Corona Effect

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The efficiency of any power system is estimated by the losses that occur in its generating transmitting and distributing equipment. As efficiency is the main criteria, measures are taken to reduce losses and improve the efficiency. However, there are losses which are sometimes advantageous and which are not eliminated completely for these advantages. One such loss is corona loss.

??????????????????????? Corona? is the phenomena of ionization of the air? which surrounds the power conductors. This phenomena when occurs, creates power loss? in the power system. Though?? there are many? disadvantages? in the occurrence of corona, due? to???? its advantages of protecting the power system, no attempts are made to eliminate it completely. However, only few measures to reduce it to some optimum value are employed.

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????????????????? Corona phenomenon is the ionization of air surrounding the power conductor. It is caused by the electric field next to an object, exceeding breakdown value for air. It is self-sustaining electric discharge in which the field intensified ionization is localized only over a portion of the distance between the electrodes.

??????????????????? Free electrons are present in free space because of radio activity and cosmic rays. As potential between? the conductors is increased, the gradient around the surface of the conductor increases.The free electrons will move with certain velocity, depending on field strength.These electrons will collide with molecules of air,thereby releasing more electrons.This process is cumulative and ultimately forms an electron avalanche.Since corona is fundamentally a breakdown phenomenon, it follows Pachen?s law, the voltage is a function of potential difference.

????????????????????? The corona starting voltage is typically 30 KV/cm. Dust or water particles on surface of the object reduce the corona starting voltage, probably by providing local areas of higher curvature and hence higher field stress.



?????????????????????????? The ions produced by electric field result in space charges which move round the conductor. This energy is derived from supply system. In order to maintain
flow of energy, additional energy is required, it is necessary to supply additional ions from supply referred to as corona loss. Peek made experiments? to study the effect of various parameters on the corona loss and gave the following empirical relation.

?Corona? loss, P=241 X 10-5 (f+25)/8?? v---(r/d)(Vp-Vo) 2? KW/km/Ph , where
Vp = Operating voltage in kV,
Vo = Critical disruptive voltage.
This relation gives correct results only if :

  1. ?f ? lies between 25 to 120 HZ.
  2. Radius is greater than 0.25 cm and
  3. Vp/Vo is greater than 1.8

Also a small error in m o, the irregularity factor, will lead to wrong results when using this formula.



?Many factors affect corona loss on overhead transmission lines, mainly
1)? Electrical factors:
Depends on the field around the conductor.
2) Atmospheric factors:

    1. Pressure and tempterature: Expression for critical disruptive voltage,

?????????? Vo=21.1mo&oln(d/r) KV
Where &o= air density correction factor, lower the value of &, higher is????
the loss because loss is directly proportional to (V-Vo) 2 .

    1. Dust, rain, snow and hail effect: The presence and occurance of these bad??

atmospheric conditions reduce the critical disrupted voltage and hence increase the corolla loss.
3)? Factors connected with conductors:
a) Diameter of the conductor : Loss is directly proportional to square root of
(r/d). Loss is also proportional to (V-Vo) 2. But it is found in practice that
the effect of second proportionality is much more than the first. Therefore,
larger the size of conductor, lower the corona loss.
b) Number of conductors per phase: Corona loss is solved by using two or????
more conductors called bundling of conductors.
c) Profile of conductor.
d) Surface conditions.


  1. Power loss is more during bad weather conditions.
  2. When corona is present effective capacitance increases, there by increasing the flow of charging current. This gives raise to a voltage of triple frequency in an ungrounded system. These triple frequency currents interfere with the communication circuits due to electro magnetic and electro static induction effects.

It reduces the magnitude of high voltage steep fronted waves due to lightning or switching by partially dissipating a corona loss. In this way, it acts as a safety valve to some extent.


  1. Maintaining possibly low frequencies.
  2. Distance of the conductors from the ground should be more.
  3. Spacing between the lines should be regular.
  4. Maintaining higher air density correction factor.
  5. Avoiding hilly areas.
  6. Large diameter conductors, hollow conductors, bundled conductors, cylindrical conductors should be employed.

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