Corona Effect

Everyone is talking about coronavirus across the globe; we are here to introduce you with the corona effect in electrical terms. This article covers an important phenomenon in designing transmission line of electrical power system. Do not mix it with novel corona virus in medical terms. STAY SAFE.

Definition of Corona

The phenomenon of violent glow, hissing sound and production of ozone gas in overhead transmission line is called corona.
–V.K Mehta

Cause of Corona

When the potential difference between two conductors are increased to a certain threshold, called critical disruptive voltage, a violet glow with hissing sound are observed. This phenomenon is called corona. This is due to the ionization of air around the conductor. Further increase in the potential difference of two conductors (keeping the distance and diameter same) up to breakdown voltage, flash between conductors may occur. This is due to breakdown of insulation of air between the conductors.

Ionization and Corona

There is always ionization present in air because of, UV radiations and radioactivity. Therefore, normally, the air around the conductors contains some ionized particles (i.e. free electrons and positive ions) and neutral molecules. When there is p.d between the conductors, the existing free electrons get higher velocities.

Potential Gradient and Formation of Corona

The greater the applied voltage, the greater the potential gradient and higher the velocities of free electrons.
Potential Gradient is:
$$g= \frac{V}{r log \frac{d}{r}} volts/cm$$
where V is voltage, r is radius of conductor and d is distance between two conductors.

When the potential gradient at surface of conductor is 30 kV/cm (peak) or 21.2 kV/cm (rms) the free electrons have the velocity to take out electrons from a neutral molecule. Consequently, in production of ion and one or more free electrons, and become chain reaction. So, either there is corona effect or sparks takes places between two conductors.

Critical Disruptive Voltage (CDV)

From above expression of potential gradient, clearly, we can calculate the critical disruptive voltage for corona. For normal condition at 25$^o$C and pressure of 76 cmHg, potential gradient is 30kV/cm. Therefore, we can calculate CDV:
$$V = g \times r \times log_e\frac{d}{r}$$

Factors Affecting Corona

From above equation we have now idea of majors factors that can cause and affect the corona formation. Therefore following factors that affect the corona in transmission lines:

  • Surrounding Conditions: As explained earlier that corona is formed due to ionization in air. Therefore, conditions of surrounding environment plays important role.
    As potential gradient is measured at 25$^o$C and nominal pressure. Therefore, change in atmosphere will affect the ionization. Like snow on transmission lines, humidity, rain will increase the corona.
  • Size of Conductors: irregular and smaller size conductor will give rise to the corona.
  • Spacing between Conductors: Greater the separation between the conductors, less electro-static charges will be there. Hence less corona.
  • Line Voltage: Again, from the equation greater the line voltages, greater the electrostatic force and greater corona.

This was a little introduction of corona on transmission lines.
Check advantages and disadvantages of corona and method of reducing corona
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