ADDENDUM (CORONA FILTER #1 & #4)
INTRODUCTION
The CoronaFinder is extremely easy to use. The CoronaFinder is supplied with two
filters, Corona Filter #1 and Corona Filter #4. The CoronaFinder must NEVER be
operated without one of the Corona Filters installed. Although either Corona Filter may
be used, the following will give some guidelines about when to use #1 or #4.
WHAT IS CORONA
Corona discharge is a luminous partial discharge from conductors and insulators due to
ionization of the air, where the electrical field exceeds a critical value. A high local
electric field ionizes the air and causes a partial discharge. This process causes the
excitation and breakdown of Nitrogen molecules, leading to emission of UV radiation.
Some molecules are not ionized but excited – emitting photons on relaxation. Buildup of
ionization occurs only if the electrons are going fast enough, i.e. if the electric field
exceeds a critical value.
Thus a problem or defect in a component creating a local high electric field will show
corona activity. This defect may be a sharp edge or point on a conductor. It may be a
discontinuity on a bare cable (dirt or broken strand). It could be a nick, break or
contaminant on an insulator, or a thinned area on a dielectric coating. It could be caused
by a cracked, chipped or dirty bushing. Or it may be loss of insulator caused by a SF6
leak.
Utilities are typically made aware of corona by complaints of faulty radio or television
signals. Because corona is invisible in daylight with the naked eye, maintenance crews
will investigate by aiming devices such as a corona camera or radio antenna at suspected
areas, and track corona.
EFFECTS OF CORONA
Corona has many damaging and environmental disturbing effects. One of these is the
generation of corrosive materials, like ozone and nitrogen oxides. These combine with
water vapor under conditions of high humidity to form nitric acid. This very corrosive
material can shorten the lifespan of high voltage lines and substation components.
Corona causes damage to high voltage insulators, especially non-ceramic (NCI)
insulators. Corona can cause radio interference (RI/RFI) mainly to AM transmissions.
Corona can cause audio noise; humming, sizzling and crackling sounds.
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