High Voltage Engineering. Fundamentals. Second edition. E. Kuffel. Dean Emeritus,. University of Manitoba,. Winnipeg, Canada. W.S. Zaengl. Professor. Dharm N-high/nvrehs.info5 THIS PAGE IS BLANK Preface to the Second Edition “ High Voltage Engineering” has been written for the undergraduate students in. High-voltage engineering covers the application, the useful use and proper ways of generating high voltages with examples of AC sources (50/60 Hz), DC.
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M S Naidu Department of High Voltage Engineering Indian Institute of Science Bangalore V Kamaraju Department of Electrical Engineering College of. High Voltage Engineering - Fundamentals. Pages · · in probability and statistics for students in engineering and applied sciences. No previous. In the textbook basic fundamentals of high voltage engineering are given: processes of charge particles generation, moving and recombination under influence.
There are various ways in which this energy can be supplied to release the electron. At room temperature, the conduction electrons of the metal do not have suffi- cient thermal energy to leave the surface. However, the experimentally obtained value of A is lower than what is predicted by the equation above.
The discrep- ancy is due to the surface imperfections and surface impurities of the metal. The gas present between the electrode affects the thermionic emission as the gas may be absorbed by the metal and can also damage the electrode surface due to continuous impinging of ions.
Also, the work function is observed to be lowered due to thermal expansion of crystal structure. Normally metals with low work function are used as cathode for thermionic emission. Field Emission: This can be ex- plained only through quantum mechanics at these high surface gradients, the cathode surface barrier becomes very thin and quantum tunnelling of electrons occurs which leads to field emission even at room temperature.
Electron Emission by Positive Ion and Excited Atom Bombardment Electrons may be emitted by the bombardment of positive ion on the cathode surface. This is known as secondary emission. In order to effect secondary emission, the positive ion must have energy more than twice the work function of the metal since one electron will neutralize the bombarding positive ion and the other electron will be released. The electron emission by positive ion is the principal secondary process in the Townsend spark discharge mechanism.
Townsend in his earlier investigations had observed that the current in parallel plate gap in- creased more rapidly with increase in voltage as compared to the one given by the above equation.
To explain this departure from linearity, Townsend suggested that a second mechanism must be affecting the current. He postulated that the additional current must be due to the presence of positive ions and the photons.
The positive ions will liberate electrons by collision with gas molecules and by bombard- ment against the cathode.
Similarly, the photons will also release electrons after collision with gas molecules and from the cathode after photon impact. Let us consider the phenomenon of self-sustained discharge where the electrons are released from the cathode by positive ion bombardment. In practice positive ions, photons and metastable, all the three may participate in the process of ionization. It depends upon the experimental conditions.
If the work function of the cathode surface is low, under the same experimental conditions will produce more emission.
Theoretically the current becomes infinitely large under the above mentioned condition but practically it is limited by the resistance of the external circuit and partially by the voltage drop in the arc. Using the above equations, the following three conditions are possible.
The discharge is then said to be self-sustained as the discharge will sustain itself even if the source producing I0 is removed. Raether has observed that if the charge concentration is higher than but lower than Fig.
Whenever the concentration exceeds , the avalanche current is followed by steep rise in current and breakdown of the gap takes place. Since the electrons have higher mobility, the space charge at the head of the avalanche is considered to be negative and is assumed to be concentrated within a spherical volume. It can be seen from Fig. The field between the two assumed charge centres i. It has been observed that if the charge carrier number exceeds , the field distortion becomes noticeable.
However, if the charge carrier exceeds , the space charge field becomes almost of the same magni- tude as the main field E0 and hence it may lead to initiation of a streamer. The space charge field, therefore, plays a very important role in the mechanism of electric discharge in a non-uniform gap. Townsend suggested that the electric spark discharge is due to the ionization of gas molecule by the electron impact and release of electrons from cathode due to positive ion bombardment at the cathode.
According to this theory, the formative time lag of the spark should be at best equal to the electron transit time tr. At pressures around atmospheric and above p. Study of the photographs of the avalanche development has also shown that under certain conditions, the space charge developed in an avalanche is capable of transforming the avalanche into channels of ionization known as streamers that lead to rapid development of breakdown.
The short-time lags associated with the discharge development led Raether and independently Meek and Meek and Loeb to the advancement of the theory of streamer of Kanal mechanism for spark formation, in which the secondary mechanism results from photoionization of gas molecules and is independent of the electrodes.
Raether and Meek have proposed that when the avalanche in the gap reaches a certain critical size the combined space charge field and externally applied field E0 lead to intense ionization and excitation of the gas particles in front of the avalanche head. There is recombination of electrons and positive ion resulting in generation of photons and these photons in turn generate secondary electrons by the photoionization process.
These electrons under the influence of the electric field develop into secondary avalanches as shown in Fig. Since photons travel with velocity of light, the process leads to a rapid development of conduction channel across the gap. Meek suggested that the transition from avalanche to streamer takes place when the radial field about the positive space charge in an electron avalanche attains a value of the order of the externally applied field.
He showed that the value of the radial field can be otained by using the expression. The minimum breakdown voltage is assumed to correspond to the condition when the avalanche has crossed the gap of length d and the space charge field Er approaches the externally applied field i. A close agreement between the calculated and experimentally deter- mined values is obtained when the gaps are short or long and the pressure is relatively low. This relation does not mean that the breakdown voltage is directly proportional to product pd even though it is found that for some region of the product pd the relation is linear i.
The variation over a large range is shown in Fig. H pK c Fig. The higher the voltage the smaller the slope and therefore, this line will intersect the ionization curve at two points e.
At low values of voltage V the slope of the straight line is large and, therefore, there is no intersection between the line and the curve 1. The point C on the curve indicates the lowest breakdown voltage or the minimum sparking potential. Hence, the probability of ionization is lower unless the voltage is increased.
The point pd min. However, in practice these values are obtained through measurements and values of some of the gases are given in the following Table 1. Table 1. Since the atmospheric conditions Tem- perature, pressure vary widely from time to time and from location to location, to obtain the actual breakdown voltage, the voltage obtained from the STP condition should be multiplied by the air den- sity correction factor.
The air density correction factor is given as 3. A typical example is that of mixture of Argon in Neon. A small percentage of Argon in Neon reduces substantially the dielectric strength of pure Neon. In fact, the dielectric strength is smaller than the dielectric strengths of either pure Neon or Argon. The lowering of dielectric strength is due to the fact that the lowest excited stage of neon is metastable and its excitation potential 16 ev is about 0.
The metastable atoms have a long life in neon gas, and on hitting Argon atoms there is a very high probability of ionizing them. This phenomenon is known as Penning Effect.
However, in non-uniform fields, before the spark or breakdown of the medium takes place, there are many manifestations in the form of visual and audible discharges.
These discharges are known as Corona discharges. In fact Corona is defined as a self-sustained electric discharge in which the field intensified ionization is localised only over a portion of the distance non-uniform fields between the electrodes. The phenomenon is of particular importance in high voltage engineering where most of the fields encountered are non-uniform fields unless of course some design features are involved to make the filed almost uniform.
Corona is respon- sible for power loss and interference of power lines with the communication lines as corona frequency lies between 20 Hz and 20 kHz. This also leads to deterioration of insulation by the combined action of the discharge ion bombarding the surface and the action of chemical compounds that are formed by the corona discharge.
After operation for a short time, reddish beads or tufts form along the wire, while around the surface of the wire there is a bluish white glow. If the conductors are examined through a stroboscope, so that one wire is always seen when at a given half of the wave, it is noticed that the reddish tufts or beads are formed when the conductor is negative and a smoother bluish white glow when the conductor is positive.
The a. As corona phenomenon is initiated a hissing noise is heard and ozone gas is formed which can be detected by its chracteristic colour. When the voltage applied corresponds to the critical disruptive voltage, corona phenomenon starts but it is not visible because the charged ions in the air must receive some finite energy to cause further ionization by collisions.
For a radial field, it must reach a gradient visual corona gradient gu at the surface of the conductor to cause a gradient g0, finite distance away from the surface of the conduc- tor. The distance between g0 and gv is called the energy distance.
From this it is clear that gv is not constant as g0 is, and is a function of the size of the conductor. At sufficiently high voltage, current amplification increases rapidly with voltage upto a current of about 10—7 A, after which the current becomes pulsed with repeti- tion frequency of about 1 kHz composed of small bursts.
This form of corona is known as burst corona. The average current then increases steadily with applied voltage, leading to breakdown. With point-plane gap in air when negative polarity voltage is applied to the point and the voltage exceeds the onset value, the current flows in vary regular pulses known as Trichel pulses.
The onset voltage is independent of the gap length and is numerically equal to the onset of streamers under positive voltage for the same arrangement. The pulse frequency increases with voltage and is a function of the radius of the cathode, the gap length and the pressure. A decrease in pressure decreases the frequency of the pulses. It should be noted that the breakdown voltage with negative polarity is higher than with positive polarity except at low pressure.
Therefore, under alternating power frequency voltage the breakdown of non-uniform field gap invariably takes place during the positive half cycle of the voltage wave.
As the spacing is increased, the positive characteristics display the distinct high corona beakdown upto a pressure of about 7 bars, followed by a sudden drop in breakdown strengths.
Under the negative polarity, the corona stabilised region extends to much higher pressures. From the figure, it is clear that— i For small spacings Zone—I , the field is uniform and the breakdown voltage depends mainly on the gap spacing. The corona inception voltage mainly de- pends on the sphere diameter.
Also it depends upon the availability of an electron between the gap for initiation of the avalanche. Normally the peak value of a. Also with d. Suppose Vd is the maximum value of d. The time that elapses between the application of the voltage to a gap sufficient to cause break- down, and the breakdown, is called the time lag. In the given case shown in Fig. It consists of two components. One is the that elapses during the voltage applications until a primary electron appears to initiate the discharge and is known as the statistical time lag ts and the other is the time required for the breakdown to develop once initiated and is known as the formative time lag tf.
The statistical time lag depends upon i The amount of pre-ionization present in between the gap ii Size of the gap iii The amount of over voltage Vd1 — Vd applied to the gap. The larger the gap the higher is going to be the statistical time lag. Similarly, a smaller over voltage results in higher statistical time lag.
However, the formative time lag depends mainly on the mechanism of breakdown. In cases when the secondary electrons arise entirely from electron emission at the cathode by positive ions, the transit time from anode to cathode will be the dominant factor determining the formative time.
The formative time lag increases with increase in gap length and field non-uniformity, decreases with increase in over voltage applied. The attachment of the electron with the neutral gas molecule may occur in two ways: Thus, these processes represent an effective way of removing electrons from the space which otherwise would have contributed to form electron avalanche.
This property, therefore, gives rise to very high dielectric strength for SF6. The gas not only possesses a good dielectric strength but it has the unique property of fast recombination after the source energizing the spark is removed. The dielectric strength of SF6 at normal pressure and temperature is 2—3 times that of air and at 2 atm its strength is comparable with the transformer oil.
Although SF6 is a vapour, it can be liquified at moderate pressure and stored in steel cylinders. The gases are also used in circuit breakers for arc interruption besides providing insulation between breaker contacts and from contact to the enclosure used for contacts.
The various gases used are i air ii oxygen iii hydrogen iv nitrogen v CO2 and vi electronegative gases like sulphur hexafluoride, arcton etc. The various properties required for providing insulation and arc interruption are: This assists cooling of current carrying conductors immersed in the gas and also assists the arc-extinction process. It should have a low dissociation temperature, a short thermal time constant ratio of energy contained in an arc column at any instant to the rate of energy dissipation at the same instant and should not produce conducting products such as carbon during arcing.
Of the simple gases air is the cheapest and most widely used for circuit breaking. Hydrogen has better arc extinguishing property but it has lower di- electric strength as compared with air.
Also if hydrogen is contaminated with air, it forms an explosive mixture. Nitrogen has similar properties as air, CO2 has almost the same dielectric strength as air but is a better arc extinguishing medium at moderate currents. Oxygen is a good extinguishing medium but is chemically active.
SF6 has outstanding arc-quenching properties and good dielectric strength. Of all these gases, SF6 and air are used in commercial gas blast circuit breakers. The compressed air supply system is a vital part of an air blast C.
Moisture from the air is removed by refrigeration, by drying agents or by storing at several times the working pressure and then expanding it to the working pressure for use in the C. The relative cost of storing the air reduces with increase in pressure. Air has an advantage over the electronegative gases in that air can be compressed to extremely high pressures at room temperature and then its dielectric strength even exceeds that of these gases.
The SF6 gas is toxic and its release in the form of leakage causes environmental problems. Therefore, the electrical industry has been looking for an alternative gas or a mixture of SF6 with some other gas as an insulating and arc interrupting medium. This mixture is not only finding acceptability for providing insulation e. The mixture is not only cost effective, it is less sensitive to find non-uniformities present within the equipment. Electric power industry is trying to find optimum SF6 to N2 mixture ratio for various components of the system viz.
With this ratio, the C. The future of using SF6 with N2 or He for providing insulation and arc interruption is quite bright. For transformer, the liquid dielectric is used both for providing insulation between the live parts of the transformer and the grounded parts besides carrying out the heat from the transformer to the atmosphere thus providing cooling effect.
For circuit breaker, again besides providing insulation between the live parts and the grounded parts, the liquid dielectric is used to quench the arc developed between the breaker contacts. The liquid dielectrics mostly used are petroleum oils. Other oils used are synthetic hydrocarbons and halogenated hydrocarbons and for very high temperature applications sillicone oils and fluorinated hyrocarbons are also used.
The three most important properties of liquid dielectric are i The dielectric strength ii The dielectric constant and iii The electrical conductivity. Other important properties are viscosity, ther- mal stability, specific gravity, flash point etc. The presence of even 0. Therefore, whenever these oils are used for providing electrical insulation, these should be free from moisture, products of oxidation and other contaminants.
The main consideration in the selection of a liquid dielectric is its chemical stability. The other considerations are the cost, the saving in space, susceptibility to environmental influences etc. The use of liquid dielectric has brought down the size of equipment tremendously. In fact, it is practically impossible to construct a kV transformer with air as the insulating medium. Dielectric properties of some liquids S. Relative permittivity 50 Hz 2.
Resistivity ohm-cm — — — 2. In contrast, commercial liquids used as insulating liquids are chemically impure and contain mixtures of complex organic molecules. In fact their behaviour is quite erratic. No two samples of oil taken out from the same container will behave identically. The theory of liquid insulation breakdown is less understood as of today as compared to the gas or even solids.
Many aspects of liquid breakdown have been investigated over the last decades but no general theory has been evolved so far to explain the breakdown in liquids. Investigations carried out so far, however, can be classified into two schools of thought. The first one tries to explain the break- down in liquids on a model which is an extension of gaseous breakdown, based on the avalanche ionization of the atoms caused by electon collisiron in the applied field.
This breakdown mechanism explains breakdown only of highly pure liquid and does not apply to explain the breakdown mechanism in commercially available liquids. This is the condition nearer to breakdown. However, if the figure is redrawn starting with low fields, a current-electric field characteristic as shown in Fig.
This curve has three distinct regions as discussed above. Conduction current High field Saturation Linear a b Fig. It has been suggested that the sus- pended particles are polarizable and are of higher permittivity than the liquid.
These particles experi- ence an electrical force directed towards the place of maximum stress. With uniform field electrodes the movement of particles is presumed to be initiated by surface irregularities on the electrodes, which give rise to local field gradients. The particles thus get accumulated and tend to form a bridge across the gap which leads finally to initiation of breakdown. The impurities could also be in the form of gaseous bubbles which obviously have lower dielectric strength than the liquid itself and hence on breakdown of bubble the total breakdown of liquid may be triggered.
Electronic Breakdown Once an electron is injected into the liquid, it gains energy from the electric field applied between the electrodes. It is presumed that some electrons will gain more energy due to field than they would lose during collision.
These electrons are accelerated under the electric field and would gain sufficient energy to knock out an electron and thus initiate the process of avalanche. The permittivity of these solids E1 will always be different from that of the liquid E2.
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: 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: 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 various mechanisms are: 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.
Assume that the heat absorbed by the block is very fast and heat generated due to the electric field is utilized in raising the temperature of the block and no heat is dissipated into the surroundings.
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. Here we assume that the voltage applied is the minimum voltage for indefinite time so that the thermal breakdown takes place. 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: The four main types are: 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 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. Heat resisting PVC: 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 electron space charge so produced causes a local increase of electric field ahead. The differ- ence arises because. These may be caused by lightning strikes. The velocity of negative streamers is not known. The space stem acts as a location for the onset of the negative leader.
The anode and cathode-directed streamers appear on either side. It is assumed that the streamer—leader transition occurs by heating in the same way as in the positive case.
This shows a fairly diffuse initial streamer corona. Mechanisms of air breakdown 21 1m Figure 1. Most power systems employ alternating voltages.
This provides conditions entirely analogous to those of Figure 1. This has. The duration of voltage around the peak is thus short compared with the times required for a leader to advance a significant distance. The front of 1. Negative ions are attracted to the volume in this time. The switching. Taking as an example the positive rod—plane gap.
Of these. The leader thus has a negligible role in the breakdown process. The lightning impulse approximates to the disturbance caused by a lightning strike: In both cases. This has been shown experimentally to be true with rod—plane gaps up to 8 m [ Two important properties make the rod—plane gap. It is evident. The absence of significant leader growth and consequent lack of ambiguities makes the positive lightning impulse an important test voltage.
These differences may be expected from the differences in the discharge propagation modes. This first group of streamers may not be sufficiently extensive to cause immediate breakdown. Since appreciable leader growth cannot progress. Mechanisms of air breakdown 23 streamer corona will occur at statistically variable times during the rise of voltage to the peak.
Both of these properties arise from the lack of any negative discharge growth at the plane. This follows from Equation 1. Under negative impulse voltage. For a gap with two non-uniform field regions. The breakdown voltage Vs is correspondingly reduced on account of the fact that the gradient of the leader is lower than that of streamers.
The reason is that successive streamer coronas develop during the relatively slow rise of voltage. This behaviour results in the so-called U-curve which has been established by testing over a wide range of gaps. Experiment shows that for further increase in rise time. Consider a rod—plane gap of. Consideration of Figure 1.
Since the rate of leader growth has been shown to be approximately independent of its length . It is illustrated in Figure 1. For longer times to peak. A simple example will illustrate. For shorter times to peak. In practice. It is found that the critical time to peak shows a closely linear increase with the length of gap. It follows from this argument that the time at which the minimum occurs depends also on the gap length. The time to peak impulse voltage at which the minimum occurs for a given gap is called the critical time to peak and it defines.
It follows from these facts that. The resulting positive space charge reduces the local field at the anode so that a larger stress is ultimately needed to form the leader.
This formula must be modified for other gap configurations section 1. Mechanisms of air breakdown 25 25 m 15 m 9m kV 10 m 6m 5m 4m 3m 2m 1m 0 T1. Note the trend for the minima to occur at longer times to peak as the front of the impulse increases where d is the gap length. When the radius is increased. The existence of the U-curve was first realised about [ It is of value because the ratio holds good for nearly all lengths of gap that are of practical interest.
It is evident that. It follows also from Equation 1. The former shows the U-curve minimum clearly. Similar considerations apply for gaps of other geometries that may arise in practice.
An argument is presented in the CIGRE guidelines  in which it is shown that the total voltage across the gap for maintenance of the predischarges in mid-gap prior to sparkover is increased by the insertion of a high field region around a pointed cathode. The effect is shown in Figure 1.
This has led to the concept of the gap factor k. This effect is particularly marked where there is a highly stressed region around the negative. The presence of an insulator in the gap also affects the gap factor. The following formula has been given : It would then be expected that the gap factor must influence the critical time to peak.
More detailed empirical formulae for estimation of gap factors for a number of practical configurations are also given in the same reference. Mechanisms of air breakdown 27 configuration k rod—plane d 1 conductor—plane d 1. Temperature changes occur.
At all other conditions of pressure. As density decreases. This implies that. Changes in both air density and humidity result in changes in ionisation efficiency and.
The effect of air density change is considered first. The prebreakdown processes have been described in sections 1. The most significant changes occur in avalanche development. The foregoing remarks assume that the kinetic processes of ionisation. The effects of atmospheric changes are on avalanche initiation and development and the streamer trail. These conditions are of pressure.
In the case of density variation. Pressure and temperature changes are manifest in changes of air density. These changes are expressed in terms of the relative air density RAD which is unity at This appears to be true whether arising from pressure or temperature change within the range 0.
The results. This fact has been adopted as a reference in the IEC Standard  which specifies how high voltage measurements of dielectric strength shall be adjusted to take account of density variations. This results in a humidity coefficient of less than 1 per cent per gm per cubic metre when the leader occupies a significant part of the gap.
Where significant leader growth occurs. Further work is needed for a resolution of these problems. Testing in long gaps. Mechanisms of air breakdown 29 The subject of avalanche formation as a function of density N requires solution of the continuity equations for electron flow in which experimentally determined parameters can be used.
Where relative air density is linked to temperature change. The Standard has set out an empirical procedure for adjustment of sparkover voltages which implicitly takes account of the extent of leader growth. Under slow front impulse. Humidity change also affects avalanche and streamer development. Researches have shown that the electric field required for streamer propagation increases at the rate of about 1 per cent per gram of moisture content per cubic metre of air at the standard pressure and temper- ature of This is also the rate at which the sparkover of the rod—plane gap increases under lightning impulse  where.
Much more data is available in this area. With increasing temperature. Much work has been carried out in the past on corona and breakdown at low pres- sures see. Measurements made under both positive  and negative  polarity corona studies show that the charge injected in corona tends to increase with decreasing air density. References 50 to Discussion of the influence of atmospheric ion densities on corona initiation was given in Reference The minimum voltage for corona inception is an important parameter.
Little data is available. The change with air density of corona development after inception is also to be taken into account. This in turn depends on the local density of atmospheric negative ions. The use of power systems at very high altitudes.
The reasons for this trend have not yet been elucidated. The increase in ion density with altitude resulting from cosmic ray activity is well known and must be considered as a further variable in determining corona onset. This result is true whether the coronas are produced under impulse  or direct  voltage.
As pressure is reduced at room temperature. Mechanisms of air breakdown 31 1. Possibly overriding these influences is the effect of the increase in electric field around the head of a streamer due to the relative permittivity of the material.
Insulators for outdoor installations such as overhead lines. Further measurements have shown that the ambient electric field required to sustain streamer propagation over an insulating surface is of the order of 20 per cent higher than that needed in air. Very recently. The attachment of ions to the surface is. In the streamer trail. This has been postulated to account for the fast rate of decay of current in a corona which is propagated over a surface .
The development of avalanches and streamers in close proximity to insulating surfaces is likely to be affected in several ways.
In the experiments described. Data is available on some of these effects. The result suggests that ions of both polarity settle on the surface after the passage of the dis- charge current. This appears at variance with the generally accepted reduction in the dielectric strength of an insulator surface.
Verhaart et al. In addition to air processes. Novi Comment. New York. Franklin Inst. University of Padova. This subject is beyond the scope of this chapter.: IEE Proc. Proceedings of 5th international symposium on High voltage engineering. A Phys. IEE A. Proceedings of 9th international conference on Gas discharges and their applications. Part A. CIGRE report no. Technical brochure no. Nauk SSSR. IEEE Trans. Proceedings of 8th international symposium on High voltage engineering.
IEEE Trans.. Peter Peregrinus. Part Mechanisms of air breakdown 35 Proceedings of 9th international conference on Gas discharges and their applications. Proceedings of 14th international symposium on High voltage engineering.
IEEE Electr. Proceedings of 14th international conference on Gas discharges and their applications.. Proceedings of 5th international symposium on High voltage engineering.. SF6 is chemically stable. Depending on the nature of the defect. Although its dielectric strength is three times that of air and.
Following a discussion of the various PD diagnostic techniques that have been proposed for use in GIS. Chapter 2 SF6 insulation systems and their monitoring O. Its wide use in power equipment is promoted by the fact that.
SF6 has good heat transfer characteristics and excellent arc-quenching properties. There is. This chapter reviews the basic ionisation processes which occur in SF6. The characteristics of the partial discharges corona discharges which occur under the non-uniform field conditions associated with certain types of defect are then dis- cussed. In practical applications. In later sections of the chapter. The frequency of collisions made by a charged particle of a given energy increases with number density or pressure.
Some gases have a dielectric strength sig- nificantly greater than that of SF6 Table 2. SF6 is therefore the only dielectric that is accepted as suit- able for GIS applications. In this process. It is therefore useful at this stage to consider how the net rate of electron production in SF6 depends on the gas pressure and the applied field. This parameter is known as the ionisation coefficient.
Figure 2. SF6 insulation systems and their monitoring 39 Table 2. This means that SF6 is a relatively brittle gas in that. Breakdown under these conditions is a complex process. In this situation localised PD. As any stress-raising defect in gas-insulated equipment will result in PD activity. The critical reduced field strength is therefore: For highly divergent fields as.
Above this level. Considering a swarm that has grown to contain n x electrons at position x in a gap of width d. Integration over the interval 0 to x gives the number of electrons in the avalanche tip at that stage in its growth: When these bridge the gap. The first stage of the breakdown involves the development of an avalanche of electrons.
The bipolar space charge generated by the ionisation process results in local distortion of the applied field such that ionisation activity ahead of. The breakdown voltage is then. In itself. The growth of this avalanche from a single starter at the cathode can readily be found by computing the net electron multiplication. For exam- ple. SF6 insulation systems and their monitoring 41 are designed for relatively low field divergence and it will be useful first to consider the simple case of breakdown in SF6 under uniform field conditions.
For coaxial electrode geometry inner radius r0. Note that the breakdown voltage is a function only of the product pressure x spacing. Under these conditions. In certain situations. The voltage collapse time depends on the pressure p. There are also problems with grounding and shielding. The minimum streamer inception or breakdown level will occur when the critical avalanche size is achieved at the anode.
Sparking during closure of a disconnector switch. As indicated above. It can be seen a that the breakdown voltage can be reduced to a low level and b that there is a critical protrusion size for the onset of roughness effects.
Note that the streamer forms when the primary avalanche has developed a rela- tively short distance. In order for breakdown to occur. SF6 insulation systems and their monitoring 43 Also. One reason for this is the fact that increased ionisation occurs in the vicinity of microscopic surface protrusions surface roughness. In the relatively low divergence field in a clean GIS system only a small increase above the onset voltage is necessary to initiate breakdown.
The streamer will then propagate until the combination of the space charge field and the geometric field is unable to sustain further ionisation. For this case. These studies have shown that there are two distinct types of breakdown. At a working pressure of 5 bar. The fact that breakdown can occur. Because of surface roughness effects and other electrode phenomena such as micro discharges in charged oxide layers. In a typical GIS. With a good technical surface finish.
This is discussed in the following section on non-uniform field breakdown in SF6. Such defects can result in very low breakdown levels and. When the stress is applied relatively slowly. The shape of these curves is typical of all non-uniform field gaps with slowly varying voltage applied.
The peak in the mid-pressure range is due to the effects of space charge injected by streamer activity around the point. As the voltage is raised. The breakdown usually occurs as a result of filamentary leader discharges developing around the shielding space charge. For both cases. This space charge tends to shield the point and stabilises the field there to a level close to the onset value.
With shorter rise time surges light- ning impulse or fast transients. For a positive point. If the streamer corona is large enough. The leader propagates into the gap in steps typically of a few mm until the streamer activity is too weak for further channel steps to form. For discharge initiation to occur. SF6 insulation systems and their monitoring 47 As the pressure is increased. Before the stress is applied. This critical volume is vanishingly small at the theoretical onset level and increases with voltage.
The mechanism of the stepped leader may be summarised. If the voltage is high enough. For a fast-fronted wave. As the field along the leader channel is much lower than that in the streamer filament.
These are produced by the action of cosmic rays. For negative-point conditions. The range of this second corona determines the length of the second channel step c. In addition to the rate-of-rise of voltage. This discharge has been shown to be identical to the stepped leader discharge which is found to occur in non-uniform field gaps under fast pulse conditions. During the dark period a—b which follows the initial corona. In configurations where the background field in the low field region is falling less steeply as.
For point-plane gaps. It is important therefore to determine the low probability breakdown level when carrying out surge breakdown tests in inhomogeneous fields.
It must be emphasised that. Models have been developed [8. Depending on the statistics of initiation of the primary streamer. As p1 is typically only about 0. V minimum impulse breakdown level streamer onset p1 pc gas pressure. At such high voltages. Such defects would probably be detected only under impulse-voltage test conditions at levels close to the BIL. SF6 insulation systems and their monitoring 49 streamer leader breakdown breakdown corona-stabilised AC or DC breakdown voltage.
The breakdown voltage of the GIS may therefore be predicted on the basis of the streamer criterion: As discussed earlier. Various models have been developed for calculating surge breakdown probability on the basis of the negative-ion density distribution and the evolution of the critically-stressed volume with time during the surge [ This led to the consider- ation of mechanisms such as density reduction in the wake of the moving particle. As the stress is increased.
At each contact with the elec- trode. The crossing does not necessarily lead to breakdown. This is the situation discussed in the section on non-uniform fields. The actual reason for the disparity between the fixed and free particle data may be inferred from the earlier discussion on corona-stabilised and direct leader breakdown.
If FCPs are present in a coaxial system they become charged by the applied field and. With the free particle. The particle then behaves like a needle fixed to the HV conductor. If the particle is rod shaped.
As the voltage is increased. In coaxial electrode systems. For DC stress. For AC conditions the particles initially make small hopping excursions at the outer electrode. For the fixed particle. Techniques which may offer further improvements in GIS insulation include. Gas mixtures containing buffer gases such as N2 in concentrations of up to 80 per cent have dielectric strengths which are not much below that of SF6 under clean conditions and may be less susceptible to particulate contamination.
Present indications are that. As the minimum fixed point impulse breakdown level corresponds to breakdown by a leader mechanism. It may therefore be necessary to wait for a relatively long period 20—30 s to ensure that a test particle will not trigger breakdown at a given voltage. The use of semiconducting surface coatings to prevent the build-up of surface charge may also be beneficial.
SF6 insulation systems and their monitoring 51 In practice. It is important. In the future it appears likely that further improved GIS designs will be supplemented by quite extensive diagnostic monitoring.
These considerations have led to the development of diagnostic techniques which allow the presence of defects to be recognised as a result of their partial discharge activity so that action can be taken to remove them before a failure occurs. Even if improvements are made in one or more of the above areas.
Much progress in diagnostic techniques for GIS has been made in recent years. If in addition the GIS is connecting the output of a nuclear station to the transmission network and the breakdown leads to a reactor shutdown. Other proposals for particle control have included techniques for covering FCPs with a sticky insulating coating by post-assembly polymerisation of an appropriate additive to the SF6.
During this time the associated circuit may be out of operation and the consequential losses can be high. Modern designs of GIS. The remainder of this chapter is devoted to a discussion of these techniques.
Simple slotted trays in the outer conductor make very effective particle traps and their use in the vicinity of solid spacers can offer useful protection. In the UK. With one technique. Various diagnostic techniques have been demonstrated in the laboratory. The insulation can then be monitored for signs of any incipient weakness. SF6 insulation systems and their monitoring 53 Figure 2.
The discharge data needs to be analysed by an expert system. The results of the on-site work have been very promising. Discharges in voids. In all cases. A PD is the localised breakdown of gas over a distance of usually less than a millimetre. With the exception only of the mechanical noise from a bouncing particle.
PD is also accompanied by the emission of light from excited molecules. The PD therefore has many effects — physical. In microsparks and intense coronas. Although this is a powerful laboratory tool for finding the onset of activity from a known corona point.
Other causes of failure are discharges from any stress-raising protrusion. The radiation is primarily in the UV band. For surface defects such as small protrusions. PD detection is the basis of all dielectric diagnostics in GIS.
It therefore appears that the chemical approach is too insensitive to be considered for PD monitoring. These are the two most common diagnostic gases. The signals in GIS have a broad bandwidth.
The acoustic technique is not suited to a permanently installed monitor. This in itself gives the approximate location of the defect. Because of the rather high attenuation of the signals. It reacts further. The different propagation velocities of the wave as it passes through various materials. In small-volume laboratory tests. One advantage of acoustic measurements is that they are made non-intrusively.
Other sources of discharge may be identified in a similar way from their own characteristics . This signal can be picked up by accelerometers or acous- tic emission sensors attached to the outside of the chamber. It also has other features. As a simpler but less sensitive alternative. The alumina. SF6 insulation systems and their monitoring 55 where it can be detected this assumes. The latter is the only instance of a diagnostic signal not coming from a PD although of course the particle gen- erates a PD as well.
Those originating at the cham- ber wall propagate as flexural waves. The main decomposition product of sulphur hexafluoride is sulphur tetrafluoride SF4.
The acoustic signal from a particle bouncing on the chamber floor is characterised by a signal not correlated with the power frequency cycle. The UHF technique will later be described in detail. The tests were made in a 6 m long section of kV GIS chambers into which was placed one of the following defects: After about a microsecond or so.
The PD current pulse at the defect has a duration of less than 1 ns. The resonances are indicative of PD activity. This excites the GIS chambers into various modes of electrical reso- nance. In addition.
The pulses are attenuated and undergo multiple reflections. From then. The resonant technique was developed in the UK . To obtain the maximum sensitivity of measurement. The general conclusions of this investigation were that: To illustrate the results reported in Reference During this time. The vessel was energised by a 0— kV metalclad test transformer. SF6 insulation systems and their monitoring 57 10 15 b 10 c 5 5 a 1 0 50 U.
IEC A more detailed description of UHF theory and the generation and transmission of UHF signals will be found in the next section. The coupler must not create an additional risk of breakdown. Circular couplers are themselves resonant structures at UHF frequencies. The frequency of operation is kept below the cut-off frequencies at which higher-order transverse electric TE and transverse magnetic TM modes begin to be excited. Excitation of a purely TEM mode signal would require symmetrical excitation of the waveguide.
The design of internal couplers for detection of UHF signals in GIS involves a compromise between the conflicting requirements of minimising the field enhance- ment while maximising the UHF sensitivity. A disadvantage is that the UHF fields also tend to be weaker in these regions. These reflections can cause resonances to appear. These modes are closely related to those of the hollow cylindrical waveguide.
In the case of a GIS. When coaxial lines are used for signal transmission. UHF antennas of a form which would be desirable for good sensitivity. Circular plate couplers have proved useful.
In the absence of barriers and discontinuities. For these reasons. For small defects. A system of cylindrical coordinates r. This is because the coupling coefficients to each of the waveguide modes vary across the coaxial cross-section of the GIS. To take full advantage of the UHF technique. Because the length of the streamer itself rarely exceeds 1 mm. In the following notes.
The intensity of this electric field is therefore the primary factor affecting the signal level that can be obtained from the coupler. Some of the signal is rapidly attenuated because it is below the cut-off frequency of the mode in which it is propagating.
Externally mounted couplers e. These effects cannot be analysed theoretically. Most discontinuities inside the GIS have a complicated reflection pattern. The attenuation of the UHF signal along the GIS duct between one coupler and the next is mainly due to the confining of signal energy within the chambers by the partially reflecting discontinuities. The overall effect of dispersion is to cause the signal to appear as a long..
Different frequency components of the PD pulse propagate at different velocities. Because of the boundary conditions in this region. The relative arrival times of the wavefronts at couplers on either side of the PD source can often be used to locate the defect.
The highest frequency components travel along the coaxial lines with a velocity approaching c. UHF signals from low level PD can only be detected if they are of sufficient amplitude to be distinguished from electrical background noise.
An infinite number of these higher-order modes exists. Higher-order modes are classified as transverse electric TE or transverse magnetic TM types. SF6 insulation systems and their monitoring 61 be considered as part of the coupler.
The frequency response of the coupler should be suitable for the frequency range of the UHF signal. Each of these modes has a unique cut-off frequency. These values can be determined numerically using standard mathematical software packages. In a kV GIS. Where external couplers must be used.
Internal couplers are best from this point of view. By its nature. K is an arbitrary constant defining the amplitude of the excitation. During the early stages of propagation. Variation of the radial field components defined by Equations 2. The rectangular plane represents the position of zero electric field 2.
In the case of PD. This is analogous to the representation of a non-sinusoidal signal by a Fourier series in the time domain. The power available from each signal is proportional to the square of the field strength..
A PD source such as a protrusion excites more higher-order modes when it is at the outer conductor than at the inner conductor . The contributions to the total field are shown in Figure 2. The loss of sensitivity that results from this restriction can be illustrated by comparing the higher-order and TEM mode contributions to the electric field at a coupler for two excitation pulses having equal amplitudes but different pulse widths.
This increases the pulse width at half-amplitude from 0. The resulting radial electric field at a distance of 1. The number of modes required to adequately represent the field increases as a result. As the duration of the PD pulse decreases. A Gaussian current pulse i t at the PD source was defined as: In certain circumstances.
SF6 insulation systems and their monitoring 63 other modes to produce a field that is only non-zero in the region close to the PD source. In contrast. The configuration used to make the comparison is that of a 10 mm PD path e.
The peak amplitude of the TEM mode field remains unchanged but the width of the reflected pulses has increased accordingly. This is because the spatial variation of the modes is inherently greater at the inner conductor. The amplitude is now comparable to that of the TEM mode.
The second example. The higher-order modes make a much larger contribution to the total field in response to this pulse. The couplers are normally capacitive. The PD current has a path length of 10 mm and the peak current is 2 mA. Measurements of the PD current pulses generated by small defects such as particles and protrusions in SF6 have shown typi- cal values of less than ps for the half-amplitude width. Theoretical studies have indicated that for the short PD pulses typical of small defects.
Although the amplitude of the TEM mode field is unchanged in Figure 2. Making the coupler larger to counter- act this effect would have the undesirable consequence of increasing the amount of low frequency noise coupled from the GIS. Over the shorter timescale 10— ns. The losses tend to increase with increasing frequency. This can lead to situations where a UHF coupler at a greater distance from a PD source detects a larger signal than does one closer to it.
Various research groups have attempted to assign specific attenuation values to individual components. These interactions are dependent on the distances between the discontinuities. The two-stage procedure involves first determining an artificial pulse. SF6 insulation systems and their monitoring 65 between adjacent chambers. The reason is that the overall signal level at a given position in the GIS arises from interactions between signals reflected from discontinuities.
An on-site test is then carried out. In the region close to the outer conductor where couplers are normally mounted. In order to ensure that a UHF monitoring system has sufficient sensitivity to detect a 5 pC discharge located midway between a pair of couplers. Dissipation losses skin effect become significant in the longer timescale — ns.
As a consequence of these effects. These couplers often take the form of a metal disc insulated from the GIS enclosure by a dielectric sheet. To allow for the many types of coupler and mounting arrangement that exist. This is the radial electric field at the point where the coupler is to be mounted.
Couplers are calibrated by measuring their transfer function in terms of their output voltage in response to a defined incident electric field. Couplers can be classified according to whether they are mounted internally or externally: The calibration system see Figure 2. These couplers are suitable for periodic insulation testing of GIS for which a permanently installed monitor is not economically viable or for older GIS that cannot be retrofitted with internal couplers.
The incident field is first calibrated using a monopole probe having a known frequency response. The measurement connection is made through a hermetically sealed coaxial connector that is usually connected to the centre of the disc. Note that the coupler sensitivity has units of length and is represented by an effective height h.
This particular coupler is shown. A digitiser records the signal from the coupler under test. External couplers are sometimes less sensitive than their internal counterparts because the UHF signal is attenuated by impedance discontinuities at the surfaces of the barrier and window materials.
They may also be more prone to electrical interference signals when they are not shielded as well as internal couplers. The probe is then replaced by a mounting plate suitable for holding the coupler to be tested. For a given disc size.
This also improves the bandwidth of the coupler by increasing sensitivity at lower frequencies. Both of these changes lead to a reduction in the Q factor of the disc resonances.
SF6 insulation systems and their monitoring 67 coupler test port calibrated field signal matching source splitter unit broadband TEM cell reference coupled signal signal digitising signal unit processing unit Figure 2. MHz Figure 2. This information is summarised in Table 2.
At UHF. As the radius of the disc coupler is decreased its resonant frequencies move upwards. In some cases. The monitoring system consists of the following basic parts: The racks contain the electronics to receive and handle the streamed data from the OCUs. In these cases. Some designs are described in Reference SF6 insulation systems and their monitoring 69 UHF coupler optical converter unit equipment cabinets Figure 2.
The UHF data is then transmitted via an optical fibre link back to the equipment cabinets. Optimisation should take place in the time domain. Inception is therefore. These parameters enable typical defects such as fixed point corona. The UHF data may be displayed in any way which reveals the characteristic patterns typical of the defects causing them. Other defects occur less commonly. In the three-dimensional displays.
Metallic particles are produced. This generates a discharge pulse each time contact is made with the floor. A typical PD pattern for a free metallic particle is shown in Figure 2. These provide a positive confirmation that the defect is a floating component. The initial discharges are of very low magnitude less than 1 pC. The discharges are concentrated on the leading quadrants of the positive and negative half-cycles.
SF6 insulation systems and their monitoring 71 on the negative half-cycle when the protrusion is on the busbar. The sparking is energetic because the floating component usually has a high capacitance. They propagate in steps until either they become extinguished.
At higher voltages. At the same time. Leaders are the precursors of breakdown. This difference between the positive and negative discharges reveals whether the protrusion is on the busbar or chamber wall. Streamer discharges will not lead to breakdown. Often the gap is asymmetrical. The pulses occur randomly over the complete power frequency cycle.
Two factors combine to make this an especially serious condition which often leads to breakdown: Various artificial intelligence AI techniques may be used to classify the type of defect present. All the analytical techniques need to be trained using extensive databases of PD signals. In general. As shown in Figure 2. The real need. The expert system will need to include additional information on the severity of the defect.
This can be applied to identify the single-cycle patterns recorded by the monitoring system out- lined above. Since no single technique appears adequate to recognise all types of defect. This full risk analysis is not possible at present. This technique may itself be used in conjunction with others.
A classification technique that is used with much success is the ANN. In the case of new substations. Averaging is applied to eliminate spurious results and reveal any underlying change in the PD activity. Service experience with UHF monitoring has been excellent. The historical records are used to determine the trends in the activity of a PD source. In many cases. Several utilities have reported the identification of defects that would have resulted in major failures.
PDM systems are often specified for key installations. The availability of suitable external couplers which allow a PDM system to be retrofitted without the need for an outage is an important factor in this situation.
South America and the USA.