breakdown in solid

1. BREAKDOWN IN SOLIDS
Topic Contents:
• Intrinsic breakdown:
 Electronic breakdown
 Avalanche or streamer breakdown,
• Electromechanical breakdown,
• Thermal breakdown,
• Treeing and tracking phenomenon,
• Chemical and Electrochemical breakdown,
• Partial discharge (Internal discharge) ,
• Composite dielectric material, properties of composite dielectrics,
• Breakdown in composite dielectrics.27-02-2020 1

2. Introduction
• When breakdown occurs gases regain their dielectric strength fully, liquid partially
and solid loses their dielectric strength completely. It is permanent breakdown.
• Breakdown depends on-
o Magnitude, time of voltage applied A.C., D.C., or impulse voltage applied.
o Ambient temperature, humidity
o Impurities or structural defect, pressure applied to electrode.
• Time of application of voltage plays an important role in breakdown. According to
time of application different breakdown mechanism occur.
1. Intrinsic or Ionic breakdown
2. Electromechanical breakdown
3. Breakdown due to treeing and tracking
4. Thermal breakdown
5. Electrochemical breakdown
6. Breakdown due to internal discharges
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3. Properties:
Good dielectric should have low dielectric loss
High mechanical strength
Should be free from gaseous inclusion and moisture
Resistance to thermal and chemical detritions.
High insulation resistance.
Applications
Used in Heater, generators, circuit breakers.
Provide mechanical support to equipment.
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4. Intrinsic /Ionic Breakdown
• This theory can be applied if the dielectric strength is
only dependent on chemical composition and dielectric
properties of material.
• The mechanism of intrinsic breakdown of solid is an
electronic phenomenon and is completed in a very short
time of order of 10−8
second.
• Two types of intrinsic breakdown are been proposed by
Frohlich’s theory.
• In case of pure dielectric, the gap between valance band
and conduction band is wide.
• The electrons can not acquire enough energy to cross in
conduction band.
• To dislodge an electron from atomic structure a field of
order of 108
V/cm is needed. But normally breakdown
occur much below it.
• So in perfect dielectric there are no free electrons and
conductivity is almost zero.27-02-2020 4

5. Avalanche Breakdown
• Under the effect of electric field an electron entering conduction level of dielectric at cathode will
move towards anode.
• It gains energy in movement and loses part of energy when collision occurs. Energy gained due to
motion is more than lost during collision when mean free path is long.
• The process is continuous and finally may lead to formation of an electron avalanche and
breakdown takes place.
• In practice breakdown does not occur by formation of single avalanche itself, but occurs as a
result of many avalanches formed within dielectrics and exceeding step by step through entire
thickness of material.
Factors affecting breakdown:
1. Temperature and it reduces with temperature.
2. Addition of foreign atoms to crystal.
3. Dielectric strength decreases with increase in molecular volume.
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6. Electronic Breakdown
• Intrinsic breakdown occurs in time of order of 10−8 second and therefore is assumed to be
electronic in nature.
• The initial density of conduction (free) electrons is also assumed to be large, and electron-electron
collision occurs.
• When an electric field is applied , electrons gain energy from electric field and cross forbidden
energy gap from valance to conduction band, when this process is repeated, more and more
electrons become available in conduction band , eventually leading to breakdown.
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7. Electromechanical breakdown
• Reasons:
i. Solid dielectrics are subjected to high electric field
ii. Electrostatic compressive force > mechanical compressive strength.
• Consider solid dielectric material piece,
𝑑0--- initial thickness
V- applied voltage
d- thickness of material after applying high voltage.
Material structure gets compressed after applying high voltage.
Electrostatic compressive force- ½ ∈0∈ 𝑟 𝑉2 /𝑑2-------(1)
Mechanical compressive strength- Y ln[𝑑0/𝑑]----------(2)
To get maximum value of electric stress at time of breakdown , equate eq. (1) & (2)
½ ∈0∈ 𝑟 𝑉2
/𝑑2
=Y ln[𝑑0/𝑑]
𝑉2= 𝑑2 [ 2 Y / ∈0∈ 𝑟] ln [𝑑0/𝑑 ]
V= d. 2
𝑌
∈0∈ 𝑟
ln [𝑑0/𝑑 ]--------------(3)
When high voltage is applied there will be compressive force so structure will differ so mechanical instability will occurs.
d/𝑑0=0.6----------(4)
𝐸 𝑚𝑎𝑥 = 𝑉/𝑑0=0.6.
𝑌
∈0∈ 𝑟27-02-2020 7

8. Electromechanical breakdown
• When solid dielectrics are subjected to high electric fields, failure occurs due to electrostatic
compressive forces which can exceed the mechanical compressive strength.
• These forces cause induced charges of opposite nature on two opposite surfaces of material. So force
of attraction is developed.
• Due to this force specimen is subjected to electrostatic compressive forces.
• Material collapses due to reduction in thickness if these forces exceed mechanical withstand strength
of material hence material continues to shrink until breakdown.
• If the thickness is ‘𝑑0
′
𝑎𝑛𝑑 𝑖𝑠 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑒𝑑 to a thickness of ‘d’ under an applied voltage ‘V’ then the
electrically developed compressive strength in equilibrium.
F= ½ ∈0∈ 𝑟 𝑉2
/𝑑2
∈0∈ 𝑟 𝑉2
/2𝑑2
= Y ln[𝑑0/𝑑]
Where Y is Youngs modulus, from above equation we get,
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9. 𝑉2
= 𝑑2
[ 2 Y / ∈0∈ 𝑟] ln [𝑑0/𝑑 ]
2V dv/dt = k[ 2d ln (𝑑0/𝑑 ) - 𝑑2
.(d/ 𝑑0). (𝑑0/𝑑2
)=0
2d ln (𝑑0/𝑑 )=d
2d ln (𝑑0/𝑑 )=1/2
d/𝑑0=0.6=0.6𝑑0
The highest apparent electric stress before breakdown,
𝐸 𝑚𝑎𝑥= v/do=0.6 [Y/ (𝜖0 𝜖 𝑟)]
This theory only approximate as Y depends on mechanical stress. Also, when material
subjected to high stresses the theory of elasticity does not hold good.
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10. Thermal Breakdown
A) Reasons For breakdown
• No solid is perfect electrical insulator, hence under an electric field small conduction current always flow in a
solid.
• This flow of current causes local heating (𝐼2
𝑅 𝑙𝑜𝑠𝑠𝑒𝑠) created by collision of electrons.
• In general the breakdown voltage of solid dielectric should increase with its thickness. But this is true only up
to certain thickness above which heat generated in dielectric due to flow of current determines conduction.
• When an electric field is applied to a dielectric, conduction current, however small it may be, flows through
material. The current heats up specimen and temperature rises.
• The heat generated is transferred to surrounding medium by conduction through solid dielectric and by
radiation from its outer surface.
• Equilibrium is reached when heat radiates out, equals heat generated.
• The heat generated under dc stress E is given as,
𝑊𝑑𝑐 = 𝐸2 𝜎 W/𝑐𝑚3
Where 𝜎 is dc conductivity o specimen.
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11. Under ac fields heat generated
𝑊𝑎𝑐 = 𝐸2
𝑓 ∈ 𝑟 𝑡𝑎𝑛𝛿/1.8*1012
W/𝑐𝑚3
Where f=frequency in Hz,
𝛿 = 𝑙𝑜𝑠𝑠 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑑𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙
E=rms value
The heat dissipated is given as
𝑊𝑡 = 𝑐 𝑣 𝑑𝑇/𝑑𝑡+div(KgradT)
Where 𝑐 𝑣=specific heat of specimen
T=temperature of specimen
K=thermal conductivity of specimen
t=time over which heat is dissipated
Equlibrium reached when heat generated(𝑊𝑎𝑐 or
𝑊𝑑𝑐) becomes equal to heat dissipated.
Breakdown occurs when 𝑊𝑎𝑐 or 𝑊𝑑𝑐exceeds 𝑊𝑡.
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12. Breakdown due to treeing and tracking
• When a solid dielectric subjected to electrical stresses for a long time fails, normally two kinds o
visible markings are observed on dielectric materials, they are
a) The presence of a conducting path across surface of insulation
b) A mechanism whereby leakage current passes through conducting path finally leading to
formation of a spark. Insulation deterioration occurs as a result of these sparks.
The spreading of spark channels during tracking in form of branches of tree is called treeing.
• Tracking is formation of continuous conducting paths across surface of insulation mainly due to
surface erosion under voltage application
• Consider a system of a solid dielectric having a conducting film and two electrodes on its surface. In
practice conducting film very often is formed due to moisture.
• On film application of voltage, the film starts conducting, resulting in generation of heat and surface
starts becoming dry.
• The conducting film becomes separate due to drying and so sparks are drawn damaging dielectric
surface.
• With organic insulating materials such as paper and bakelite, the dielectric carbonizes at sparking
regions.
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13. • The carbonized portion acts as permanent conducting region resulting in an increase of stress
over rest of portion between electrodes.
• This process repeats if environment is such as to form a conducting layer again
• Tracking can occur at as low voltages as 100V and mainly caused by high current density at
spark tips. In high voltage tracking, the conducting path begins by impinging of discharge on
dielectric surface.
Methods to prevent tracking
• Insulation should be kept clean and dry.
• Clean and dry environment.
• Use of track resistance material sometimes moisture replacements gases are used.
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14. Breakdown due to treeing
• Spreading of spark channels during tracking in form
of branches of tree is called treeing.
• It occurs due to erosion of material at tips of spark.
Erosion results in roughening of surface and hence
becomes a source of dirt and contamination. This
cause increased conductivity resulting either in
formation of a conducting path bridging electrodes or
mechanical failure of dielectric.
• When dielectric material lies between two electrodes
as shown in figure.
• There is possibility for two different dielectric media
air and dielectric to come in series.
• The voltage across two media are as shown
( 𝑉1 across air gap, 𝑉2 across dielectric)
𝑉1 =
𝑉𝑑1
𝑑1+𝑑2
∈1
∈2
where, V is applied voltage. ∈2 > ∈1
• Hence more voltage is concentrated across 𝑑1 i.e.
across air gap. The sparking will occur in air-gap
and charge accumulation takes place on surface,
the insulation sometimes erodes surface of
insulation.
• This deterioration of material goes on continuing
with time and spreads in shape o tree like manner.
This form conducting channel.
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15. Treeing Tracking
Definition Spreading of spark channels during tracking in
form of branches of a tree is called treeing.
Tracking is formation of continuous
conducting path across surface of insulation
cause • Occurs due to erosion of material at tip of spark
• Occurs at very high voltage
• Occurs due to carbonization at surface
under voltage application.
• occurs at very low voltage about 100V.
• Main cause for its occurrence is gaseous
impurity.
• Causes due to air cavity and increasing stress in
it.
• Main cause for its occurrence is solid
impurities like metallic dust, salt.
• causes due to deposition of metallic dust
and industrial pollutants.
Prevention Can be prevented by having clean dry and
undamaged surface and clean environment.
Can be prevented by increasing creepage
distance and by choosing material of proper
heat resistance.
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16. Chemical & Electrochemical Deterioration &
Breakdown
• In the presence of air and other gases some dielectric materials undergo chemical changes
when subjected to continuous electrical stresses.
• Some of the important chemical reactions that occur are the following:
1. Oxidation
2. Hydrolysis
3. Chemical Action
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17.  Oxidation
• In the presence of air or oxygen, materials such as rubber and polyethylene undergo
oxidation giving rise to surface cracks.
 Hydrolysis
• Failure When moisture or water vapour is present on the surface of a solid dielectric,
hydrolysis occurs and the materials lose their electrical and mechanical properties.
• Electrical properties of materials such as paper, cotton tape, and other cellulose
materials deteriorate very rapidly due to hydrolysis.
• Plastics like polyethylene undergo changes, and their service life considerably reduces.
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18.  Chemical Action
• Even in the absence of electric fields, progressive chemical degradation of insulating
materials can occur due to a variety of processes such as chemical instability at high
temperatures, oxidation and cracking in the presence of air and ozone, and hydrolysis due
to moisture and heat.
• Since different insulating materials come into contact with each other in any practical
apparatus, chemical reactions occur between these various materials leading to reduction
in electrical and mechanical strengths resulting in failure.
• The effects of electrochemical and chemical deterioration could be minimized by carefully
studying and examining the materials.
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19. • High soda content glass insulation should be avoided in moist condition,
because soda content will cause deterioration.
• It was observed that this type of material will lose its mechanical strength
within 24 hrs, when it is exposed to atmospheres havjng 100% relative
humidity at 70℃.
• In paper insulation, even if partial discharges are prevented completely,
breakdown can occur due to chemical degradation.
• The chemical and electrochemical deterioration increases very rapidly with
temperature, and hence high temperatures should be avoided.
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20. Breakdown due to internal discharge
• 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.
• The electric field strength in the voids is higher than that across the dielectric. Therefore, even
under normal working voltages the field in the voids may exceed their breakdown value, and
breakdown may occur.
• Let us consider dielectric between two conductors as shown in figure below
•
Figure : Electrical discharge in a cavity and its equivalent circuit.
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21. • If we divide the insulation into three parts, an electrical network of 𝐶1, 𝐶2 and 𝐶3 can be
formed as shown in Figure 1b.
• 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 rest of the
dielectric.
• When the applied voltage is V, the voltage across the void, 𝑉1F1 is given by the same
equation as 1
• where 𝑑1 and 𝑑2 are the thickness of the void and dielectric respectively, having
permittivity's 𝜀0 and 𝜀1. Usually 𝑑1is less than 𝑑2.
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22. • If we assume cavity is filled with gas then,
𝑉1 = 𝑉. ∈ 𝑟
𝑑1
𝑑2
Where ∈ 𝑟 is relative permittivity of dielectric.
• When voltage V is applied, 𝑉1 reaches breakdown strength of medium in cavity (𝑉𝑖) and breakdown
occurs. Where (𝑉𝑖) is called discharge inception voltage.
• When applied voltage is ac, breakdown occurs on both half cycles and number of discharges will
depend on applied voltage.
• The voltage and discharge current waveforms are sown in figure below:
27-02-2020 22Breakdown in Solid (MM)

23. • When first breakdown occurs, voltage across cavity ( 𝑉1) becomes zero and spark gets
extinguished and again voltage rises till breakdown occurs again. This process repeats
again and again, the current pulses are obtained in both cycles as shown in above figure.
• The internal discharges (also called partial discharges) will have same effect as treeing on
insulation.
• When breakdown occurs in void, electrons and positive ions are formed. They will have
sufficient energy and when they reach void surfaces they may break chemical bonds.
• Also in each discharge there will be some heat dissipated in cavities and this will
carbonize surface of voids and will cause erosion of material.
• All these effects will result in gradual erosion of material and consequent reduction in
thickness of insulation leading to breakdown.
• The life of insulation with internal discharge depends upon applied voltage and number of
discharges. Breakdown by this process may occur in few days or take few years.
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24. Breakdown in composite dielectrics
• It is difficult to imagine a complete insulation system in an electrical equipment which does not
consist of more than one type of insulation.
• If an insulation system as a whole is considered, it will be found that more than one insulating
material is used.
• These different materials can be in parallel with each other, such as air or 𝑆𝐹6 gas in parallel with
solid insulation or in series with one another.
• Such insulation systems are called composite dielectrics.
• Composite insulating materials are generally composed of different chemical substances or they
come into contact with different compositions.
• Chemical reactions occur when voltage is applied to them and there will substantial increase in the
rate of these reactions if applied voltage is continuous and high temperature are present.
• Under these conditions, the composites undergo chemical deterioration leading to reduction in both
electrical and mechanical strength.
• The composite nature of an insulation system arises from the mechanical requirements involved in
separating electrical conductors which are at different potentials.
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25. • Also, parts of a single system that are normally composed of a single material are in fact
composite in nature.
• In actual practice, these single materials will normally have small volumes of another material
present in their bulk.
• For example, a solid will contain gas pockets or voids, while a liquid or gas will contain metallic or
dust particles, gas bubbles.
• A commonly encountered composite dielectric is the solid/liquid combination or liquid
impregnated flexible solid like thin sheets of paper or plastic.
• This type of composite dielectric is widely used in a variety of low and high voltage apparatus such
as cables, capacitors, transformers, oil-filled switchgear, bushings. In recent years solid/𝑆𝐹6 gas
technology has become more acceptable.
• In the practical system, in order to reduce the undesirable effects, composite insulation is used by
combining different dielectrics either in series or in parallel such that it is possible to obtain
superior dielectric properties than that possible for a single material of the same thickness.
• A composite dielectric generally consists of a large number of layers arranged one over the other.
This is called "the layered construction" and is widely used in cables, capacitors and transformers.
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26. • Three properties of composite dielectrics which are important to their
performance are given below.
 Effect of Multiple Layers
 Effect of Layer Thickness
 Effect of Interfaces
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27.  Effect of Multiple Layers
• The simplest composite dielectric consists of two layers of the same material.
• Here, advantage is taken of the fact that two thin sheets have a higher dielectric
strength than a single sheet of the same total thickness.
• The advantage is particularly significant in the case of materials having a wide
variation in dielectric strength values measured at different points on its surface.

28.  Effect of Layer Thickness
• Increase in layer thickness normally gives increased breakdown voltage.
• In a layered construction, breakdown channels occur at the interfaces only and not directly
through another layer.
• Also, a discharge having penetrated one layer cannot enter the next layer until a part of the
interface also attains the potential which can produce an electric field stress comparable to that
of the discharge channel.
• The interface between two dielectric surfaces in a composite dielectric system plays an important
role in determining its pre-breakdown and breakdown strengths
• Discharges usually occur at the interfaces and the magnitude of the discharge depends on the
associated surface resistance and capacitance.
• When the surface conductivity increases, the discharge magnitude also increases, resulting in
damage to the dielectric.

29.  Effect of Interfaces
• In a composite dielectric, it is essential to maintain low dielectric losses because they
normally operate at high electric stresses.
• However, even in an initially pure dielectric liquid, when used under industrial
conditions for impregnating solid dielectrics, impurities arise, resulting in increased
dielectric losses.
• The effect of various impurities in causing the breakdown of composite dielectrics .

30. Breakdown Mechanism in Composite Dielectrics
Short Term Breakdown:
• The failure of insulating material may occur without actually damaging insulating material.this
occurs due to very high electric field stresses. The failure occurs in seconds or even faster.
• When applied voltages close to breakdown value, breakdown occurs in one or more discharges.
• When stress exceeds critical stress at which discharge enters or penetrates from surface and
propagates rapidly into volume, breakdown occurs.
• Breakdown can occur after single discharge and more rapidly if electric field in insulation assists
charged particles in discharge. These charged particles then penetrates into insulation.
• Breakdown occurs more rapidly when bombarding particles are electrons, rather positive ions.
• Under high field conditions local field intensifies due to presence of impurities and variations in
thickness plays a very important role; effect depends on field insulation before discharge impinges
on it.
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31. Long Term Breakdown
A) Ageing and breakdown due to partial
discharges
• Partial discharge involves transfer of electric
charge between two points to cause discharge of
local capacitance i.e. void or gas filled cavity. The
charge on dielectric surface produce a
deterioration of insulating properties depending on
geometry of cavity and nature of dielectric.
• The degree of ageing depends upon
1. Inception voltage V1.
2. Discharge magnitude
V1 depends on:
1. Permittivity of dielectric ∈ 𝑟
2. Thickness of coating
𝑉𝑖 =
𝐸 𝑔
𝐸 𝑟
(t+∈ 𝑟g)
𝐸𝑔- breakdown stress of cavity air gap
𝐸𝑟- permittivity of dielectric
g- thickness of gap
t- thickness of dielectric in series with cavity,
(g+t) is constant
𝑉𝑖 =
𝐸 𝑔
𝐸 𝑟
[ (𝐸𝑟-1)g +C]
Differentiate
𝑑𝑉 𝑖
𝑑 𝑔
=
∈ 𝑟−1
∈ 𝑟
[𝐸𝑔 + ( 𝑔 +
𝐶
∈ 𝑟−1
)
𝑑𝐸
𝑑𝑔
Eg is positive and dE/dg is negative or zero.
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32. • Assumptions:
1. Cavity stress Eg= ∈𝑟.E
2. With an cavity stress Egmax/Eg=1
with these assumptions Paschen's law can be applied or small cavities. The inception voltage
decreases as cavity depth increases following Paschen's curve of gas breakdown.
• The corrosion of cavity walls will not affect insulation conditions. i.e. breakdown will not occur
and life of insulation will be long.
• If applied voltage is greater than 2V1, erosion is greater. Causing rapid ageing of insulation and
insulation life is reduced.
• There is a capacitance formed due to cavity. The discharge through this cavity takes place due
to field stress. The entire capacitance of cavity does not discharge at once but many small
discharges take place. Each discharge takes place in small cavities.
• The entire cavity then become conductive.
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33. B) Ageing and breakdown due to accumulation of charges
on insulator surfaces.
• During discharge in composite dielectric interfaces, electron(charge) gets deposited on solid
insulation surface.
• It remain there for a sufficient long period (several days/weeks) causing increase in surface
conductivity. The magnitude and frequency of occurrence of discharges thus increase.
• This causes damage to insulation and quality and life of insulation degrades.
 Discharging chara. Changes with life of insulation for clean surface V1- discharge chara.
depends on nature of dielectric, its size and shape and discharge consists of small number of
discharges.
 after some time erosion causes discharges to decrease in number and also in magnitude.
With the passage of time phenomenon becomes complex because of charges on surface
induced conductively add to charge accumulation in bulk due to partial discharge.
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