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Breakdowngass
1. SUBJECT:- HIGH VOLTAGE ENGINEERING(2160904)
UNIT : 02
FACULTY:- RAKESH VASANI
1
Electrical Breakdown
In Gases
2. • The simplest and most commonly found
dielectrics are gases.
• Most of the electrical apparatus use air as
the insulating medium.
• Few other gaseous dielectrics are
nitrogen, carbon dioxide, Freon, sulphur
hexafloride……
3. • When the applied voltage is low, small
currents flow and the insulation retains its
properties.
• If the applied voltages are large, the current
flowing through the insulation increases very
sharply and electrical breakdown occurs.
• The maximum voltage applied to the insulation
at the time of breakdown is called the
breakdown voltage.
4. • The build-up of high currents in a breakdown
is due to the process known as ionization.
• At present, two theories are present which can
explain the mechanism of breakdown under
different conditions.
– Townsend Theory
– Streamer Theory
5. Types of Collision:
• The electrical discharge is normally created
from un-ionized gas by collision process.
• These processes occur mainly due to collision
between charged particles and gas atoms or
molecules.
• These are of two types,
– Elastic Collisions
– Inelastic Collisions
6. • Elastic Collisions:
• When they occur, no change takes place in the
internal energy of the particles but only their
kinetic energy gets re-distributed.
• Electrons are very light in weight as compared
to gas molecules and are accelerated due to
electric field.
• When they collide with gas molecules, they
transfer only a part of their kinetic energy.
7. • Inelastic Collisions:
• Here internal changes in energy take place
within an atom or molecule at the expense of
total kinetic energy of the colliding particle.
• The collision often results in a change in
structure of the atom.
• All collisions that occur in practice are
inelastic collisions.
8. Mobility of Ions and Electrons:
• Electric force on an electron / ion of charge e
is eE, with the resulting acceleration being
eE/m.
• The drift velocity for ion (Wi) is proportional to
the electrical field intensity E and may be
expressed as,
• Wi = μi E
9. • Here μi is the mobility of ions.
• And its characterizes as how quickly an
electron can move through a metal or
semiconductor, when pulled by an electric
field.
10. Diffusion:
• When particles possessing energy, which is
exhibited as a random motion, are distributed
unevenly through a space, then they tend to
redistribute themselves uniformly throughout
the space.
• This process is known as diffusion.
11. The Mean Free Path (λ):
• The mean free path is defined as the average
distance between collisions.
• Depending on the internal energy of the
colliding electron, the distance between the
two collisions vary.
• The average of this is the mean free path.
12. • The mean free path can be expressed as
λ = k/p cm
where k is a constant and p is the gas pressure
in microns(torr).
13. Ionization Process:
• A gas in its normal state is almost a perfect
insulator.
• However, when a high voltage is applied
between the two electrodes immersed in a
gaseous medium, the gas becomes a conductor
and an electrical breakdown occurs.
14. • The processes that are primarily
responsible for the breakdown of a gas
are
–Ionization by collision
–Photo-ionization and
–The secondary ionization processes
15. Ionization by Collision:
• The process of liberating an electron from a
gas molecule with the simultaneous production
of a positive ion is called ionization.
• In the process of ionization by collision, a free
electron collides with a neutral gas molecule
and gives rise to a new electron and a positive
ion.
16. • An electric field E is applied across two plane
parallel electrodes, as shown in Fig.
• Any electron starting at the cathode will be
accelerated more and more between collisions
with other gas molecules during its travel
towards the anode.
17.
18. • If the energy (E) gained during this travel
between collisions exceeds the ionization
potential, Vi, which is the energy required to
dislodge an electron from its atomic shell, then
ionization takes place.
• This process can be represented as
• Where A is an atom, A+ is positive ion and e- is
the electron
19. Photo Ionization:
• The phenomena associated with ionization by
radiation, or photo-ionization, involves the
interaction of radiation with matter.
• Photo-ionization occurs when the amount of
radiation energy absorbed by an atom or molecule
exceeds its ionization potential.
• Just as an excited atom emits radiation when the
electron returns to the lower state or to the ground
state, the reverse process takes place when an atom
absorbs radiation.
20. • Ionization occurs when
h is the Planck's constant,
c is the velocity of light,
λ is the wavelength of the incident
radiation
Vi is the ionization energy of the
atom.
21. • Consider a parallel plate capacitor having gas as an
insulating medium and separated by a distance d as
shown in Fig.
• When no electric field is set up between the plates, a state
of equilibrium exists between the state of electron and
positive ion generation due to the decay processes.
• This state of equilibrium will be disturbed at a moment a
high electric field is applied.
• The variation of current as a function of voltage was
studied by Townsend.
• He found that the current at first increased proportionally
as the voltage is increased and then remains constant.
Townsend’s first ionization coefficient
22. • To explain the exponential rise in current,
Townsend introduced a coefficient α
known as Townsend’s first ionization
coefficient
• and is defined as the number of electrons
produced by an electron per unit length of
path in the direction of field.
• Let n0 be the number of electrons leaving
the cathode and when these have moved
through a distance x from the cathode,
these become n.
• Now when these n electrons move
through a distance dx produce additional
dn electrons due to collision.
23. • The term eαd is called the electron avalanche and it represents
the number of electrons produced by one electron in travelling
from cathode to anode.
24. SECONDARY EFFECTS
• Work function: The energy required to knock out
an electron from a Fermi level is known as the
work function.
• Thermionic emission
• Field emission
• Photo emission
• Metastable Atoms: Neutral excited atoms or
molecules (metastable) incident upon the cathode
surface are also capable of releasing electron from
the surface.
25. • Electron Emission by Positive Ion:
Electrons may be emitted by the bombardment
of positive ion on the cathode surface.
• This is known as secondary emission.
• In order to effect secondary emission, the
positive ion must have energy more than twice
the work function of the metal since one
electron will neutralize the bombarding
positive ion and the other electron will be
released.
26. • Thermionic Emission: If the metals are heated
to temperature 1500°K and above, the
electrons will receive energy from the violent
thermal lattice in vibration sufficient to cross
the surface barrier and leave the metal.
• Field Emission: If a strong electric field is
applied between the electrodes, the effective
work function of the cathode decreases.
28. 1. The positive ions liberated may have
sufficient energy to cause liberation of
electrons from the cathode when they
impinge on it.
2. The excited atoms or molecules in avalanches
may emit photons, and this will lead to the
emission of electrons due to photo-emission.
3. The metastable particles may diffuse back
causing electron emission.
29. • The secondary ionization coefficient γ is
defined in the same way as α, as the net
number of secondary electrons produced per
incident positive ion, photon, or excited
particle and the total value of γ is the sum of
the individual coefficients due to the three
different processes, i.e., γ = γ1 + γ2 + γ3.
• γ is called the Townsend‘s secondary
ionization coefficient and is a function of the
gas pressure p and E/p.
30. • n+ = n’0 the number of electrons released from the
cathode due to positive ion bombardment or secondary
process and n the number of electrons reaching the
anode.
• Let ν, known as Townsend second ionization co-efficient
be defined as the number of electrons released from
cathode per incident positive ion or secondary process ,
• Then , n = (n0 + n+)eαd
• Now total number of electrons released from the cathode
is n”0 = (n0 + n+) and those reaching the anode are n,
• therefore, the number of electrons released from the gas
= n – (n0 + n+) ,
31. The above equation gives the total current in the gap before the breakdown
32. The above equation gives the total current in the gap before the breakdown
33. Townsend’s Criterion for Breakdown
• The previous equation gives the total current in
the gap before the breakdown.
• As the distance between the electrodes d is
increased, the denominator of the equation
tends to zero, and at some critical distance
d = ds.
35. • For a given gap spacing and at a given pressure
the value of the voltage V which gives the values
of α and γ satisfying the breakdown criterion is
called the spark breakdown voltage Vs and the
corresponding distance ds is called the sparking
distance.
• The Townsend mechanism explains the
phenomena of breakdown only at low pressures.
• The condition γeαd = 1 defines the condition for
beginning of spark and is known as the Townsend
criterion for spark formation or Townsend
breakdown criterion.
36. (1) γeαd = 1
• The number of ion pairs produced in the gap by the passage of arc
electron avalanche is sufficiently large and the resulting positive
ions are able to release one secondary electron and so cause a
repetition of the avalanche process. The discharge is then said to be
self-sustained as the discharge will sustain itself even if the source
producing I0 is removed.
• Therefore, the condition γeαd = 1 defines the threshold sparking
condition.
(2) γeαd > 1
• Here ionization produced by successive avalanche is cumulative.
The spark discharge grows more rapidly the more γeαd exceeds
unity.
(3) γeαd < 1
• Here the current I is not self-sustained i.e., on removal of the
source the current I0 ceases to flow.
37. Example
• A steady current of 600μA flows through
the plane electrodes separated by a
distance of 0.5 cm when a voltage of
10kV is applied. Determine the
Townsend’s first ionization constant
(coefficient) if a current of 60μA flows
when the distance of separation is
reduced to 0.1 cm and the field is kept
constant at previous value.
40. STREAMER OR KANAL MECHANISM :
•Raether has observed that if the charge
concentration is higher than 106 but lower than
108 the growth of an avalanche is weakened.
•Whenever the concentration exceeds 108, the
avalanche current is followed by steep rise in
current and breakdown of the gap takes place.
41. STREAMER OR KANAL MECHANISM :
•The weakening of the avalanche at lower
concentration and rapid growth of avalanche at
higher concentration have been attributed to the
modification of electric field E0 due to the space
charge field.
•If the charge carrier exceeds 108, the space charge
field becomes almost of the same magnitude as the
main field E0 and hence it may lead to initiation of a
streamer (channels of ionization).
44. • when the avalanche in the gap reaches a certain critical
size the combined space charge field and externally
applied field E0 lead to intense ionization and
excitation of the gas particles in front of the avalanche
head.
• There is recombination of electrons and positive ion
resulting in generation of photons and these photons in
turn generate secondary electrons by the
photoionization process.
• These electrons under the influence of the electric field
develop into secondary avalanches as shown in Fig.
• Since photons travel with velocity of light, the process
leads to a rapid development of conduction channel
across the gap.
45. Paschen’s Law:
• The Townsend’s Criterion v(eαd -1) = 1
enables the evaluation of breakdown voltage of
the gap by the use of appropriate values of α/p
and ν corresponding to the values E/p when
the current is too low to damage the cathode
and also the space charge distortions are
minimum.
γ
γ
46. • An expression for the breakdown voltage for
uniform field gaps as a function of gap length
and gas pressure can be derived from the
threshold equation by expressing the ionization
coefficient α/p as a function of field strength E
and gas pressure p i.e.,
Substituting this, we have
47.
48. • This shows that the breakdown voltage of a uniform
field gap is a unique function of the product of gas
pressure and the gap length for a particular gas and
electrode material. This relation is known as
Paschen’s law.
Fig.: Paschen’s law curve
49. Penning Effect
• Paschen’s law does not hold good for many
gaseous mixtures. A typical example is that of
mixture of Argon in Neon.
• A small percentage of Argon in Neon reduces
substantially the dielectric strength of pure
Neon. In fact, the dielectric strength is smaller
than the dielectric strengths of either pure
Neon or Argon.
50. • The lowering of dielectric strength is due to
the fact that the lowest excited stage of neon is
metastable and its excitation potential (16 ev)
is about 0.9 ev greater than the ionization
potential of Argon.
• The metastable atoms have a long life in neon
gas, and on hitting Argon atoms there is a very
high probability of ionizing them. This
phenomenon is known as Penning Effect.
51. Corona Discharges:
• If the electric field is uniform and if the field is
increased gradually, just when measurable
ionization begins, the ionization leads to complete
breakdown of the gap.
• However, in non-uniform fields before the spark or
breakdown of the medium takes place, there are
many manifestations in the form of visual and
audible discharges. These discharges are known as
Corona discharges.
• Corona is defined as a self-sustained electric
discharge in which the field intensified ionization
is localized only over a portion of the distance
(non-uniform fields) between the electrodes.
52. • Corona is responsible for power loss and
interference of power lines with the
communication lines as corona frequency lies
between 20 Hz and 20 kHz.
• This also leads to deterioration of insulation by
the combined action of the discharge ion
bombarding the surface and the action of
chemical compounds that are formed by the
corona discharge.
53. • When a voltage higher than the critical voltage is
applied between two parallel polished wires, the
glow is quite even.
• After operation for a short time, reddish beads or
tufts form along the wire, while around the
surface of the wire there is a bluish white glow.
• As corona phenomenon is initiated a hissing noise
is heard and ozone gas is formed which can be
detected by its characteristic color.
54. • Critical Disruptive voltage: Minimum phase –
neutral voltage when corona occurs, given by,
• Visual Critical Voltage: Mini. phase –neutral
voltage at which corona glows appears all
along the line conductors, given as,
55. • Power loss due to corona: In form of light,
heat, sound and chemical action given as
Advantages:
1. Virtual dia Increases so stress b/w conductor
reduces.
2. Reduces the transients due to surge.
56. • Dis-Advantages.:
1. Loss of energy hence Efficiency Reduces.
2. Ozone produced might be corrosive.
3. Interference with commu. Line coz of non-
sinusoidal nature of current drawn by corona.
57. • When the voltage applied corresponds to the
critical disruptive voltage, corona phenomenon
starts but it is not visible because the charged
ions in the air must receive some finite energy
to cause further ionization by collisions.
• Visual disruptive voltage- The minimum
voltage at which the corona just becomes
visible is called visual critical voltage. The
corona glow is brightest at those surface of the
conductor where the conductor surface is
rough or dirty.
58. Breakdown in Electronegative Gases
• SF6, has excellent insulating strength because of
its affinity for electrons (electronegativity) i.e.,
whenever a free electron collides with the neutral
gas molecule to form negative ion, the electron is
absorbed by the neutral gas molecule.
• The attachment of the electron with the neutral
gas molecule may occur in two ways:
SF6 + e ----→ SF6
–
SF6 + e ----→ SF5
– + F
59. • The negative ions formed are relatively heavier
as compared to free electrons and, therefore,
under a given electric field the ions do not attain
sufficient energy to lead cumulative ionization in
the gas.
• Thus, these processes represent an effective way
of removing electrons from the space which
otherwise would have contributed to form
electron avalanche.
• This property, therefore, gives rise to very high
dielectric strength for SF6.
• The gas not only possesses a good dielectric
strength but it has the unique property of fast
recombination after the source energizing the
spark is removed.
60. • The dielectric strength of SF6 at normal pressure
and temperature is 2–3 times that of air and at 2
atm its strength is comparable with the
transformer oil.
• Although SF6 is a vapour, it can be liquefied at
moderate pressure and stored in steel cylinders.
• Even though SF6 has better insulating and arc-
quenching properties than air at an equal pressure,
• it has the important disadvantage that it can not be
used much above 14 kg/cm2 unless the gas is
heated to avoid liquefaction.
61. • (a) Particle exchange mechanism
• (b) Field emission mechanism
• (c) Clump theory
Vacuum Breakdown
66. Application of Gases in Power System
• The gases find wide application in power system
to provide insulation to various equipments and
substations.
• The gases are also used in circuit breakers for arc
interruption besides providing insulation between
breaker contacts and from contact to the enclosure
used for contacts.
• The various gases used are
(i) air (ii) oxygen (iii) hydrogen (iv) nitrogen (v)
CO2 and (vi) electronegative gases like sulphur
hexafluoride, arcton etc.
67. • The various properties required for providing
insulation and arc interruption are:
(i) High dielectric strength.
(ii) Thermal and chemical stability
(iii) Non-inflammability.
(iv) High thermal conductivity.
(v) Arc extinguishing ability.
(vi) Commercial availability at moderate cost.
