Hv ppt

Ankit Rupareliya
ankitrupareliya09@gmail.com

CONDUCTION AND
BREAKDOWN IN GASES

Electric Field Stress

Introduction
 In mechanical designs, criterion for design
depends on the mechanical strength of the
materials and the stresses that are
generated during their operation.
 Same way, in high voltage applications, the
dielectric strength of insulating materials
and the electric field stresses developed in
them are important factors.

Introduction
 The electric stress to which an insulating
material is subjected to is numerically equal
to the voltage gradient, and is equal to the
electric field intensity E.

Gases as Insulating
Media

Introduction
 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 and sulphur
hexafloride.

Introduction
 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.

Introduction
 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

Introduction
 Ionization process also depends upon
various physical condition of gases like
pressure, temperature, nature of electrode
surfaces, availability of initial conducting
particles etc.

Collision Process

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

Types of Collision
 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.

Types of Collision
 Elastic Collisions:
 Hence electrons gain very high energy and
speeds than the heavier gas ions and
electrons play a major role in such type of
collisions.

Types of Collision
 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.

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

Mobility of Ions and Electrons
 From the kinetic theory, the electron drift
velocity We is given as,
 We = Ee/3ma2 d/dc (1c2)
 Where 1 is an equivalent mean free path of
an electron with speed c.

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.

Diffusion
 Both diffusion and mobility result in mass
motion described by a drift velocity caused
either by unbalanced collision forces or by
the electric field itself.

Collision Cross Section
 It is defined as the area of contact between
two particles during collision.
 It is also the total area of impact.
 For simultaneously occurring processes
such as ionization, excitation, charge
transfer, chemical reactions etc. the
effective cross section is obtained by simple
addition.
 qt = qi + qe + qc + ….

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.

The Mean Free Path (λ)
 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.

Ionization Process

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.

Ionization Process
 The processes that are primarily
responsible for the breakdown of a gas are
 Ionization by collision
 Photo-ionization and
 The secondary ionization processes

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.

 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.

 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 atom, A+ is positive ion and e- is
the electron

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.

Photo Ionization
 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.

Photo Ionization
 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.
 Ionization occurs when

Townsend’s Current
Growth Equation

Reference Figure

Townsend’s Current Growth
Eqn
 Referring to Fig. let us assume that n0
electrons are emitted from the cathode.
 When one electron collides with a neutral
particle, a positive ion and an electron are
formed.
 This is called an ionizing collision.

Eqn
 Let α be the average number of ionizing
collisions made by an electron per
centimeter travel in the direction of the field
(α depends on gas pressure p and EIp, and
is called the Town send's first ionization
coefficient).
 At any distance x from the cathode, let the
number of electrons be nx.

Eqn
 When these nx electrons travel a further
distance of dx they give rise to (αnx dx)
electrons.
 At x = 0, nx = n0

Eqn
 Therefore the average current in the gap,
which is equal to the number of electrons
travelling per second will be
 The number of new electrons created, on
the average, by each electron is
where I0 is the initial current at the cathode.

Current Growth in the
presence of
Secondary Processes

Curr. Growth in presence of
S.P.
 The single avalanche process described in
the previous section becomes complete
when the initial set of electrons reaches the
anode.
 However, since the amplification of
electrons [exp(αd)] is occurring in the field,
the probability of additional new electrons
being liberated in the gap by other
mechanisms increases, and these new
electrons create further avalanches.

S.P.
 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.

S.P.
 Following Townsend's procedure for current
growth, let us assume
 n’0 = number of secondary electrons
produced due to secondary (γ) processes.
 n”0 = total number of electrons leaving the
cathode.
 Then n”0 = n0 + n’0

S.P.
 The total number of electrons n reaching
anode becomes,

S.P.
 Eliminating n’0
 The above equation gives the total current
in the gap before the breakdown.

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.

Breakdown
 For values of d < ds, I is approximately
equal to I0, and if the external source for the
supply of I0 is removed, I becomes zero.
 If d = ds, I → ∞ and the current will be
limited only by the resistance of the power
supply and the external circuit.
 This condition is called Townsend's
breakdown criterion and can be written as

Breakdown
 Normally, exp(αd) is very large, and hence
the above equation reduces to,

Breakdown
 For a given gap spacing and at a give
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.

Breakdown in Electronegative
gases
 It has been recognized that one process
that gives high breakdown strength to a gas
is the electron attachment in which free
electrons get attached to neutral atoms or
molecules to form negative ions.
 Since negative ions like positive ions are too
massive to produce ionization due to
collisions, attachment represents an
effective way of removing electrons which
otherwise would have led to current growth
and breakdown at low voltages.ankitrupareliya09@gmail.com

gases
 The gases in which attachment plays an
active role are called electronegative gases.
 The most common attachment processes
encountered in gases are
 (a) the direct attachment in which an electron
directly attaches to form a negative ion, and
 (b) the dissociative attachment in which the gas
molecules split into their constituent atoms and
the electronegative atom forms a negative ion.

gases
 A simple gas of this type is oxygen. Other
gases are sulphur hexafluoride, freon,
carbon dioxide, and fluorocarbons.
 With such gases, the Townsend current
growth equation is modified to include
ionization and attachment.

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