In electricity, a corona discharge
is an electrical discharge brought on by the ionization of a fluid surrounding
a conductor that is electrically energized. The discharge will occur when the
strength (potential gradient) of the electric field around the conductor is
high enough to form a conductive region, but not high enough to cause
electrical breakdown or arcing to nearby objects.
Problems caused by corona discharge
Corona discharge is generally
undesirable. It represents
- Power loss
- Audible noise
- Electromagnetic interference
- Purple glow
- Ozone production
- Insulation damage
Also Coronas can generate audible
and radio-frequency noise.
Mechanism of corona discharge
Corona discharge results when the
electric field is strong enough to create a chain reaction: electrons in the
air collide with atoms hard enough to ionize them, creating more electrons
which ionize more atoms. The process is:
1. A neutral atom or
molecule, in a region of strong electric field (such as the high potential
gradient near the curved electrode) is ionized by a natural environmental event
(for example, being struck by an ultraviolet photon or cosmic ray particle), to
create a positive ion and a free electron.
2. The electric field
accelerates these oppositely charged particles in opposite directions,
separating them, and preventing their recombination, and imparting to each of
them kinetic energy.
3. The electron has a
much higher charge/mass ratio and so is accelerated to a higher velocity than
the ion. It gains enough energy from the field that when it strikes another
atom it ionizes it, knocking out another electron, and creating another positive
ion. These electrons are accelerated and collide with other atoms, creating
further electron/positive-ion pairs, and these electrons collide with more
atoms, in a chain reaction process called an electron avalanche. Both positive and negative coronas rely on
electron avalanches. In a positive corona all the electrons are attracted
inward toward the nearby positive electrode and the ions are repelled outwards.
In a negative corona the ions are attracted inward and the electrons are
repelled outwards.
4. The glow of the
corona is caused by electrons recombining with positive ions to form neutral
atoms. When the electron falls back to its original energy level, it releases a
photon of light. The photons serve to ionize other atoms, maintaining the creation
of electron avalanches.
5. At a certain
distance from the electrode, the electric field becomes low enough that it no
longer imparts enough energy to the electrons to ionize atoms when they
collide. This is the outer edge of the corona. Outside this the ions move
through the air without creating new ions. The outward moving ions are
attracted to the opposite electrode and eventually reach it and combine with
electrons from the electrode to become neutral atoms again, completing the
circuit.
Thermodynamically, a corona is a
very non equilibrium process, creating a non-thermal plasma. The avalanche
mechanism does not release enough energy to heat the gas in the corona region
generally and ionize it, as occurs in an electric arc or spark. Only a small
number of gas molecules take part in the electron avalanches and are ionized,
having energies close to the ionization energy of 1 - 3 ev, the rest of the
surrounding gas is close to ambient temperature.The onset voltage of corona or
corona inception voltage (CIV) can be found with Peek's law (1929), formulated from empirical observations. Later
papers derived more accurate formulas.