HIGH VOLTAGE ENGINEERING EET 413 EET413 HIGH VOLTAGE ENGINEERING 1
CHAPTER 2 CONDUCTION & BREAKDOWN IN GASES EET413 HIGH VOLTAGE ENGINEERING 2
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 EET413 HIGH VOLTAGE ENGINEERING 3
TOPIC OUTLINE 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 EET413 HIGH VOLTAGE ENGINEERING 4
INTRODUCTION The most commonly gases are Nitrogen (N 2 ), Carbon dioxide (CO 2 ), Freon (CCl 2 F 2 ) and sulphur hexafluoride (SF 6 ). 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. EET413 HIGH VOLTAGE ENGINEERING 5
CONT.. 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. EET413 HIGH VOLTAGE ENGINEERING 6
CONT.. 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. EET413 HIGH VOLTAGE ENGINEERING 7
5.1 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. EET413 HIGH VOLTAGE ENGINEERING 8
5.1.1 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. EET413 HIGH VOLTAGE ENGINEERING 9
CONT.. EET413 HIGH VOLTAGE ENGINEERING 10
CONT.. The process can be represented as; where A is the atom, A + is the positive ion and e - is the electron. ε : energy gained V i : ionization potential EET413 HIGH VOLTAGE ENGINEERING 11
5.1.2 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 a) positive ions) with sufficient energy Irradiation of surface of metal by short wave-radiation, b) h f > work function Superposition of strong external electric field (field c) emission) Heating the cathode can increase the kinetic energy d) and velocity of electrons ( thermo-ionic emission) EET413 HIGH VOLTAGE ENGINEERING 12
5.1.2 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. EET413 HIGH VOLTAGE ENGINEERING 13
CONT.. Ionization occurs when Radiation having a wavelength of 1250 Å is capable of causing photoionization of almost all gases. EET413 HIGH VOLTAGE ENGINEERING 14
5.2.3 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 γ i which is called the Townsend’s secondary ionization coefficient due to positive ions. γ i increases with ion velocity and depends on the kind of gas and electrode material used. EET413 HIGH VOLTAGE ENGINEERING 15
CONT.. 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; v h is known as the threshold frequency. ϕ is the work function (eV) of the metallic electrode. EET413 HIGH VOLTAGE ENGINEERING 16
CONT.. 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). 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). EET413 HIGH VOLTAGE ENGINEERING 17
5.2 Breakdown Mechanism of Townsend TOWNSEND’S CURRENT GROWTH EQUATION n 0 : 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. n x : number of electrons at any distance x from the cathode. at x = 0, n x = n 0 x n n e dn x 0 x n also (2.1) x dx EET413 HIGH VOLTAGE ENGINEERING 18
CONT.. Then, number of electrons reaching the anode (x = d) is d n n e d 0 The number of new electrons created on the average by each electron is, n n d d 0 (2.2) e 1 n 0 EET413 HIGH VOLTAGE ENGINEERING 19
CONT.. Average current in the gap = the number of electrons travelling per second d (2.3) I I e 0 I 0 = initial current at the cathode. EET413 HIGH VOLTAGE ENGINEERING 20
CURRENT GROWTH IN THE PRESENCE OF SECONDARY PROCESS Since the amplification of electrons e 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. EET413 HIGH VOLTAGE ENGINEERING 21
CONT.. 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 n 0 ’ = number of secondary electrons produced due to secondary processes. n 0 ” = total number of electrons leaving the cathode. Then n 0 ” = n 0 + n 0 ’ EET413 HIGH VOLTAGE ENGINEERING 22
CONT.. Total number of electrons n reaching the anode becomes, d d " ' n n e n n e 0 0 0 Since ' ' n n n n 0 0 0 d n e 0 n (2.4) d 1 e 1 d or I e 0 I (2.5) d 1 e 1 EET413 HIGH VOLTAGE ENGINEERING 23
Recommend
More recommend
