Whenever corona occurs, due to electrostatic stress on the atmospheric air surrounding the conductors of the transmission line. The air becomes ionized and produces ions. These ions present in air experience a force upon them and move around the conductor. In order to maintain the motion, the ions draw some energy from the conductor (i.e., from the supply).
This additional energy taken by the ions is dissipated in the form of light (faint luminous glow), heat, sound (hissing noise), and chemical action (production of ozone gas), thus resulting in power loss. The additional energy drawn during the phenomenon of the corona is called corona loss in transmission lines.
Equation For Power Loss Due to Corona :
When the disruptive voltage is exceeded (disruptive voltage is the minimum voltage at which corona occurs), the power loss due to corona under fair weather conditions is given by Peek's formula i.e.,
Where,- f = Supply frequency in Hz
- Vph = Phase to neutral voltage (RMS)
- Vc = Critical disruptive voltage (RMS) per phase
- δ = Air density factor
- d = Spacing between the conductors
- r = Radius of conductors.
However, Peek's formula is applicable for fair weather conditions when the ratio V/Vc is greater than 1.8. When the ratio V/Vc is less than 1.8, the power loss due to corona is given by Peterson's formula which is given as,
Where F is called the corona loss function and it depends on the ratio V/Vc.
Factors Affecting Power Loss Due to Corona :
In transmission lines, the power loss due to corona occurs due to ionization of the air surrounding the line conductors. Hence, it is affected by the physical state of the atmosphere as well as the conditions of the line. The various factors affecting power loss due to corona are,
Atmospheric Condition :
The corona loss or power loss due to corona is highly variable with respect to weather conditions i.e., its value will be different under different weather conditions. This is because corona loss is nothing but the energy taken by the ions in order to maintain their motion.
The number of ions presents around the conductor will be different under different weather conditions i.e., if the weather is rainy or if there is a snowfall, then the number of ions will be more, hence the energy taken from the supply will be more.
On the other hand, the number of ions will be less under fair weather conditions and hence the energy taken from the supply will be less. Hence, we can say that the corona loss will be different in different weather conditions.
Condition of the Conductor :
- The spacing between the two conductors affects the formation of the corona. If the spacing of the transmission line conductors is large, the electrostatic stress between the conductors is less, hence the effect of corona and corona loss decreases.
- Corona loss is more in the case of stranded conductors as compared to solid conductors.
- Corona is also more if the surface of the conductors is rough and dirty, whereas it is less for smooth conductors.
Effect of Line Voltage :
At low voltages, there is no effect of corona and no power loss due to corona. But at higher line voltages, the electric field intensity of the conductor is high and hence corona will appear thereby decreasing the transmission efficiency due to power loss. Also, the power loss increases with the increase in transmission line voltage.
Effect of Supply Frequency :
From the above equation, the power loss due to corona is directly proportional to the supply frequency. The increase in frequency increases the power loss. Thus we can say that corona loss is high in the case of alternating current supply compared to the direct current supply.
Effect of Conductor Radius :
The electric field intensity of the conductor depends upon the size of the conductor. The higher the size of the conductor, the lower will be the electric field intensity, which in turn decreases the corona effect. Hence corona losses will be less for conductors of larger diameter.