Corona and its Effects

Corona is a form of Partial discharge which is luminous and occassionally visible to the Naked Eye.

Corona are caused when the air around an energized conductor gets ionized, causing a discharge. Corona are caused when defects are present in the conductors such as jagged edges or cracks which cause high local electric field. A hissing sound can also be heard during the occurence of Corona.

The Nitrogen molecules in the air get excited and result and cause ultraviolet radiation. Corona is sometimes visible as a hazy blue light around conductors, especially during the night.

Effects of Corona
  1. Corona is accompanied by the creation of ozone and nitrogen oxides. Nitrogen oxides may react with moisture in the air and form nitric acid a potentially corrosive substance.
  2. Corona can cause damage to insulators used in High voltage applications.
  3. Corona emissions are accompanied by the generation of radio waves which can interfere with commercial radio transmission. Sometimes, Corona can also result in noise which may disturb the neighborhood.
  4. It can sometimes cause carbon deposits which may later result in arcing.
Corona can also be used to identify problems. Corona in power transmission lines indicate the presence of dirt or other substances on power lines and may require cleaning.

Corona after commission an equipment may indicate improper installation.

The degree of corona will vary with the humidity. In high humidity conditions, corona can develop into a flashover which can cause trippings and damage to equipment.

Prevention of Corona
One of the means of reducing corona in transmission lines has been to increase the radius of the conductors.  This leads to decreased electrostatic field stress on the air around the conductor and prevents the initation of the corona.  ACSR conductors ( see article on ACSR conductors) which have a greater radii than steel conductors suffer less from the effects of corona.