What is Corona Discharge?
Corona Discharge (also known as the Corona Effect) is an electrical discharge created by the ionization of a fluid surrounding an electrically charged conductor, such as air. In high-voltage systems, the corona effect will arise unless care is taken to restrict the strength of the surrounding electric field.
As the air around the conductors is ionized by the corona discharge, it can make an audible hissing or cracking noise. This is a regular occurrence in high-voltage power transmission cables. A violet light, ozone gas formation around the conductor, radio interference, and electrical power loss can all be caused by the corona effect.
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What is Corona Effect in Transmission Lines ?
Because air is not a perfect insulator and contains many free electrons and ions under normal conditions, the corona effect happens naturally. When an electric field is created in the air between two conductors, the free ions and electrons in the air feel a force. Ions and free electrons are accelerated and moved in the opposite direction as a result of this phenomenon.
Electrical
During their motion, the charged particles collide with one another as well as with slow-moving uncharged molecules. As a result, the quantity of charged particles rapidly grows. If the electric field is strong enough, air will dielectrically break down and an arc will emerge between the conductors.
The mass transfer of electrical energy from generating units located many kilometers away from major consumption areas or cities is referred to as electric power transmission. As a result, long-distance transmission cables are critical for effective power transfer— which, unsurprisingly, results in significant system losses.
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Factors Affecting Corona Discharge:
The phenomena of electric discharge associated with electrified electrical devices, especially transmission lines, causes power loss, lowering transmission line efficiency. The degree of the Corona Effect can be influenced by the following factors:
- Supply Voltage
- Air Density Factor
- Spacing between Conductors
- Conductor Surface
- Atmosphere
Supply Voltage
As the electrical corona discharge mostly depends upon the electric field intensity produced by the applied system voltage. As a result, if the applied voltage is too high, the corona discharge will result in significant corona loss in the transmission lines. The corona, on the other hand, is negligible in low-voltage transmission lines due to the insufficient amount of electric field required for air breakdown.
Air Density Factor
The corona loss in transmission lines is also determined by the air density factor. Corona loss is proportional to the air density factor. Power loss is considerable due to corona in transmission lines traveling through a mountainous location because the density of air is low in a hilly area.
Spacing between Conductors
After doing a comprehensive investigation, design engineers compute the spacing between the two conductors in the transmission line. The conductor spacing affects the occurrence of corona discharge. The corona effect may not occur if the distance between two conductors is very vast in comparison to the diameter of the conductor. Because the distance between conductors is greater, the electro-static stress at the conductor surface is reduced, and corona production is avoided.
Conductor Surface
The corona effect is determined by the form, material, and operating circumstances of the conductors. The rough and irregular surface, or unevenness, reduces the value of the breakdown voltage. The increase in the corona effect is caused by a drop in breakdown voltage caused by a concentrated electric field at rough places. The roughness of a conductor is typically produced by the accumulation of dirt, dust, and scratches. Surface imperfections caused by raindrops, snow, fog, and moisture on the conductor surface can also increase corona.
Atmosphere
Corona is affected by the physical state of the atmosphere since it is created by the ionization of air surrounding the conductors. The number of ions is higher in stormy weather than in regular weather. The quantity of ions increases as the value of the breakdown voltage decreases. As a result, corona occurs at a significantly lower voltage than the breakdown voltage value in clear weather.
Effects of Corona Discharge
Corona effect is a very unprofitable thing in electrical theory, and failing to control it can lead to a variety of problems.
One of the most serious issues is power outages. Because a considerable amount of electrostatic energy is wasted owing to air insulation breakdown, power loss in transmission lines is also quite high at that time.
Furthermore, because the frequency of this noise is so high, it can interfere with surrounding radio, television, or other forms of frequencies. Corona discharge generates ozone gas in the surrounding environment, which can shorten the life of both the conductor and the insulator.
To avoid harm, the corona effect must be managed and kept to a minimum. It is impossible to totally eradicate it, but it can be significantly slowed to prevent more harm.
How Can We Control Corona Effect?
The following methods can be used to reduce the corona effect.
- By increasing conductor spacing
- By Increasing Conductor Size
- By Using Corona Ring
By increasing conductor spacing
The corona effect can be avoided by increasing the distance between conductors, which raises the voltage at which corona occurs. However, the cost of supporting structure, which includes larger cross arms and supports to follow the increase in conductor spacing, limits the increase in conductor spacing.
By Increasing Conductor Size
By increasing the size of the conductor, the voltage at which corona develops can be raised. As a result, the corona effect could be decreased. One of the reasons ACSR conductors with a greater cross-sectional area are utilised in transmission lines is because of this.
By Using Corona Ring
At the location where the conductor curvature is sharp, the intensity of the electric field is strong. Corona discharge thus happens first at sharp points, edges, and corners. Corona rings are used at the terminals of very high voltage equipment to minimise electric fields.
What is Corona Rings?
Corona rings are toroidal-shaped metallic rings that are attached to the ends of bushings and insulator strings. Because of its smooth round form, this metallic ring disperses the charge over a larger region, lowering the potential gradient at the conductor’s surface below the crucial disruptive value and preventing corona discharge.
Advantages & Disadvantages of Corona Effect
Advantages:
- The sheath of air surrounding the conductor becomes conductive as a result of corona across the conductor, thereby increasing the conductor diameter. The maximum potential gradient or maximum electrostatic stress is reduced by this virtual increase in conductor diameter. Thus, the probability of flash-over is lowered.
- The corona effect also reduces the effects of transients caused by lightning or electrical surges. As a result of the corona loss, the charges created on the line by surge or other causes will be partially dissipated. Corona, therefore, preserves transmission lines by decreasing the effect of transients caused by voltage surges.
Disadvantages
- Due to electromagnetic transients and electrostatic induction effects, a non-sinusoidal voltage drop occurs in the transmission line due to non-sinusoidal corona current, causing interference with nearby communication circuits.
- The production of corona produces ozone gas, which chemically reacts with the conductor and causes corrosion.
- Corona loss is the amount of energy lost in a system as a result of the corona effect. Corona power loss is both undesired and uneconomical. Because of the loss of power or energy, the transmission line’s efficiency is greatly diminished.