In the world of high-voltage power and electrical engineering, there is a phenomenon that is both an enemy and a useful tool. We are talking about a corona discharge - a type of independent gas discharge that occurs when the electric field is sharply inhomogeneous near electrodes with low surface curvature. This process is often accompanied by a characteristic glow and hissing sound, which in the professional environment is called “corona”.
For energy engineers and power specialists high voltage equipment Understanding the mechanisms by which this phenomenon arises is critically important. On the one hand, an uncontrolled discharge leads to colossal power losses, insulation wear and radio interference. On the other hand, it is this effect that underlies the operation of electric precipitators, ozonizers and gas purification systems.
To effectively manage the process or prevent its negative consequences, it is necessary to clearly understand the physical conditions under which a gas loses its dielectric properties and enters a conducting state. In this article we will analyze in detail the criteria, formulas and practical aspects that determine the moment of breakdown.
Physical nature and mechanism of ionization
The basis for the occurrence of a corona discharge is the process of impact ionization. When the electric field strength in a gas reaches a critical value, free electrons, always present in the medium due to cosmic radiation or natural radioactivity, begin to accelerate. Gaining kinetic energy, they collide with neutral gas molecules, knocking out new electrons from them.
This avalanche-like process leads to the formation of plasma - a conducting channel, which we observe as a glow. However, simply high field strength is not enough to maintain the discharge. The key factor is electrode geometry. At points, edges, and thin wires, the electric field lines become concentrated, creating local areas of extremely high voltage, even if the overall voltage in the system is relative.
⚠️ Attention: Ignoring the tip effect when designing high-voltage lines can lead to premature equipment failure due to local overheating and chemical destruction of insulation by ozonation reaction products.
It is important to understand that corona discharge is not an instantaneous breakdown of the entire interelectrode gap. It is localized in a narrow zone near the active electrode. Outside this zone, the field weakens and ionization stops, which distinguishes a corona from a spark or arc discharge. It is this localization that allows the phenomenon to be used for technical purposes without the risk of a complete short circuit.
To reduce the level of corona on power lines, split wires are often used, consisting of several current-carrying wires, which increases the equivalent radius of the wire and reduces the field strength on its surface.
Critical tension and Pasha's formula
Determining the exact conditions for the occurrence of a discharge is based on Pasch's law, which relates the breakdown voltage of a gas to the product of pressure and the distance between the electrodes. However, for a corona discharge, where the distance to the external electrode is large, the concept of critical field strength at the surface of the conductor is more relevant.
According to Pick's empirical formula, the critical field strength $E_{kr}$ depends on the wire radius and air density. A decrease in the radius of the wire or a decrease in air density (for example, in the mountains) leads to a decrease in the threshold for the occurrence of a discharge. This means that on high-altitude power lines, conditions for corona formation occur at lower voltages than at sea level.
Let's consider the main parameters that influence the threshold of occurrence:
- 🌪️ Air Density: depends on temperature and atmospheric pressure, directly affecting the electron mean free path.
- ⚡ Radius of curvature: The sharper the angle or thinner the wire, the higher the local field strength.
- 💧 Humidity and pollution: drops of water or dust on the surface of the insulator distort the field and serve as ionization centers.
Engineering calculations often require consideration of surface roughness. Even microscopic irregularities on seemingly smooth metal can become sources of discharge initiation. Therefore, in high-voltage technology, the quality of surface treatment is subject to the strictest requirements.
☑️ Checking conditions to prevent corona
Influence of electrode geometry on discharge
The geometric configuration of the electrode system is a determining factor. In technology, the most common systems are “wire-wire”, “wire-plane” or “needle-plane”. In each of these configurations, the potential distribution is different, which dictates its own conditions for the start of ionization.
The most unfavorable system from the point of view of the occurrence of an unwanted corona is a system with a sharply inhomogeneous field. If one of the electrodes has a small radius of curvature (tip, thin wire), then when a certain voltage is reached, a luminous shell forms around it. In systems with a uniform field (for example, between two wide parallel plates), corona discharge practically does not occur; instead, spark breakdown occurs immediately.
To minimize risks in high-voltage equipment, special shielding rings and smooth contours are used. These elements increase the effective radius of curvature, “smearing” the field lines and reducing its maximum intensity. Usage toroidal shapes instead of corner connections - standard practice in the design of transformers and arresters.
| System type | Field uniformity | Character of the discharge | Application |
|---|---|---|---|
| Shar-Shar (close) | Homogeneous | Spark breakdown | Arresters, voltage standards |
| Needle-Plane | Sharply heterogeneous | Corona discharge | Electrostatic precipitators, ozonizers |
| Wire-Wire (Power Line) | Heterogeneous | Local crown | Electricity transmission |
| Coaxial cable | Cylindrical | In case of insulation breakdown | High voltage cables |
Particular attention should be paid to the places where the wires are fastened and connected. Any loose nut, protruding bolt or damage to a conductor can become a source of permanent corona, which will eventually destroy the adjacent insulation. Regular visual inspection and thermal imaging help to identify such defects at an early stage.
Why does corona sound like a crackling sound?
A crackling or hissing sound occurs due to pulsating discharge current. With alternating current, the discharge lights up and goes out twice per period (at each half-wave), creating an acoustic wave with a frequency of 100 Hz (with a network frequency of 50 Hz), which is perceived by the human ear as characteristic noise.
The role of dielectric constant and state of the medium
The properties of the gas environment in which the electrodes are located play an important role. Air is the most common dielectric, but its properties vary depending on external conditions. Dielectric strength air is not a constant value, but a variable, depending on many factors.
An increase in air humidity, oddly enough, can either increase or decrease the threshold for the occurrence of corona, depending on the geometry. Water condensing on the tips increases their effective radius, which temporarily suppresses the discharge. However, the flowing drops create field instability, causing pulsed discharges. Air pollution with industrial emissions containing easily ionized particles significantly reduces the onset of corona voltage.
In conditions of high dust or smoke, free electrons are actively captured by neutral particles, forming heavy negative ions. These ions have lower mobility than electrons, which changes the nature of the conductivity of the discharge gap. It is on this principle that electric precipitators work: charged dust particles rush to the collecting electrodes.
⚠️ Attention: When working with high-voltage equipment in conditions of increased dustiness or chemically aggressive environments, the threshold values for the onset of corona voltage can be reduced to 30-40% of the calculated values for clean air.
The temperature regime also makes its own adjustments. Heating a gas reduces its density, which, according to Pasch's law, reduces the electrical strength. Therefore, in systems where overheating of current-carrying parts is possible, the risk of a discharge increases in proportion to the temperature increase.
Direct and alternating current: differences in conditions
The nature of the supply voltage radically changes the discharge pattern. At constant current, the corona discharge has a stationary character. Depending on the polarity of the electrode on which the discharge occurs (anode or cathode), positive and negative corona are distinguished. The mechanisms of their development are different: the positive corona develops through the streamer mechanism, and the negative corona through the mechanism of repeated impact processes.
In alternating current (AC) systems, corona discharge is pulsating in nature. A discharge occurs at moments when the instantaneous voltage value exceeds the critical value. This causes the discharge to light up and go out 100 times per second (for a 50 Hz network). This cyclicity causes specific energy losses and the generation of high-frequency interference.
Let's compare the key aspects:
- ⚡ DC current: Stable glow, less radio interference, but transfer of substance from one electrode to another is possible (the “wind” effect).
- 📡 AC current: Pulsating glow, high level of radio interference, high corona losses in wet weather.
- 🔌 Pulse voltage: During lightning or switching overvoltages, corona can occur briefly, but at voltages significantly higher than operating ones.
For direct current (HVDC) power lines, the problem of corona is less severe in terms of energy loss, but requires consideration of the effect of electric wind, which can cause the wires to sway. In AC networks, the main enemy remains active power losses and generated interference.
Prevention and technical control methods
The fight against unwanted corona discharge in the energy sector has been going on for decades. The main method is to reduce the electric field strength at the surface of the conductors. As mentioned earlier, increasing the radius of curvature is the most effective way. In practice, this is realized through the use of wires of increased diameter or split phases.
The second important aspect is keeping the equipment clean. Regular washing of insulators, removal of bird nests, and trimming of tree branches approaching the wires eliminate local field distortions. Usage smooth contact surfaces and special coatings that prevent oxidation and dust adhesion also have a positive effect.
To monitor the condition of lines and equipment, various diagnostic methods are used:
- Visual inspection at night (the crown is clearly visible in the dark).
- Acoustic monitoring using ultrasonic detectors.
- Radio interference (measurement of the level of interference in a given frequency range).
- Thermal imaging control (discharge zones often have elevated temperatures).
Timely detection of corona lesions helps prevent the development of more serious defects, such as insulation breakdown or wire breakage. Modern monitoring systems use sensors that transmit data on the level of corona in real time.
The main principle of preventing corona is to maximize the equalization of the electric field and eliminate any sharp edges or contaminants on the surface of high-voltage parts.
Practical application of corona discharge
Despite the fact that the energy sector is fighting corona, this effect is being successfully exploited in other industries. Controlled corona discharge is the basis for the operation of many useful devices. Understanding the conditions under which it occurs allows engineers to create efficient installations for air purification, coating and document copying.
For example, in electric precipitators of thermal power plants, corona discharge is used to charge ash particles, after which they are deposited on the electrodes. In the printing and plastic film industries, corona treatments improve surface adhesive properties, allowing paint or adhesives to adhere better. Ozonizers that use corona to disinfect water and air are also based on this physical phenomenon.
Thus, knowledge of the conditions under which corona discharge occurs is necessary not only to protect equipment, but also to create new technologies. The balance between preventing destructive effects and using the beneficial properties of a phenomenon is a task for a competent engineer.
How does humidity affect the onset of corona discharge?
High humidity usually lowers the threshold for corona occurrence. Water vapor reduces the mean free path of electrons, but at the same time, water drops on the surface of the electrodes sharply distort the electric field, creating local zones of ultra-high voltage, which provokes a discharge at lower voltages.
Can a corona discharge turn into an arc discharge?
Yes, if the corona current increases so much that the thermal ionization process covers the entire gap between the electrodes, or if the distance between the electrodes is small. At this moment, the gap resistance drops sharply, and a transition to an arc discharge occurs with the release of a huge amount of heat.
Why is corona dangerous for radio communications?
Corona discharge is pulsed in nature, especially on alternating current. Surges of current generate a wide range of electromagnetic waves (radio interference) that can interfere with broadcast, television, and communications signals in the long to ultra-short wavelength ranges.