1. HIGH VOLTAGE ENGINEERING
EET 417

2. CONDUCTION & BREAKDOWN IN GASES
CHAPTER 2
CONDUCTION & BREAKDOWN IN GASES

3. On completion of this lesson, a student should be able to:
Ability to analyze the various breakdown mechanism and applications of vacuum, liquid, solid and composite dielectrics

4. TOPIC OUTLINE 2.1 Ionization Process
2.2 Breakdown Mechanism of Townsend
2.3 Breakdown in Electronegative Gases
2.4 Streamer Theory of Breakdown in Gases
2.5 Paschen’s Law
2.6 Breakdown in Non-uniform Fields and Corona Discharges
2.7 Post Breakdown Phenomena and Applications
2.8 Practical Consideration in Using Gases and Gas Mixture for Insulation Purposes
2.9 Vacuum Insulation

5. 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.

6. INTRODUCTION 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.

7. INTRODUCTION
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.

8. 2.1 IONIZATION PROCESS
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.

9. 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 2.1) 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.

10. 2.1.1 Ionization by Collision
where A is the atom, A+ is the positive ion and e- is the electron.
Fig. 2.1: Two plane parallel electrodes of a Townsend discharge

11. 2.1.1 Ionization by Collision
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

12. 2.1.1 Ionization by Collision
Electric field strength, E

13. 2.1.2 Photo-ionization
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 of energy called the work function.
Bombardment of surface of metal by particles (like positive ions) with sufficient energy
Irradiation of surface of metal by short wave-radiation, hf > work function
Superposition of strong external electric field (field emission)
Heating the cathode can increase the kinetic energy and velocity of electrons ( thermo-ionic emission)

14. 2.1.2 Photo-ionization
The phenomena associated with ionization by radiation, or photo-ionization, 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 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.

15. 2.1.2 Photo-ionization Ionization occurs when
Radiation having a wavelength of 1250 Å is capable of causing photoionization of almost all gases.

16. Thermal Ionization
Thermal ionization means all the process of ionization caused by thermal condition
of a gas.
The term thermal ionization, in general applies to ionizing action of molecular
collisions, radiation and electron collision occurring in gasses at high tempera-
ture. If a gas is heated to sufficiently high temperature many of the gas atoms or
molecules acquire sufficiently high velocity to cause ionization on collision with the
atoms or molecules.
At high temperature the following possibilities of ionization exist:
Ionization due to collision between molecules of a gas which move with high velocities at high temperatures.
Photo ionization on account of thermal radiation by a heated gas,
Ionization under collision of molecules with electrons formed as result of the first
two processes.

17. Ionization on the Surface of Electrode
Liberation of electron from the thickness of metal also requires a definite a mount
of work to be done called the energy of liberation, which is different for different
metals and depends upon the condition of their surface.
Energy for liberation ( ev ) of electrons
Kinds of Metal
Energy of liberation
Aluminum
1.8
Copper
3.9
Copper oxide
5.34
Iron
Silver
3.1.
Platinum
3.6
Barium oxide
1.0

18. 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 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.

19. 2.1.3 Secondary Ionization Processes
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.

20. 2.1.3 Secondary Ionization Processes
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).

21. Breakdown Characteristic in Gases Dielectric
Mechanisms of Breakdown
Townsend’s Mechanism
Streamer Mechanism

22. 2.2 BREAKDOWN MECHANISM OF TOWNSEND’S
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

23. 2.2 BREAKDOWN MECHANISM OF TOWNSEND’S
Then, number of electrons reaching the anode (x = d) is
The number of new electrons created on the average by each electron is,

24. 2.2 BREAKDOWN MECHANISM OF TOWNSEND’S
Average current in the gap = the number of electrons travelling per second
I0 = initial current at the cathode.

25. CURRENT GROWTH IN THE PRESENCE OF SECONDARY PROCESS
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.

26. CURRENT GROWTH IN THE PRESENCE OF SECONDARY PROCES
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’

27. CURRENT GROWTH IN THE PRESENCE OF SECONDARY PROCES
Total number of electrons n reaching the anode becomes, or

28. TOWNSEND’S CRITERION FOR BREAKDOWN
Normally , the above equation reduces to
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.
Townsend breakdown criterion

29. EXPERIMENTAL DETERMINATION Of COEFFICIENTS α AND γ
Experimental arrangement is shown in Figure 2.2

30. EXPERIMENTAL DETERMINATION Of COEFFICIENTS α AND γ
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.

31. EXPERIMENTAL DETERMINATION Of COEFFICIENTS α AND γ

32. EXPERIMENTAL DETERMINATION Of COEFFICIENTS α AND γ
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.

33. Example 1
In an experiment in a certain gas it was found that the steady state current is 5.5 x 10-8 A at 8 kV at a distance of 0.4 cm between the plane electrodes. Keeping the field constant and reducing the distance to 0.1 cm results in a current of 5.5 x 10-9 A. Calculate Townsend’s primary ionization coefficient, α.

34. Solution The current at the anode I is given by d1 = 0.4 cm d2= 0.1 cm
I1 = 5.5 x 10-8 A I2 = 5.5 x 10-9 A

35. Example 2
In Example 1, if the breakdown occurred when the gap distance was increased to 0.9 cm, what is the value of γ?
Solution breakdown occurs when

36. 2.3 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.

37. 2.3 BREAKDOWN IN ELECTRONEGATIVE GASES
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.

38. 2.3 BREAKDOWN IN ELECTRONEGATIVE GASES
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)

39. 2.3 BREAKDOWN IN ELECTRONEGATIVE GASES
The Townsend breakdown criterion for attaching gases;
(2.9)

40. 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.

41. TIME LAGS FOR BREAKDOWN
Total time lag, t = ts + tf , as shown in Figure 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.

42. TIME LAGS FOR BREAKDOWN

43. 2.4 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 s, while in actual practice breakdown was observed to occur at very short times of the order of s.
iii) Townsend mechanism predicts a very diffused form of discharge, but in actual practice, discharges were found to be filamentary and irregular.

44. 2.4 STREAMER THEORY OF BREAKDOWN IN GASES
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 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.

45. 2.4 STREAMER THEORY OF BREAKDOWN IN GASES
Consider Figure 2.11

46. 2.4 STREAMER THEORY OF BREAKDOWN IN GASES
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.

47. 2.4 STREAMER THEORY OF BREAKDOWN IN GASES
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.

48. 2.4 STREAMER THEORY OF BREAKDOWN IN GASES
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.

49. 2.5 PASCHEN’S LAW The breakdown criterion in gases is given as; (2.10)
α and γ are functions of E/p.
Also

50. 2.5 PASCHEN’S LAW
From equation 2.9, by letting η = 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.

51. 2.5 PASCHEN’S LAW
Fig 2.13 shows the relationship between breakdown voltage and pd.

52. 2.5 PASCHEN’S LAW
The Paschen’s curve is shown in Figure 2.14 for three gases CO2, air and H2.

53. 2.5 PASCHEN’S LAW
Substituting α and γ in terms of pd gives rise to gap distance, d as

54. 2.5 PASCHEN’S LAW
f1 and f2 being some functions and may be assumed to follows and exponential function and may be written as
Substituting for α and retaining γ
Breakdown voltage

55. 2.5 PASCHEN’S LAW
The minimum value for V
e = 2.718

56. Example 3
What will be the breakdown voltage of a spark gap in a gas at pr = 760 torr at 25 °C if A = 15/cm, B = 360/cm, d = 1 mm and γ = 1.5 x 10-4?
Solution
pd = 760 torr x 0.1 cm = 76 torr cm
The breakdown voltage

57. Example 4
What is the minimum spark over voltage of the gap in Example 3 if γ = 1x10-4 with all other parameters remaining the same?
Solution

58. 2.5 PASCHEN’S LAW
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;

59. 2.5 PASCHEN’S LAW
Equation for coaxial cylinders, whose inner cylinder has a radius r
m = the surface irregularity factor which become unity for highly polished smooth wires
d = the relative air density correction factor
b = the atmospheric pressure in torr
T = temperature in °C

60. 2.6 BREAKDOWN IN NON-UNIFORM FIELDS AND 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.

61. 2.6.1 Corona Discharges
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.

62. 2.6.1 Corona Discharges
Corona inception and breakdown voltages of the sphere-plane arrangement are shown in Figure
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.

63. 2.6.1 Corona Discharges
The study of corona and non-uniform field breakdown is very complicated and investigations are still under progress.

64. Breakdown and corona inception characteristics for
spheres of different diameters in sphere - plane gap
From figure above can be seen that
At small spacing (region I), the field is uniform, and the breakdown voltage mainly
depends on the spacing.
b. At rather large spacing (region II), the field is non uniform, and the breakdown voltage
depends both on the sphere diameter and the spacing and
c. At large spacing (region III), the field is non uniform, and the breakdown is preceded
by corona and is controlled only by spacing.
The corona inception voltage mainly depends on the sphere diameter.

65. 2.6.2 Breakdown in Non-uniform Field
αd in Townsend’s criterion is rewritten by replacing αd by
And becomes;
(2.14)

66. 2.6.2 Breakdown in Non-uniform Field
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.

67. 2.6.2 Breakdown in Non-uniform 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

68. 2.7 POST-BREAKDOWN PHENOMENA
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)

69. 2.8 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.
d) low temperature of condensation.
e) good heat transfer.
f) ready availability at moderate cost.

70. 2.8 PRACTICAL CONSIDERATION IN USING GASES FOR INSULATION PURPOSES
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.

71. 2.8 PRACTICAL CONSIDERATION IN USING GASES FOR INSULATION PURPOSES
Figure 2.21 shows the dielectric strength of gases, comparable with solid and liquid dielectrics.

72. Lightning Impulse Breakdown Strength of SF6 /Other Gas mixture
SF6/N2 mixtures is the one that has been found to be a good replacement for SF6.

73. BD Voltage as a function of pressure

74. 2.9 VACUUM INSULATION
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 20 °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.

75. 2.9 VACUUM INSULATION 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

76. 2.9 VACUUM INSULATION (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.

77. 2.9 VACUUM INSULATION
Figure 2.22 shows the particle-exchange mechanism. The breakdown will occur if the coefficients of production of secondary electrons exceeds unity;
(AB + CD) > (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).

78. 2.9 VACUUM INSULATION

79. 2.9 VACUUM INSULATION
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) > (2.17)
E and F represent the coefficients for the negative and positive ion liberation by positive and negative ions.

80. 2.9 VACUUM INSULATION
b) Field emission theory
Anode heating mechanism

81. 2.9 VACUUM INSULATION
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.

82. 2.9 VACUUM INSULATION
ii) Cathode heating mechanism

83. 2.9 VACUUM INSULATION
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.

84. 2.9 VACUUM INSULATION
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.

85. 2.9 VACUUM INSULATION

86. 2.9 VACUUM INSULATION
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.