Cl wadhwa free download

Let us assume these particles to be sphere of radisus r. In a uniform electric field which usually can be developed by a small sphere gap, the field is the strongest in the uniform field region. Therefore, the particles will be dragged into the uniform field region. Since the permittivity of the particles is higher than that of the liquid, the presence of particle in the uniform field region will cause flux concentration at its surface.

Other particles if present will be attracted towards the higher flux concentration. If the particles present are large, they become aligned due to these forces and form a bridge across the gap. The field in the liquid between the gap will increase and if it reaches critical value, brakdown will take place. If the number of particles is not sufficient to bridge the gap, the particles will give rise to local field enhancement and if the field exceeds the dielectric strength of liquid, local breakdown will occur near the particles and thus will result in the formation of gas bubbles which have much less dielectric strength and hence finally lead to the breakdown of the liquid.

The movement of the particle under the influence of electric field is oposed by the viscous force posed by the liquid and since the particles are moving into the region of high stress, diffusion must also be taken into account. It has been found that liquid with solid impurities has lower dielectric strength as compared to its pure form.

Also, it has been observed that larger the size of the particles impurity the lower the overall dielectric strength of the liquid containing the impurity. The higher the hydrostatic pressure, the higher the electric strength, which suggests that a change in phase of the liquid is involved in the breakdown process. In fact, smaller the head of liquid, the more are the chances of partially ionized gases coming out of the gap and higher the chances of breakdown.

This means a kind of vapour bubble formed is responsible for the breakdown. The following processes might lead to formation of bubbles in the liquids: i Gas pockets on the surface of electrodes. The bubble under the influence of the electric field E0 elongates keeping its volume constant.

When the field Eb equals the gaseous ioni- zation field, discharge takes place which will lead to decomposition of liquid and breakdown may follow. From the expres- sion it can be seen that the breakdown strength depends on the initial size of the bubble which of course depends upon the hydrostatic pressure above the bubble and temperature of the liquid. Since the above formation does not take into account the production of the initial bubble, the experimental values of breakdown were found to be much less than the calculated values.

Electroconvection Breakdown It has been recognized that the electroconvection plays an important role in breakdown of insulating fluids subjected to high voltages.

When a highly pure insulating liquid is subjected to high voltage, electrical conduction results from charge carriers injected into the liquid from the electrode surface. The resulting space charge gives rise to coulombic forces which under certain conditions causes hydro- dynamic instability, yielding convecting current.

It has been shown that the onset of instability is asso- ciated with a critical voltage. As the applied voltage approaches the critical voltage, the motion at first exhibits a structure of hexagonal cells and as the voltage is increased further the motion becomes turbulent.

Thus, interaction between the space charge and the electric field gives rise to forces creating an eddy motion of liquid. The charge transport will be largely by liquid motion rather than by ionic drift.

The criterion for instability is that the local flow velocity should be greater than drift velocity. Oil, besides being a good insulating medium, it allows better dispersion of heat. It allows transfer and absorption of water, air and residues created by the ageing of the solid insulation.

In order to achieve operational requirements, it must be treated to attain high degree of purity. Whatever be the nature of impurities whether solid, liquid or gaseous, these bring down the dielectric strength of oil materially.

Similarly, air dissolved in oil produces a risk of forming bubble and reduces the dielectric strength of oil. Air Absorption: The process of air absorption can be compared to a diffusing phenomenon in which a gaseous substance in this case air is in contact with liquid oil here.

If the viscosity of the liquid is low, the convection movements bring about a continuous inter- mixing whereby a uniform concentration is achieved. This phenomenon can, for example, be checked in a tank where the air content or the water content measured both at the top and the bottom are approximately equal. The oil is degassed and dried with the help of the vacuum pump 1 and then introduced into the installation until the desired pressure is reached.

A part of this air is absorbed by the oil, the pressure being maintained at a constant 2 Value by reducing the volume in absorption meter 3 Thus, air content of oil by volume can be measured. Precision manometer 4 is used to calibrate the absorption meter.

Phosphorus pentaoxide trap 5 takes in the remainder of the water vapour. In case of a completely degassed oil i. In order that water molecule takes the place of oil molecule and is dissolved in the mixture, it is necessary to provide this molecule with a quantity of energy E in the form of heat. Let N be the number of oil molecule n, the number of water molecules. Some of the methods used to remove these impurities have been described below. Filtration and Treatment Under Vacuum: Different types of filters have been used.

Filter press with soft and hard filter papers is found to be more suitable for insulating oil. Due to hygroscopic properties of the paper, oil is predried before filtering. Therefore, this oil can not be used for high voltage insulation. The subsequent process of drying is carried out in a specially, designed tank under vacuum. Through this process, both the complete drying and degassing are achieved simultaneously. By suitable selection of the various components of the plant e. The oil from a transformer or a storage tank is prefiltered 1 so as to protect the feeder pump 2.

The degassing tank is evacuated by means of vacuum pump 6 whereas the second vacuum pump 7 is either connected with the degassing tank in parallel with pump 6 or can be used for evacuating the transformer tank which is to be treated.

The operating temperature depends upon the quality and the vapour pressure of oil. In order to prevent an excessive evaporation of the aromatics, the pressure should be greater than 0. The filteration should be carried out at a suitable temperature as a higher temperature will cause certain products of the ageing process to be dissolved again in the oil. Centrifugal Method: This method is helpful in partially extracting solid impurities and free water. It is totally ineffective as far as removal of water and dissolved gases is concerned and oil treated in this manner is even over-saturated with air as air, is thoroughly mixed into it during the process.

However, if the centrifugal device is kept in a tank kept under vacuum, partial improvement can be obtained. But the slight increase in efficiency of oil achieved is out of proportion to the additional costs involved. Adsorption Columns: Here the oil is made to flow through one or several columns filled with an adsorbing agent either in the form of grains or powder. Following adsorbing agents have been used: i Fuller earth ii Silica gel iii Molecular sieves Activated Fuller earths absorb carbonyl and hydroxyl groups which from the principal ageing products of oil and small amount of humidity.

Best results of oil treatment are obtained by a combina- tion of Fuller earth and subsequent drying under vacuum. Molecular sieves are synthetically produced Zeolites which are activated by removal of the crystallisation water. Their adsorption capacity remains constant upto saturation point.

The construction of an oil drying plant using molecular sieves is, therefore, simple. The plant consists of an adsorption column containing the sieves and of an oil circulating pump. The adsorption cycle is followed by a desorption cycle once the water content of the sieves has exceeded 20 per cent. It has been found that the two processes adsorption and desorption are readily reversible. Electrostatic Filters: The oil to be treated is passed between the two electrodes placed in a container.

The electrostatic field charges the impurities and traces of water which are then attracted and retained by the foam coated electrodes. This method of drying oil is found to be economical if the water content of the oil is less than 2 ppm. It is, therefore, essential that the oil is dried before hand if the water content is large.

Also, it is desirable that the oil flow should be slow if efficient filtering is required. Therefore, for industrial application where large quantity of oil is to be filtered, large number of filters will have to be connected in parallel which may prove uneconomical. The electrodes are polished spheres of A suitable gauge is used to adjust the gap. While preparing the oil sample, the test-cell should be thoroughly cleaned and the moisture and suspended particles should be avoided.

The voltmeter is connected on to the primary side of the high voltage transformer but calibrated on the high voltage side. The voltage is increased gradually and continuously till a flash over of the gap is seen or the MCB operates. Note down this voltage. This voltage is known as rapidly-applied voltage. The breakdown of the gap has taken place mainly due to field effect.

The thermal effect is minimal as the time of application is short. See if the gap has broken. If not, increase the voltage everytime by 2. Start again with zero voltage and increase the voltage to a value just obtained in the previous step and wait for a minute. It is expected that the breakdown will take place. A few trials around this point will give us the breakdown value of the dielectric strength. The acceptable value is 30 kV for 4 mm applied for one minute. In fact these days transformer oils with 65 kV for 4 mm 1 minute value are available.

If it is less than 30 kV, the oil should be sent for reconditioning. It is to be noted that if the electrodes are immersed vertically in the oil, the dielectric strength measured may turn out to be lower than what we obtained by placing the electrodes in horizontal position which is the normal configura- tion.

It is due to the fact that when oil decomposes carbon particles being lighter rise up and if the electrodes are in vertical configuration, these will bridge the gap and the breakdown will take place at a relatively lower value. Also it provides cooling effect to the apparatus placed within the enclosure. Besides providing insulation, the oil helps the C. The gases liberated are approx.

The temperature about the arc is too high for the three last-named gases to exist and the arc itself runs into a mixture of hydrogen, carbon and copper vapour at temperature above K. The hydrogen being a diatomic gas gets dissociated into the atomic state which changes the characteristics of the arc on account of its associated change in its thermal conductivity.

The outcome of this is that the discharge suddenly contracts and acquires an appreciably higher core temperature. In certain cases, the thermal ionization may be so great that the discharge runs with a lower voltage which may stop the ionization due to the electric field strength. The transition from the field ionization to thermal ionization is most marked in hydrogen and, therefore, in oil circuit breakers.

The separation of the C. Initially when the contacts just begin to separate the magnitude of current is very large but the contact resistance being very small, a small voltage appears across them. But the distance of separation being very very small, a large voltage gradient is set up which is good enough to cause ionization of the particles between the contacts. Also it is known that with the copper contacts which are generally used for the circuit breakers very little thermal ionization can occur at temperature below the melting point.

From this it is clear that the arc is initiated by the field emission rather than the thermal ioniza- tion. This high voltage gradient exists only for a fraction of a micro-second. But in this short period, a large number of electrons would have been liberated from the cathode and these electrons while reach- ing anode, on their way would have collided with the atoms and molecules of the gases.

Thus, each emitted electron tends to create others and these in turn derive energy from the field and multiply. In short, the work done by the initially-emitted electrons enables the discharge to be maintained. Finally, if the current is high, the discharge attains the form of an arc having a temperature high enough for thermal ionization, which results in lower voltage gradient. Thus, an arc is initiated due to field effect and then maintained due to thermal ionization.

The solid insulation not only provides insulation to the live parts of the equipment from the grounded structures, it sometimes provides mechanical support to the equipment. In general, of course, a suitable combination of solid, liquid and gaseous insulations are used. The processes responsible for the breakdown of gaseous dielectrics are governed by the rapid growth of current due to emission of electrons from the cathode, ionization of the gas particles and fast development of avalanche process.

When breakdown occurs the gases regain their dielectric strength very fast, the liquids regain partially and solid dielectrics lose their strength completely. The breakdown of solid dielectrics not only depends upon the magnitude of voltage applied but also it is a function of time for which the voltage is applied.

Roughly speaking, the product of the breakdown voltage and the log of the time required for breakdown is almost a constant i. Variation of Vb with time of application The dielectric strength of solid materials is affected by many factors viz. The mechanism of breakdown in solids is again less understood.

However, as is said earlier the time of application plays an important role in break- down process, for discussion purposes, it is convenient to divide the time scale of voltage application into regions in which different mechanisms operate. The intrinsic strength, therefore, depends mainly upon the structural design of the material i. In order to obtain the intrinsic dielectric strength of a material, the samples are so prepared that there is high stress in the centre of the specimen and much low stress at the corners as shown in Fig.

The intrinsic breakdown is obtained in times of the order of 10—8 sec. The intrinsic strength is generally assumed to have been reached when electrons in the valance band gain sufficient energy from the electric field to cross the forbidden energy band to the conduction band. In pure and homogenous materials, the valence and the conduction bands are separated by a large energy gap at room temperature, no electron can jump from valance band to the conduction band.

The impurity atoms may act as traps for free electrons in energy levels that lie just below the conduction band is small. An amorphous crystal will, therefore, always have some free electrons in the conduction band. At room temperature some of the trapped electrons will be excited thermally into the conduction band as the energy gap between the trapping band and the conduction band is small.

As an electric field is applied, the electrons gain energy and due to collisions between them the energy is shared by all electrons.

In an amorphous dielectric the energy gained by electrons from the electric field is much more than they can transfer it to the lattice. Therefore, the temperature of electrons will exceed the lattice temperature and this will result into increase in the number of trapped electrons reaching the conduction band and finally leading to complete breakdown.

When an electrode embeded in a solid specimen is subjected to a uniform electric field, breakdown may occur. An electron entering the conduction band of the dielectric at the cathode will move towards the anode under the effect of the electric field. During its movement, it gains energy and on collision it loses a part of the energy. If the mean free path is long, the energy gained due to motion is more than lost during collision.

The process continues and finally may lead to formation of an electron avalanche similar to gases and will lead finally to breakdown if the avalanche exceeds a certain critical size. The possibility of instability occuring for lower average field is ignored i.

Similarly whenever a solid material has some impurities in terms of some gas pockets or liquid pockets in it the dielectric strength of the solid will be more or less equal to the strength of the weakest impurities. As a result, the gas breaks down at a relatively lower voltage. The charge concentration here in the void will make the field more non-uniform. These charge concentrations at the voids within the dielectric lead to breakdown step by step and finally lead to complete rupture of the dielectric.

Since the breakdown is not caused by a single discharge channel and assumes a tree like structure as shown in Fig. The treeing phenomenon can be readily demonstrated in a laboratory by applying an impulse voltage between point plane electrodes with the point embedded in a transparent solid dielectric such as perspex.

The treeing phenomenon can be observed in all dielectric wherever non-uniform fields prevail. Suppose we have two electrodes separated by an insulating material and the assembly is placed in an outdoor environment. Some contaminants in the form of moisture or dust particles will get deposited on the surface of the insulation and leakage current starts between the electrode through the contaminants say moisture.

The current heats the moisture and causes breaks in the moisture films. These small films then act as electrodes and sparks are drawn between the films. The sparks cause carbonization and volatilization of the insulation and lead to formation of permanent carbontracks on the surface of insulations. Therefore, tracking is the formation of a permanent conducting path usually carbon across the surface of insulation.

For tracking to occur, the insulating material must contain organic substances. For this reason, for outdoor equipment, tracking severely limits the use of insulation having organic substances. The rate of tracking can be slowed down by adding filters to the polymers which inhibit carbonization. The conductivity of the material increases with increase in termperature and a condition of instability is reached when the heat generated exceeds the heat dissipated by the material and the material breaks down.

Unstable equilibrium exists for field E2 at T2, and for field E3 the state of equilibrium is never reached and hence the specimen breaks down thermally. Cubical speciman—Heat flow In order to obtain basic equation for studying thermal breakdown, let us consider a small cube Fig. Therefore, to obtain solution of the equation, we make certain practical assumptions and we consider two extreme situations for its solution.

We obtain, therefore, an expression for what is known as impulse thermal breakdown. However, the critical field is independent of the critical temperature due to the fast rise in temperature. For this, we assume that we have a thick dielectric slab that is sub- jected to constant ambient temperature at its surface by using sufficiently large electrodes as shown in Fig.

As a result after some time, a temperature distribution will be set up within the specimen with maximum temperature Tm at its centre and it decreases as we approach the surface. In order to calculate maximum thermal voltage, let us consider a point inside the dielectric at a distance x from the central axis and let the voltage and temperature at the point are Vx and Tx, respec- tively.

We further assume that all the heat generated in the dielectric will be carried away to its sur- roundings through the electrodes. However, Vm is independent of the thickness of the insulating material but for thin specimens the thermal breakdown becomes touch- ing asymptotically to a constant value for thick specimen.

In fact, higher the frequency the lower the thermal breakdown voltage. Ceramics HV Steatite — 9. When the gas in the cavity breaks down, the surfaces of the specimen provide instantaneous anode and cathode. Some of the electrons dashing against the anode with sufficient energy shall break the chemical bonds of the insulation surface.

Similarly, positive ions bombarding against the cathode may increase the surface temperature and produce local thermal instability. Similarly, chemical degra- dation may also occur from the active discharge products e.

The net effect of all these processes is a slow erosion of the material and a consequent reduction in the thickness of the specimen. Normally, it is desired that with ageing, the dielectric strength of the specimen should not decrease. This is the main reason why high a. In fact, these days very low frequency testing is being suggested 0.

The breakdown of solid dielectric due to internal discharges or partial discharges has been elaborately explained in section 6. High insulation resistance 2. High dielectric strength 3. Good mechanical properties i. It should not be affected by chemicals around it 5. It should be non-hygroscopic because the dielectric strength of any material goes very much down with moisture content Vulcanized rubber : Rubber in its natural form is highly insulating but it absorbs moisture readily and gets oxidized into a resinous material; thereby it loses insulating properties.

When it is mixed with sulphur alongwith other carefully chosen ingredients and is subjected to a particular temperature it changes into vulcanized rubber which does not absorb moisture and has better insulating properties than even the pure rubber. It is elastic and resilient. The electrical properties expected of rubber insulation are high breakdown strength and high insulation resistance.

In fact the insulation strength of the vulcanized rubber is so good that for lower voltages the radial thickness is limited due to mechanical consideration. The physical properties expected of rubber insulation are that the cable should withstand nor- mal hazards of installation and it should give trouble-free service. Vulcanized rubber insulated cables are used for wiring of houses, buildings and factories for low-power work.

There are two main groups of synthetic rubber material : i general purpose synthetics which have rubber-like properties and ii special purpose synthetics which have better properties than the rubber e.

The four main types are: i butyl rubber, ii silicon rubber, iii neoprene, and iv styrene rubber. Butyl rubber: The processing of butyl rubber is similar to that of natural rubber but it is more difficult and its properties are comparable to those of natural rubber.

Butyl rubber compound can be so manufactured that it has low water absorption and offers interesting possibilities for a non-metallic sheathed cable suitable for direct burial in the ground. Silicone rubber: It is a mechanically weak material and needs external protection but it has high heat resistant properties. The raw materials used for the silicon rubber are sand, marsh gas, salt, coke and magnesium.

Neoprene: Neoprene is a polymerized chlorobutadiene. Chlorobutadiene is a colourless liquid which is polymerized into a solid varying from a pale yellow to a darkish brown colour. Neoprene does not have good insulating properties and is used upto V a. Styrene rubber: Styrene is used both for insulating and sheathing of cables.

It has properties almost equal to the natural rubber. Polyvinyl Chloride PVC It is a polymer derived generally from acetylene and it can be produced in different grades depending upon the polymerization process. For use in cable industry the polymer must be compounded with a plasticizer which makes it plastic over a wide range of temperature. The grade of PVC depends upon the plasticizer. PVC is inferior to vulcanized in respect of elasticity and insulation resistance. PVC material has many grades.

General purpose type: It is used both for sheathing and as an insulating material. In this com- pound monomeric plasticizers are used. It is to be noted that a V. Hard grade PVC: These are manufactured with less amount of plasticizer as compared with general purpose type.

Hard grade PVC are used for higher temperatures for short duration of time like in soldering and are better than the general purpose type. Hard grade can not be used for low continu- ous temperatures.

PVC compounds are normally costlier than the rubber compounds and the polymeric plasticized compounds are more expensive than the monomeric plasticized ones.

PVC is inert to oxygen, oils, alkalis and acids and, therefore, if the environmental conditions are such that these things are present in the atmosphere, PVC is more useful than rubber. Polythene This material can be used for high frequency cables. This has been used to a limited extent for power cables also.

The thermal dissipation properties are better than those of impregnated paper and the impulse strength compares favourably with an impregnated paper-insulated device. Cross-linked polythene: The use of polythene for cables has been limited by its low melting point. The polythene is inert to chemical reactions as it does not have double bonds and polar groups.

Therefore, it was thought that polythene could be cross-linked only through special condition, e. Many irradiation processes have been developed in the cable making industry even though large amounts of high energy radiations are required and the procedure is expensive.

Polythene can also be irradiated with ultraviolet light, after adding to it a smal quantity of ultra- violet sensitive material such as benzophenone. Under the influence of ultraviolet light on benzophenone, a radical is formed of the same type as in the decomposition of peroxide by the radical mechanism. Organic peroxides have also been used successfully to crosslink the polythene. Impregnated paper A suitable layer of the paper is lapped on the conductor depending upon the operating voltage.

It is then dried by the combined application of heat and vacuum. This is carried out in a hermetically sealed steam heated chamber.

After the device is dried, an insulating compound having the same temperature as that of the chamber is forced into the chamber. All the pores of the paper are completely filled with this compound. After impregna- tion the device is allowed to cool under the compound so that the void formation due to compound shrinkage is minimized.

In case of pre-impregnated type the papers are dried and impregnated before they are applied on the conductor. The compound used in case of impregnated paper is a semifluid and when the cables are laid on gradients the fluid tends to move from higher to lower gradient. This reduces the compound content at higher gradients and may result in void formation at higher gradients. This is very serious for cables operating at voltages higher than 3.

In many cases, the failures of the cables have been due to the void formation at the higher levels or due to the bursting of the sheath at the lower levels because of the excessive internal pressure of the head of compound. Insulating press boards. If the thickness of paper is 0. When many layers of paper are laminated with an adhesive to get desired thickness, these are known as press boards and are used in bushings, transformers as insulating barriers or supporting materials.

The electrical properties of press boards varies depending upon the resin content. The application of these press boards depends upon the thickness and density of paper used. For high frequency capacitors and cables usually low density paper 0. The electric strength of press board is higher than that of resins or porcelain. Mica consists of crystalline mineral silicates of alumina and potash.

It has high dielectric strength, low dielectric losses and good mechanical strength. Thin layers of mica are laminated with a suitable resin or varnish to make thick sheets of mica. Mica can be mixed with the required type of resin to obtain its application at different operating temperatures. Mica is used as a filler in insulating materials to im- prove their dielectric strength, reduce dielectric loss and improve heat resistance property.

Ceramics materials are produced from clay containing aluminium oxide and other inorganic materials. The specific insulation resistance of ceramics is comparatively low. The breakdown strength of porecelain compared to other insulating material is low but it remains unaffected over a wide range of temperature variation. Porcelain is chemically insert to alkalies and acids and, therefore, corrosion resistant and does not get contaminated.

Alumina Al2O3 has replaced quartz because of its better thermal conductivity, insulating property and mechanical strength. It is used for the fabrication of high current vacuum circuit breakers. Glass is a thermoplastic inorganic material consisting of silicondioxide SiO2 , which is available in nature in the form of quartz. Different types of metal oxides could be used for producing different types of glasses but for use in electrical engineering only non-alkaline glasses are suitable having alkaline content less than 0.

The dielectric constant of glass varies between 3. Glass is used for X-ray equipments, electronic valves, electric bulbs etc. Epoxy Resins. Epoxy resins are low molecular but soluble thermosetting plastics which exhibit sufficient hardening quality in their molecules. The chemical cross-linking of epoxy resins is normally carried out at room temperatures either by a catalytic mechanism or by bridging across epoxy molecule through the epoxy or hydroxyl group.

Epoxy resins have high dielectric and mechanical strength. They can be cast into desired shapes even at room temperature. They are highly elastic and it is found that when it is subjected to a pressure of psi, it returned to its original shape after the load is removed. The dielectric constant varies between 2. As filler materials, the inorganic substances like quartz powder SiO2 are used for casting applications. These are found to be more compatible to the decomposed products of SF6 by partial discharge and arcing discharges.

It is to be noted that the cast or encapsulation should not contain voids or humidity especially in high voltage applications and the material is desired to be homogeneous. It is, therefore, desirable to dry and degas the individual components of the mixture and casting is preferably carried out in vacuum.

The epoxy resins casts are inert to ether, alcohol and benzol. It is for this reason that they are not found suitable for applications in filled transformers. There are certain application which require insulating materials to operate between a high range of temperature e.

Some of the applications are space shuttle solar arrays, capaci- tors, transformers high speed locomotive, microprocessor chip carriers, cryogenic cables and other applications at cryogenic temperatures.

For this some thermoplastic polymer films are used which have unique combination of electrical, mechanical and physical quantities and these materials are able to retain these properties over a wide range of temperatures where other insulating materials may fail.

These films are used under extreme conditions of temperature and environment. These films are used for insulation on high temperature wires, cables, motor coils phase and ground insulation and for capacitors. This is also used as a substrate for flexible printed circuits and flexible cables.

Another insulating film in which has the best thermal properties in this category of insulating materials is polyimide film under the trade name of Kapton manufactured by DuPont of America. It has high dielectric and tensile strength. The disadvantages of the film are i high moisture absorption rate and ii it is affected by alkalies and strong inorganic acids.

Kepton films can be used capacitors, transformers formed coil insulation, motor state insulation and flexible printed circuits. The film is selectively costlier and is mainly used where its unique charac- teristics makes it the only suitable insulation. The use of this insulation for motors reduces the overall dimensions of the motors for the same ratings. It is, therefore, used in almost all situations whose space is a serious problem and the other nature insulation result in bigger dimension.

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