Download HIGH VOLTAGE INSULATION AND ELECTRICAL DISCHARGE

HIGH VOLTAGE INSULATION AND ELECTRICAL
DISCHARGE
1.0
INTRODUCTION
•
High voltages used for a wide variety of applications covering the power
systems, industry and research laboratories.
•
High voltage apparatus necessitate careful design of its insulation and the
electrostatic field profiles.
•
The principle media of insulation used are gases, vacuum, solid and
liquid, or a combination of these.
1.1
ELECTRIC FIELD SRESSES
• In high voltage applications, the dielectric strength of insulating materials
and the electric field stresses developed in them when subjected to high
voltages are the important factors in high voltage systems.
• The electric stress to which an insulating material is subjected to is
numerically equal to the voltage gradient, and is equal to the electric field
intensity.
E = - ∇ϕ
E : electric field intensity
ϕ : applied voltage
∂
∂
∂
∇ ≡ ax
+ ay
+ az
∂z
∂x
∂y
• Most important material used in a high voltage apparatus is the
insulation.
• The dielectric strength of an insulating material can be defined as the
maximum dielectric stress which the material can withstand.
1
• The electric breakdown strength of insulating materials depends on
pressure, temperature, humidity, field configurations, nature of applied
voltage, imperfections in dielectric materials, material of electrodes,
surface condition of electrodes, etc.
• Most common cause of insulation failure is the presence of discharges.
1.2
GAS/VACUUM INSULATION
• Air at atmospheric pressure is the most common gaseous insulation.
• Breakdown occurs in gases due to the process of collisional ionization.
• Practical insulating gases used are carbon dioxide (CO2),
dichlorodifluoromethane (CCl2F2) (freon), sulphur hexafluoride (SF6) and
nitrogen (N2).
• SF6 has been found to maintain its insulation superiority compared to any
other gases.
• Ideally, vacuum is the best insulator with field strengths up to 107 V/cm.
•
1.3
Under high vacuum conditions, where the pressures are below 10-4 torr,
the breakdown cannot occur due to collisional processes like in gases,
and hence the breakdown strength is quite high.
LIQUID INSULATION
• Liquids are used in high voltage equipment to serve the dual purpose of
insulation and heat conduction.
• Highly purified liquids have dielectric strength as high as 1 MV/cm. The
breakdown strength can reduce due to the presence of impurities.
• Petroleum oils are the commonest insulating liquids. However, askarels,
fluorocarbons, silicones and organic esters including castor oil are used
in significant quantities.
2
• The important electrical properties of the liquid are dielectric strength,
conductivity, flash point, gas content, viscosity, dielectric constant,
dissipation factor, stability, etc.
• Askarels and silicones are particularly useful in transformers and
capacitors and can be used at temperatures of 200°C and higher. Castor
oil is a good dielectric for high voltage energy storage capacitors because
of its high corona resistance, high dielectric constant, non-toxicity and
high flash point
•
1.4
In practical applications, liquids are normally used at voltage stresses of
about 50 - 60 kV/cm when the equipment is continuously operated.
SOLID BREAKDOWN
• The breakdown stress in solid will be as high as 10 MV/cm if the solid
insulating material is truly homogeneous and free from imperfections.
• The breakdown occurs due to many mechanisms such as intrinsic or
ionic breakdown, electromechanical breakdown, treeing and tracking,
thermal breakdown, electrochemical breakdown and breakdown due to
internal discharges. These mechanisms of breakdown vary depending on
the time of application of voltages.
• In general, the breakdown occurs over the surface than in the solid itself,
and the surface insulation failure is the most frequent cause of trouble in
practice.
• The failure of solid insulation by discharges, which may occur in the
internal voids and cavities of the dielectric, is called partial discharges.
• The effect of the partial discharges can be minimized by vacuum
impregnation of the insulation.
•
High voltage switchgear, bushings, cables and transformers are typically
devices for which partial discharge effects should be considered in
design.
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1.5
ESTIMATION AND CONTROL OF ELECTRIC STRESS
• The electric field distribution is usually governed by the Poisson’s eqn,
i.e.;
∇ 2ϕ = −
ρ
ε0
ϕ : potential,
ρ : space charge density,
εo : vacuum permittivity
For space charges are not present, potential distribution is governed by
the Laplace’s eqn, i.e.;
∇ 2ϕ = 0
• Most commonly methods for determining the potential distribution are,
a) the electrolytic tank method
b) the numerical method
• Figure 1.0 shows the methods of stress control at an electrode edge. In
the design of hv apparatus, the electric field intensities have to be
controlled, otherwise higher stresses will trigger or accelerate the aging
of the insulation.
• Numerical methods are employed for the calculation of electric fields,i.e.,
a)
b)
c)
d)
Finite Difference Method (FDM)
Finite Element Method (FEM)
Charge Simulation Method (CSM)
Surface Charge Simulation Method (SSM) or Boundary Element
Method (BEM)
4
Fig 1.0 Control of stress at an electrode edge.
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2.
CONDUCTION AND BREAKDOWN IN GASES
2.0
INTRODUCTION
• 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.
• 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.
• There are two types of breakdown mechanism under different conditions,
viz.
i) Townsend theory
ii) Streamer theory
•
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.
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2.1
IONIZATION PROCESSES
• 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.
2.1.1 Ionization by Collision
• 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 2.0) then, any electron starting at the cathode will be
accelerated more and more between collisions with other gas molecules
during its travel towards the anode.
• The process can be represented as;
-
e +A
ε > Vi
e- + A+ + e-
where A is the atom, A+ is the positive ion and e- is the electron.
ε : energy gained
Vi : ionization potential
7
Fig 2.0 Arrangement for study of a Townsend discharge
2.1.2 Photo-ionization
• The phenomena associated with ionization by radiation, or photoionization, involves the interaction of radiation with matter. Photoionization occurs 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.
•
h
c
λ
Vi
•
h
Vi
: Planck’s constant = 6.62517 x 10-34 Js
: velocity of light
: wavelength of the incident radiation
: ionization energy of the atom (eV), 1 eV = 1.6021 x 10-19 J.
Ionization occurs when
λ ≤ c.
Radiation having a wavelength of 1250 Å is capable of causing photoionization of almost all gases.
8
2.1.3 Secondary Ionization Processes
• Secondary ionization processes by which secondary electrons are
produced are the one which sustain a discharge after it is established due
to ionization by collision and photo-ionization.
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 kinectic 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.
b) Electron Emission due to Photons
• To cause an electron to escape from a metal, the enough energy is given
to overcome the surface potential barrier. The energy is in the form of a
photon of ultraviolet light of suitable frequency.
ϕ
,
h
is known as the threshold frequency. ϕ is the work function (eV) of the
metallic electrode.
• The frequency (ν) is given by the relationship; ν =
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 (10-8
s).
9
• 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.
•
2.2
Neutral atoms in the ground state also give rise to secondary electron
emission if their kinetic energy is high (≈ 1000 eV).
TOWNSEND’S CURRENT GROWTH EQUATION
no : 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,
n x = no
also
dn x
= αn x
dx
∴
nx = no exp(αx)
(2.1)
Then, number of electrons reaching the anode (x = d) is
nd = no exp(αd)
The number of new electrons created on the average by each electron is,
exp ( αd ) - 1 =
nd - n o
no
(2.2)
Average current in the gap = the number of electrons travelling per second
∴
I = Io exp(αd)
(2.3)
10
Io = initial current at the cathode.
2.3 CURRENT GROWTH IN THE PRESENCE OF SECONDARY
PROCESS
• Since the amplification of electrons [exp(αd)] 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.
• 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 E/p.
•
Assume no’ = number of secondary electrons produced due to
secondary processes.
no” = total number of electrons leaving the cathode.
Then
no” = no + no’
Total number of electrons n reaching the anode becomes,
n = no” exp(αd) = (no + no’) exp(αd),
and
no’ = γ [n - (no + no’)]
11
∴
or
2.4
n =
n o exp ( αd )
1 - γ [exp ( αd ) - 1]
(2.4)
I =
I o exp ( αd )
1 - γ [exp ( αd ) - 1]
(2.5)
TOWNSEND’S CRITERION FOR BREAKDOWN
From equation (2.5);
γ [ exp(αd) - 1 ] = 1 ⇒ Townsend’s breakdown criterion
(2.6)
Normally exp(αd) >> 1, then equation (2.6) reduces to;
γ exp(αd) = 1
•
(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.
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2.5 EXPERIMENTAL DETERMINATION OF COEFFICIENTS α
AND γ
• Experimental arrangement is shown in Figure 2.1.
Fig 2.1 Experimental arrangement to measure ionization coefficients
• 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 (no).
• Typical current growth curve in a Townsend discharge is shown in Figure
2.2. In the regions T1 and T2 the current increases steadily due to the
Townsend mechanism.
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Fig 2.2 Typical current growth curve in a Townsend discharge
•
For determining the α and γ, the V-I characteristics for different gap
settings are obtained. A log I/Io versus gap distance plot is obtained under
constant field (E) conditions as shown in Figure 2.3. 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.
Fig 2.3 Townsend type log (I/Io) vs d plot
14
2.6
BREAKDOWN IN ELECTRONEGATIVE GASES
• 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.
• The most common attachment processes are;
a) Direct attachment;
b) Dissociative attachment;
AB + e
AB + e
AB- + hν
A + B- + e
• 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.
• 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;
⎤ ⎡ η ⎤
⎡ α
⎢ (α - η ) exp(α - η )d ⎥ - ⎢ (α - η ) ⎥
⎦
⎦ ⎣
I = Io ⎣
⎧
α
[ exp(α - η )d - 1]⎫⎬
1 - ⎨γ
⎩ (α - η )
⎭
(2.8)
15
• The Townsend breakdown criterion for attaching gases;
γ
≈
•
2.7
α
( α - η)
γ
exp ( α - η)d - 1 = 1
α
exp ( α - η)d = 1
( α - η)
(2.9)
Normally γ is very small (≤ 10-4). When α = η, this condition puts a limit
for E/p below which no breakdown is possible irrespective of the value of
d, and the limit value is called the critical E/p. Critical E/p for SF6 is 117
V cm-1 torr-1, and for CCl2F2 it is 121 V cm-1 torr-1 (both at 20 °C).
TIME LAGS FOR BREAKDOWN
• 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 of 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.
• Total time lag, t = ts + tf , as shown in Figure 2.4. 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.
16
Fig 2.4
2.8
STREAMER THEORY OF BREAKDOWN IN GASES
• 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 observed 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.
• 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.
17
• The streamer theories predict the development of 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.
• Consider Figure 2.5;
Fig 2.5 Field distortion in a gap due to space charge
*
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.
18
*
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 occured.
• The field Er produced by the space charge at the radius r is given by;
E r = 5.27 x 10 -7
α exp(αx)
1/ 2
⎛⎜ x ⎞⎟
⎝ p⎠
V/cm
α : Townsend’s first ionization coef.
p : gas pressure in torr.
x : distance to which the streamer has extended in the gap.
•
2.9
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.
PASCHEN’S LAW
• The breakdown criterion in gases is given as;
γ [ exp(αd) - 1 ] = 1
(2.10)
α and γ are functions of E/p.
α
⎛E⎞
= f1 ⎜⎜ ⎟⎟ ;
p
⎝p⎠
⎛E⎞
⎝p⎠
γ = f 2 ⎜⎜ ⎟⎟
Also, E = V/d
• From equation (2.9), by letting η = 0, the equation can be rewrite as;
⎧
⎛ V ⎞⎡
⎛ V ⎞⎫ ⎤
f 2 ⎜⎜ ⎟⎟ ⎢ exp⎨pd f1 ⎜⎜ ⎟⎟⎬ - 1⎥ = 1
⎝ pd ⎠ ⎣
⎝ pd ⎠⎭ ⎦
⎩
(2.11)
19
Equation (2.11) shows relationship between V and pd.
∴
V = f (pd)
(2.12)
Equation (2.12) is known as Paschen’s law.
Fig 2.6 shows the relationship between breakdown voltage and pd.
Fig 2.6 Breakdown voltage-pd curve (Paschen’s law)
• The Paschen’s curve is shown in Figure 2.7 for three gases CO2, air and
H2.
Fig 2.7
20
• The cathode materials also affect the breakdown values. This is shown in
Figure 2.8.
Fig 2.8 Dependence of breakdown voltage on the cathode materials
• 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;
⎡ 293 pd ⎤
⎡ 293 pd ⎤
V = 24.22 ⎢
+
6.08
⎢⎣ 760 T ⎥⎦
⎣ 760 T ⎥⎦
1/ 2
(2.13)
21
2.10 BREAKDOWN IN NON-UNIFORM FIELDS AND CORONA
DISCHARGES
2.10.1
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
inteference.
• Voltage gradient required to produce visual a.c. corona in air at a
conductor surface is called the corona inception field.
• 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.
• Corona inception and breakdown voltages of the sphere-plane
arrangement are shown in Figure 2.9.
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.
22
Fig 2.9 Breakdown and corona inception characteristics
•
The study of corona and non-uniform field breakdown is very
complicated and investigations are still under progress.
2.10.2
Breakdown in non-uniform fields
αd in Townsend’s criterion is rewritten by replacing αd by
d
∫ α dx
and
0
becomes;
⎧
⎡d
⎣0
⎤
⎦
⎫
γ ⎨ exp ⎢ ∫ α dx ⎥ - 1 ⎬ = 1
⎩
(2.14)
⎭
When applied the non-uniform field breakdown process to streamer theory,
the field produced by space charge is modified as;
x
5.27 x 10 α x exp( ∫ α dx)
-7
Er =
0
⎛⎜ x ⎞⎟
⎝ p⎠
1/2
V/cm
(2.15)
αx : value of α at the head of avalanche.
23
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 spheresphere gap are of great importance, as they are used for the protection of
electrical apparatus and for the measurement of high voltage.
2.11 POST-BREAKDOWN PHENOMENA
• The phenomena that occur in the region CG (as shown in Figure 2.10) are
the post-breakdown phenomena (glow discharge, CE and arc discharge,
EG)
Fig 2.10 d.c. voltage-current characteristics of an electrical discharge with
electrodes having no sharp points or edges
2.12 PRACTICAL CONSIDERATION IN USING GASES FOR
INSULATION PURPOSES
• 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.
24
d) low temperature of condensation.
e) good heat transfer.
f) ready availability at moderate cost.
• 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. Figure 2.11 shows the dielectric
strength of gases, comparable with solid and liquid dielectrics.
Fig 2.11 d.c. breakdown strength of different insulations in uniform fields
25
2.13 VACUUM INSULATION
2.13.1 Introduction
• 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
b) very high vacuum
c) ultra high vacuum
: 1 x 10-3 to 1 x 10-6 Torr
: 1 x 10-6 to 1 x 10-8 Torr
: 1 x 10-9 and below
For electrical insulation purposes, the range of vacuum generally used in the
high vacuum.
2.13.2
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
26
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 of 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.
• Figure 2.12 shows the particle-exchange mechanism. The breakdown will
occur if the coefficients of production of secondary electrons exceeds
unity;
(AB + CD) > 1
where :
(2.16)
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).
Fig 2.12 Particle exchange mechanism of vacuum breakdown
27
• 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.
b)
i)
Field emmision theory
Anode heating mechanism
Fig. 2.13 Electron beam anode heating mechanism of vacuum breakdown
• 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.
28
ii)
Cathode heating mechanism
Fig. 2.14 Breakdown in vacuum caused by the heating of a microprojection on the
cathode
• 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.
29
iii)
Clump mechanism
Fig. 2.15 Clump mechanism of vacuum breakdown
• 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.
30
• 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 observations.
•
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.
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