Electrical Corona Effect | Explained

• July 1, 2020
• Gaurav J

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Have you ever noticed the humming noise while passing by a substation or high and extra high voltage transmission line? Have you ever noticed the violet glow that occurs on high & extra high voltage lines? What is all of this ? Well, in electrical engineering this is called as corona effect or corona discharge. In this tutorial, we’ll learn about the corona discharge or corona effect in detail. So, let’s start.

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In this video, we’ll learn about these things

1. What is corona effect & how it is produced
2. What are the effects or disadvantages of corona
3. How we can minimise corona

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What is Electrical corona effect & How it is produced

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Let’s say, we have two conducting electrodes in parallel with each other, separated in air by distance D. As we know, air is not a perfect dielectric. Or in simple language air is not the perfect insulator. Even in the normal condition air has lot of free ions & electrons. Now, let’s say these electrodes are carrying 25kV. As these electrodes are carrying electricity, each electrode will generate it’s own electric field, as shown above.

Now, the free electrons in the air will acquire motion and they start maintaining very small current. This current is almost negligible if the electric field intensity is less than 30kV per cm. The value depends up on many other factors which we’ll see in this tutorial.
So, if the electric filed intensity is less than 30kV/cm then the value of current flowing between these two is negligible and you can ignore it there is no problem.

But, what will happen if we increase the voltage from 25kV to 45kV? Well, of course, you guessed it correctly, electric filed intensity between the electrodes will increase. Let’s say, it is now 40 kV/cm.

Now, this is enough to break the insulation properties of air. The air will loose it’s insulation property. Complete electric breakdown occurs and an arc will stuck between these two electrodes.

Same thing happens with the transmission lines carrying high & extra high voltage. The only difference is distance between the two conductors is more. When the conductor is carrying high or extra high voltage , air surrounding the conductor becomes ionised due to electric field around it. Intensity of the electric field is maximum at the surface of conductor and as we move away from conductor it decreases.

It is same like magnet and a metal clip. As you bring the clip near to the magnet, you’ll feel more force. And as you take it away from the magnet the force will decrease. Here also the same case. Intensity is maximum at the surface and as you move away from the conductor is keep on decreasing. As the intensity at the surface of conductor is maximum the air near to the surface will become ionised. If the electric intensity is high enough, and because of the breakdown of air a faint violet glow will appear, and at the same time humming noise will be heard, and this is nothing but the Corona effect in electrical engineering.

If the electric field intensity increases, the glow and the noise will also increase until a spark over take place. Further, if the surface of conductor is not smooth and it has some rough points there the glow will be brighter as compare to the smooth areas.

Critical Voltage of Corona effect

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The voltage value at which the breakdown of air happens or the corona starts is called as Critical voltage. In our case of two electrodes, critical value was 40kV/cm. This value depends up on the many things like,

• Temperature
• Humidity
• Pollution level in air
• Condition of the conductor surface

Please remember that, corona effect is generally seen in the lines carrying 245kV or higher. So, if you are passing by a 11kV or 36kV line you may not see the violet glow and the humming noise will not be there.

So this is how the corona happens. Now, you may ask, let the corona happen what is bad in that, anyways it is happening at a height so what wrong it will do? Well, there are few things which corona causes and for which an electrical engineer should be concerned.

Effects of Electrical Corona

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Power loss over the entire length of line

Because of the corona discharge, power loss take place over the entire length of line, even in the fair weather conditions. This loss increase when the weather becomes humid. So let’s say if the line length is 100km and and the power loss of 0.5kW per KM. Over the entire length of line we’ll lose approx. 50kW of power. And if this loss is not controlled, power available at receiving end will be lower than the sending end.

Interference with Radio & Televisions

As we have seen corona emits a humming noise and this noise is of a very high frequency which can interact with the nearby radio receivers or TV sets.

Production of Ozone gas

Yes, corona also produces ozone gas. Which can reduce the life of insulator and the conductor.

And hence, ignoring the corona is not at all a good idea. So, what to do? Of course we cannot 100% avoid corona but we can definitely minimise it. Let’s see what are the ways to minimise the corona.

How to minimise Electrical Corona Effect

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What is causing corona? The electric field around the conductors. Therefore, to reduce the corona we should try to reduce the electric field around the conductor surface. This can be done either by increasing the diameter of the conductor or arranging the conductors in sets of two, three or may be four per phase. This arrangement helps in reducing the electric field around the conductor and hence the corona effect. And this is the reason why you’ll see bundling of the conductors.

Not only transmission lines, but also the high & extra high voltage equipment like bushing of transformer or circuit breaker, disconnectors etc, are likely to face the corona effect. In case of such equipment, the corona discharge can happen where there is a sharp edge or the points which has small curvature. This is because, electric field intensity is more at such points. This can be avoided by using the corona rings. Corona rings are nothing but the rings made up of thick conducing material.

Here, you can see in the above photo, it’s a circuit breaker and the round thing highlighted is corona ring. So, what corona ring does is that, it reduces the electric field intensity by giving a nice curvature shape at the sharp points. In this way, the current does not ionise the air, instead it flows through the round corona ring. Again, this effect is generally seen in equipment which are used above 245kV voltage levels.

These are the ways by using which, you can minimise the corona effect.

Summary

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So, let’s summarise this tutorial.

• Corona is an effect in which, due to ionisation of air around the conductor, a violet glow and humming noise along with the ozone gas produces. It is generally seen in transmission lines & equipment of 245kV & above.

• Corona can cause lot of power loss. It produces high frequency noise which can interfere with the nearby radio receivers and TV sets. Production of ozone gas can reduce the life of insulator and conductor.

• Electrical Corona effect can be minimised by increasing the diameter of the conductor. By bundling the conductor in sets of 2,3 or 4 per phase. In case of high & extra high voltage equipment, corona can be reduced by using corona rings.