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Case study CONDUCTION and BREAKDOWN IN GASES

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CONDUCTION and BREAKDOWN IN GASES
Gases as Insulating Media
The most common dielectrics are gases. Many electrical apparatus use air as the insulating medium, in a few
cases other gases such as N
2
, CO
2
, CCl
2
F
2
(freon) and SF
6
(hexafluoride) are used.
Various phenomena occur in gaseous dielectrics when a voltage is applied.
-When low voltage is applied, small current flow between the electrodes and the insulation retains its electrical
properties.
-If the applied voltage is large, the current flowing through the insulation increases very sharply and an electrical
breakdown occur. A strongly conducting spark formed during breakdown, practically produces a short circuit
between the electrodes. The maximum voltage applied to the insulation at the moment of breakdown is called
the breakdown voltage.
In order to understand the breakdown phenomenon in gases, the electrical properties of gases should be
studied. The processes by which high currents are produced in gases is essential. The electrical discharges in
gases are of two types;
i) non-sustaining discharges
ii) self-sustaining types.
The breakdown in a gas (spark breakdown) is the transition of a non-sustaining discharges into a self-sustaining
discharge. The build up of high currents in a breakdown is due to the ionization in which electrons and ions are
created from neutral atoms or molecules, and their migration to the anode and cathode respectively leads to
high currents. Townsend theory and Streamer theory are the present two types of theories which explain the
mechanism of breakdown under different conditions as pressure, temperature, electrode field configuration,
nature of electrode surfaces and availability of initial conducting particles.
Ionization Process
Consider a simple electrode arrangement as shown in the fig 2.1, having two parallel plate electrodes separated
by a distance d and immersed in a gas at pressure p. A uniform E is applied between two electrodes. Due to
any external radiation (ultra violet illumination) free electrons are liberated at the cathode. When an electron, e
is placed in an E it will be accelerated with a force eE towards the anode, and it gains an energy
2
2
1
mveExu
(eqn. 2.1) where x is the distance traveled by the electron from the cathode, m is the mass
and v is the velocity of the electron.
This electron collides with the other gas molecules while it is traveling towards the anode. If the energy of the
electron is sufficiently large (about 12.2 eV for N
2
or 15.5 eV for O
2
), on collision it will cause a break-up of the
atom or molecule into positive ion and electron, so the new electrons and positive ions are created. Thus
created electrons form a group or an avalanche and reach the anode. This is the electric current and if it is
sufficiently large it results in the formation of a conducting path between the electrodes resulting in the
breakdown of the gap.
Townsend conducted experiments on the growth of these currents which led to breakdown under d.c. voltage
conditions, and he proposed a theory to explain the phenomenon.

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Townsend’s Current Growth Equation
Assuming n
0
electrons are emitted from the cathode and when one electron collides with a neutral particle, a
positive atom and electron formed. This is called an ionization collision. Let
be the average number of
ionizing collisions made by an electron per centimeter travel in the direction of the field where it depends on gas
pressure p and E/p, and is called the Townsend’s first ionization coefficient. At any distance x from the cathode
when the number of electrons, n
x
, travel a distance of dx they give rise to (
dxn
x
) electrons. Then, the number
of electrons reaching the anode at x=d, n
d
will be
0
0
x
x
nn
(eqn. 2.2)
x
x
n
dx
dn
or
x
x
enn
0
(eqn. 2.3) and
x
d
enn
0
(eqn. 2.4) at x=d.
The number of new electrons created, on the average, by each electron is
(eqn. 2.5).
Therefore the average current in the gap, which is equal to the number of electrons traveling per second will be
d
eII
0
(eqn. 2.6) where I
0
is the initial current at the cathode.
Current Growth in the Presence of Secondary Processes
When the initial set of electrons reaches the anode the single avalanche process is completed. Since the
amplification of electrons
d
e
is occurring in the field, the probability of additional new electrons being liberated
by other mechanisms increases, and created further avalanches and are called as secondary electrons. The
other mechanisms are
i) The positive ions liberated may have sufficient energy to cause liberation of electrons from the cathode when
they impinge on it.
ii) The exited atoms or molecules in avalanches may emit photons, and this will lead to the emission od
electrons due to photo-emission.
iii) the metastable particles may diffuse back causing electron emission.
Defining the Townsend’s secondary ionization coefficient
in the same way as
, then the net number of
secondary electrons produced per incident positive ion, photon, excited particle or metastable particle and the
total value of
due to the three different processes is
321
and is function of gas pressure p and
E/p.
Following Townsend’s procedure for current growth, let us assume
'
0
n
number of secondary electrons produced due to secondary
processes.
Let
''
0
n
total number of electrons leaving the cathode.
Anode
cathode
+
+
_
A
_
V
B
Adjustable HV source
R Current limiting
resistor
Ultraviolet light
_
I
Fig. 2.1 Arrangement for study of a Townsend discharge
I
0
d

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CONDUCTION and BREAKDOWN IN GASES Gases as Insulating Media The most common dielectrics are gases. Many electrical apparatus use air as the insulating medium, in a few cases other gases such as N2, CO2, CCl2F2 (freon) and SF6 (hexafluoride) are used. Various phenomena occur in gaseous dielectrics when a voltage is applied. -When low voltage is applied, small current flow between the electrodes and the insulation retains its electrical properties. -If the applied voltage is large, the current flowing through the insulation increases very sharply and an electrical breakdown occur. A strongly conducting spark formed during breakdown, practically produces a short circuit between the electrodes. The maximum voltage applied to the insulation at the moment of breakdown is called the breakdown voltage. In order to understand the breakdown phenomenon in gases, the electrical properties of gases should be studied. The processes by which high currents are produced in gases is essential. The electrical discharges in gases are of two types; i) non-sustaining discharges ii) self-sustaining types. The breakdown in a gas (spark breakdown) is the transition of a non-sustaining discharges into a self-sustaining discharge. The build up of high currents in a breakdown is due to the ionization in which electrons and ions are created from neutral atoms or molecules, and their migration to the anode and cathode respectively leads to high currents. Townsend theory and Streamer theory are the ...
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