There are numerous factors such as system frequency, system voltage, air conductivity, air density, conductor radius, surface of conductor, load current and atmospheric conditions, on which the corona loss depends.
Factor # 1. Effect of System Frequency:
From Eqs. (7.8), (7.9) and (7.10) it is obvious that corona loss varies directly as the system frequency.
Factor # 2. Effect of System Voltage:
The electric field in the space around the conductors depends mainly on the potential difference between the conductors. Greater the potential difference, greater is the electric field and, therefore, greater is the power loss due to corona. In the region near the disruptive critical voltage, the rate of increase of corona power loss with the increase in system voltage is small but when Vph is large as compared with Vd0 corona loss increases at a very fast rate with the increase in system voltage. This fact is quite evident from Eqs. (7.8), (7.9) and (7.10).
Factor # 3. Effect of Conductivity of Air:
The conductivity of air depends on the number of ions per unit volume of air, the size and charge per ion. Both of these factors vary with altitude and atmospheric conditions. During rain and thunderstorms, ion content increases and, therefore, atmosphere becomes more conducting. Higher conductivity leads to greater corona loss.
Factor # 4. Effect of Density of Air:
Corona loss increases with the decrease in the density of air. The corona loss of a transmission line passing through a hilly area is higher than that of a similar line in plains due to reduced value of δ at high altitudes.
Factor # 5. Effect of Conductor Radius:
The electric field intensity decreases with the increase in radius of conductor. Hence with conductors of large radius, electric field intensity decreases resulting in lower corona power loss.
Factor # 6. Effect of Conductor Surface:
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The potential gradient at the surface of a stranded conductor is greater than that for the equivalent solid conductor. So breakdown voltage is low and corona loss is more for stranded conductors.
Roughness of the surface of the conductor causes a field distortion and gives rise to high potential gradient causing higher corona loss.
Factor # 7. Effect of Atmospheric Conditions:
Corona loss, particularly local corona discharges, increases in rain and bad atmospheric conditions, such as fog, sleet and snowstorms. The effect is more pronounced if the conductor is of larger diameter because on smaller diameter conductors the radius of a rain drop may not be very much smaller than the radius of the conductor itself.
Factor # 8. Effect of Load Current:
The heating of the conductor due to flow of load current through it has an indirect reducing effect on the corona loss. Heating of conductor prevents deposition of dew or snow on the surface of the conductor and reduces corona loss.
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During rains, the heating of conductor has no effect on the corona loss but, after the rain it accelerates the drying of the conductor surface. The time duration for which the drops remain on the surface is reduced and so the corona loss is reduced.
For long transmission lines passing through routes of varying altitudes, the average value of corona loss is determined by determining the corona loss per km at a number of points and then taking out an average.
Factor # 9. Bundling of Conductors:
A bundled conductor consists of two or more parallel sub- conductors at a spacing of several diameters. These groups of conductors form the phase conductors. Thus the effective diameter of the bundled conductor is much larger than that of the equivalent single conductor. The value of large diameter, as obvious from the equation for corona loss, will reduce the corona loss.