Corona Effect in Transmission Lines

It can be noticed that near the overhead lines, there exists a hissing noise and sometimes a faint violet glow. The effect due to which such phenomenon exists surrounding the overhead lines is called the corona effect.

When a normal alternating voltage is applied across two conductors with enough spacing in between, then there is no change in the atmospheric conditions surrounding the conductors. But if the voltage applied exceeds a particular limiting value then the air surrounding the conductors gets ionized due to which a hissing noise or a faint violet glow appears.

Thus the phenomenon of hissing noise, faint violet glow, and production of ozone gas surrounding the overhead lines, due to ionization of air is called corona.

The corona effect takes place when the applied voltage exceeds a particular value which is called critical disruptive voltage. As this voltage increases, the glow and hissing noise also increases. If the voltage increases up to the breakdown value then the flashover occurs between the conductors, due to the breakdown of air.

Similarly, if the ratio of the spacing between the conductors to the radius of the conductor is less than 15 then the flashover occurs before the corona. In practice, this ratio is very high and there is no possibility of a flashover.

When a voltage more than the critical disruptive voltage is applied, the faint violet glow is even between the two parallel polished conductors and can be seen all along the length of the conductors. At the rough points, it appears a little bit brighter.

In the case of d.c. voltage, reddish beads are formed near the negative conductor while a smoother bluish-white uniform glow is formed near the positive conductor. The hissing noise can be easily heard while the formation of ozone gas can be detected from its odour.

Basic Principle of Corona

In the free space surrounding the conductors, some ionization is always present due to radioactivity, cosmic rays, and ultraviolet rays. Thus the air near the conductors consists of some free electrons and ionized particles along with the neutral molecules. When the potential is applied to the conductors, a potential gradient is developed in the air. This potential gradient is maximum around the surface of the conductors.

Due to the potential gradient, the free electrons will start moving with a certain velocity which depends on the field strength. The greater the applied voltage, the higher the potential gradient and the velocity acquired by free electrons. These electrons moving with high speed collide with the neutral molecules in the air. If the potential gradient is about 30 kV/cm (maximum) then striking free electrons dislodge the electrons from the natural molecules.

This increases the number of free electrons which also acquire high velocities and start colliding with other neutral molecules. The process of ionization is cumulative which finally results in an electron avalanche. This ionization of air surrounding the conductors gives rise to the corona effect. 

Factors Affecting Corona and Corona Loss

The various factors affecting corona and corona loss are:-

1. Electrical factors: Corona loss depends on the supply frequency. The higher the supply frequency, the higher the corona loss. Thus D.C. corona loss is less compared to A.C. corona loss. Due to the corona effect third harmonic components are generated hence actually corona loss is higher.

2. Line voltage: The line voltage directly affects the corona and corona loss. For lower line voltage corona may be absent. But for voltages higher than disruptive voltage, the corona starts. The higher the line voltage, the higher the corona loss. 

3. Atmospheric conditions: The most important atmospheric factors are pressure and temperature. The pressure and temperature together decide the value of δ which affects the disruptive voltage and the corona loss. The lower the value of δ, the higher the corona loss hence δ is a very important factor regarding corona.

For lower pressures and higher temperatures, the value of δ is small and the corona effect and loss are dominant. Hence in mountain areas, the corona loss is high.

Similarly in stormy conditions, dusty and rainy conditions, the number of free electrons is more hence disruptive voltage is lower. This increases the corona loss considerably.

4. Size of the conductor: The corona loss is directly proportional to the square root of the radius of the conductor. So it appears that loss is more if the size of the conductor is more. But for large-size conductors, V_d is more, and hence the term (V_ph – V_d) is less. Thus loss is less. The effect of V_d is more dominating than the factor √r hence higher the size of the conductor, the lower the corona loss.

5. Surface conditions: The corona depends on the surface conditions. For rough and uneven surfaces, the value of disruptive voltage is less, and the corona effect is dominant. Similarly, corona loss is also more for rough and dirty surfaces.

6. Number of conductors per phase: For higher voltages a single conductor per phase produces large corona loss. Hence bundled conductors are used due to which self GMD of the conductor δ increases, which increases the disruptive voltage, reducing corona loss.

7. Spacing between conductors: If the spacing is made very large, the corona can be absent. Practically the spacing is selected so that the corona is tolerable.

8. Shape of conductors: The shape of the conductors like flat, oval, cylindrical, etc. affects the corona loss. For a uniform cylindrical shape, corona loss is less compared to any other shape.

9. Clearance from the ground: The height of the conductors from the ground also affects the corona loss. The smaller the clearance of the conductors from the ground, the higher the corona loss.

10. Effect of load current: As the load current increases, the temperature of the conductors increases. This does not allow snow, dew, and dirt to deposit on the surface. This reduces the corona loss.

Due to all these factors for the long transmission lines, the corona loss per km of line at various points is obtained. And net corona loss is obtained by taking an average of all the values.

Advantages of Corona

The various advantages of the corona effect are:-

• Due to the corona, the air surrounding the conductor is ionized and becomes conducting. This increases the virtual diameter of the conductor.
• Corona reduces the effects produced by the surges and the conductor is saved from the possibility of lightning. It acts as a safety device.

Disadvantages of Corona

The various disadvantages of corona are:-

• The corona power loss is the biggest disadvantage which reduces the transmission efficiency.
• The third harmonic component produced due to the corona makes the current nonsinusoidal. This increases the corona loss.
• The ozone gas formed due to the corona chemically reacts with the conductor and can cause corrosion.

Methods of Reducing Corona Effect in Transmission Lines

The corona effects are observed when the operating voltage level is more than 33 kV. The following methods are used to reduce the corona effect.

1. Increasing the conductor size: By increasing the size of the conductors, disruptive voltage is raised. Hence the corona effects are reduced. Thus conductors with large cross-sectional areas like ACSR conductors are used to reduce the corona effect.

2. Increasing the conductor spacing: The increased spacing increases the voltage level at which the corona starts. Thus the corona effects are reduced. But practically spacing can not be increased beyond certain limits as the cost of the supporting structure increases with the increased spacing.

3. Using hollow and bundled conductors: The hollow and bundled conductors reduce the corona effects.

Radio Interference due to Corona Effect in Transmission Lines

The corona discharge produces radiation which may introduce noise signals in the communication lines, carrier signals, radio and television receivers, navigation signals, etc. Such noise signals which adversely affect the wireless signals, produced by corona are called radio interference.

Radio interference can cause problems to communication lines that are a few kilometers away from the corona. The radio interference is significant for the voltage levels greater than 200 kV.

The radio interference is measured in terms of an electric field in microvolts per meter at any distance from the transmission line. The radio interference field is inversely proportional to the radio frequency hence transmission like radar, television, FM broadcasting, etc. are less affected by the radio interference.

The radio interference increases gradually till the system voltage produces measurable corona loss. After this voltage level, radio interference increases rapidly and becomes very high. The rate of increase in radio interference depends on the nature of the conductor surface and the diameter of the conductors.

For smooth surface conductors, in good weather conditions, the radio interference is small. While designing the transmission line, the radio interference plays an important role.