This document summarizes key concepts in high voltage engineering related to insulation breakdown in gaseous dielectrics. It discusses:
1) Different ionization processes that can occur when electrons collide with gas molecules, including simple collision ionization, excitation, double electron impact ionization, and photoionization.
2) Townsend breakdown process where free electrons cause an "electron avalanche" through successive ionizing collisions, leading to exponential growth in the number of electrons.
3) Mathematical analysis of Townsend breakdown using ionization coefficients to describe the number of electrons/ions produced per unit length.
This document discusses the mechanisms of breakdown in gases. It explains that at high electric fields, free electrons in gas can gain enough energy between collisions to cause ionization when striking other molecules. This leads to an electron avalanche effect where the number of electrons increases rapidly. The document outlines various types of ionization processes and theories of breakdown proposed by Townsend, including his first and second ionization coefficients. Townsend's theory of electron avalanches explains the exponential rise in current during breakdown. The document provides mathematical equations to describe current growth based on these coefficients.
The document discusses electrical breakdown in gases. It explains that gases are commonly used as dielectric mediums in electrical apparatus due to their insulating properties. However, when high voltages are applied, electrical breakdown can occur through ionization. The Townsend theory and streamer theory are presented as explanations for the breakdown mechanism under different conditions. Collision processes, mobility of ions and electrons, diffusion, and mean free path are also discussed. The document further explains the ionization process and Townsend's criteria for electrical breakdown in gases.
1. Gases can act as insulating media in electrical apparatus due to their ability to undergo ionization when subjected to electric fields. This document discusses various ionization processes in gases and their role in electrical breakdown.
2. Townsend developed equations to model the exponential growth of current in a gas due to electron avalanches caused by ionization collisions. The current is dependent on primary and secondary ionization coefficients.
3. Breakdown occurs when the current becomes infinitely large, as defined by Townsend's criterion. Alternative mechanisms like streamers can also lead to spark formation in gases.
Stanley A Meyer Legacy Back up Secret Docs Save all Protect Spread print and give to schools NEVER STOP!!!!!!! Join Support here https://www.patreon.com/securesupplies/shop
Stanley A Meyer Legacy Back up Secret Docs Save all Protect Spread print and give to schools NEVER STOP!!!!!!! Join Support here https://www.patreon.com/securesupplies/shop
The document discusses band theory of solids, which explains the electrical, thermal, and magnetic properties of solids. It begins by covering classical and quantum free electron theories, before introducing band theory. Band theory states that the motion of free electrons in solids is characterized by allowed energy bands separated by forbidden bands. The width of bands and size of gaps depends on factors like the periodic potential of the lattice and strength of scattering. Semiconductors have a small forbidden band gap, allowing electrical conductivity to be controlled by doping with impurities.
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
The document discusses ionization processes in gases that lead to electrical breakdown. It introduces several key concepts:
- Ionization occurs through collisional processes that give electrons enough energy to liberate other electrons, creating an avalanche.
- Townsend's theory describes the exponential growth of this avalanche current as more electrons ionize more atoms. The growth depends on the ionization coefficient α.
- Secondary processes like photon emission can produce additional "seed" electrons, captured by the secondary coefficient γ.
- Together α and γ determine the current amplification and eventual self-sustaining breakdown when enough charges cross the gap.
This document discusses the mechanisms of breakdown in gases. It explains that at high electric fields, free electrons in gas can gain enough energy between collisions to cause ionization when striking other molecules. This leads to an electron avalanche effect where the number of electrons increases rapidly. The document outlines various types of ionization processes and theories of breakdown proposed by Townsend, including his first and second ionization coefficients. Townsend's theory of electron avalanches explains the exponential rise in current during breakdown. The document provides mathematical equations to describe current growth based on these coefficients.
The document discusses electrical breakdown in gases. It explains that gases are commonly used as dielectric mediums in electrical apparatus due to their insulating properties. However, when high voltages are applied, electrical breakdown can occur through ionization. The Townsend theory and streamer theory are presented as explanations for the breakdown mechanism under different conditions. Collision processes, mobility of ions and electrons, diffusion, and mean free path are also discussed. The document further explains the ionization process and Townsend's criteria for electrical breakdown in gases.
1. Gases can act as insulating media in electrical apparatus due to their ability to undergo ionization when subjected to electric fields. This document discusses various ionization processes in gases and their role in electrical breakdown.
2. Townsend developed equations to model the exponential growth of current in a gas due to electron avalanches caused by ionization collisions. The current is dependent on primary and secondary ionization coefficients.
3. Breakdown occurs when the current becomes infinitely large, as defined by Townsend's criterion. Alternative mechanisms like streamers can also lead to spark formation in gases.
Stanley A Meyer Legacy Back up Secret Docs Save all Protect Spread print and give to schools NEVER STOP!!!!!!! Join Support here https://www.patreon.com/securesupplies/shop
Stanley A Meyer Legacy Back up Secret Docs Save all Protect Spread print and give to schools NEVER STOP!!!!!!! Join Support here https://www.patreon.com/securesupplies/shop
The document discusses band theory of solids, which explains the electrical, thermal, and magnetic properties of solids. It begins by covering classical and quantum free electron theories, before introducing band theory. Band theory states that the motion of free electrons in solids is characterized by allowed energy bands separated by forbidden bands. The width of bands and size of gaps depends on factors like the periodic potential of the lattice and strength of scattering. Semiconductors have a small forbidden band gap, allowing electrical conductivity to be controlled by doping with impurities.
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
The document discusses ionization processes in gases that lead to electrical breakdown. It introduces several key concepts:
- Ionization occurs through collisional processes that give electrons enough energy to liberate other electrons, creating an avalanche.
- Townsend's theory describes the exponential growth of this avalanche current as more electrons ionize more atoms. The growth depends on the ionization coefficient α.
- Secondary processes like photon emission can produce additional "seed" electrons, captured by the secondary coefficient γ.
- Together α and γ determine the current amplification and eventual self-sustaining breakdown when enough charges cross the gap.
This document summarizes a seminar on energy bands and gaps in semiconductors. It discusses the introduction of energy bands, including valence bands, conduction bands, and forbidden gaps. It describes how materials are classified as insulators, conductors, or semiconductors based on their band gap energies. Direct and indirect band gap semiconductors are also defined. Intrinsic, n-type, and p-type semiconductors are classified based on their majority charge carriers.
Electrical and Magnetic Properties of MaterialsAbeni9
Properties of a material which determine its response to an electric field.
Materials are classified based on their electrical properties as conductors, semiconductors and insulators and newly super conductors.
Ionization potential and electron affinityAqsa Manzoor
1) The document discusses trends in ionization potential and electron affinity across the periodic table. Ionization potential generally increases from left to right in a period as nuclear charge increases and atomic radius decreases. It also decreases down a group as atomic size increases.
2) Electron affinity generally increases from left to right as nuclear charge increases but decreases down a group as atomic size increases. Elements in the lower left of the periodic table tend to have lower ionization potentials and are more metallic.
3) Factors that influence ionization potential and electron affinity include effective nuclear charge, atomic size, shielding effects, and stability of electron configurations.
The document discusses atomic structure and mass spectrometry. It defines key terms like mass number, atomic number, and isotope. It explains the process of mass spectrometry, including ionization, acceleration, deflection, and detection of ions. Graphs of ionization energies are analyzed to determine electronic configurations and periodic trends. Successive ionization energies are explained by electron shielding effects. Radioactive decay and half-life are also defined.
This document provides an overview of the topics covered in the high voltage engineering course 19EE702. It discusses the need for high voltages in power transmission and laboratories. It also covers breakdown phenomena in gases, including ionization processes. Townsend's theory of gas breakdown and limitations are explained. The document discusses insulation types and applications of high voltages in components like cables and bushings. It also covers electrical breakdown mechanisms and time lags associated with breakdown.
This document discusses different types of electron emission from metal surfaces. There are four principal types: thermionic emission, where heating provides the energy for electrons to overcome the work function; field emission, where a strong electric field pulls electrons from the surface; photoelectric emission, where light energy is transferred to electrons; and secondary emission, where high-velocity electrons striking the surface knock out more electrons. Thermionic emission is described in more detail, including the Richardson-Dushman equation that relates emission current density to temperature and work function, and examples are provided to calculate emission currents and determine metal work functions.
This document discusses different types of electron emission from metal surfaces. Thermionic emission occurs when heat is applied to a metal, increasing the kinetic energy of electrons and allowing them to overcome the surface barrier. Common thermionic emitters discussed are tungsten, thoriated tungsten, and oxide coatings, with their respective work functions and operating temperatures listed. The Richardson-Dushman equation describes how emission current density increases exponentially with temperature but depends on the work function of the emitter material.
Principles of Electronics chapter - IIAmit Khowala
This document discusses electron emission from thermionic emitters. It describes the process of thermionic emission where heating a metal provides electrons with enough energy to overcome the surface barrier and be emitted. Common thermionic emitters mentioned are tungsten, thoriated tungsten, and oxide coated cathodes. Thoriated tungsten has a lower work function than pure tungsten, allowing emission at lower temperatures. Oxide coated cathodes operate at even lower temperatures but cannot withstand high voltages. The Richardson-Dushman equation governs thermionic emission current density as a function of temperature and work function.
1. The document discusses the discovery of the electron through cathode ray experiments and the determination of the charge to mass ratio of electrons.
2. Rutherford's alpha particle scattering experiments showed that the atom has a small, dense nucleus containing positive charge and mass, surrounded by electrons. This led to the development of the Rutherford model of the atom.
3. The document also discusses the discovery of protons and neutrons, atomic number and mass number, isotopes, drawbacks of the Rutherford model, wave-particle duality of light, and Planck's quantum theory.
1. Electric charges can be positive or negative, and electric forces cause like charges to repel and unlike charges to attract according to Coulomb's Law.
2. Atoms contain protons with a positive charge and electrons with a negative charge; objects are neutral when they contain equal numbers of protons and electrons.
3. Electric fields are created by electric charges and describe the interaction of electric forces; electric fields can be used to accelerate electrons in devices like x-ray machines and televisions.
4. Capacitors are used to store electric charge and consist of conductive plates separated by an insulator; the amount
1) Experiments with cathode ray tubes led to the discovery of the electron as a negatively charged fundamental particle.
2) Further experiments showed that atoms are mostly empty space and contain a small, dense nucleus made up of protons and neutrons, around which electrons orbit.
3) The photoelectric effect showed that light behaves as a particle (photon) rather than just a wave, transferring its energy in discrete quantized amounts to electrons and ejecting them from metal surfaces.
The document discusses the principles and physics of welding. It covers topics such as fusion welding processes, characteristics of heat sources like welding arcs, arc structures, and potential drop characteristics. The key points are:
1) In fusion welding, material around the joint is melted to join two parts together. Important factors include the heat source, arc characteristics, filler material deposition, and heat flow.
2) A welding arc is a sustained electrical discharge through an ionized gas that produces heat. It is maintained by thermionic emission and ionization between the electrodes.
3) The voltage drop across a welding arc depends on factors like the electrode material, spacing and current. There is an optimal arc length that produces maximum power
Lasers & semiconductors 2008 prelim solutionsJohn Jon
Electrons and holes are urged towards the junction region when the diode is in forward bias. This results in the reduction of the depletion region, thus allowing a current to flow. When a p-d is applied across a p-n junction diode in the forward bias mode, the width of the depletion layer decreases, allowing charge carriers to flow more easily across the junction. In reverse bias mode, the depletion layer width increases, preventing charge flow and making the diode act as an open switch.
This document provides an overview of intrinsic and extrinsic semiconductors. It begins with an introduction to crystalline solids and classifications of solids as conductors, insulators, or semiconductors. It then discusses intrinsic semiconductors, how increasing temperature generates electron-hole pairs, and how conductivity increases with temperature. Extrinsic or doped semiconductors are introduced, including n-type and p-type semiconductors created by adding donor or acceptor impurities. The document explains how doping increases the number of charge carriers and conductivity.
Ajit Lulla is an expert physics faculty with over 11 years of experience teaching at IIT Bombay and Allen Career Institute. He has helped many students achieve top ranks in JEE and NEET, with some students ranking as high as AIR 69, 81, and 121. AT24 offers unlimited access to Ajit Lulla's structured courses, personalized guidance, test analysis, specialized study material, a customized study planner with bi-weekly reviews, and study booster workshops led by exam experts.
Ohm's law relates current, voltage, and resistance. Resistivity is a material property independent of geometry, while conductivity is the inverse of resistivity and indicates how easily a material conducts electricity. Materials are classified as conductors, insulators, or semiconductors based on conductivity. Semiconductors have applications in electronics due to their sensitivity to impurities and ability to be "doped" to control conductivity. Their band structure results in varying conductivity depending on temperature and doping.
In your previous class you have already studies about the structure of an atom but some of the exception you can learn here in this chapter how the structure of an atom is fully defined
Atomic Structure from A level chemistry.saqibnaveed9
This document provides an overview of atomic structure and the discovery of subatomic particles like electrons. It discusses J.J. Thomson's cathode ray tube experiments in the late 1800s that led to the discovery of electrons and determination of their charge to mass ratio (e/m). It also describes Millikan's oil drop experiment from 1909 that precisely measured the charge of individual electrons as 1.6022×10-19 coulombs and calculated the mass of an electron as 9.1095×10-31 kg.
Townsend ’s theory
Introduction
Ionization by collision
Townsend’s current growth equation
Current Growth in the Presence of Secondary Processes
Townsend’s secondary ionization coefficient
Townsend’s Criterion for Breakdown
Breakdown in Electronegative Gases
This document summarizes a seminar on energy bands and gaps in semiconductors. It discusses the introduction of energy bands, including valence bands, conduction bands, and forbidden gaps. It describes how materials are classified as insulators, conductors, or semiconductors based on their band gap energies. Direct and indirect band gap semiconductors are also defined. Intrinsic, n-type, and p-type semiconductors are classified based on their majority charge carriers.
Electrical and Magnetic Properties of MaterialsAbeni9
Properties of a material which determine its response to an electric field.
Materials are classified based on their electrical properties as conductors, semiconductors and insulators and newly super conductors.
Ionization potential and electron affinityAqsa Manzoor
1) The document discusses trends in ionization potential and electron affinity across the periodic table. Ionization potential generally increases from left to right in a period as nuclear charge increases and atomic radius decreases. It also decreases down a group as atomic size increases.
2) Electron affinity generally increases from left to right as nuclear charge increases but decreases down a group as atomic size increases. Elements in the lower left of the periodic table tend to have lower ionization potentials and are more metallic.
3) Factors that influence ionization potential and electron affinity include effective nuclear charge, atomic size, shielding effects, and stability of electron configurations.
The document discusses atomic structure and mass spectrometry. It defines key terms like mass number, atomic number, and isotope. It explains the process of mass spectrometry, including ionization, acceleration, deflection, and detection of ions. Graphs of ionization energies are analyzed to determine electronic configurations and periodic trends. Successive ionization energies are explained by electron shielding effects. Radioactive decay and half-life are also defined.
This document provides an overview of the topics covered in the high voltage engineering course 19EE702. It discusses the need for high voltages in power transmission and laboratories. It also covers breakdown phenomena in gases, including ionization processes. Townsend's theory of gas breakdown and limitations are explained. The document discusses insulation types and applications of high voltages in components like cables and bushings. It also covers electrical breakdown mechanisms and time lags associated with breakdown.
This document discusses different types of electron emission from metal surfaces. There are four principal types: thermionic emission, where heating provides the energy for electrons to overcome the work function; field emission, where a strong electric field pulls electrons from the surface; photoelectric emission, where light energy is transferred to electrons; and secondary emission, where high-velocity electrons striking the surface knock out more electrons. Thermionic emission is described in more detail, including the Richardson-Dushman equation that relates emission current density to temperature and work function, and examples are provided to calculate emission currents and determine metal work functions.
This document discusses different types of electron emission from metal surfaces. Thermionic emission occurs when heat is applied to a metal, increasing the kinetic energy of electrons and allowing them to overcome the surface barrier. Common thermionic emitters discussed are tungsten, thoriated tungsten, and oxide coatings, with their respective work functions and operating temperatures listed. The Richardson-Dushman equation describes how emission current density increases exponentially with temperature but depends on the work function of the emitter material.
Principles of Electronics chapter - IIAmit Khowala
This document discusses electron emission from thermionic emitters. It describes the process of thermionic emission where heating a metal provides electrons with enough energy to overcome the surface barrier and be emitted. Common thermionic emitters mentioned are tungsten, thoriated tungsten, and oxide coated cathodes. Thoriated tungsten has a lower work function than pure tungsten, allowing emission at lower temperatures. Oxide coated cathodes operate at even lower temperatures but cannot withstand high voltages. The Richardson-Dushman equation governs thermionic emission current density as a function of temperature and work function.
1. The document discusses the discovery of the electron through cathode ray experiments and the determination of the charge to mass ratio of electrons.
2. Rutherford's alpha particle scattering experiments showed that the atom has a small, dense nucleus containing positive charge and mass, surrounded by electrons. This led to the development of the Rutherford model of the atom.
3. The document also discusses the discovery of protons and neutrons, atomic number and mass number, isotopes, drawbacks of the Rutherford model, wave-particle duality of light, and Planck's quantum theory.
1. Electric charges can be positive or negative, and electric forces cause like charges to repel and unlike charges to attract according to Coulomb's Law.
2. Atoms contain protons with a positive charge and electrons with a negative charge; objects are neutral when they contain equal numbers of protons and electrons.
3. Electric fields are created by electric charges and describe the interaction of electric forces; electric fields can be used to accelerate electrons in devices like x-ray machines and televisions.
4. Capacitors are used to store electric charge and consist of conductive plates separated by an insulator; the amount
1) Experiments with cathode ray tubes led to the discovery of the electron as a negatively charged fundamental particle.
2) Further experiments showed that atoms are mostly empty space and contain a small, dense nucleus made up of protons and neutrons, around which electrons orbit.
3) The photoelectric effect showed that light behaves as a particle (photon) rather than just a wave, transferring its energy in discrete quantized amounts to electrons and ejecting them from metal surfaces.
The document discusses the principles and physics of welding. It covers topics such as fusion welding processes, characteristics of heat sources like welding arcs, arc structures, and potential drop characteristics. The key points are:
1) In fusion welding, material around the joint is melted to join two parts together. Important factors include the heat source, arc characteristics, filler material deposition, and heat flow.
2) A welding arc is a sustained electrical discharge through an ionized gas that produces heat. It is maintained by thermionic emission and ionization between the electrodes.
3) The voltage drop across a welding arc depends on factors like the electrode material, spacing and current. There is an optimal arc length that produces maximum power
Lasers & semiconductors 2008 prelim solutionsJohn Jon
Electrons and holes are urged towards the junction region when the diode is in forward bias. This results in the reduction of the depletion region, thus allowing a current to flow. When a p-d is applied across a p-n junction diode in the forward bias mode, the width of the depletion layer decreases, allowing charge carriers to flow more easily across the junction. In reverse bias mode, the depletion layer width increases, preventing charge flow and making the diode act as an open switch.
This document provides an overview of intrinsic and extrinsic semiconductors. It begins with an introduction to crystalline solids and classifications of solids as conductors, insulators, or semiconductors. It then discusses intrinsic semiconductors, how increasing temperature generates electron-hole pairs, and how conductivity increases with temperature. Extrinsic or doped semiconductors are introduced, including n-type and p-type semiconductors created by adding donor or acceptor impurities. The document explains how doping increases the number of charge carriers and conductivity.
Ajit Lulla is an expert physics faculty with over 11 years of experience teaching at IIT Bombay and Allen Career Institute. He has helped many students achieve top ranks in JEE and NEET, with some students ranking as high as AIR 69, 81, and 121. AT24 offers unlimited access to Ajit Lulla's structured courses, personalized guidance, test analysis, specialized study material, a customized study planner with bi-weekly reviews, and study booster workshops led by exam experts.
Ohm's law relates current, voltage, and resistance. Resistivity is a material property independent of geometry, while conductivity is the inverse of resistivity and indicates how easily a material conducts electricity. Materials are classified as conductors, insulators, or semiconductors based on conductivity. Semiconductors have applications in electronics due to their sensitivity to impurities and ability to be "doped" to control conductivity. Their band structure results in varying conductivity depending on temperature and doping.
In your previous class you have already studies about the structure of an atom but some of the exception you can learn here in this chapter how the structure of an atom is fully defined
Atomic Structure from A level chemistry.saqibnaveed9
This document provides an overview of atomic structure and the discovery of subatomic particles like electrons. It discusses J.J. Thomson's cathode ray tube experiments in the late 1800s that led to the discovery of electrons and determination of their charge to mass ratio (e/m). It also describes Millikan's oil drop experiment from 1909 that precisely measured the charge of individual electrons as 1.6022×10-19 coulombs and calculated the mass of an electron as 9.1095×10-31 kg.
Townsend ’s theory
Introduction
Ionization by collision
Townsend’s current growth equation
Current Growth in the Presence of Secondary Processes
Townsend’s secondary ionization coefficient
Townsend’s Criterion for Breakdown
Breakdown in Electronegative Gases
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Design and optimization of ion propulsion dronebjmsejournal
Electric propulsion technology is widely used in many kinds of vehicles in recent years, and aircrafts are no exception. Technically, UAVs are electrically propelled but tend to produce a significant amount of noise and vibrations. Ion propulsion technology for drones is a potential solution to this problem. Ion propulsion technology is proven to be feasible in the earth’s atmosphere. The study presented in this article shows the design of EHD thrusters and power supply for ion propulsion drones along with performance optimization of high-voltage power supply for endurance in earth’s atmosphere.
Comparative analysis between traditional aquaponics and reconstructed aquapon...bijceesjournal
The aquaponic system of planting is a method that does not require soil usage. It is a method that only needs water, fish, lava rocks (a substitute for soil), and plants. Aquaponic systems are sustainable and environmentally friendly. Its use not only helps to plant in small spaces but also helps reduce artificial chemical use and minimizes excess water use, as aquaponics consumes 90% less water than soil-based gardening. The study applied a descriptive and experimental design to assess and compare conventional and reconstructed aquaponic methods for reproducing tomatoes. The researchers created an observation checklist to determine the significant factors of the study. The study aims to determine the significant difference between traditional aquaponics and reconstructed aquaponics systems propagating tomatoes in terms of height, weight, girth, and number of fruits. The reconstructed aquaponics system’s higher growth yield results in a much more nourished crop than the traditional aquaponics system. It is superior in its number of fruits, height, weight, and girth measurement. Moreover, the reconstructed aquaponics system is proven to eliminate all the hindrances present in the traditional aquaponics system, which are overcrowding of fish, algae growth, pest problems, contaminated water, and dead fish.
An improved modulation technique suitable for a three level flying capacitor ...IJECEIAES
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What is artificial intelligence? Artificial intelligence is the ability of a computer or computer-controlled robot to perform tasks that are commonly associated with the intellectual processes characteristic of humans, such as the ability to reason.
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Artificial intelligence (AI) | Definitio
1. Course code : EEE-4123
High Voltage Engineering
Lecture-02
Topics
Insulation: Gasious dielectric
Presented by
Mst. Tajmun Nahar
Lecturer, Dept. of EEE, NUBTK
2. Objectives
In this course you will learn the following:
Insulating materials and their necessity
Necessity of studying breakdown of insulation
Different ionization process
Town sends breakdown process
3. Insulating materials
An electrical insulator is a material whose internal electric charges do not
flow freely, and therefore make it nearly impossible to conduct an electric
current under the influence of an electric field. These materials thus offer a
very high resistance to the passage of direct currents. However, they cannot
withstand an infinitely high voltage. When the applied voltage across the
dielectric exceeds a critical value the insulation will be damaged. The
dielectrics may be gaseous, liquid or solid in form.
Break down in insulating materials:
Gaseous dielectrics in practice are not free of electrically charged
particles, including free electrons. The electrons, which may be caused by
irradiation or field emission, can lead to a breakdown process to be
initiated.
These free electrons, are accelerated from the cathode to the anode by
the electric stress applying a force on them. They acquire a kinetic energy
(½ mu2) as they move through the field. These free electrons, moving
towards the anode collide with the gas molecules present between the
electrodes.
4. In these collisions, part of the kinetic energy of the electrons is lost and
part is transmitted to the neutral molecule. If this molecule gains sufficient
energy, it may ionise by collision.
The newly liberated electron and the impinging electron are then
accelerated in the field and an electron avalanche is set up. Further
increase in voltage results in additional ionising processes and breakdown
occurs.
In uniform fields, the ionisation present at voltages below breakdown
is normally too small to affect engineering applications. In non-uniform
fields, however, considerable ionisation may be present in the region of
high stress, at voltages well below breakdown.
5. Why do we need to study the breakdown of
insulating medium?
With ever increasing demand of electrical energy, the power
system is growing both in size and complexities. The generating
capacities of power plants and transmission voltage are on the
increase because of their inherent advantages. If the
transmission voltage is doubled, the power transfer
capability of the system becomes four times and the line losses
are also relatively reduced. As a result, it becomes a stronger and
economical system.
A system (transmission, switchgear, etc.) designed
for 400 kV and above using conventional insulating
materials is both bulky and expensive and, therefore, newer and
newer insulating materials are being investigated to bring down
both the cost and space requirements.
6. The electrically live conductors are supported on insulating
materials and sufficient air clearances are provided to avoid
flashover or short circuits between the live parts of the system
and the grounded structures.
Sometimes, a live conductor is to be immersed in
an insulating liquid to bring down the size of the container and
at the same time provide sufficient insulation between the live
conductor and the grounded container. In electrical
engineering all the three media, viz. the gas, the liquid
and the solid are being used and, therefore, we study
here the mechanism of breakdown of these media.
7. Ionization process:
Ionization is the process by which an electron is removed from an
atom, leaving the atom with a net positive charge. Since an
electron in the outermost orbit is subject to the least attractive
force from the nucleus, it is the easiest removed by any of the
collision processes. The energy required to remove an outer
electron completely from its normal state in the atom to a distance
well beyond the nucleus is called the first ionization potential
Ionization process in gas discharge:
01. Ionization by simple collusion: When the kinetic energy of
an electron (½ mu²), in collision with a neutral gas molecule
exceeds the ionisation energy (Ei = e Vi) of the molecule, then
ionisation can occur.
8. 02. Excitation: In the case of simple collision, the neutral gas
molecule does not always gets ionised on electron impact. In such
cases, the molecule will be left in an excited state M*, with energy
Ec.
This excited molecule can subsequently give out a photon of
frequency v with energy emitted h v. The energy is given out when
the electron jumps from one orbit to the next.
03. Ionization by double electron impact: If a gas molecule is
already raised to an excited state (with energy Ee) by a previous
collision, then ionisation of this excited molecule can occur by a
collision with a relatively slow electron.
9. 04. Photo-ionization: A molecule in the ground state can be
ionised by a photon of frequency v provided that the quantum of
energy emitted hv (by an electron jumping from one orbit to
another), is greater than the ionisation energy of the molecule
05. Electron attachment: If a gas molecule has unoccupied energy
levels in its outermost group, then a colliding electron may take up
one of these levels, converting the molecule into a negative ion M-.
M + e- → M-
06. Electron detachment: This occurs when a negative ion gives
up its extra electron, and becomes a neutral molecule.
M- → M + e-
10. There are two mechanism of breakdown of gases:
1. Avalance breakdown
2. Streamer breakdown
Avalance or Townsend breakdown process:
It is based on the generation of successive secondary avalanches to
produce breakdown.
11. Suppose a free electron exists (caused by some external effect
such as radio-activity or cosmic radiation) in a gas where an
electric field exists. If the field strength is sufficiently high, then
it is likely to ionize a gas molecule by simple collision resulting
in 2 free electrons and a positive ion. These 2 electrons will be
able to cause further ionization by collision leading in general to
4 electrons and 3 positive ions. The process is cumulative, and
the number of free electrons will go on increasing as they
continue to move under the action of the electric field. The
swarm of electrons and positive ions produced in this way is
called an electron avalanche.
13. Mathematical Analysis
When the voltage applied across a pair of electrodes is increased, the
current throughout the gap increases slowly, as the electrons emitted
from the cathode move through the gas with an average velocity
determined by their mobility for the field strength existing for the
particular value of voltage. Now let,
n0 = number of electrons/second emitted from the cathode,
nx = number of electrons/second moving at a distance x from the
cathode
α = number of ionising collisions, on average, made by one
electron per unit drift in the direction of the field. [Townsend's first
ionisation coefficient]
1/ α = average distance traversed in the field direction between
ionising collisions.
14. Consider a parallel plate capacitor having gas as insulating material
and separated by a distance d. A laminar of thickness dx at a distance
x from the cathode.
To explain the exponential rise in current, Townsend introduced a
coefficient α known as “Townsend’s first ionization coefficient” and is
defined as the number of electrons produced by an electron per unit
length of path in the direction of field.
let,
n0 = number of electrons/second emitted from the cathode
n= number of electrons/second moved through a distance x
from the cathode
15. Mathematical Analysis
When these n electron move through a distance dx, they
produce another dn electrons due to collision. Therefore:
x
e
n
n
16. In the steady state, the number of positive ions arriving at the
cathode/second must be exactly equal to the number of newly formed
electrons arriving at the anode. Thus the circuit current will be given
by
where I0 is the initial photo-electric current at the cathode and
Is called electron avalance which represents the number of electron
produced by one electron in traveling from cathod to anode
d
e
18. n0 = number of electrons/second emitted from the
cathode
n+ = number of electrons/second released from the
cathode due to positive ion bombardment
n = number of electrons/second reaching the anode
d
e
n
n
n
)
( 0
19. Total number of electron released from the cathode is
and total electron reached in anode is n.
Hence number of electron released from the gas is
Which is equal to the positive ion produced in the gas
and will bombard on the cathode
)
( 0
n
n
)
( 0
n
n
n
20. Here comes Townsends second ionization co-efficient
which is defiened as the number of electron released from
the cathode per incident positive ion.
Hence
Substituting this value in
d
e
n
n
n
)
( 0