What is the Corona Effect in Transmission Lines? How Engineers Overcome it?

There is a hissing noise with violet glow phenomenon termed as corona effect which is commonly observed in high voltage transmission lines. The corona effects leads to high voltage drop and energy loss along with release of ozone gas. There is a need to be aware of this phenomenon and its effects on the transmission system. Read more as we cover the factors contributing to corona effect, its disadvantages & also some advantages and methods to reduce the corona effect.

What is Corona Discharge? 

DEFINITION: The ionization of air surrounding the high voltage transmission lines causing the conductors to glow, producing a hissing noise, is called Corona Discharge or Corona Effect.

Corona Effect in Transmission Lines:

This phenomenon occurs when the electrostatic field across the transmission line conductors produces the condition of potential gradient. The air gets ionized when the potential gradient at the conductor surface reaches the value of 30kV/cm at normal pressure and temperature.

In transmission lines, conductors are surrounded by the air. Air acts as a dielectric medium. When the electric field intensity is less than 30kV/cm, the induced current between the conductor is not sufficient to ionize the air. However, when the voltage of air surrounding the conductor exceeds the value of 30kV/cm, the charging current starts to flow through the air, that is air has been ionized. The ionized air act as a virtual conductor, producing a hissing sound with a luminous violet glow.

Advantages & Disadvantages of Corona Effect 

1. Advantages: 

To every disadvantage, there is a corresponding advantage. Corona effect may highly affect the efficiency of transmission lines, however, it also provides safety to the transmission line.

The main advantages of corona effects are:

• Due to corona across the conductor, the sheath of air surrounding the conductor becomes conductive which rises the conductor diameter virtually. This virtual increase in the conductor diameter reduces the maximum potential gradient or maximum electrostatic stress. Thus, the probability of flash-over is reduced.
• Effects of transients produced by lightning or electrical surges are also reduced due to the corona effect. As, the charges induced on the line by surge or other causes, will be partially dissipated as a corona loss. In this way, corona protects the transmission lines by reducing the effect of transients that are produced by voltage surges.

Factors Affecting Corona Discharge:

The phenomenon of electric discharge associated with energized electrical devices, including transmission lines results in a power loss, reducing the efficiency of the transmission lines. The following factors can change the magnitude of the Corona Effect: 

1. Supply Voltage: As the electrical corona discharge mainly depends upon the electric field intensity produced by the applied system voltage. Therefore, if the applied voltage is high, the corona discharge will cause excessive corona loss in the transmission lines. On contrary, the corona is negligible in the low-voltage transmission lines, due to the inadequate amount of electric field required for the breakdown of air.
2. Conductor Surface: The corona effect depends upon the shape, material and conditions of the conductors. The rough and irregular surface i.e., unevenness of the surface, decreases the value of breakdown voltage. This decrease in breakdown voltage due to concentrated electric field at rough spots, give rise to more corona effect. The roughness of conductor is usually caused due to the deposition of dirt, dust and scratching. Raindrops, snow, fog and condensation accumulated on the conductor surface are also sources of surface irregularities that can increase corona.
3. Air Density Factor: Air density factor also determines the corona loss in transmission lines. The corona loss in inversely proportional to air density factor. Power loss is high due to corona in Transmission lines that are passing through a hilly area because in a hilly area the density of air is low.
4. Spacing between Conductors: Design engineers calculate the spacing between the two conductors in the transmission line after careful and extensive research. As the phenomenon of corona discharge is affected by the conductor spacing. If the distance between two conductors is very large as compared to the diameter of conductor, the corona effect may not happen. It is because the larger distance between conductors reduces the electro-static stress at the conductor surface, thus avoiding corona formation.
5. Atmosphere: As corona is formed due to ionization of air surrounding the conductors, therefore, it is affected by the physical state of atmosphere. In the stormy weather, the number of ions is more than normal weather. The decrease in the value of breakdown voltage is followed by the increase in the number of ions. As a result of it, corona occurs at much less voltage as compared to the breakdown voltage value in fair weather.

How Corona Effect is Reduced:

It has been observed that the intense corona effects are observed at a working voltage of 33 kV or above. On the sub-stations or bus-bars rated for 33 kV and higher voltages, highly ionized air may cause flash-over in the insulators or between the phases, causing considerable damage to the equipment, if careful designing is not made to reduce the corona effect.The corona effect can be reduced by the following methods:

1. By Increasing Conductor Size: The voltage at which corona occurs can be raised by increasing conductor size. Hence, the corona effect may be reduced. This is one of the reasons that ACSR conductors which have a larger cross-sectional area are used in transmission lines.
2. By Increasing Conductor Spacing: The corona effect can be eliminated by increasing the spacing between conductors, which raises the voltage at which corona occurs. However, increase in conductor spacing is limited due to the cost of supporting structure as bigger cross arms and supports to accompany the increase in conductor spacing, increases the cost of transmission system.
3. By Using Corona Ring: The intensity of electric field is high at the point where the conductor curvature is sharp. Therefore, corona discharge occurs first at the sharp points, edges, and corners. In order to, mitigate electric field, corona rings are employed at the terminals of very high voltage equipment.

Corona rings are metallic rings of toroidal shaped, which are fixed at the end of bushings and insulator strings. This metallic ring distributes the charge across a wider area due to its smooth round shape which significantly reduces the potential gradient at the surface of the conductor below the critical disruptive value and thus preventing corona discharge.

Important Parameters in Corona Analysis: 

In the design of an overhead transmission line, the phenomenon of corona plays an important role. Therefore, the following terms are used in the analysis of corona effects:

1. Critical Disruptive Voltage: The minimum phase-neutral voltage at which corona occurs is known as 'Critical disruptive voltage'. 

Now, consider two conductors having radius of 'r' cm separated from each other by 'd' cm. Potential gradient 'g' at the conductor surface is given by:

Where, V is phase-neutral potential. The value of g must be made equal to the breakdown strength of air, for the formation of corona.The breakdown strength of air at 76 cm pressure and temperature of 25ºC is 30 kV/cm (max) or 212 kV/cm (r.m.s.) and is denoted by g_o.If Vc is the phase-neutral potential required to produce corona under these conditions, then,

  Therefore, Critical disruptive voltage is:

The value of g_o is directly proportional to air density.Thus, the breakdown strength of air at a barometric pressure of b cm of mercury and temperature of t ⁰C becomes:

Under standard conditions, the value of δ = 1.

∴ Critical disruptive voltage, Vc = go δ r loge (d/r) 

The corona effect also depends upon the surface condition of the conductor. Thus, the irregularity factor mo is accounted by multiplying the above expression. 

∴ Critical disruptive voltage, Vc = mo go δ r loge (d/r) kV/phase 

Where, value of mo is given as:

Where m _v is another irregularity factor. Its value is 1 when conductors' surface is polished, and 0.72 to 0.82 for rough conductors.

3. Power Loss Due to Corona: The formation of corona is always accompanied by the loss of energy which is dissipated in the form of light, heat, sound and chemical action. When disruptive voltage is exceeded, the power loss due to corona is given by:

Where,

f = supply frequency in Hz

V = phase-neutral voltage (r.m.s.)

Vc = disruptive voltage (r.m.s.) per phase

In all, electrical corona discharge is an important factor in transmission and sub-transmission systems which should be taken into account to ensure both reduction in energy loss and increment in system safety. Corona effect causes corrosion at conductor's surface, and pose a threat to the signal integrity of data communication. Several techniques have been implemented to reduce the corona effect to some extent. Such as some present day methods include increasing the conductor's diameter, spacing between the transmission line conductors, using hollow conductors and corona rings.

Generally, it is recommended to use Aluminum corona rings at the conductor end of the string insulators for lines above 230 kV and on both ends of the insulator for 500 kV. Having a good understanding of the nature of conductor material is essential for engineers, so that they can develop proper techniques to mitigate adverse effects caused by electrical corona. In the end, it is emphasized to consider the configuration of the conducting line as well as the factors affecting corona discharge such as, resistance of the AC line and the current capacity. 

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