Chapter 02- Breakdown in Gases (part-2)
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This document provides a summary of key concepts regarding electrical breakdown and conduction in gases: - Gases can act as insulating or conducting media depending on the applied voltage. Low voltages allow small currents, while higher voltages cause electrical breakdown through ionization processes. - Breakdown occurs through the formation of a conductive spark between electrodes. It involves transitions from non-sustaining to self-sustaining discharges. - Ionization processes like collisional ionization and photoionization generate free electrons and ions, leading to current growth. Secondary processes like positive ion bombardment and photon emission further sustain the discharge. - The Townsend theory and streamer theory describe the mechanisms of breakdown under different conditions involving
Chapter 02- Breakdown in Gases (part-2)
- 1. CONDUCTION AND BREAKDOWN IN GASES Dated : 07-11-2017
- 2. Gases as insulating media • The simplest and the most commonly found dielectrics are gases. Most of the electrical apparatus use air as the insulating medium, and in a few cases other gases such as nitrogen, carbon dioxide, freon and sulphur hexafluoride are also used. • Various phenomena occur in gaseous dielectrics when a voltage is applied. When the applied voltage is low, small currents flow between the electrodes and the insulation retains its electrical properties.
- 3. Gases as insulating media • On the other hand, if the applied voltages are large, the current flowing through the insulation increases very sharply, and an electrical breakdown occurs. • A strongly conducting spark formed during breakdown practically produces a short circuit between the electrodes.
- 4. Gases as insulating media • The maximum voltage applied to the insulation at the moment of breakdown is called the breakdown voltage. • In order to understand the breakdown phenomenon in gases, a study of the electrical properties of gases and the processes by which high currents are produced in gases is essential.
- 5. Gases as insulating media • The electrical discharges in gases are of two types, i.e. (i) non-sustaining discharges, and (ii) self-sustaining types. • 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.
- 6. Gases as insulating media • At present two types of theories, viz. (i) Townsend theory, and (ii) Streamer theory are known which explain the mechanism for breakdown under different conditions. • 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.
- 7. Ionization Processes • A gas in its normal state is almost a perfect insulator. However, 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.
- 8. Ionization by collision • The process of liberating an electron from a gas molecule with the simultaneous production of a positive ion is called ionisation. • In the process of ionisation by collision, a free electron collides with a neutral gas molecule and gives rise to a new electron and a positive ion. • If we consider a low pressure gas column in which an electric field E is applied across two plane parallel electrodes, as shown in Figure1.
- 9. Ionization by collision Figure 1: Arrangement for study of a Townsend discharge.
- 10. Ionization by collision • Any electron starting at the cathode will be accelerated more and more between collisions with other gas molecules during its travel towards the anode. • If the energy (E) gained during this travel between collisions exceeds the ionisation potential, Vi, which is the energy required to dislodge an electron from its atomic shell, then ionisation takes place.
- 11. Ionization by collision • This process can be represented as • Where, A is the atom, 𝐴+ is the positive ion and 𝑒− is the electron.
- 12. Ionization by collision • A few of the electrons produced at the cathode by some external means, say by ultra-violet light falling on the cathode, ionise neutral gas particles producing positive ions and additional electrons. • The additional electrons, then, themselves make 'ionising collisions' and thus the process repeats itself. This represents an increase in the electron current, since the number of electrons reaching the anode per unit time is greater than those liberated at the cathode. • In addition, the positive ions also reach the cathode and on bombardment on the cathode give rise to secondary electrons.
- 13. Photo Ionization • The phenomena associated with ionisation by radiation, or photo-ionisation, involves the interaction of radiation with matter. • Photo-ionisation occurs when the amount of radiation energy absorbed by an atom or molecule exceeds its ionisation potential. • Just as an excited atom emits radiation when the electron returns to the lower state or to the ground state, the reverse process takes place when an atom absorbs radiation. This reversible process can be expressed as
- 14. Secondary Ionisation Processes Dated : 13-11-2017
- 15. Secondary Ionization Process • Secondary ionisation processes by which secondary electrons are produced are the one which sustain a discharge after it is established due to ionisation by collision and photo-ionization. (a) Electron Emission due to Positive Ion Impact (b) Electron Emission due to Photons (c) Electron Emission due to Metastable and Neutral Atoms
- 16. Electron Emission due to Positive Ion Impact • Positive ions are formed due to ionisation by collision or by photo-ionisation, and being positively charged, they travel towards the cathode. • A positive ion approaching a metallic cathode can cause emission of electrons from the cathode by giving up its kinetic energy on impact.
- 17. Electron Emission due to Positive Ion Impact • If the total energy of the positive ion, namely, the sum of its kinetic energy and the ionisation energy, is greater than twice the work function of the metal, then one electron will be ejected and a second electron will neutralise the ion. • The probability of this process is measured as 𝛾𝑖 which is called the Townsend's secondary ionisation coefficient due to positive ions.
- 18. Electron Emission due to Photons • To cause an electron to escape from a metal, it should be given enough energy to overcome the surface potential barrier. The energy can also be supplied in the form of a photon of ultraviolet light of suitable frequency. Electron emission from a metal surface occurs at the critical condition. ℎ. 𝑣 ≥ 𝜑 • where 𝜑 is the work function of the metallic electrode. The frequency ( 𝑣 ) is given by the relationship 𝑣 = 𝜑 ℎ is known as the threshold frequency.
- 19. Electron Emission due to Metastable & 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). • Electrons 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. • This process is most easily observed with metastable atoms, because the lifetime of other excited states is too short for them to reach the cathode and cause electron emission, unless they originate very near to the cathode surface.
- 20. Electron Attachment Process • The types of collisions in which electrons may become attached to atoms or molecules to form negative ions are called attachment collisions. Electron attachment process depends on the energy of the electron and the nature of the gas and is a very important process from the engineering point of view. • Atom +𝑒− → 𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒 𝑎𝑡𝑜𝑚𝑖𝑐 𝑖𝑜𝑛 + (𝐸 𝑎 + 𝐾) • The energy liberated as a result of this process is the kinetic energy K plus the electron affinity 𝐸 𝑎.
- 21. Electron Attachment Process • In the attaching or insulating gases, the atoms or molecules have vacancies in their outermost shells and, therefore, have an affinity for electrons. • The attachment process plays a very important role in the removal of free electrons from an ionised gas when arc interruption occurs in gas-insulated switchgear.
- 22. Townsend ‘s Current Growth Equation Dated : 13-11-2017
- 23. • Referring to Fig. 1 let us assume that n0 electrons are emitted from the cathode. • When one electron collides with a neutral particle, a positive ion and an electron are formed. This is called an ionizing collision. Figure 1: Arrangement for study of a Townsend discharge.
- 24. • Let 𝛼 be the average number of ionizing collisions made by an electron per centimetre 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). • Details are available in conduction and breakdown in gas chapter(page no.33-34)
- 25. Current Growth in a Townsend’s Discharge
- 26. Time Lags for Breakdown Dated : 20-11-2017
- 27. • In the previous section, the mechanism of spark breakdown is considered as a function of ionization processes under uniform field conditions. But in practical engineering designs, the breakdown due to rapidly changing voltages or impulse voltages is of great importance. • Actually, there is a time difference between the application of a voltage sufficient to cause breakdown and the occurrence of breakdown itself. This time difference is called the time lag.
- 28. • The Townsend criterion for breakdown is satisfied, only if at least one election is present in the gap between the electrodes. • In the case of applied d.c. or slowly varying (50 Hz a.c) voltages, there is no difficulty in satisfying this condition. • However, with rapidly varying voltages of short duration (≅ 10−6 s), the initiatory electron may not be present in the gap, and in the absence of such an electron breakdown cannot occur.
- 29. • The time t which lapses between the application of the voltage sufficient to cause breakdown and the appearance of the initiating electron is called a statistical time lag (𝑡 𝑠) of the gap. • The appearance of electrons is usually statistically distributed. After the appearance of the electron, a time 𝑡𝑡 is required for the ionization processes to develop fully to cause the breakdown of the gap, and this time is called the formative time lag (𝑡𝑡 ). • The total time 𝑡 = 𝑡 𝑠 + 𝑡𝑡 is called the total time lag.
- 30. Streamer Theory of Breakdown in Gases Dated : 20-11-2017
- 31. • 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 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.
- 32. • Also, 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, Raether and, Meek and Loeb independently proposed the Streamer theory. • The 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.
- 33. • A single electron starting at the cathode by ionization builds up an avalanche that crosses the gap. • The electrons in the avalanche move very fast compared with the positive ions. • By the time the electrons reach the anode the positive ions are virtually in their original positions and form a positive space charge at the anode. • This enhances the field, and the secondary avalanches are formed from the few electrons produced due to photo ionization in the space charge region.
- 34. • This occurs first near the anode where the space charge is maximum. This results in a further increase in the space charge. • This process is very fast and the positive space charge extends to the cathode very rapidly resulting in the formation of a streamer. • 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 the spark breakdown has occurred.
- 37. Post Breakdown Phenomena Dated : 21-11-2017
- 38. • This is the phenomenon which occurs after the actual breakdown has taken place and is of technical importance. • Glow and arc discharges are the post- breakdown phenomena, and there are many devices that operate over these regions. • In a Townsend discharge (see Fig.1) the current increases gradually as a function of the applied voltage.
- 39. Figure 1: D.C. voltage-current characteristic of an electrical discharge with electrodes having no sharp points or edges
- 40. • Further to this point (B) only the current increases, and the discharge changes from the Townsend type to Glow type (BC). • Further increase incurrent results in a very small reduction in voltage across the gap (CD) corresponding to the normal glow region. • The gap voltage again increases (DE), when the current is increased more, but eventually leads to a considerable drop in the applied voltage. • This is the region of the arc discharge (EG). The phenomena that occur in the region CG are the post-breakdown phenomena consisting of glow discharge (CE) and the arc discharge (EG)
- 41. Glow Discharge • A glow discharge is characterized by a diffused luminous glow. The colour of the glow discharge depends on the cathode material and the gas used. • The glow discharge covers the cathode partly and the space between the cathode, and the anode will have intermediate dark and bright regions. This is called normal glow. • If the current in the normal glow is increased such that the discharge covers the entire cathode surface, then it becomes abnormal glow.
- 42. Arc Discharge • If the current in the gap is increased to about 1 A or more, the voltage across the gap suddenly reduces to a few volts (20-50 V). • The discharge becomes very luminous and noisy (region EG in Figure 1). This phase is called the arc discharge. • Arcing is associated with high temperatures, ranging from 1000℃ to several thousand degrees Celsius. The discharge will contain a very high density of electrons and positive ions, called the arc plasma.
- 43. Arc Discharge • The study of arcs is important in circuit breakers and other switch contacts. It is a convenient high temperature high intensity light source. It is used for welding and cutting of metals.
- 44. Thank You… Dated : 21-11-2017