Corona Discharge: How to Reduce The Corona Effect

Corona Discharge, also referred to as the Corona Effect, is an electrical discharge phenomenon that occurs when a conductor carrying high voltage ionizes the surrounding fluid, often air. The corona effect will occur in high-voltage systems unless sufficient care is taken to limit the strength of the surrounding electric field.

It’s since corona discharge involves a loss of energy, engineers seek to reduce corona discharge to minimize electrical power loss, production of ozone gas, and radio interference.

Corona discharge can cause an audible hissing or cracking noise as it ionizes the air around the conductors. This is common in high-voltage electric power transmission lines. The corona effect can also produce a violet glow, the production of ozone gas around the conductor, radio interference, and electrical power loss.

What is the Corona Effect?

The corona effect occurs naturally because air is not a perfect insulator—containing many free electrons and ions under normal conditions. When an electric field is established in the air between two conductors, the free ions and electrons in the air will experience a force. Due to this effect, the ions and free electrons get accelerated and moved in the opposite direction.

The charged particles during their motion collide with one another and also with slow-moving uncharged molecules. Thus the number of charged particles increases rapidly. If the electric field is strong enough, a dielectric breakdown of air will occur and an arc will form between the conductors.

Electric power transmission deals with the bulk transfer of electrical energy, from generating stations situated many kilometers away from the main consumption centers or the cities. For this reason, long-distance transmission conductors are of utmost necessity for effective power transfer – which in-evidently results in huge losses across the system.

Minimizing these energy losses has been a major challenge for power engineers. Corona discharge can significantly reduce the efficiency of EHV (Extra High Voltage) lines in power systems.

Two factors are important for corona discharge to occur:

1. Alternating electrical potential differences must be supplied across the line.

2. The spacing of the conductors must be large enough compared to the line diameter.

When an alternating current is made to flow across two conductors of a transmission line whose spacing is large compared to their diameters, the air surrounding the conductors (composed of ions) is subjected to dielectric stress.

At low values of the supply voltage, nothing occurs as the stress is too small to ionize the air outside. But when the potential difference increases beyond some threshold value (known as the critical disruptive voltage), the field strength becomes strong enough for the air surrounding the conductors to dissociate into ions—making it conductive. This critical disruptive voltage occurs at approximately 30 kV.

The ionized air results in electric discharge around the conductors (due to the flow of these ions). This gives rise to a faint luminescent glow, along with the hissing sound accompanied by the liberation of ozone.

This phenomenon of electric discharge occurring in high-voltage transmission lines is known as the corona effect. If the voltage across the lines continues to increase, the glow and hissing noise becomes more and more intense – inducing a high power loss into the system.

Factors Affecting Corona Loss

The line voltage of the conductor is the main determining factor for corona discharge in transmission lines. At low values of voltage (lesser than the critical disruptive voltage), the stress on the air is not high enough to cause dielectric breakdown—and hence no electrical discharge occurs.

With increasing voltage, the corona effect in a transmission line occurs due to the ionization of atmospheric air surrounding the conductors – it is mainly affected by the conditions of the cable as well as the physical state of the atmosphere. The main factors affecting corona discharge are:

• Atmospheric Conditions

• Condition of Conductors

• Spacing Between Conductors

Let us look into these factors in greater detail:

Atmospheric Conditions

The voltage gradient for the dielectric breakdown of air is directly proportional to air density. Consequently, on stormy days, the number of ions surrounding the conductor increases due to continuous airflow, making electrical discharge more likely than on clear weather days.

The voltage system must be designed to accommodate these extreme conditions.

Condition of Conductors

Corona’s impact is highly dependent on the conductors and their physical condition. The phenomenon is inversely proportional to the diameter of the conductors, implying that an increase in diameter considerably reduces the corona effect.

Moreover, the presence of dirt or roughness on the conductor reduces the critical breakdown voltage, making the conductors more susceptible to corona losses. This factor is particularly significant in cities and industrial areas with high pollution, where mitigation strategies are essential to counter its negative effects on the system.

Spacing Between Conductors

The spacing between conductors is a crucial element for corona discharge. For corona discharge to occur, the spacing between the lines should be much larger than its diameter.

However, if the spacing is excessively large, the dielectric stress on the air decreases, reducing the corona effect. If the spacing is too large, corona might not occur at all in that region of the transmission line.

Strategies to Reduce Corona Discharge

Given that corona discharge invariably leads to power loss in the form of light, sound, heat, and chemical reactions, it’s crucial to employ strategies to minimize its occurrence in high-voltage networks.

Corona discharge can be reduced by:

• Increasing the conductor size: A larger conductor diameter results in a decrease in the corona effect.

• Increasing the distance between conductors: Increasing conductor spacing decreases the corona effect.

• Using bundled conductors: Bundled conductors increase the effective diameter of the conductor – hence reducing the corona effect.

• Using corona rings: Electric fields are stronger at points of sharp conductor curvature, hence corona discharge first occurs at sharp points, edges, and corners. Corona rings, which are electrically connected to the high-voltage conductor, encircle the points where the corona effect is most likely to occur. They effectively ‘round out’ the conductors, reducing the sharpness of the conductor surface, and distributing the charge over a wider area, thereby reducing corona discharge. Corona rings are used at the terminals of very high-voltage equipment (such as at the bushings of high-voltage transformers).

Corona Discharge and Current

A closer look at the relationship between corona discharge and current reveals further insights into this phenomenon’s impact on high-voltage systems.

The Role of Current in Corona Discharge

The flow of electric charge plays a vital role in the occurrence of corona discharge. When a high voltage is applied to a transmission line, the current flowing through the conductors creates an electric field around them.

This electric field ionizes the air molecules surrounding the conductors, leading to the corona effect.

The magnitude of the current flowing through the transmission line is proportional to the severity of corona discharge. Higher current levels generate a stronger electric field, leading to more ionization and a higher likelihood of corona discharge.

Current and Corona Losses

The interaction between current and corona discharge contributes significantly to power losses in transmission systems. As the current increases, the corona discharge becomes more intense, leading to more power being lost in the form of light, heat, sound, and ozone production.

Current Management to Mitigate Corona Discharge

Given this relationship, managing the current in high-voltage systems is an effective strategy to control and mitigate corona discharge.

Techniques to regulate current can range from using circuit components that limit current to maintaining the overall health and integrity of the transmission system.

Direct Current (DC) Corona Discharge

Direct Current (DC) corona discharge presents unique characteristics and considerations compared to its Alternating Current (AC) counterpart.

Fundamentals of DC Corona Discharge

DC corona discharge is an observed experimental phenomenon that occurs when a high voltage is applied to an electrode system, typically consisting of an active electrode (usually sharp) and a passive electrode (often a flat plane), in an environment with gaseous molecules, such as air. The polarity of the DC voltage can be either positive or negative, which in turn affects the nature of the corona discharge.

Differences Between Positive and Negative Corona Discharge

The positive corona discharge typically forms a smooth, faintly visible glow around the active electrode.

On the other hand, a negative corona discharge results in a brighter, filamentary glow, primarily concentrated in the regions of the highest electric field strength.

DC Corona Discharge Characteristics

One of the defining characteristics of a DC corona discharge is its current-voltage relationship. As the voltage applied to the electrode system is increased beyond the onset voltage (the minimum voltage required for the corona discharge to occur), the corona current begins to rise.

Notably, the onset voltage for a DC corona discharge is influenced by several factors, including the shape and size of the electrodes, the gap distance between the electrodes, and the properties of the gas in which the discharge occurs.

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