Breakdowns Mechanism of di-electric.pptx

Breakdown Mechanism of Dielectric Medium

1. Breakdown of Di-electrics Instructor Engr. Muhammad Yaseen

2. Gaseous Breakdowns Gas/Vacuum as Insulator Air at atmospheric pressure is the most common gaseous insulation. The breakdown of air is of considerable practical importance to the design engineers of power transmission lines and power apparatus. Breakdown occurs in gases due to the process of collisional ionization. Electrons get multiplied in an exponential manner, and if the applied voltage is sufficiently large, breakdown occurs. In some gases, free electrons are removed by attachment to neutral gas molecules; the breakdown strength of such gases is substantially large.

3. Gaseous Breakdowns An example of such a gas with larger dielectric strength is sulphur hexafluoride (SF6) . The breakdown strength of gases increases steadily with the gap distance between the electrodes; but the breakdown voltage gradient reduces from 3MV/m for uniform fields and small distances to about 0.6MV/m for large gaps of several meters. For very large gaps as in lightning, the average gradient reduces to 0.1 to 0.3 MV/m. High pressure gas provides a flexible and reliable medium for high voltage insulation. Nitrogen (N 2 ) was the gas first used at high pressures because of its inertness and chemical stability, but its dielectric strength is the same as that of air.

4. Gaseous Breakdowns Other important practical insulating gases are carbon dioxide(CO 2 ), dichlorodifluoro-methane (CCl 2 F 2 ) (popularly known as Freon), and sulphur hexafluoride (SF 6 ). SF 6 has been found to maintain its insulation superiority, about 2.5 times over N 2 and CO 2 at atmospheric pressure, the ratio increasing at higher pressures. SF 6 gas was also observed to have superior arc quenching properties over any other gas. The breakdown voltage at higher pressures in gases shows an increasing dependence on the nature and smoothness of the electrode material. Under high vacuum conditions, where the pressures are below 10 -4 torr, the breakdown cannot occur due to collisional processes like in gases, and hence the breakdown strength is quite high.

5. Gaseous Breakdowns Ionization by Formation of Electron Avalanche This process is also called as cumulative means total result of separate facts. Assume two electrodes of opposite polarity say (+ve) and (-ve) are separated by gas space, as shown in figure

6. Gaseous Breakdowns Here the atom is placed near the two electrodes in such a manner that the electrons from cathode are attracted towards the anode which has got the opposite polarity as compare the cathode, and thus leaving behind the +ve ion (A+). The gradual increase in flow of electron from cathode to anode in groups (in shape of clouds) is known as “Electron Avalanche” In figure, electrons leaving the cathode are denoted by n o , these electrons are emitted from cathode by means of ultra violet radiation. And the n x represents the electrons at distance “x” from the cathode.

7. Gaseous Breakdowns The process of emitting the electrons from the cathode towards anode by ultraviolet radiation is called as primary process and the avalanche produced during this process is called as “primary avalanche”. Now the process of liberating the electron in gap by means of any other process such as the bombardment of +ve ion from cathode, photo-ionization, and detachment is called as secondary process, and the avalanche producing during this process is called as “Secondary avalanche”. Figure shows the formation of both the primary and secondary avalanches in gas space.

8. Gaseous Breakdowns Townsend Criterion In the absence of electric field, the number of electron and number of ions in a dielectric is equalized by decay process and accomplish a state of equilibrium. This state of equilibrium is disturbed by the electric field. This produces the variation of current. This varied current between two electrodes was first found by the scientist named as a Townsend. Townsend found that when high voltages are applied, initially the current is increased properly (up to V 1 ) and after some time it undergoes the constant value of the current say i o (Saturation current). Now if the voltages are further increased then current increases exponentially. This V/I characteristics is shown below in figure.

9. Gaseous Breakdowns Townsend described that above the voltage V 2 , the electron leaves the cathode and collides with the neutral atom of the gas dielectric thus causes the ionization by electron collision.

10. Gaseous Breakdowns Townsend First Ionization Coefficient ( α ) In order to measure the increase in current with the applied voltages, the Townsend introduces a quantity α which is known as “Townsend first ionization coefficient”. It is defined as “ The no of electrons produced by an electron per unit length in the direction of field ”. The electron produced can be shown as:

11. Gaseous Breakdowns d n =electron produced e ad = electron avalanche The probability of ionization ( α /p) is the function of (E/P) depending upon the gas pressure and temperature. α /P = f(E/P) α = P f(E/P)

12. Gaseous Breakdowns Townsend Second Ionization Coefficient ( γ ) According to the equation (i.e. i = i o e ad ), the graph of log i (i.e. log i = i o e ad ) against gap distance length (d) should form the linear line if E is constant. According to the first experiment of Townsend, when the voltage between two electrode is increased the current increases at a more rapid time than i = i o e ad . The curves of log i against the gap length (d) at constant E, are shown below in fig:

13. Gaseous Breakdowns To analyze this departure of linearity of the current the Townsend stated the second mechanism. In second mechanism he first considered the liberation of electron in the gas dielectric by the collision of +ve ions, and later he considered the liberation of electron from the cathode by +ve ion bombardment. Another cause of departure of linearity of the current was secondary electron emission at the cathode by photo ionization. On these basis Townsend deduces the equation for the current for self sustained discharge, where the electrons are produced at the cathode by +ve ion bombardment. Let, n = no of electrons released from cathode and reaching to anode. n o = no of electrons released from cathode by UV illumination. n + = no of electrons released from cathode by +ve ion bombardment. γ = 2 nd ionization coefficient = no of electrons released per incident +ve ion

14. Gaseous Breakdowns Thus the value of γ can be calculated from above equation by measuring the values of the current in gap, field strength, gap length and the appropriate value of α

15. Gaseous Breakdowns Streamer mechanism in gases breakdown 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 Sphere gap Transformer Impulse voltage mechanism predicts time lags of the order of 1(T5S, while in actual practice breakdown was observed to occur at very short times of the order of 1(T8S. Also, while the Townsend mechanism predicts a very diffused form of discharge, in actual practice, discharges were found to be filamentary (Strands) 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 ( Plasma is one of the four fundamental states of matter, and was first described by chemist Irving Langmuir in the 1920s. It consists of a gas of ions – atoms which have some of their orbital electrons removed – and free electrons ) streamer. Consider Figure. A single electron starting at the cathode by ionization builds up an avalanche that crosses the gap.

16. Gaseous Breakdowns

17. Gaseous Breakdowns Streamer mechanism in gases breakdown (Cont…) 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. 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. Comparatively narrow luminous tracks occurring at breakdown at high pressures are called streamers. 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. The three successive stages in the development of the streamer are shown diagrammatically in Fig. 2.13 in which (a) shows the stage when avalanche has crossed the gap, (b) shows that the streamer has crossed half the gap length, and (c) shows that the gap has been bridged by a conducting channel.

18. Gaseous Breakdowns where a is Townsend's first ionization coefficient, p is the gas pressure in torr, and x is the distance to which the streamer has extended in the gap. According to Meek, the minimum breakdown voltage is obtained when Er = E and x = d in the above equation. Streamer mechanism in gases breakdown (Cont…) Meek proposed a simple quantitative criterion to estimate the electric field that transforms an avalanche into a streamer. The field Er produced by the space charge, at the radius r, is given by:

19. Gaseous Breakdowns Partial Discharge Partial discharge (PD) is a localized (limited) dielectric breakdown of a small portion of a fluid electrical insulation system under high voltage stress . The distance between two electrode is only partially bridged. Partial discharge is responsible for reducing the insulation strength . Partial discharge can occur in a gaseous, Liquid or solid insulating medium . PD usually begins within voids, cracks, or inclusion within a solid dielectric , at conductor-dielectric interfaces within solid or liquid dielectrics, or in bubbles within liquid dielectrics. PD can also occur along the boundary between different insulating materials or hybrid(combination of two types of di-electric materials, it means solid + liquid) type of di-electric material.

20. Gaseous Breakdowns Partial Discharge (Continue….) The basic definition according to IEC6020 (International electro-technical commission), the partial discharge can be define as follows: “ In localized electrical discharge that only partially bridges the insulation between conductor and which can or can not occur adjust in to the conductor.” OR “It is the partial breakdown into the insulation between two active conductive is known as partial discharges.” OR “An electrical discharge that only partially bridges the dielectric or insulating medium between two conductors.” Examples are: internal discharges, surface discharges and corona discharges.

21. Gaseous Breakdowns Partial Discharge (Continue….) Actually partial discharge generally appears as a pulse with a typical dielectric material which is less than the duration of 1 micro second. Usually the partial discharge can be measure in terms of mili volts or Pico coulombs. Condition of Partial Discharge: Partial discharge can happen at any point in the insulation system, where the electric field strength exceeds the breakdown strength of that portion of the insulating material. Examples are: internal discharges, surface discharges and corona discharges. Internal discharges are discharges in cavities or voids which lie inside the volume of the dielectric or at the edges of conducting inclusions in a solid or liquid insulating media

22. Gaseous Breakdowns Partial Discharge (Continue….) Surface discharges are discharges from the conductor into a gas or a liquid medium and form on the surface of the solid insulation not covered by the conductor. Partial discharge which in course of time reduce the strength of insulation leading to a total or partial failure or breakdown of insulation.

23. Gaseous Breakdowns Mathematical Modeling of Partial Discharge The upper capacitor of the divider represents parallel combination of the capacitance in series with the voids, similarly the lower capacitor is represents the capacitance of void. All together it is representing the capacity voltage divider. The parallel capacitor represents the remaining unvoid area

24. Gaseous Breakdowns Classification of Partial Discharge

25. Gaseous Breakdowns Reason For Partial Discharge

26. Liquid Breakdowns Liquid Breakdown Liquids are used in high voltage equipment to serve the dual purpose of insulation and heat conduction. They have the advantage that a puncture path is self-healing (Automatic recovered). Temporary failures due to overvoltages are reinsulated quickly by liquid flow to the attacked area. However, the products of the discharges may deposit on solid insulation supports and may lead to surface breakdown over these solid supports.

27. Liquid Breakdowns Highly purified liquids have dielectric strengths as high as 1MV/cm. Under actual service conditions, the breakdown strength reduces considerably due to the presence of impurities. The breakdown mechanism in the case of very pure liquids is the same as the gas breakdown, but in commercial liquids, the breakdown mechanisms are significantly altered by the presence of the solid impurities and dissolved gases. Petroleum oils are the commonest insulating liquids. However, askarels, fluorocarbons, silicones, and organic esters including castor oil are used in significant quantities. A number of considerations enter into the selection of any dielectric liquid.

28. Liquid Breakdowns The important electrical properties of the liquid include the dielectric strength, conductivity, flashpoint, gas content, viscosity, dielectric constant, dissipation factor, stability, etc. Because of their low dissipation factor and other excellent characteristics, polybutanes are being increasingly used in the electrical industry. Askarels and silicones are particularly useful in transformers and capacitors and can be used at temperatures of 200 o c and higher. Castor oil is a good dielectric for high voltage energy storage capacitors because of its high corona resistance, high dielectric constant, non toxicity, and high flash point. In practical applications liquids are normally used at voltage stresses of about 50-60kV/cm when the equipment is continuously operated. On the other hand, in applications like high voltage bushings, where the liquid only fills up the voids in the solid dielectric, it can be used at stresses as high as 100-200kV/cm.

29. Liquid Breakdowns Breakdown of commercial liquids When a difference of potential is applied to a pair of electrodes immersed in an insulating liquid, a small conduction current is first observed. If the voltage is raised continuously, at a critical voltage a spark passes between the electrodes. The passage of a spark through a liquid involves the following. Flow of a relatively large quantity of electricity, determined by the characteristics of the circuit, A bright luminous path from electrode to electrode, The evolution of bubbles of gas and the formation of solid products of decomposition (if the liquid is of requisite chemical nature). Formation of small pits(cavity or hole) on the electrodes, An impulsive pressure through the liquid with an accompanying explosive sound.

30. Liquid Breakdowns Breakdown of commercial liquids Tests on highly purified transformer oil show that Breakdown strength has a small but definite dependence on electrode material, breakdown strength decreases with increase in electrode spacing, breakdown strength is independent of hydrostatic pressure for degassed oil, but increases with pressure if oil contains gases like nitrogen or oxygen in solution. In the case of commercial insulating liquid, which may not be subjected to very detail purifying treatment, the breakdown strength will depend more upon the nature of impurities it contains than upon the nature of the liquid itself.

31. Liquid Breakdowns Breakdown due to liquid globules If an insulating liquid contains in suspension a globule of another liquid, then breakdown can result from instability of the globule in the electric field. Consider a spherical globule of liquid of permittivity immersed in a liquid dielectric of permittivity When it is subjected to an electric field between parallel electrodes, the field inside the globule would be given by where E0 is the field in the liquid in the absence of the globule. The electrostatic forces cause the globule to elongate and take the shape of a prolate (elliptic) spheroid (i.e. an elongated spheroid). As the field is increased, the globule elongates so that the ratio ᵧ of the longer to the shorter diameter of the spheroid increases. For the same field E, the ratio ᵧ is a function of

32. Liquid Breakdowns Breakdown due to liquid globules When (generally when ), and the field exceeds a critical value, no stable shape exists, and the globule keeps on elongating eventually causing bridging of the electrodes, and breakdown of the gap.When the critical field at which the globule becomes unstable no longer depends on the ratio, and is given by E crit .

33. Liquid Breakdowns Theory of pseudo bubbles in liquid Insulation In liquid dielectrics, "pseudo bubbles" typically refer to regions within the liquid where gas or vapor is present. These are not true bubbles in the sense of a distinct gas-liquid interface; instead, they represent areas where gas is dissolved or entrained within the liquid dielectric material . Liquid dielectrics are used in various electrical devices, such as transformers and capacitors, to provide electrical insulation. The presence of gas within the liquid dielectric can affect its dielectric strength and overall performance. Pseudo bubbles can be formed due to various reasons, including the presence of dissolved gases, decomposition of the liquid dielectric, or the release of gas during electrical stress.

34. Liquid Breakdowns Theory of pseudo bubbles in liquid Insulation (cont…) The understanding of pseudo bubbles in liquid dielectrics is crucial because the presence of gas can impact the breakdown voltage and the insulation performance of the system. Researchers and engineers study factors such as gas content, distribution, and the dynamics of gas within the liquid dielectric to optimize the design and operation of electrical equipment . Mitigating (reduce) the effects of pseudo bubbles may involve the purification of the liquid dielectric, degassing procedures, and careful design considerations to minimize the formation and impact of gas in the system.

35. Liquid Breakdowns Factors effecting breakdown in liquids 1. Temperature 2. Humidity 3. Pressure 4. Solid impurities 5. Gas bubbles 6. Electrode geometry 7. Voltage type, application duration 8. Frequency

36. Solid Breakdowns Solid Breakdown Solid dielectric materials are used in all kinds of electrical circuits and devices to insulate one current carrying part from another when they operate at different voltages. A good dielectric should have low dielectric loss, high mechanical strength, should be free from gaseous inclusions, and moisture, and be resistant to thermal and chemical deterioration. Solid dielectrics have higher breakdown strength compared to liquids and gases. Solid insulating materials, which are generally used in practice, are of two types, namely organic materials, such as paper, wood, rubber, and the inorganic material, such as mica, glass and porcelain, and synthetic polymers, such as PVC, epoxy resins.

37. Solid Breakdowns Solid Breakdown Studies of the breakdown of solid dielectrics are of extreme importance in insulation studies. When breakdown occurs, solids get permanently damaged while gases fully and liquids partly recover their dielectric strength after the applied electric field is removed. The mechanism of breakdown is a complex phenomena in the case of solids, and varies depending on the time of application of voltage as shown in Figure.

38. Solid Breakdowns

39. Solid Breakdowns Solid Breakdown The various breakdown mechanisms can be classified as follows: Intrinsic or ionic breakdown, Electromechanical breakdown, failure due to treeing and tracking, Thermal breakdown, Electrochemical breakdown

40. Solid Breakdowns Intrinsic or ionic breakdown When voltages are applied only for short durations of the order of 10 -8 S the dielectric strength of a solid dielectric increases very rapidly to an upper limit called the intrinsic (inherent) electric strength. Experimentally, this highest dielectric strength can be obtained only under the best experimental conditions when all extraneous (irrelevant) influences have been isolated and the value depends only on the structure of the material and the temperature.

41. Solid Breakdowns Intrinsic or ionic breakdown (Cont…) The maximum electrical strength recorded is 15 MV/cm for polyvinyl-alcohol. The maximum strength usually obtainable ranges from 5 MV/cm to10MV/cm. Intrinsic breakdown depends upon the presence of free electrons which are capable of migration through the lattice of the dielectric. Usually, a small number of conduction elections are present in solid dielectrics, along with some structural imperfections and small amounts of impurities.

42. Solid Breakdowns Intrinsic or ionic breakdown (Cont…) The impurity atoms, or molecules or both act as traps for the conduction electrons up to certain ranges of electric fields and temperatures. When these ranges are exceeded, additional electrons in addition to trapped electrons are released, and these electrons participate in the conduction process. Based on this principle, two types of intrinsic breakdown mechanisms have been proposed. Electronic Breakdown. Avalanche or Streamer Breakdown.

43. Solid Breakdowns Electronic Breakdown Intrinsic breakdown occurs in time of the order of 10 -8 s and therefore is assumed to be electronic in nature. The initial density of conduction (free)electrons is also assumed to be large, and electron-electron collisions occur. When an electric field is applied, electrons gain energy from the electric field and cross the forbidden energy gap from the valence to the conduction band. When this process is repeated, more and more electrons become available in the conduction band, eventually leading to breakdown.

44. Solid Breakdowns Avalanche or Streamer Breakdown This is similar to breakdown in gases due to cumulative ionization. Conduction electrons gain sufficient energy above a certain critical electric field and cause liberation of electrons from the lattice atoms by collisions. Under uniform field conditions, if the electrodes are embedded in the specimen, breakdown will occur when an electron avalanche bridges the electrode gap. An electron within the dielectric, starting from the cathode will drift towards the anode and during this motion gains energy from the field and loses it during collisions. When the energy gained by an electron exceeds the lattice ionization potential, an additional electron will be liberated due to collision of the first electron.

45. Solid Breakdowns Avalanche or Streamer Breakdown (Cont..) This process repeats itself resulting in the formation of an electron avalanche. Breakdown will occur, when the avalanche exceeds a certain critical size. In practice, breakdown does not occur by the formation of a single avalanche itself, but occurs as a result of many avalanches formed within the dielectric and extending step by step through the entire thickness of the material as shown in Figure2. This can be readily demonstrated in a laboratory by applying an impulse voltage between point-to-plane electrodes with point embedded in a transparent solid dielectric.

46. Solid Breakdowns Avalanche or Streamer Breakdown (Cont..)

47. Solid Breakdowns Electro Mechanical breakdown When solid dielectrics are subjected to high electric fields, failure occurs due to electrostatic compressive forces which can exceed the mechanical compressive strength. If the thickness of the specimen is d and is compressed to a thickness d under an applied voltage V, then the electrically developed compressive stress in equilibrium. where Y is the Young's modulus.

48. Solid Breakdowns Thermal breakdown In general, the breakdown voltage of a solid dielectric should increase with its thickness. But this is true only up to a certain thickness above which the heat generated in the dielectric due to the flow of current determines the conduction. When an electric field is applied to a dielectric, conduction current may be flows through the material. The current heats up the specimen and the temperature rises. The heat generated is transferred to the surrounding medium by conduction through the solid dielectric and by radiation from its outer surfaces. Equilibrium is reached when the heat used to raise the temperature of the dielectric, plus the heat radiated out, equals the heat generated. The heat generated under d.c. stress E is given as

49. Solid Breakdowns Thermal breakdown (Cont…) where, σ is the conductivity of the specimen. Under A.C. fields, the heat generated

50. Solid Breakdowns Thermal breakdown (Cont…) The heat dissipated (W T ) Equilibrium is reached when the heat generated (W AC or W DC becomes equal to the heat dissipated W T ) In actual practice there is always some heat that is radiated out.

51. Solid Breakdowns Breakdown due to treeing and tracking Polymeric insulation is widely used for many engineering applications as they are tough, light in weight and possess excellent dielectric properties. Polymeric insulation can be easily fabricated in any complicated shape as required in practical use. However, their life when used in high voltage systems gets severely reduced by the degradation processes. When a solid dielectric subjected to electrical stresses for a long time fails, normally two kinds of visible markings are observed on the dielectric materials. They are:

52. Solid Breakdowns Breakdown due to treeing and tracking (Cont…) The presence of a conducting path across the surface of the insulation. A mechanism whereby leakage current passes through the conducting path finally leading to the formation of a spark. Insulation deterioration occurs as a result of these sparks. The spreading of spark channels during tracking, in the form of the branches of a tree is called treeing. Tracking is the formation of a continuous conducting paths across the surface of the insulation mainly due to surface erosion under voltage applications. While in use, the insulator progressively gets coated with moisture that causes increased conducting leading to the formation of surface tracks.

53. Solid Breakdowns Breakdown due to treeing and tracking (Cont…) Consider a system of a solid dielectric having a conducting film and two electrodes on its surface. In practice, the conducting film very often is formed due to moisture. On application of voltage, the film starts conducting, resulting in generation of heat, and the surface starts becoming dry. The conducting film becomes separate due to drying, and so sparks are drawn damaging the dielectric surface. With organic insulating materials, the dielectric carbonizes at the region of sparking, and the carbonized regions act as permanent conducting channels resulting in increased stress over the rest of the region. This is a cumulative process, and insulation failure occurs when carbonized tracks bridge the distance between the electrodes. This phenomena, called tracking is common between layers of paper and similar dielectrics built of laminates.

54. Solid Breakdowns Breakdown due to treeing and tracking (Cont…) Treeing occurs due to the erosion of material at the tips of the spark. Erosion results in the roughening of the surfaces, and hence becomes a source of dirt and contamination. This causes increased conductivity resulting either in the formation of a conducting path bridging the electrodes or in a mechanical failure of the dielectric. Under a.c. voltage conditions treeing can occur in a few minutes or several hours. Hence, care must be taken to see that no series air gaps or other weaker insulation gaps are formed. Usually, tracking occurs even at very low voltages of the order of about 100 V, whereas treeing requires high voltage. For testing of tracking, low and medium voltage tracking tests are specified. These tests are done at low voltages but for times of about 100 hr or more

55. Solid Breakdowns Breakdown due to treeing and tracking (Cont…) The insulation should not fail. Sometimes the tests are done using 5 to 10 kV with shorter durations of 4 to 6 hr. The numerical value of voltage that initiates or causes the formation of a track is called the "tracking index" and this is used to qualify the surface properties of dielectric materials. Treeing can be prevented by having clean, dry, and undamaged surfaces and a clean environment. The materials chosen should be resistant to tracking. Sometimes moisture repellant greases are used. But this needs frequent cleaning and re-greasing. Increasing creepage distances should prevent tracking, but in practice the presence of moisture films defeat the purpose.

56. Solid Breakdowns Electro Chemical Breakdown In the presence of air and other gases some dielectric materials undergo chemical changes when subjected to continuous electrical stresses. Some of the important chemical reactions that occur are the following: Oxidation Hydrolysis Chemical Action

57. Solid Breakdowns Electro Chemical Breakdown (Cont..) Oxidation In the presence of air or oxygen, materials such as rubber and polyethylene undergo oxidation giving rise to surface cracks. Hydrolysis Failure When moisture or water vapour is present on the surface of a solid dielectric, hydrolysis occurs and the materials lose their electrical and mechanical properties. Electrical properties of materials such as paper, cotton tape, and other cellulose materials deteriorate very rapidly due to hydrolysis. Plastics like polyethylene undergo changes, and their service life considerably reduces.

58. Solid Breakdowns Electro Chemical Breakdown (Cont..) Chemical Action Even in the absence of electric fields, progressive chemical degradation of insulating materials can occur due to a variety of processes such as chemical instability at high temperatures, oxidation and cracking in the presence of air and ozone, and hydrolysis due to moisture and heat. Since different insulating materials come into contact with each other in any practical apparatus, chemical reactions occur between these various materials leading to reduction in electrical and mechanical strengths resulting in failure. The effects of electrochemical and chemical deterioration could be minimized by carefully studying and examining the materials.

59. Solid Breakdowns Chemical Action (Cont..) High soda content glass insulation should be avoided in moist condition, because soda content will cause deterioration. It was observed that this type of material will lose its mechanical strength within 24 hrs, when it is exposed to atmospheres having 100% relative humidity at 70℃. In paper insulation, even if partial discharges are prevented completely, breakdown can occur due to chemical degradation. The chemical and electrochemical deterioration increases very rapidly with temperature, and hence high temperatures should be avoided.

60. SOLID DIELECTRICS USED IN PRACTICE

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63. SOLID DIELECTRICS USED IN PRACTICE

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65. SOLID DIELECTRICS USED IN PRACTICE

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68. Insulation Material

69. Electrode Material

70. Raychem Kit

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