EET 413 EET413 HIGH VOLTAGE ENGINEERING 1 CHAPTER 2 CONDUCTION - - PowerPoint PPT Presentation

EET 413

HIGH VOLTAGE ENGINEERING

1 EET413 HIGH VOLTAGE ENGINEERING

CHAPTER 2

EET413 HIGH VOLTAGE ENGINEERING 2

CONDUCTION & BREAKDOWN IN GASES

On completion of this lesson, a student should be able to:

EET413 HIGH VOLTAGE ENGINEERING 3

Ability to analyze the various breakdown

mechanism and applications of vacuum, liquid, solid and composite dielectrics

TOPIC OUTLINE

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5.1 Ionization Process 5.2 Breakdown Mechanism of Townsend 5.3 Breakdown in Electronegative Gases 5.4 Streamer Theory of Breakdown in Gases 5.5 Paschen’s Law 5.6 Breakdown in Non-uniform Fields and Corona Discharges 5.7 Post Breakdown Phenomena and Applications 5.8 Practical Consideration in Using Gases and Gas Mixture for Insulation Purposes 5.9 Vacuum Insulation

INTRODUCTION

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The most commonly gases are Nitrogen (N2),

Carbon dioxide (CO2), Freon (CCl2F2) and sulphur hexafluoride (SF6).

Various phenomena occur in gaseous dielectric

when a voltage is applied. When the applied voltage is low, small currents flow between the electrodes and the insulation retains its electrical properties. If the applied voltages are large, the current flowing through the insulation increases very sharply, and an electrical breakdown occurs.

CONT..

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The electrical discharges in gases are of two types, i.e. i) non-sustaining discharges ii) self-sustaining discharge

The breakdown in a gas, called spark breakdown is

the transition of a 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 currents.

CONT..

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The various physical conditions of gases, namely,

pressure, temperature, electrode field configuration, nature of electrode surfaces and the availability of initial conducting particles are known to govern the ionization processes.

5.1 IONIZATION PROCESS

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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.

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.

5.1.1 Ionization by Collision

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Ionization - The process of liberating an electron

from a gas molecule with the simultaneous production of a positive ion.

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.

When electric field E is applied across two plane

parallel electrodes (as shown in Figure 2.1) then, any electron starting at the cathode will be accelerated more and more between collisions with

• ther gas molecules during its travel towards the

anode.

CONT..

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CONT..

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The process can be represented as;

where A is the atom, A+ is the positive ion and e- is the electron. ε : energy gained Vi : ionization potential

5.1.2 Photo-ionization

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 Before we go into photo-ionization, it is important to

understand how electron can appear in gas by emission from the cathode. The process require a definite amount

• f energy called the work function.

a)

Bombardment of surface of metal by particles (like positive ions) with sufficient energy

b)

Irradiation of surface of metal by short wave-radiation, hf > work function

c)

Superposition of strong external electric field (field emission)

d)

Heating the cathode can increase the kinetic energy and velocity of electrons ( thermo-ionic emission)

5.1.2 Photo-ionization

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The phenomena associated with ionization by

radiation, or photoionization, involves the interaction of radiation with matter. Photoionization

• ccurs when the amount of radiation energy

absorbed by an atom or molecule exceeds its ionization potential.

The processes by which radiation can be absorbed by

atoms or molecules are; i) excitation of the atom to a higher energy state. ii) continuous absorption by direct excitation of the atom or dissociation of diatomic molecule or direct ionization etc.

CONT..

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Ionization occurs when Radiation having a wavelength of 1250 Å is

capable of causing photoionization of almost all gases.

5.2.3 Secondary Ionization Processes

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 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. a) Electron Emission due to Positive Ion Impact

 Positive ions are formed due to ionization process and

travel towards the cathode. These positive ions can cause emission of electrons from the cathode by giving up its kinetic energy on impact.

 The probability of the process is measured as γi which is

called the Townsend’s secondary ionization coefficient due to positive ions. γi increases with ion velocity and depends on the kind of gas and electrode material used.

CONT..

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b) Electron Emission due to Photons

To cause an electron to escape from a metal,

enough energy should be given to overcome the surface potential barrier. The energy is in the form of a photon of ultraviolet light of suitable frequency.

The frequency (ν) is given by the relationship;

is known as the threshold frequency. ϕ is the work function (eV) of the metallic electrode.

h v  

CONT..

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c) Electron Emission due to Metastable and Neutral Atoms

 A metastable atom or molecule is an excited particle

whose lifetime is very large (10-3 s) compared to the lifetime of an ordinary particle (10-8s).

 Electron can be ejected from the metal surface by the

impact of excited (metastable) atoms, provided that their total energy is sufficient to overcome the work function.

 Neutral atoms in the ground state also give rise to

secondary electron emission if their kinetic energy is high (≈ 1000 eV).

5.2 Breakdown Mechanism of Townsend

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TOWNSEND’S CURRENT GROWTH EQUATION

n0 : electrons emitted from the cathode. α : average number of ionizing collisions made by an electron per cm travel in the direction of the field. α depends on gas pressure p and E/p, and is called the Townsend’s first ionization coefficient. nx : number of electrons at any distance x from the cathode. at x = 0, nx = n0 also (2.1)

x x

n dx dn  

x x

e n n



 

CONT..

19 EET413 HIGH VOLTAGE ENGINEERING

Then, number of electrons reaching the anode (x

= d) is

The number of new electrons created on the

average by each electron is, (2.2)

d d

e n n



 1 n n n e

d d

  



CONT..

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Average current in the gap = the number of electrons

travelling per second (2.3)

I0 = initial current at the cathode.

d

e I I





CURRENT GROWTH IN THE PRESENCE OF SECONDARY PROCESS

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Since the amplification of electrons eαd is occurring

in the field, the probability of additional new electrons being liberated in the gap by other mechanisms increases, ie; i) The positive ions liberated may have sufficient energy to cause liberation of electrons from the cathode when they impinge on it. ii) The excited atoms or molecules in avalanches may emit photons, and this will lead to the emission of electrons due to photo-emission. iii) The metastable particles may diffuse back causing electron emission.

CONT..

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The electrons produced by these processes are

called secondary electrons, and the secondary ionization coefficient γ is defined in the same way as α.

γ is called the Townsend’s secondary ionization

coefficient and is a function of the gas pressure p and

Assume n0’ = number of secondary electrons

produced due to secondary processes. n0”= total number of electrons leaving the cathode.

Then n0” = n0 + n0’

p E

CONT..

EET413 HIGH VOLTAGE ENGINEERING 23

Total number of electrons n reaching the anode

becomes,

Since

(2.4)

• r

(2.5)

 

d d

e n n e n n

 

' "   

 

 

' ' n n n n   

 

1 1    

d d

e e n n

 



 

1 1   

d d

e e I I

 



TOWNSEND’S CRITERION FOR BREAKDOWN

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Normally , the above equation reduces to (2.7)

For a given gap spacing and at a give pressure, the

value of the voltage 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 sparking distance.

 

1 1   

d

e 

Townsend breakdown criterion (2.6)

1 

d

e 1 

d

e





EXPERIMENTAL DETERMINATION Of COEFFICIENTS α AND γ

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Experimental arrangement is shown in Figure 2.2

CONT..

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The electrode system is placed in an ionization

• chamber. The chamber is evacuated to a very

high vacuum of the order of 10-4 to 10-6 torr. Then it is filled with desired gas.

Cathode is irradiated using an ultra-violet lamp

in order to produce initiatory electrons (n0).

Typical current growth curve in a Townsend

discharge is shown in Figure 2.4. In the regions T1 and T2 the current increases steadily due to the Townsend mechanism.

CONT..

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CONT..

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 For determining the α and γ, the V-I characteristics for

different gap settings are obtained. A log I/I0 versus gap distance plot is obtained under constant field (E) conditions as shown in Figure 2.5. The slope of initial portion of the curves gives the value of α. Then by using equation (2.5), γ can be found using points on the upcurving portion of the graph.

5.3 BREAKDOWN IN ELECTRONEGATIVE GASES

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The process that give high breakdown strength

to a gas is the electron attachment. Free electrons get attached to neutral atoms or molecules to form negative ions.

Electron attachment represents an effective

ways of removing electrons which otherwise would have led to current growth and breakdown at low voltage.

The gases in which attachment plays an active

role are called electronegative gases.

CONT..

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The most common attachment processes are; The gases that the attachment process occured

are SF6, O2, freon, CO2 and fluorocarbon.

Townsend current growth equation is modified to

include ionization and attachment.

CONT..

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An attachment coefficient (η) is defined as the

number of attaching collisions made by one electron drifting one cm in the direction of the field.

Under these conditions, the current reaching the

anode can be written as; (2.8)

   

 

            

 

1 1

d n d n

e n e n I I

 

       

CONT..

EET413 HIGH VOLTAGE ENGINEERING 32

The Townsend breakdown criterion for attaching

gases; (2.9)

 

 

 

  1

1 1      

  d n d n

e n e n

 

     

5.4 TIME LAGS FOR BREAKDOWN

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Time lag is a time difference between the

application of a voltage sufficient to cause breakdown and the occurrence of breakdown itself

The time which lapses between the application

• f the voltage sufficient to cause breakdown and

the appearance of the initiating electron is called statistical time lag, ts.

The time required for the ionization process to

develop fully to cause the breakdown is called formative time lag, tf.

CONT..

EET413 HIGH VOLTAGE ENGINEERING 34

Total time lag, t = ts + tf , as shown in Figure 2.8.

Statistical time lag depends upon the amount of pre-ionization present. Formative time lag depend mostly on the mechanism of the avalanche grow.

Formative time lag is usually much shorter than

the statistical time lag.

CONT..

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5.4 STREAMER THEORY OF BREAKDOWN IN GASES

EET413 HIGH VOLTAGE ENGINEERING 36

Townsend mechanism when applied to breakdown at

atmospheric pressure was found to have certain drawbacks, i.e.

i) 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.

ii) The mechanism predicts time lags of the order of

10-5 s, while in actual practice breakdown was

• bserved to occur at very short times of the order of

10-8 s.

iii) Townsend mechanism predicts a very diffused

form of discharge, but in actual practice, discharges were found to be filamentary and irregular.

CONT..

EET413 HIGH VOLTAGE ENGINEERING 37

The Townsend mechanism failed to explain all

these observed phenomena and as a result, around 1940, Raether, Meek and Loeb independently proposed the streamer theory.

The streamer theories predict the development

• f a spark discharge directly from a single

avalanche in which the space charge developed by the avalanche itself is said to transform the avalanche into a plasma streamer.

CONT..

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Consider Figure 2.11

CONT..

EET413 HIGH VOLTAGE ENGINEERING 39

A single electron starting at the cathode by

ionization builds up an avalanche that crosses the gap.

Electrons in the avalanche move very fast compared

with the positive ions. This enhances the field, and the secondary avalanches are formed due to photo- ionization in the space charge region. This occurs first near the anode when the space charge is

• maximum. This results in a further increase in the

space charge.

The process is very fast and the positive space

charge extends to the cathode very rapidly resulting in the formation of streamer.

CONT..

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As soon as the streamer tip approaches the cathode,

a cathode spot is formed and a stream of electrons rush from the cathode to neutralize the positive space charge in the streamer, the result is a spark and breakdown has occurred.

The field Er produced by the space charge at the

radius r is given by; α : Townsend’s first ionization coeficient. p : gas pressure in torr. x : distance to which the streamer has extended in the gap.

2 1 7

10 27 . 5        



p x e E

x r 



CONT..

EET413 HIGH VOLTAGE ENGINEERING 41

Generally, for pd values below 1000 torr-cm and

gas pressure varying from 0.01 to 300 torr, the Townsend mechanism operates, while at higher pressures and pd values, the streamer mechanism plays the dominant role in explaining the breakdown phenomena.

5.5 PASCHEN’S LAW

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The breakdown criterion in gases is given as;

(2.10) α and γ are functions of E/p. Also

  1

1   

d

e                    p E f p E f p

2 1

;  d V E 

CONT..

EET413 HIGH VOLTAGE ENGINEERING 43

From equation 2.9, by letting = 0, the

equation can be rewrite as (2.11) Equation (2.11) shows relationship between V and pd. (2.12) Equation (2.12) is known as Paschen’s law.



1 1

1

2

                 

pd V pdf

e pd V f

 

pd f V  

CONT..

EET413 HIGH VOLTAGE ENGINEERING 44

Fig 2.13 shows the relationship between

breakdown voltage and pd.

CONT..

EET413 HIGH VOLTAGE ENGINEERING 45

The Paschen’s curve is shown in Figure 2.14 for

three gases CO2, air and H2.

CONT..

EET413 HIGH VOLTAGE ENGINEERING 46

For the effect of temperature, the Paschen’s law

is generally stated as V = f(Nd), where N is a density of the gas molecules. The pressure of the gas changes with temperature according to the gas law pν = NRT , where ν is a volume of the gas, T is the temperature and R is a constant.

Based on the experimental results, the

breakdown potential of air is expressed as; (2.13)

2 1

760 293 08 . 6 760 293 42 . 2               T pd T pd V

5.6 BREAKDOWN IN NON-UNIFORM FIELDS AND CORONA DISCHARGES

EET413 HIGH VOLTAGE ENGINEERING 47

5.6.1 Corona Discharges

 If the field is non-uniform, an increase in voltage will first

cause a discharge in the gas to appear at points with highest electric field intensity. This form of discharge is called a corona discharge and can be observed as a bluish luminescence.

 The corona discharge is accompanied by a hissing noise,

and the air surrounding the corona region becomes converted into ozone.

 Corona is responsible for considerable loss of power from

high voltage transmission lines, deterioration of insulation and rise its radio interference.

 Voltage gradient required to produce visual a.c. corona in

air at a conductor surface is called the corona inception field.

CONT..

EET413 HIGH VOLTAGE ENGINEERING 48

 Electric field to produce visual ac corona in air at a conductor

surface are called corona inception field.

 For the case of parallel wires  For the case of coaxial cylinders  Where r is the radius of conductor, m is the surface irregularity

factor which becomes equal to unity and d is the relative air density correction factor given by

 b is the atmospheric pressure (in torr)  T is the temperature in ºC

        dr md Ew 301 . 1 30

        dr md Ew 308 . 1 31

 

T b d   273 392 .

CONT..

EET413 HIGH VOLTAGE ENGINEERING 49

There is a distinct difference in the visual

appearance of the corona under positive and negative polarities of the applied voltage.

• When the voltage is positive - corona appears as

a uniform bluish white sheath over the entire surface of the conductor.

• When voltage is negative - like reddish glowing

spot distributed along the length of wire.

CONT..

EET413 HIGH VOLTAGE ENGINEERING 50

Corona inception and breakdown voltages of the

sphere-plane arrangement are shown in Figure 2.15. a) Region I (small spacing) - the field is uniform. Breakdown voltage depends on the spacing. b) Region II (fairly large spacing) - field is non-

• uniform. Breakdown voltage depends both on

the sphere diameter and the spacing. c) Region III (large spacing) - the field is non-

• uniform. Breakdown is preceded by corona. The

corona inception voltage mainly depends on the sphere diameter.

CONT..

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The study of corona and non-uniform field

breakdown is very complicated and investigations are still under progress.

CONT..

EET413 HIGH VOLTAGE ENGINEERING 52

5.6.2 Breakdown in non-uniform fields

αd in Townsend’s criterion is rewritten by

replacing αd by And becomes; (2.14)



d

dx 

  1

1

dx

 



d

e





CONT..

EET413 HIGH VOLTAGE ENGINEERING 53

 When applied the non-uniform field breakdown process to

streamer theory, the field produced by space charge is modified as; (2.15) αx : value of α at the head of avalanche.

 When space charge field, Er = applied field at the head of

avalanche - formation of streamer is reached.

 From the practical engineering point of view, rod-rod gap

and sphere-sphere gap are of great importance, as they are used for the protection of electrical apparatus and for the measurement of high voltage.

        



p x e E

x

x r

0 dx

7

10 27 . 5





5.7 POST-BREAKDOWN PHENOMENA

EET413 HIGH VOLTAGE ENGINEERING 54

The phenomena that occur in the region CG (as

shown in Figure 2.20) are the post-breakdown phenomena (glow discharge, CE and arc discharge, EG)

5.8 PRACTICAL CONSIDERATION IN USING GASES FOR INSULATION PURPOSES

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Generally, the preferred properties of a gaseous

dielectric for high voltage application are; a) high dielectric strength b) thermal stability and chemical inactivity towards material of construction. c) non-flammability and physiological inertness. d) low temperature of condensation. e) good heat transfer. f) ready availability at moderate cost.

CONT..

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SF6 - possess most of the above requirement has

higher dielectric strength and low liquification temperature - can be used in wide range has excellent arc-quenching properties.

Additional of 30% SF6 to air - increases the

dielectric strength of air by 100%. One of qualitative effect of mixing SF6 to air is to reduce the overall cost of the gas, and attaining relatively high dielectric strength or simply preventing the onset of corona.

CONT..

EET413 HIGH VOLTAGE ENGINEERING 57

 Figure 2.21 shows the dielectric strength of gases, comparable

with solid and liquid dielectrics.

5.9 VACUUM INSULATION

EET413 HIGH VOLTAGE ENGINEERING 58

 In the absence of any particles, as in the case of perfect

vacuum, there should be no conduction. However in practice, the presence of metallic electrodes and insulating surfaces within the vacuum, a sufficiently high voltage will cause a breakdown.

 In vacuum systems, the pressure is always measured in

terms of mm mercury (Hg). 1 mm Hg = 1 Torr Standard atmospheric pressure = 760 mm Hg at 0 °C.

 Vacuum may be classified as;

a) high vacuum : 1 x 10-3 to 1 x 10-6 Torr b) very high vacuum : 1 x 10-6 to 1 x 10-8 Torr c) ultra high vacuum : 1 x 10-9 and below For electrical insulation purposes, the range of vacuum generally used in the high vacuum.

CONT..

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Vacuum Breakdown

In a high vacuum, an electron crosses the gap

without encountering any collisions. Therefore the current growth prior to breakdown cannot be due to the formation of electron avalanches. However if a gas is liberated in the vacuum gap, then the breakdown can occur by the Townsend process.

Three categories of the mechanisms for breakdown

in vacuum. a) Particle exchange mechanism b) Field emission mechanism c) Clump theory

CONT..

EET413 HIGH VOLTAGE ENGINEERING 60

(a) Particle exchange mechanism

A charge particle would be emitted from one

electrode under the action of the high electric field, and when it impinges on the other electrode, it liberates oppositely charged particles.

The particles are accelerated by the applied voltage

back to the first electrode where they release more

• f the original type of particles. When this process

becomes cumulative, a chain reaction occurs which leads to the breakdown of the gap.

The particle-exchange mechanism involves

electrons, positive ions, photons and the absorbed gases at the electrode surfaces.

CONT..

EET413 HIGH VOLTAGE ENGINEERING 61

Figure 2.22 shows the particle-exchange mechanism.

The breakdown will occur if the coefficients of production of secondary electrons exceeds unity; (AB + CD) > 1 (2.16) where : A : released positive ions from the impact of charged particle (electron) at anode. B : liberated electrons from the impact of each positive ion (A). C : photons - from the impact of charged particle (electrons) at anode. D : liberated electrons from the impact of each photon (C).

CONT..

EET413 HIGH VOLTAGE ENGINEERING 62

CONT..

EET413 HIGH VOLTAGE ENGINEERING 63

Trump and Van de Graff showed that the

coefficients in equation (2.16) were too small for the process of breakdown to take place. Then the theory was modified to allow for the presence of negative ions, and the criterion for breakdown becomes; (AB + EF) > 1 (2.17)

E and F represent the coefficients for the

negative and positive ion liberation by positive and negative ions.

CONT..

EET413 HIGH VOLTAGE ENGINEERING 64

b) Field emission theory

i)

Anode heating mechanism

CONT..

EET413 HIGH VOLTAGE ENGINEERING 65

Electrons produced at small micro-projections on the

cathode due to field emission bombard the anode causing a local rise in temperature and release gases and vapours into the vacuum gap. These electrons ionize the atoms of the gas and produce positive ions.

These positive ions arrive at the cathode, increase

the primary electron emission due to space charge formation and produce secondary electrons by bombarding the surface. The process continues until a sufficient number of electrons are produced to give rise to breakdown.

CONT..

EET413 HIGH VOLTAGE ENGINEERING 66

ii) Cathode heating mechanism

CONT..

EET413 HIGH VOLTAGE ENGINEERING 67

Sharp points on the cathode surface are

responsible for the existence of the pre- breakdown current. These current causes resistive heating at the tip of a point and when a critical current density is reached, the tip melts and explodes, thus initiating vacuum discharge.

Experimental evidence shows that breakdown

takes place by this process when the effective cathode electric field is of the order of 106 to 107 V/cm.

CONT..

EET413 HIGH VOLTAGE ENGINEERING 68

iii) Clump mechanism Basically this theory has been developed on the following assumptions; i) A loosely bound particle (clump) exists on one of the electrode surfaces. ii) This particle get charged when high voltage is applied, and get detached from the mother electrode and is accelerated across the gap. iii) The breakdown occurs due to a discharge in the vapour or gas released by the impact at the target electrode.

CONT..

EET413 HIGH VOLTAGE ENGINEERING 69

CONT..

EET413 HIGH VOLTAGE ENGINEERING 70

Although there has been a large amount of work

done on vacuum breakdown phenomena, so far, no single theory has been able to explain all the available experimental measurements and

• bservations.

The most significant experimental factors which

influence the breakdown mechanisms are; gap length, geometry and material of the electrodes, surface uniformity and treatment of the surface, presence of extraneous particles and residual gas pressure in the vacuum gap.