Breakdown in solids, liquids and gases

AJITH VIJAYAN, ASST PROFESSOR, EEE DEPT., GEC WAYANAD


MECHANISM OF BREAKDOWN IN GASES, LIQUIDS AND SOLIDS

MECHANISM OF BREAKDOWN IN GASES

At normal temperature and pressure, the gases

are excellent insulators. The current conduction

is of the order of 1010 A/cm2. This current

conduction is due to the ionization. At higher

fields, charged particles may gain sufficient

energy between collisions to cause ionization

on impact with neutral molecules

Explanation

Gaseous dielectrics contain free electrons which

may be caused by irradiation or field emission

and this can lead to a breakdown process. On

the application of an electric field these free

electrons are accelerated from the cathode to

the anode. They acquire a kinetic energy (

mv2) as they move through the field. These free

electrons, moving towards the anode collide

with the gas molecules present between the

electrodes. In these collisions, part of the

kinetic energy of the electrons is lost and part is

transmitted to the neutral molecule. This

molecule gains sufficient energy it may ionize

by collision. The newly liberated electron and

the impinging electron are then accelerated in

the field and an electron avalanche is set up.

Further increase in voltage results in additional

ionizing processes. Ionization increases rapidly

with voltage.

Ionization can occur in any of the following

ways:

(i) Ionization by simple collision: When the kinetic energy of an electron ( mu), in collision with a neutral gas molecule exceeds the ionization energy of the molecule, then ionization can occur.

M + e- ( mu) M

+ + 2 e

-

(ii) Ionization by Double electron impact : If a

gas molecule is already raised to an excited

state (with energy Ee) by a previous collision,

then ionization of this excited molecule can

occur by a collision with a relatively slow

electron. This electron would need less energy

than the ionization energy, but the energy must

exceed the additional energy required to attain

the ionization energy.

(iii)Photo-ionization: This ionization by

radiation or photons 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

energy and is represented as

A + h A+ + e

where A represents a neutral atom or molecule

in the gas and h the photon energy.

(iv) Electron Attachment : If a gas molecule has unoccupied energy levels in its outermost group, then a colliding electron may take up one of these levels, converting the molecule into a negative ion M-.

M + e- M-

(vi) Electron detachment: This occurs when a negative ion gives up its extra electron, and becomes a neutral molecule.

M- M + e- (vi) Thermal Ionization: The term thermal

ionization is the ionizing actions of molecular

collisions, radiation and electron collisions

occurring in gases at high temperatures. When

a gas is heated to high temperature, some of

the gas molecules acquire high kinetic energy

and these particles after collision with neutral



particles ionize them and release electrons.

These electrons and other high-velocity

molecules in turn collide with other particles

and release more electrons. Thus, the gas gets

ionized.

In short: if the applied voltage/field is large, the

current flowing through the insulation increases

very sharply an electrical breakdown occurs. A strongly conducting spark formed during

breakdown practically produces a short circuit

between the electrodes. The breakdown in a

gas, called spark is the transition of non-

sustaining discharge into a self-sustaining

discharge. The build-up of high currents in a

breakdown is due to the process known as

ionization in which electrons and ions are

created from neutral atoms or molecules' and

their migration to the anode and cathode

respectively leads to high current. There are

two theories related to breakdown of gases

which are of importance.

(i) Electron Avalanche Mechanism (Townsend

Breakdown Process)

(ii) Streamer theory

TOWNSEND BREAKDOWN MECHANISM:

It is based on the generation of successive

secondary avalanches to produce breakdown.

Suppose a free electron exist in a gas which is

under the influence of an electric field. If the

field strength is sufficiently high, then it is likely

to ionize a gas molecule by simple collision

resulting in 2 free electrons and a positive ion.

These 2 electrons will be able to cause further

ionization by collision leading in general to 4

electrons and 3 positive ions. The process is

cumulative, and the number of free electrons

will go on increasing as they continue to move

under the action of the electric field. The swarm

of electrons and positive ions produced in this

way is called an electron avalanche. In the

space of a few millimeters, it may grow until it

contains many millions of electrons.

Townsend's first ionization coefficient

Consider a parallel plate capacitor having gas as an insulating medium and separated by a distance d.

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 when 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, at I 0 which corresponds to the saturation current. At still higher voltages, the current increases exponentially. To explain the exponential rise in current, Townsend introduced a coefficient known as Townsends 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.



When the voltage applied across a pair of

electrodes is increased, the current throughout

the gap increases slowly as the electrons

emitted from the cathode move through the

gas. Let

n0 = number of electrons/second emitted from the cathode,

nx = number of electrons/second moving at a distance x from the cathode

[nx > n0 due to ionizing collisions in gap]

= number of ionizing collisions [Townsend's first ionization coefficient]

1/ = average distance traversed in the field direction between ionizing collisions.

Consider a laminar of thickness dx at a distance

x from the cathode. The nx electrons entering

the laminar will traverse in the presence of the

applied field E. The ionizing collisions generated

in the gas gap will be proportional to both dx

and to nx. Say dnx electrons are formed due to

collisions.

Thus dnx nx

Also dnx dx

Therefore dnx = . nx . dx

Rearranging and integrating gives

If the anode is at a distance x = d from the

cathode, then the number of electrons nd

striking the anode per second is given by

nd = n0 . ed

In the steady state, the number of positive ions

arriving at the cathode/second must be exactly

equal to the number of newly formed electrons

arriving at the anode. Thus the circuit current

will be given by

I = I0 . ed

where I0 is the initial photo-electric current at

the cathode. The term ed is called the electron

avalanche and it represents the number of

electrons produced by one electron in travelling

from cathode to anode.

Townsend's second ionization coefficient

We have the current growth equation,

I = I0 . ed

Taking log on both the sides, ln I = ln I0 + d

This is a straight line equation with slope and

intercept (ln I0) if pressure p and E is kept

constant as shown in fig.

Townsend in his earlier investigations had

observed that the current in parallel plate gap

increased more rapidly with increase in voltage

as compared to the one given by the above

equation. To explain this departure from

linearity, Townsend suggested that a second

mechanism must be affecting the current. He

postulated that the additional current must be

due to the presence of positive ions and the



photons. The positive ions will liberate

electrons by collision with gas molecules and by

bombardment against the cathode. Similarly,

the photons will also release electrons after

collision with gas molecules and from the

cathode after photon impact. Let

= number of secondary electrons (on average)

produced at the cathode per ionizing collision in

the gap. [Townsend's second ionization

coefficient]

n0 = number of primary photo-electrons/second emitted from the cathode

n0' = number of secondary electrons/second produced at the cathode

n0" = total number of electrons/second leaving the cathode

Then n0 = n0 + n0

On an average, each electron leaving the

cathode produces [ed - 1] collisions in the gap,

giving the number of ionizing collisions/second

in the gap as n0" (ed - 1). Thus by definition

Similar to the case of the primary process (with

only), we have

Thus, in steady state, the circuit current I will be given by

This equation describes the growth of average current in the gap before spark breakdown occurs.

As the applied voltage increases, ed and ed increase until ed 1 , and the denominator of the circuit current expression becomes zero and the current I . In this case, the current in practice is limited only by the resistance of the power supply and the conducting gas.

This condition may thus be defined as the breakdown and can be written as

.(ed - 1) = 1

This condition is known as the Townsend criteria for spark breakdown.

When ed > 1, the ionization produced by successive avalanche is cumulative. The spark discharge grows more rapidly

When ed < 1, the current I is not self-sustained i.e., on removal of the source the current I0 ceases to flow

Determination of Townsend's Coefficients

and

Townsend's coefficients are determined in an

ionization chamber which is first evacuated to a

very high vacuum of the order of 10-4 and 10-6

torr before filling with the desired gas at a

pressure of a few torr. The applied direct

voltage is about 2 to 10 kV, and the electrode

system consists of a plane high voltage

electrode and a low voltage electrode

surrounded by a guard electrode to maintain a



uniform field. The low voltage electrode is

earthed through an electrometer amplifier

capable of measuring currents in the range 0.01

pA to 10 nA. The cathode is irradiated using an

ultra-violet lamp from the outside to produce

the initiation electron. The voltage current

characteristics are then obtained for different

gap settings. At low voltage the current growth

is not steady. Afterwards the steady Townsend

process develops as shown in figure

From the Townsend mechanism, the discharge

current is given by

I = I0 . ed

This can be written in logarithmic form as

ln I = d + ln I0

y = m x + c

From a graph of ln I vs d, the constants and I0

can be determined from the gradient and the

intercept respectively.

Once I0 and are known, can be determined

from points on the upward region of the curve.

PASCHENS LAW

When electrons and ions move through a gas in

a uniform field E and gas pressure p, their mean

energies attain equilibrium values dependant

on the ratio E/p

For a uniform field gap, the electric field E =

V/d. Thus applying Townsend's criterion for

spark breakdown of gases gives

which may be written in terms of the functions

as

This equation shows that the breakdown

voltage V is an implicit function of the product

of gas pressure p and the electrode separation

d.

In the above derivation the effect of

temperature on the breakdown voltage is not

taken into account. Using the gas equation

pressure . volume = mass. R . Temperature

(PV=nRT)

Or pressure = density . R . Temperature. (RT)

Thus the correct statement of the above

expression is V = f(.d). This shows that the

breakdown voltage of a uniform field gap is a

unique function of the productof gas pressure

and the gap length for a particular gas and

electrode material.

This is the statement for Paschens law.



With very low products of (pressure x spacing),

a minimum breakdown voltage occurs, known

as the Paschen's minimum.

The breakdown voltage varies linearly with the

product pd. The variation over a large range is

shown in fig

(ii) STREAMER OR KANAL MECHANISM

This type of breakdown mainly arises due to the

added effect of the space-charge field of an

avalanche and photo-electric ionization in the

gas volume.

Townsend mechanism when applied to

breakdown at atmospheric pressure was found

to have certain drawbacks.

Firstly, according to the Townsend

theory, current growth occurs as a

result of ionization processes, only. But

in practice, breakdown voltages were

found to depend on the gas pressure

and the geometry of the gap.

Secondly, the mechanism predicts time

lags(The time that elapses between the

application of the voltage to a gap

sufficient to cause breakdown) of the

order of 10-5 s, while in actual practice

breakdown was observed to occur at

very short times of the order of 10-8 s

While the Townsend mechanism

predicts a very diffused form of

discharge, in actual practice, discharges

were found to be filamentary and

irregular.

The Townsend mechanism failed to explain all

these observed phenomena and as a result,

around 1940, Rather and, meek and. Loeb

independently proposed the Streamer theory.

The term 'Kanal' is taken from German language

which means a canal or a channel.

The growth of charge carriers in an avalanche in

a uniform field is described by ed. This is valid

only as long as the influence of the space charge

due to ions is very small compared to the

applied field. In his studies on the effect of

space charge on avalanche growth, Rather

observed that when charge concentration was

between 106 and 108, the growth of the

avalanche became weak. On the other hand,

when the charge concentration was higher than

108, the avalanche current was followed by a

steep rise in the current between the electrodes

leading to the breakdown of the gap.

For simplicity, the space charge at the head of

the avalanche is assumed to have a spherical

volume (See fig below) containing negative

charge at its top because of the higher electron

mobility. The space charge produced in the

avalanche causes sufficient distortion of the

electric field that those free electrons move

towards the avalanche head, and in so doing

generate further avalanches in a process that

rapidly becomes cumulative. As the electrons

advance rapidly, the positive ions are left

behind in a relatively slow-moving tail. The field

will be enhanced in front of the head. Just

behind the head the field between the

electrons and the positive ions is in the opposite



direction to the applied field and hence the

resultant field strength is less. Again between

the tail and the cathode the field is enhanced.

Due to the enhanced field between the head

and the anode, the space charge increases,

causing a further enhancement of the field

around the anode. The process is very fast and

the positive space charge extends to the

cathode very rapidly resulting in the formation

of a streamer. The breakdown process is shown

below.

Thorough experimental investigation developed

an empirical relation for the streamer spark

criterion of the form

where Er is the radial field due to space charge

and E0 is the externally applied field.

BREAKDOWN IN LIQUID DIELECTRICS

Liquid dielectrics are used for filling

transformers, circuit breakers and as

impregnates in high voltage cables and

capacitors. For circuit breaker, again besides

providing insulation between the live parts and

the grounded parts, the liquid dielectric is used

to quench the arc developed between the

breaker contacts. The most important factors

which affect the dielectric strength of oil are

the, presence of fine water droplets and the

fibrous impurities.

Liquids which are chemically pure, structurally

simple and do not contain any impurity even in

traces of 1 in 109, are known as pure liquids.Eg

Paraffin Hydrocarbons, n-Heptane.. In contrast,

commercial liquids used as insulating liquids are

chemically impure and contain mixtures of

complex organic molecules.

There are two schools of thought regarding the

breakdown in liquids.

The first one tries to explain the

breakdown in liquids on a model which

is an extension of gaseous breakdown,

based on the avalanche ionization of

the atoms caused by electron collision

in the applied field. It has been

observed that conduction in pure

liquids at low electric field (1 kV/cm) is

largely ionic due to dissociation of

impurities and increases linearily with

the field strength. At moderately high

fields the conduction saturates but at



high field (electric), 100 kV/cm the

conduction increases more rapidly and

thus breakdown takes place. Fig. 1.11

(a) shows the variation of current as a

function of electric field for hexane.

This is the condition nearer to

breakdown.

The type of breakdown process in pure

liquids is called the electronic

breakdown, involves emission of

electrons at fields greater than

100kV/cm.

The second school of thought

recognizes that the dielectric strength

of liquid dielectrics is affected by the

presence of foreign particles in liquid

insulations( like gas bubbles, suspended

particles as in commercial liquids).

When breakdown occurs in these

liquids, additional gases and gas

bubbles are evolved and solid

decomposition products are formed.

The breakdown mechanism depends on

the nature and condition of the

electrodes, physical properties of the

liquid and impurities and gases present

in the liquid.

The breakdown mechanism is classified

into 4 (for commercial liquids) :

1. Suspended Particle Mechanism

2. Cavitation and Bubble Mechanism

3. Thermal Mechanism

4. Stressed oil volume theory

ELECTRONIC BREAKDOWN (IN PURE LIQUIDS) :

Once an electron is injected into the liquid, it

gains energy from the electric field applied

between the electrodes. It is presumed that

some electrons will gain more energy due to

field than they would lose during collision.

These electrons are accelerated under the

electric field and would gain sufficient energy to

knock out an electron and thus initiate the

process of avalanche. The threshold condition

for the beginning of avalanche is achieved when

the energy gained by the electron equals the

energy lost during ionization (electron emission)

and is given by

e E = C.hv

where is the mean free path, hv is the energy

of ionization and C is a constant

1.SUSPENDED SOLID PARTICLE MECHANISM

Commercial liquids will always contain solid

impurities either as fibers or as dispersed solid

particles. The permittivity of these solids (1)

will always be different from that of the liquid

(2).

Let us assume these particles to be sphere of

radius r. These particles get polarized (acquire

polarity) in an electric field E and experience a

force which is given as

The force will tend the particle to move towards

the strongest region of the field. If the particles

present are large, they become aligned due to

these forces and form a bridge across the gap.

The field in the liquid between the gap will

increase and if it reaches critical value,

breakdown will take place.



If the number of particles is not sufficient to

bridge the gap, the particles will give rise to

local field enhancement and if the field exceeds

the dielectric strength of liquid, local

breakdown will occur near the particles and

thus will result in the formation of gas bubbles

which have much less dielectric strength and

hence finally lead to the breakdown of the

liquid.

Liquids with solid impurities have lower

dielectric strength as compared to its pure

form. The larger the size of the particle impurity

the lower is the overall dielectric strength of the

liquid.

2. CAVITATION AND BUBBLE MECHANISM:

The dielectric strength of liquid depends upon

the hydrostatic pressure above the gap length.

The higher the hydrostatic pressure, the higher

the electric strength.

The smaller the head of liquid, the more are the

chances of partially ionized gases coming out of

the gap and higher the chances of breakdown.

This means a kind of vapour bubble formed is

responsible for the breakdown. The following

processes lead to formation of bubbles in the

liquids:

a) Gas pockets on the surface of

electrodes.

b) Changes in temperature and pressure.

c) Gaseous products due to the

dissociation of liquid molecules by

electron collisions.

d) Vapourization of the liquid by corona

type discharge due to irregular surface

of electrodes.

The bubble will elongate in the direction of the

electric field under the influence of electrostatic

forces. When the field Eb equals the gaseous

ionization field, discharge takes place which will

lead to decomposition of liquid and breakdown

occurs. An expression for the breakdown

field/bubble breakdown strength is given as

o is the surface tension of the liquid,

o E0 is the field in the liquid in absence of

the bubble.

o 2 and 1 are the permittivities of the

liquid and bubble respectively,

o r - the initial radius of the bubble

o Vb- the voltage drop in the bubble.

This theory does not take into account the

production of the initial bubble and hence the

results given by this theory do not agree well

with the experimental result.

3. THERMAL BREAKDOWN:

Extremely large currents are produced just

before breakdown. The high current pulses

originate from the tips of the microscopic

projections on the cathode surface with

densities of the order of 1 A/cm3. These high

density current pulses give rise to localized

heating of the oil which may lead to the

formation of vapour bubbles.

When a bubble is formed, breakdown follows,

either because of its elongation to a critical size

or when it completely bridges the gap between

the electrodes. In either case it will result in the

formation of a spark. The breakdown strength

depends on the pressure and the molecular

structure of the liquid. The theory doesnt

explain the reduction in breakdown strength

with increased gap length.

4. STRESSED OIL VOLUME THEORY:

In commercial liquids where minute traces of

impurities are present, the breakdown strength

is determined by the largest possible impurity

or weak link. The electrical breakdown



strength of the oil is defined by the weakest

region in the oil, that is, the region which is

stressed to the maximum and by the volume of

oil included in that region. In non-uniform

fields, the stressed oil volume is taken as the

volume which is contained between the

maximum stress contour (Emax) and (0.9 Emax)

contour. According to this theory the

breakdown strength is inversely proportional to

the stressed oil volume.

The breakdown voltage is highly influenced by

the gas content in the oil, the viscosity of the

oil, and the presence of other impurities. These

being uniformly distributed, increase in the

stressed oil volume consequently results in a

reduction in the breakdown voltage. The

variation of the breakdown voltage stress with

the stressed oil volume is shown in fig below

BREAKDOWN IN SOLID DIELECTRICS

Solid insulating materials are used almost in all

electrical equipments, be it an electric heater or

a 500 MW generator or a circuit breaker, solid

insulation forms an integral part of all electrical

equipments especially when the operating

voltages are high. When breakdown occurs the

gases regain their dielectric strength very fast,

the liquids regain partially and solid dielectrics

lose their strength completely. The breakdown

of solid dielectrics not only depends upon the

magnitude of voltage applied but also it is a

function of time for which the voltage is

applied. the product of the breakdown voltage

and the log of the time required for breakdown

is almost a constant, i.e.

Vb.ln tb = constant

The various mechanisms of break down are

classified based on the time scale of voltage

application.

The various mechanisms are:

1. Intrinsic Breakdown

2. Electromechanical Breakdown

3. Breakdown due to Treeing and Tracking

4. Thermal Breakdown

5. Electrochemical Breakdown

6. Chemical deterioration

1. INTRINSIC BREAKDOWN:

If the voltage is applied for a very short time of

the order of 108 second, the dielectric strength

of the specimen increases rapidly to an upper

limit known as intrinsic dielectric strength. The

intrinsic strength, therefore, depends mainly

upon the structure of the material and

temperature. The stresses required are of the

order of 5-10 million volt/cm.

Intrinsic breakdown depends upon the

presence of free electron which is capable of

migration through the lattice of the dielectric.



Usually small numbers of conduction electrons

are present, with some structural imperfections

and small amounts of impurities. The impurity

atoms or molecules act as traps for the

conduction electrons up to certain ranges of

electric fields and temperatures. When these

ranges are exceeded, additional electrons are

trapped and released which participate in the

conduction process.

There are two types of intrinsic breakdown

mechanisms:

i. Electronic breakdown:

The intrinsic breakdown is obtained in times

of the order of 108 sec and therefore is

considered to be electronic in nature. The

intrinsic strength is generally assumed to have

been reached when electrons in the valance

band gain sufficient energy from the electric

field to cross the forbidden energy band to the

conduction band. As an electric field is applied,

the electrons gain energy and due to collisions

between them the energy is shared by all

electrons. Finally the temperature of electrons

will exceed the lattice temperature and this will

result into increase in the number of trapped

electrons reaching the conduction band and

finally leading to complete breakdown.

ii. Avalanche or streamer breakdown:

This is similar to breakdown in gases due to cumulative ionization. Conduction electrons gain sufficient energy above a certain critical electric field and cause liberation of electrons from the lattice atom by collisions.

The electrons moving from cathode to anode will gain energy from the field and losses it during collisions. When the energy gained by an electron exceeds the ionization potential, an additional electron will be liberated due to collision of the first electron. This process repeats itself resulting in the formation of an electron avalanche, and breakdown will occur

when the avalanche exceeds a certain critical value. In practice, breakdown does not occur by the formation of a single avalanche, but occurs as a result of many avalanches formed and extending step by step through the entire thickness of the material.

2. ELECTROMECHANICAL BREAKDOWN: When a dielectric material is subjected to an electric field, charges of opposite nature are induced on the two opposite surfaces of the material and hence a force of attraction is developed and the specimens is subjected to electrostatic compressive forces and when these forces exceed the mechanical withstand strength of the material, the material collapses. If the initial thickness of the material is d0 and is compressed to a thickness d under the applied voltage V then the compressive stress developed due to electric field is

where r is the relative permittivity of the specimen. If is the Youngs modulus, the mechanical compressive strength is

Equating the two under equilibrium condition, we have

On differentiating the above equation w.r.t d and then solving we get

For any real value of voltage V, the reduction in thickness of the specimen cannot be more than 40%. If the ratio V/d is less than the intrinsic strength of the specimen, a further increase in V shall make the thickness unstable and the specimen collapses. The highest apparent stress is obtained as



3.BREAKDOWN DUE TO TREEING AND

TRACKING (SURFACE BREAKDOWN)

Treeing : The strength of a chain is given by the

strength of the weakest link in the chain.

Similarly whenever a solid material has some

impurities in terms of some gas pockets or

liquid pockets in it the dielectric strength of the

solid will be more or less equal to the strength

of the weakest impurities.

Suppose some gas pockets are trapped in a

solid material during manufacture, the gas has a

relative permittivity of unity and the solid

material r, the electric field in the gas will be r

times the field in the solid material. As a result,

the gas breaks down at a relatively lower

voltage. The charge concentration here in the

void will make the field more non-uniform. The

charge concentration in such voids is found to

be quite large to give fields of the order of 10

MV/cm which is higher than even the intrinsic

breakdown.

These charge concentrations at the voids within

the dielectric lead to breakdown step by step

and finally lead to complete rupture of the

dielectric. Since the breakdown is not caused by

a single discharge channel and assumes a tree

like structure as shown in fig below, it is known

as breakdown due to treeing.

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

through the insulation in an irregular tree and

leading to the formation of conducting channel.

Tracking: Suppose we have two electrodes

separated by an insulating material and the

assembly is placed in an outdoor environment.

Some contaminants in the form of moisture or

dust particles will get deposited on the surface

of the insulation and leakage current starts

between the electrodes through the

contaminants. The current heats the moisture

and causes breaks in the moisture films. These

small films then act as electrodes and sparks are

drawn between the films. The sparks cause

carbonization and volatilization of the insulation

and lead to formation of permanent carbon

tracks on the surface of insulations.

Thus, tracking is the formation of a permanent

conducting path usually carbon across the

surface of insulation. The conduction (carbon

path) results from degradation of the insulation

itself leading to a bridge between the

electrodes.

For tracking to occur, the insulating material must contain organic substances. The rate of tracking can be slowed down by adding filters to the polymers which inhibit carbonization. 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. 4. THERMAL BREAKDOWN:

When an insulating material is subjected to an electric field, the material gets heated up due to conduction current and dielectric losses due to polarization. The conductivity of the material increases with increase in temperature and a



condition of instability is reached when the heat generated exceeds the heat dissipated by the material and the material breaks down.

In practice, although the heat lost may be considered somewhat linear, the heat generated increases rapidly with temperature, and at certain values of electric field no stable state exists where the heat lost is equal to the heat generated so that the material breaks down thermally.

The variation of heat generated by a device for 2 different applied fields and the heat lost from the device with temperature is shown below.

For the field E2, a stable temperature A exists. For the field E1, the heat generated is always greater than the heat lost so that the temperature would keep increasing until breakdown occurs.

Heat generated is proportional to the frequency and hence thermal breakdown is more serious at high frequency. Thermal breakdown stresses (MV/cm) are lower under a.c. condition then under d.c.

5.Electrochemical breakdown:

Whenever cavities are formed in solid

dielectrics, the dielectric strength in the

specimen decreases. Some of the electrons

dashing against the anode with sufficient

energy shall break the chemical bonds of the

insulation surface. Similarly, positive ions

bombarding against the cathode may increase

the surface temperature and produce local

thermal instability. Similarly, chemical

degradation may also occur from the active

discharge products e.g., O3, NO2 etc. formed in

air. The net effect of all these processes is a

slow erosion of the material and a consequent

reduction in the thickness of the specimen.

6. Chemical deterioration :

Progressive chemical degradation of insulating

materials can occur in the absence of electric

stress from a number of causes.

a) Oxidation: In the presence of air or oxygen,

especially ozone, materials such as rubber

and polyethylene undergo oxidation giving

rise to surface cracks, particularly if

stretched and exposed to light. Polythene

also oxidizes in strong day light unless

protected by opaque filler.

b) Hydrolysis : 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. Polyethylene

film may lose its mechanical strength in a

few days if kept at 100 % relative humidity.

c) Chemical Instability: Many insulating

materials, especially organic materials,

show chemical instability. Under normal

operating conditions, this process is very

slow, but the process is strongly

temperature dependant. The logarithm of

the life t of paper insulation can be

expressed as an inverse function of the

absolute temperature .

log10 t = A/ + B

where A & B are constants.

Engineering

Breakdown in solids, liquids and gases