor geometry. Proper field grading can significantly improve corona performance.
Corona Effect vs Flashover
Corona and flashover are not the same phenomenon.
Corona is a localized partial discharge around a conductor or electrode where the surrounding air becomes ionized.
Flashover, on the other hand, is a much more extensive electrical discharge that can bridge an insulation path or gap.
In simple words:
- Corona: Localized ionization and partial discharge.
- Flashover: Complete disruptive discharge across an insulation path or gap.
Understanding this difference is important for electrical engineering students and power-system professionals.
Corona Effect in AC and DC Systems
Corona can occur in both AC and DC high-voltage systems.
In AC systems, the electric field changes continuously with the alternating voltage, and corona behavior can differ between positive and negative half-cycles.
In DC systems, corona behavior depends on polarity, conductor configuration, surface condition and atmospheric conditions.
Therefore, corona analysis for HVDC and HVAC transmission systems requires consideration of the specific electrical and environmental conditions of the line.
How is Corona Detected?
Corona can be detected using several methods depending on the application.
Common indicators include:
- Audible hissing or crackling
- Visible glow under suitable conditions
- Radio-frequency interference
- Ultraviolet detection
- High-frequency electrical measurements
- Specialized corona cameras and inspection systems
Modern condition-monitoring techniques can help locate corona activity around transmission-line components and high-voltage equipment.
Frequently Asked Questions About Corona in Electrical System
What is corona in an electrical system?
Corona is a localized electrical discharge caused by ionization of air around a high-voltage conductor when the electric-field intensity becomes sufficiently high.
What causes corona in transmission lines?
High voltage, small conductor diameter, rough conductor surfaces, reduced air density, unfavorable weather conditions and unsuitable conductor geometry can contribute to corona.
What are the main disadvantages of corona?
The major disadvantages are power loss, audible noise, radio and electromagnetic interference, ozone formation and possible deterioration of materials.
How can corona loss be reduced?
Corona can be reduced by using larger conductors, bundled conductors, smooth conductor surfaces, suitable conductor spacing and proper electric-field grading using devices such as corona rings.
Does corona occur only in AC systems?
No. Corona can occur in both AC and DC high-voltage systems. Its characteristics depend on voltage polarity, conductor geometry, surface condition and atmospheric conditions.
Why are bundled conductors used in EHV transmission lines?
Bundled conductors increase the effective radius of the phase conductor and reduce electric-field stress at the conductor surface. This helps control corona and its associated effects.
Conclusion
Corona in electrical systems is an important high-voltage phenomenon that occurs when the electric field around a conductor becomes strong enough to ionize the surrounding air.
Although corona can provide some natural dissipation of high-voltage transients, excessive corona is undesirable because it can cause corona power loss, audible noise, electromagnetic interference and ozone formation.
For this reason, engineers use larger conductors, bundled conductors, suitable conductor spacing, smooth surfaces and corona or grading rings to control electric-field stress.
For electrical engineering students, understanding corona effect, corona loss, critical disruptive voltage, factors affecting corona and methods of reducing corona is especially important for power-system and transmission-line studies.
In practical EHV transmission design, corona is not treated as just a theoretical topic—it is an important factor affecting efficiency, electromagnetic compatibility, noise and reliable operation of high-voltage networks.
Also Read: More Electrical Engineering topics, power system concepts, transmission line fundamentals and electrical interview questions on our blog.