MYSTERIOUS CORONA EFFECTS IN TRANSMISSION LINES

Fig-Corona Effect in Transmission Line Visible at Night

What Are Corona's effects on electrical energy transmission lines?

If you look along the high voltage power transmission line in a clear sky at night, you will observe the dreamiest violet glow phenomenon embedded with hissing noise along the transmission line conductors. This indefinable phenomenon causes nothing but a production of ozone gas surrounding overhead electrical energy transmission lines known as the Corona or Corona effect.

 Usually, you will find noticeable corona effects in EHV or Extra High Voltage transmission lines, like- 230kV, 400kV, or higher grading. You may discover corona effects on normal voltage lines if the line is overloaded or carries overestimated loads.

Theory and History of  Mysterious Corona

The mysterious corona factor equation was first derived by F.W. Peek and published in 1911. In a later a little modified from the original the Corona effect equation is published where he showed that the total amount of power loss in a wire due to the Corona effect was equal to the equation below:   

where the parameter and the sample values are as below table:

Parameter		Sample Value
k0	Fixed constant	241
g0	Disruptive Gradient in Air	21.1 kv/cm
kd	Normalized Air Density Factor 1 (25 °C, 76 cm pressure)	1
a	Radius of Conductor	3.5 cm (see Fig. 2)
d	Conductor Spacing	1000 cm
f	Frequency	60 Hz
ki	Wire Irregularity Factor	0.95 (weathered wires)
V0	Line Voltage to Neutral (1/1.73 x Voltage Between Conductors)	442 kV (765 kV/1.73)
	Disruptive Critical Voltage (g0 ki a kd ln(d/a))	397 kv
	Corona Loss kW/km/line	25kW/km
	Corona Loss % (1000km line at 2.25 GW)	3.3%

What are the Advantages and Disadvantages of Corona Effects?

The advantages of corona effects are: I) Because of corona formation surrounding the conductor, the conductor diameter becomes larger virtually. As the diameter increases the electrostatic stress between the conductor decreases; ii) it reduces the surge voltage that is created because of switching and thundering.

The disadvantages of corona effects on transmission lines are-

1. It reduces the transmission line’s efficiency;
2. Ozone created by this effect because of corrosion of the conductors, a health risk for humans and animals;
3.  Due to the corona effect non-sinusoidal voltage drop occurs across the line which causes inductive interference with neighboring communication lines, etc.

Features of Corona

• Conductor Size: The surface of the conductors decreases the breakdown voltage. Therefore, solid conductors are widely used for transmission lines;
• Line voltage: if the line voltage exceeds its limit or the line become overloaded, a breakdown will occur which creates Corona;
• Conductor’s spacing: To reduce corona more spacing is required between conductors;
• Atmospheric limitations: In Rainy, Foggy, or Stormy weather Corona occurs at much less voltage at fair weather.

Factors Affecting Corona

The phenomenon of corona is affected by the physical state of the atmosphere as well as by the conditions of the line. The following are the factors upon which  corona depends :

(i) Atmosphere: As a corona is formed due to the ionization of air surrounding the conductors, therefore, it is affected by the physical state of the atmosphere. In stormy weather, the number of ions is more than normal, and as such corona occurs at much less voltage as compared with fair weather.

(ii) Conductor size: The corona effect depends upon the shape and conditions of the conductors. The rough and irregular surface will give rise to more corona because unevenness of the surface decreases the value of breakdown voltage. Thus a stranded conductor has ir regular surface and hence gives rise to more corona than a solid conductor.

(iii) Spacing between conductors: If the spacing between the conductors is made very large as compared to their diameters, there may not be any corona effect. This is because a larger distance between conductors reduces the electro-static stresses at the conductor surface, thus avoiding corona formation.

(iv) Line voltage: The line voltage greatly affects the corona. If it is low, there is no change in the condition of air surrounding the conductors, and hence no corona is formed. However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, then a corona is formed.

Difference between the Corona Effect and Skin Effect in Electrical Energy Transmission Lines

Corona is an electrical discharge caused by a liquefied ionization surrounding a conductor that is electrically charged. Corona is an electrical discharge caused by electrical overstress on the conductor. Corona which produces spark is one of the power losses during transmission due to the ionization of air surrounding the conductors.

Skin effect is the tendency of an AC or Alternating Current to become more distributed over the conductor's skin than inside the conductor. As a result, the current density is largest near the surface of the conductor and decreases with greater depths in the conductor. Because of this the resistance of the alternating current carrying conductor increases which increases the power loss.