The corona effect

   The "corona effect" is a term that designates partial discharges around the conductors of a power transmission line under some conditions.

    Intense electric fields (peak value close to 30 kV/cm in the air) may occur at the surfaces of conductors and other "live" components of high voltage systems. In some circumstances, this leads to ionization and electrical breakdown of the air immediately surrounding the conductor. This effect is known as corona discharge, or simply "corona". The St. Elmo's fire is an example of naturally occurring corona

     The presence on the conductor surface of even small protrusions, such as water drops, snow flakes, insects, or the raised edges of nicks in the metal, produce strong local enhancements of the field. The corona activity consequently varies markedly with surface and atmospheric conditions.

    Corona on conductors is minimized by using either large conductors or  bundle conductors and also by not having the different phases too close together or to the ground. Particular phase arrangement for double-circuit lines or the use of non symmetrical bundles can also reduce corona generation.

    It is not possible to eliminate corona altogether; however, it is not normally encountered on systems below about 200 kV.