Factors Affecting Corona and Corona Loss

Factors Affecting Corona and Corona Loss

AU : Dec.-04, 16, May-11, 13, 17

The various factors affecting corona and corona loss are,

1. Electrical factors : From the equation of corona loss it can be observed that it depends on the supply frequency. Higher the supply frequency, higher is the corona loss. Thus d.c. corona loss is less compared to a.c. corona loss. Due to corona effect third harmonic components are generated hence actually corona loss is higher.

2. Line voltage : The line voltage directly affects the corona and the corona loss. For lower line voltage corona may be absent. But for voltages higher than disruptive voltage, corona starts. Higher the line voltage, higher is 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. Lower the value of δ, higher is the corona loss hence δ is very important factor regarding corona. For lower pressures and higher temperatures, value of δ is small and corona effect and loss is dominant. Hence in mountain areas the corona loss is high.

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

4. Size of the conductor : The corona loss is directly proportional to square root of radius of conductor. So it appears that loss is more if size of 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 Vr. Hence higher the size of the conductor, lower is 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 corona effect is dominent. 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 8 increases, which increases the disruptive voltage, reducing corona loss. 

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

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

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

1. Methods of Reducing Corona Effect

The corona effects are observed when the operating voltage level is more than 33 kV.

The following methods are used for reducing 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 area like ACSR conductors are used to reduce corona effect.

2. Increasing the conductor spacing : The increased spacing increases the voltage level at which corona starts. Thus the corona effects are reduced. But practically spacing cannot 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.

Review Questions

1. Discuss the factor which affect the corona loss.

2. State the methods of reducing corona.