What is corona loss
When the corona initiated on the transmission lines, certain real power loss will takes place. The amount of real power loss is called as corona loss.
Corona power loss formula:
Formation of corona is always accompanied by energy loss which is dissipated in the form of light, heat, sound and chemical action. When disruptive voltage is exceeded, the corona power loss is given by
P = 241 x 10-5 (f + 25/ δ) √ r/d ( V p - V d )2 kw/phase/km
Where f is the supply frequency in Hz
δ is the air density factor
r is the radius of the conductor in cm
d is the distance of separation in m.
Vp is the operating voltage / phase/rms
Vd is the critical disruptive voltage rms/phase
When the operating voltage is more than critical disruptive voltage i.e. V p > V d , then only the corona will be initiated. Otherwise there is no corona.
Factors affecting corona loss:
1. Electrical factors:
- Supply frequency increases the corona loss will increase, because P is proportional to (f +25).
- The corona loss of the Ac transmission line is more than Dc transmission line for the same operating voltage.
- The corona loss at positive polarity conductor at a Dc transmission line is more than negative polarity conductor. At the positive polarity conductor the electron will move at a faster rate due to positive electric field intensity and negative polarity electron. So the critical disruptive voltage is less and corona loss will be high.
- When the sinusoidal waveform is having distortion consists of harmonics, the corona loss will increase.
- Lower the height at the power conductor, higher will be the corona loss.Due to lower height, the capacitance of the conductor will increase so that the operating voltage will increases. The increased operating voltage will provide high electric field intensity so that the critical disruptive voltage is less which in turn increases the corona loss.
- When the transmission lines are arranged in equilateral form, the capacitance or phase will be same so the operating voltage will be same, which in than the critical disruptive voltage will be same for the three conductors. So the corona loss will be same. In case of unsymmetrical conductor configuration, the capacitance of the middle conductor is high, so the critical disruptive voltage is less and the corona loss is high.
2. Atmospheric factors:
Temperature and pressure. Both are related to air density factor.
δ = (3.92xh) / (273 + t)
P ∝ ( V p - V d )2 or V d ∝ δ
As δ increases, the corona loss will decrease. As δ increases V d will increase. So ( V p - V d )2 will reduce which in turn reduce corona loss. In hill area, the corona loss is high compared to sea level area. In hill area the falling of pressure is very high when compared to falling of temperature. So δ will be less in hill area and corona loss will be high.
Deposition of impurities like dust, ice, snow and fur will reduce the dielectric strength of air so that the critical disruptive voltage Vd will reduce and the corona loss will increase.
3. Factors related to the size of the conductor:
Use larger diameter of the conductor.
P ∝ √ r
P ∝ ( V p - V d )2
V d ∝ r
From the above three equations, As the diameter increases, there is a possibility to increase the corona as per equation.
By using more number of sub conductor per phase the self distance will increase i.e GMR. So the critical disruptive voltage V d will increase and the corona loss is reduced.
By using cylindrical shape of conductor the field is more uniformly distributed throughout the cross section at the conductor. So the V d will increases and the corona loss will be reduced.
How to reduce corona effect:
It has been seen that intense corona effects are observed at a working voltage of 33 kv or above. Therefore, careful design should be made to avoid corona on the sub stations or bus bars rated for 33 kv and higher voltages otherwise highly ionized air may cause flash over in the insulators or between the phases, causing considerable damage to the equipment. The corona effects may be reduced by the following methods.
- By increasing conductor size: By increasing conductor size, the voltage at which the corona occurs is raised and hence corona effects are considerably reduced. This is one of the reasons that ACSR conductors which have a larger cross sectional area are used in transmission lines.
- By increasing conductor spacing: By increasing the spacing between the conductors, the voltage at which corona occurs is raised and hence corona effects can be eliminated. However, spacing cannot be increased too much otherwise the cost of supporting structure may increase to a considerable extent.
- Use hollow conductor.
- Use bundle conductor.
Corona loss will be high on a stranded conductor and it will be less on hollow conductor. However the corona will exist beyond 275 kv transmission system only. So bundle conductors which are again stranded conductors are employed.