Mechanism for breakdown for

Presentation
on
Mechanism for Breakdown for
Dielectric solids

 Introduction
 Breakdown in solid dielectrics
 Intrinsic breakdown
 Electromechanical breakdown
 Breakdown due to treeing and tracking
 Thermal breakdown
 Electrochemical breakdown

 Solid dielectric materials are used in all kinds of electrical
circuits and devices to insulate one current carrying part
from another when they operate at different voltages.
 A good dielectric should have low dielectric loss, high
mechanical strength, should be free from gaseous
inclusion, and moisture, and be resistant to thermal and
chemical deterioration.
 When breakdown occurs, solids get permanently damaged
while gases fully and liquids partly recover their dielectric
strength after the applied electric field removed.
 The mechanism of breakdown is a complex phenomenon
in the case of solids, and varies depending on the time of
application of voltage .

 Variation of Vb with time of application
Breakdown in solid dielectrics

BREAKDOWN IN
SOLID
DIELECTRICS
Intrinsic breakdown Electromechanical
breakdown
Breakdown due to
treeing and
tracking
Thermal
breakdown
Electrochemical
breakdown
Breakdown due to
internal
discharges

 When voltages are applied only for short durations of the order
of 10-8 s the dielectric strength of a solid dielectric increases
very rapidly to an upper limit called the intrinsic electric
strength.
 The maximum electrical strength recorder is 15 MV/cm for
polyvinyl-alcohol at -1960C. The maximum strength usually
obtainable ranges from 5 MV/cm.
 Intrinsic breakdown depends upon the presence of free
electrons which are capable of migration through the lattice of
the dielectric.
 The impurity atoms, or molecules or both act as traps for the
conduction electrons up to certain ranges of electric fields and
temperatures..

 When these ranges are exceeded, additional electrons in
addition to trapped electrons are released, and these
electrons participate in the conduction process.
 Based on this principle, two types of intrinsic breakdown
mechanisms have been proposed.
 Electronic Breakdown
 Avalanche or Streamer Breakdown

• When a dielectric material is subjected to an electric
field charges of opposite nature are induced on two
opposite surfaces of the material.
• Hence a force of attraction is developed and the
material is compressed.
• When these electrostatic compressive forces exceed
the mechanical withstand strength of the material the
material collapse.
• Normally these kinds of breakdown take place in soft
materials where ionic polarization is predominant.

 Spreading of spark channels during tracking in the form of
the branches of a tree is called treeing.
 In practice, the surface of solid dielectric material always
having the conducting film, which is formed due to
moisture. On application of voltage, the film starts
conducting, resulting in generation of heat, and the surface
starts becoming dry.
 With organic insulating material, the dielectric carbonizes
at the region of sparking, and carbonized regions act as
permanent conducting channels.
 This is a cumulative process, and insulation failure occurs
when carbonized tracks bridge the distance between the
electrodes. This phenomenon is called tracking.

 Treeing occurs due to the erosion of material at the tips of
the spark and results the roughening of the surface and
becomes dirt and contamination. Breakdown channels
spread thru the insulation in an irregular “tree” and leading to
the formation of conducting channel.
 Under a.c. conditions, treeing can occurs in a few minutes or
several hours. Hence care must be taken to see that no
series air gaps or other weaker insulation gaps are formed.
 Usually tracking occurs even at very low voltages, whereas
treeing requires high voltage.
 The numerical value of voltage that initiates or causes the
formation of a track is called the “tracking index” and this is
used to qualify the surface properties of dielectric material.
 Treeing can be prevented by having clean, dry and
undamaged surfaces and clean environment. Usually treeing
phenomena is observed in capacitors and cables

Breakdown channels in Perspex between point-
plane electrodes

 When an electric field is applied to a dielectric,
conduction current flows through the material.
 Current heats up the specimen and the temperature
rises.
 Heat generated is transferred to the surrounding medium
by conduction and radiation.
 Equilibrium is reached when the heat generated (Wdc or
Wac) is equal to heat dissipated (WT). Breakdown occurs
when Wdc or Wac exceeds WT.
 where, Wdc = E2σ W/cm2

 f : frequency in Hz.
 δ : loss angle of the dielectric material
 E : rms value
• CV : specific heat of the specimen
• T : temperature of the specimen
• K : thermal conductivity of the specimen
• t : time over which the heat is dissipated

• When electric field is applied to a solid specimen heat is
produced due to dielectric losses in the specimen.
• The losses are due to :
 Ohmic losses
 Dipole oscillations
 Partial discharges due to voids
• Due to losses, heat is generated

 In presence of air and other gases, dielectric materials
undergo chemical changes when subjected to
continuous electrical stresses. Chemical reactions that
occur are:
 Oxidation: In the presence of air or oxygen, materials
such as rubber and polyethylene undergo oxidation
giving rise to surface cracks.
 Hydrolysis: When moisture or water vapour is present on
the surface of the solid dielectric, hydrolysis occurs and
the materials lose their electrical and mechanical
properties. Materials like paper, cotton tape and other
cellulose materials deteriorate very rapidly due to
hydrolysis.

 Chemical Action: Progressive chemical degradation can
occur due to a variety of processes such as chemical
instability at high temperature, oxidation, cracking and
hydrolysis.
 Chemical and electrochemical deterioration increases
very rapidly with temperature.

 Solid insulating materials, and to a lesser extent liquid dielectrics
contain voids or cavities within the medium or at the boundaries
between the dielectric and the electrodes.
 These voids are generally filled with a medium of lower dielectric
strength, and the dielectric constant of the medium in the voids is
lower than that of the insulation.
 Therefore, even under normal working voltages the field in the
voids may exceed their breakdown value, and breakdown may
occur.
 Let us consider a dielectric between two conductors as shown in
Fig. If we divide the insulation into three parts, an electrical
network of C1 ,C2 ,and C3 can be formed as
 shown in Fig. In this, C1 represents the capacitance of the void or
cavity, C2 is the capacitance of the dielectric which is in series with
the void, and C3 is the capacitance of the dielectric.

• When a voltage V is applied, V1 reaches the breakdown strength of the
medium in the cavity (Vi) and breakdown occurs. Vi is called the “discharge
inception voltage”.
• When the applied voltage is a.c., breakdown occurs on both the half
cycles and the number of discharges will depend on the applied voltage.
• When once the voltage V1 becomes zero, the spark gets extinguished and
again the voltage rises till breakdown occurs again.
• Also, in each discharge there will be some heat dissipated in the cavities,
and this will carbonize the surface of the voids and will caused erosions of
the material. Channels and pits formed on the cavity surfaces increase the
conduction.

Mechanism for breakdown for

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