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Summary TESTING OF MATERIALS
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DIELECTRIC BREAKDOWN, TESTING OF MATERIALS
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Unit 02: A) Dielectric Breakdown:Concept of Primary and Secondary Ionization of Gases (descriptive
treatment only),
Breakdown Voltage, Breakdown Strength,
Factors affecting Breakdown Strengths of Solid, Liquid and
Gaseous dielectric materials
Unit 02: B) Testing of Materials
With objectives, equipment required, circuit diagrams and
observations to be taken.
Measurement of dielectric loss tangent (tan δ) by Schering Bridge-
IS 13585-1994
Measurement of dielectric strength of solid insulating material as
per IS.
Measurement of dielectric strength of liquid insulating material -IS
6798.
Measurement of dielectric strength of gaseous insulating material
as per IS.
Primary and Secondary Ionization of Gases
Ionization: The ionization caused by the passing particle is the primary
ionization, the ionization caused by δ rays is secondary ionization. • The
amount of secondary ionization depends on the material; it can be up to 10
times the amount of primary ionization.
Primary Ionization of Gases
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 occur. The processes
that are primarily responsible for the breakdown of a gas are ionization by
collision, photo-ionization, and the secondary ionization processes. In insulating
gases (also called electron-attaching gases) the process of attachment also plays
an important role.
,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. If we consider a low-pressure gas column in
which an electric field£ is applied across two plane parallel electrodes, as shown
in Fig. then, 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 (ε)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 the atom, A + is the positive ion and e - is the electron.
A few of the electrons produced at the cathode by some external means, say by
ultra-violet light falling on the cathode, ionize neutral gas particles producing
positive ions and additional electrons. The additional electrons, then, themselves
make ‘ionizing collisions’ and thus the process repeats itself. This represents an
increase in the electron current, since the number of electrons reaching the
anode per unit time is greater than those liberated at the cathode. In addition,
the positive ions also reach the cathode and on bombardment on the cathode
give rise to secondary electrons.
2.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.
There are several processes by which radiation can be absorbed by atoms or
molecules. They are
(a) Excitation of the atom to a higher energy state, and
, (b) Continuous absorption by direct excitation of the atom or dissociation of
diatomic molecule or direct ionization, etc.
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. This reversible process can be expressed as
Ionization occurs when
where, h is the Planck’s constant, c is the velocity of light, λ is the wavelength of the
incident radiation and Vi is the ionization energy of the atom. Substituting for h
and c, we get
Where, Vi is in electron volts ( eV). The higher the ionization energy, the shorter will
be the wavelength of the radiation capable of causing ionization. It was observed
experimentally that a radiation having a wavelength of 1250 Å is capable of causing
photo-ionization of almost all gases.
Secondary Ionization Processes
Secondary ionization processes by which secondary electrons are produced are the
one which sustain a discharge after it is established due to ionization by collision
and photo-ionization.
Electron Emission due to Positive Ion Impact
Positive ions are formed due to ionization by collision or by photo-ionization, and
being positively charged, they travel towards the cathode. A positive ion
approaching a metallic cathode can cause emission of electrons from the cathode by
giving up its kinetic energy on impact. If the total energy of the positive ion, namely,
the sum of its kinetic energy and the ionization energy, is greater than twice the
work function of the metal then one electron will be ejected and a second electron
will neutralize the ion. The probability of this process is measured as γi which is
called the Townsend’s secondary ionization coefficient due to positive ions and is
defined as the net yield of electrons per incident positive ion. γi increases with ion
velocity and depends on the kind of gas and electrode material used.
(b) Electron Emission due to Photons
To cause an electron to escape from a metal, it should be given enough energy to
overcome the surface potential barrier. The energy can also be supplied in the form
of a photon of ultraviolet light of suitable frequency. Electron emission from a metal
surface occurs at the critical condition
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