What is called a dielectric. Basic electrical properties of dielectrics
The dielectric constant can be dispersive.
A number of dielectrics exhibit interesting physical properties.
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- Virtual Fund of Natural Science and Scientific and Technical Effects "Effective Physics"
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See what "Dielectrics" are in other dictionaries:
DIELECTRICS, substances that poorly conduct electric current (resistivity of the order of 1010 Ohm? M). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of the dielectric. In some solid ... ... Modern encyclopedia
Dielectrics - DIELECTRICS, substances that poorly conduct electric current (resistivity of the order of 1010 Ohm · m). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of the dielectric. In some solid ... ... Illustrated Encyclopedic Dictionary
Substances, poorly conducting electric current (specific electrical resistance 108 1012 Ohm cm). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of dielectrics. In some solid dielectrics ... ... Big encyclopedic Dictionary
- (English dielectric, from the Greek dia through, through and English electric electric), substances that poorly conduct electricity. current. The term "D." introduced by Faraday to designate in in, in which electric penetrates. field. D. yavl. all gases (non-ionized), some ... Physical encyclopedia
DIELECTRICS - DIELECTRICS, non-conductors, or body insulators, poorly or completely non-conductive. Such bodies are, for example. glass, mica, sulfur, paraffin, ebonite, porcelain, etc. For a long time in the study of electricity ... ... Big medical encyclopedia
- (insulators) non-conductive substances. Examples of dielectrics: mica, amber, rubber, sulfur, glass, porcelain, various types of oils, etc. Samoilov KI Marine dictionary. M. L .: State Naval Publishing House of the Union NKVMF ... Marine Dictionary
The name given by Michael Faraday to non-conductive bodies, otherwise, badly conductive electricity, such as air, glass, various resins, sulfur, etc. Such bodies are also called insulators. Before Faraday's research in the 30s ... ... Encyclopedia of Brockhaus and Efron
DIELECTRICS - substances that practically do not conduct electric current; are solid, liquid and gaseous. In an external electric field, D. are polarized. They are used to isolate electrical devices, in electrical capacitors, in quantum ... ... Big Polytechnic Encyclopedia
Substances that do not conduct electricity well. The term "D." (from the Greek diá through and English electric electric) was introduced by M. Faraday (See Faraday) to denote substances through which electric fields penetrate. In any substance, ... ... Great Soviet Encyclopedia
Substances that poorly conduct electric current (dielectric conductivity 10 8 10 17 Ohm 1 · cm 1). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of dielectrics. In some solid ... ... encyclopedic Dictionary
Books
- Dielectrics and Waves, A.R. Hippel. The author of the monograph offered to the readers of the monograph, a well-known researcher in the field of dielectrics, American scientist A. Hippel has repeatedly appeared in periodicals and in ...
- Effect of laser radiation on polymer materials. Scientific foundations and applied problems. In 2 books. Book 1. Polymer materials. Scientific bases of laser action on polymer dielectrics, BA Vinogradov, KE Perepelkin, GP Meshcheryakova. The proposed book contains information about the structure and basic thermal and optical properties of polymeric materials, the mechanism of exposure to laser radiation in the infrared, visible ...
Dielectric materials in electronic equipment are separated electrically, and solid materials are separated mechanically by conductors under different electric potentials. They are used for electrical insulation of equipment elements, for accumulating electric field energy (capacitors), for manufacturing structural parts, as well as in the form of coatings on the surface of parts, for gluing parts.
Dielectric properties of materials
The main property of a dielectric is not to conduct electric current. The SPECIFIC VOLUME RESISTANCE of dielectrics is large: from 108 to 1018 Ohm, since there are almost no free carriers of electric charge in them. Some conductivity is caused by impurities and structural defects.
There are always more impurities and defects on the surface of any body, therefore, for dielectrics, the concept of surface conductivity and the parameter SPECIFIC SURFACE RESISTANCE s are introduced, defined as the resistance measured between two linear conductors 1 m long each, located parallel to each other at a distance of 1 m on the dielectric surface ... The value of s strongly depends on the method of obtaining (processing) the surface and its condition (dustiness, moisture, etc.). Since the surface electrical conductivity usually significantly exceeds the bulk conductivity, measures are taken to reduce it.
The dielectric is an insulator only in relation to direct voltage. In an alternating electric field, a current flows through the dielectric due to its polarization.
POLARIZATION is the process of displacement of bound charges over a limited distance under the action of an external electric field.
The electrons of the atoms are displaced towards the positive pole, the nuclei of the atoms - towards the negative. The same happens with ions in ionic crystals, with molecules or parts of molecules with an uneven distribution of charged particles in the volume they occupy. As a result of polarization, an intrinsic internal field is formed in the dielectric, its vector is smaller in magnitude and opposite in direction to the vector of the external field. The electrical capacitance between electrodes with a dielectric is greater than between the same electrodes without a dielectric by a factor of, where is the RELATIVE DIELECTRIC PERMEABILITY of the DIELECTRIC.
During ELECTRONIC POLARIZATION, the electron shells of atoms of a substance are deformed under the action of an external electric field. It is characterized by a short (about 10-15 s) settling time and therefore it is inertialess for radio frequencies, does not depend on frequency, weakly depends on temperature and occurs practically without losses. Substances with predominantly electronic polarization (weakly polar dielectrics) have a low dielectric constant: from 1.8 to 2.5. This type of polarization is inherent in all substances.
IONIC POLARIZATION occurs in ionic solids, has a settling time of the order of 10-13 s, therefore, practically does not depend on the field frequency, weakly depends on temperature. The magnitude for most ionic polarized materials is between 5 and 10.
DIPOLE (ORIENTATION) POLARIZATION is manifested as orientation under the action of the field of polar molecules or groups of atoms. Polar, for example, are water molecules, in which the hydrogen atoms are located asymmetrically relative to the oxygen atom, or vinyl chloride (polyvinyl chloride monomer) H2C-CHCl. To overcome the interaction of molecules and friction forces, the field energy is spent, which is converted into thermal energy, therefore, the dipole polarization is inelastic, relaxation in nature. Due to the large sizes and masses of the dipoles participating in the dipole polarization, its inertia is significant and manifests itself in the form of a strong dependence of the dielectric constant and energy losses on frequency.
MIGRATION POLARIZATION is caused by inelastic movements of weakly bound impurity ions over short distances. In terms of consequences (energy loss, frequency dependence), this polarization is similar to dipole.
The energy loss in the dielectric during polarization is estimated by the TANGENS OF THE LOSS ANGLE tg. A dielectric with losses in an electrical circuit is represented in the form of an equivalent circuit: an ideal capacitor and a loss resistance connected in parallel to it. The angle complements up to 90o the angle of shift between current and voltage on the vector diagram of such a two-port network. Good (weakly polar) dielectrics have tg10-3, which is not very frequency dependent. Bad dielectrics have tg, measured in tenths of a unit or even more, strongly dependent on frequency.
Special types are formed by polarization under the action of mechanical stresses observed in PIEZOELECTRICS, as well as SPONTANEOUS POLARIZATION in PYROELECTRICS and FERROELECTRICS. Such dielectrics are called ACTIVE and are used in special devices: in resonators, filters, piezoelectric generators and transformers, radiation converters, capacitors of high specific capacity, etc.
ELECTRIC STRENGTH - the ability of a dielectric to maintain a high resistivity in high-tension circuits. It is estimated by the breakdown field strength Ep \u003d Upr / d, where Upr is the voltage causing the breakdown, d is the dielectric thickness. Dimension Епр - В / м. For different dielectrics Epr \u003d 10 ... 1000 MV / m, and even for one material, this value fluctuates widely depending on the thickness, shape of the electrodes, temperature and a number of other factors. The reason for this is the variety of breakdown processes. ELECTRIC BREAKDOWN is caused by the tunneling transition of electrons to the conduction band from the valence band, from impurity levels or metal electrodes, as well as their avalanche multiplication due to impact ionization in high-intensity fields. ELECTROTHERMAL BREAKDOWN is caused by an exponential increase in the electrical conductivity of a dielectric with an increase in its temperature. At the same time, the leakage current increases, which even more warms up the dielectric, a conducting channel is formed in its thickness, the resistance drops sharply, melting, evaporation, and destruction of the material occur in the thermal effect zone. ELECTROCHEMICAL BREAKDOWN is caused by the phenomena of electrolysis, ion migration and, as a result, changes in the composition of the material. IONIZATION BREAKDOWN occurs due to partial discharges in a dielectric with air inclusions. The electric strength of air is lower, and the field strength in these inclusions is higher than in a dense dielectric. This type of breakdown is typical for porous materials. SURFACE BREAKDOWN (OVERLAPPING) of a dielectric occurs due to impermissibly high surface currents. With sufficient power of the current source, surface breakdown develops through the air and turns into an arc breakdown. Conditions contributing to this breakdown: cracks, other irregularities and contamination on the surface of the dielectric, humidity, dustiness, low atmospheric pressure.
For the reliable operation of any electrical device, the operating voltage of its insulation Uwork must be significantly less than the breakdown voltage Upr. The ratio Upr / Urab is called the RESERVE FACTOR OF THE ELECTRIC STRENGTH OF THE INSULATION.
Lecture 1.3.1. Polarization of dielectrics
Dielectric materials
Dielectrics are substances that can polarize and maintain an electrostatic field. This is a wide class of electrical materials: gaseous, liquid and solid, natural and intellectual, organic, inorganic and organoelement. According to their functions, they are divided into passive and active. Passive dielectrics are used as electrical insulating materials. In active dielectrics (ferroelectrics, piezoelectrics, etc.), electrical properties depend on control signals that can change the characteristics of electrical devices and devices.
According to the electrical structure of molecules, non-polar and polar dielectrics are distinguished. Non-polar dielectrics consist of non-polar (symmetric) molecules in which the centers of positive and negative charges coincide. Polar dielectrics are composed of asymmetric molecules (dipoles). A dipole molecule is characterized by a dipole moment - p.
During the operation of electrical devices, the dielectric heats up, since part of the electrical energy in it is dissipated in the form of heat. Dielectric losses are highly dependent on the frequency of the current, especially in polar dielectrics, so they are low frequency. Non-polar dielectrics are used as high-frequency ones.
The main electrical properties of dielectrics and their characteristics are given in table. 3.
Table 3 - Electrical properties of dielectrics and their characteristics
Polarization is the limited displacement of bound charges or the orientation of dipole molecules in an electric field. Under the influence of the lines of force of the electric field, the charges of the dielectric are displaced in the direction of the acting forces, depending on the magnitude of the intensity. In the absence of an electric field, the charges return to their previous state.
There are two types of polarization: instantaneous polarization, quite elastic, without release of scattering energy, i.e. without heat release, during 10 -15 - 10 -13 s; polarization does not occur instantly, but increases or decreases slowly and is accompanied by energy dissipation in the dielectric, i.e. it heats up - this is a relaxation polarization for a time from 10 -8 to 10 2 s.
The first type includes electronic and ionic polarization.
Electronic polarization (C e, Q e) - elastic displacement and deformation of the electron shells of atoms and ions during 10 -15 s. Such polarization is observed for all types of dielectrics and is not associated with energy loss, and the dielectric constant of a substance is numerically equal to the square of the refractive index of light n 2.
Ionic polarization (C u, Q u) is characteristic of solids with an ionic structure and is caused by the displacement (vibration) of elastically bound ions in the nodes of the crystal lattice during 10 -13 s. As the temperature rises, the displacement also increases as a result of the weakening of the elastic forces between the ions, and the temperature coefficient of the dielectric constant of ionic dielectrics turns out to be positive.
All relaxation polarizations are referred to the second type.
Dipole relaxation polarization (C dr, r dr, Q dr) associated with the thermal motion of the dipoles with a polar bond between molecules. The rotation of the dipoles in the direction of the electric field requires overcoming some resistance, the release of energy in the form of heat (r etc). The relaxation time here is of the order of 10 -8 - 10 -6 s - this is the time interval during which the ordering of the dipoles oriented by the electric field after the removal of the field will decrease due to the presence of thermal motions by 2.7 times of the initial value.
Ion relaxation polarization (C ir, r ir, Q ir) observed in inorganic glasses and in some substances with loose ion packing. Weakly bound ions of a substance under the influence of an external electric field among chaotic thermal motions receive excessive surges in the direction of the field and shift along power line his. After removing the electric field, the orientation of the ions weakens exponentially. Relaxation time, activation energy and frequency natural vibrations occurs within 10 -6 - 10 -4 s and is related by law
where f is the frequency of natural vibrations of particles; v is the activation energy; k is the Boltzmann constant (8.63 10 -5 EV / deg); T - absolute temperature according to K 0.
Electronic relaxation polarization (C er, r er, Q er)arises due to the excited thermal energies of excess, defective electrons or "holes" in a time of 10 -8 - 10 -6 s. It is characteristic of dielectrics with high refractive indices, high internal field and electronic conductivity: titanium dioxide with impurities, Ca + 2, Ba + 2, a number of compounds based on metal oxides of variable valence - titanium, niobium, bismuth. With this polarization, there is a high dielectric constant and at negative temperatures there is a maximum in the temperature dependence of e (dielectric constant). e for titanium-containing ceramics decreases with increasing frequency.
Structural polarizations distinguish:
Migration polarization (C m, r m, Q m)occurs in solids of an inhomogeneous structure with macroscopic inhomogeneities, layers, interfaces, or the presence of impurities in a time of the order of 10 2 s. This polarization manifests itself at low frequencies and is associated with significant energy dissipation. The reasons for this polarization are conductive and semiconducting inclusions in technical, complex dielectrics, the presence of layers with different conductivity, etc. At the interfaces between the layers in the dielectric and in the electrode layers, the charges of slowly moving ions are accumulated - this is the effect of interlayer or structural high-voltage polarization. For ferroelectrics, there are spontaneous or spontaneous polarization, (C cn, r cn, Q cn), when there is a significant dissipation of energy or the release of heat due to domains (separate regions of rotating electron shells) displaced in an electric field, i.e., even in the absence of an electric field, there are electric moments in the substance, and at a certain strength of the external field saturation occurs and increasing polarization.
Classification of dielectrics by the type of polarization.
The first group consists of dielectrics with instantaneous electronic and ionic polarizations. The structure of such materials consists of neutral molecules, can be weakly polar and is characteristic of solid crystalline and amorphous materials such as paraffin, sulfur, polystyrene, as well as liquid and gaseous materials such as benzene, hydrogen, etc.
The second group - dielectrics with electronic and dipole relaxation polarizations - are polar organic liquid, semi-liquid, solid substances such as oil rosin compounds, epoxy resins, cellulose, chlorinated hydrocarbons, etc. materials.
The third group - solid inorganic dielectrics, which are divided into two subgroups, differing in electrical characteristics - a) dielectrics with electronic and dipole relaxation polarizations, such as quartz, mica, rock salt, corundum, rutile; b) dielectrics with electronic and ionic relaxation polarizations are glasses, materials with a glassy phase (porcelain, micalex, etc.) and crystalline dielectrics with loose ion packing.
The fourth group consists of dielectrics with electronic and ionic instantaneous and structural polarizations, which is typical of many positional, complex, layered and ferroelectric materials.
Classification by molecular structure
Chemical classification
Classification by production method
Physical state classification
Active and passive dielectrics
Determination of dielectric materials
Classification and fields of use of dielectric materials
Dielectrics are substances whose main electrical property is the ability to polarize in an electric field.
Insulating materials are dielectric materials designed to create electrical insulation of live parts of electrical installations.
An insulator is a product made of electrical insulating material, the tasks of which are to fasten and isolate conductors from each other at different potentials (for example, insulators of an overhead power line).
Electrical insulation refers to the electrical insulation system of a specific electrical product made of one or more electrical insulating materials.
Dielectrics used as insulating materials are called passive dielectrics. Currently, the so-called active dielectrics are widely used, the parameters of which can be controlled by changing the electric field strength, temperature, mechanical stresses and other parameters of the factors affecting them.
For example, a capacitor, in which a piezoelectric serves as a dielectric material, changes its linear dimensions under the action of an applied alternating voltage and becomes a generator of ultrasonic vibrations. The capacity of an electric capacitor made of a nonlinear dielectric - ferroelectric varies depending on the strength of the electric field; if such a capacitance is included in an oscillatory LC circuit, then its tuning frequency also changes.
Dielectric materials are classified:
By state of aggregation: gaseous, liquid and solid;
By the method of obtaining: natural and synthetic;
By chemical composition: organic and inorganic;
Molecular structure: neutral and polar.
GASEOUS DIELECTRICS
Gaseous dielectrics include: air, nitrogen, hydrogen, carbon dioxide, SF6 gas, freon (freon), argon, neon, helium, etc. They are used in the manufacture of electrical devices (air and SF6 switches, arresters)
The most widely used electrical insulating material is air. Air contains: water vapor and gases: nitrogen (78%), oxygen (20.99%), carbon dioxide (0.03%), hydrogen (0.01%), argon (0.9325%), neon (0 , 0018%), as well as helium, krypton, and xenon, which in total amount to ten thousandths of a percent.
Important properties of gases are their ability to restore electrical strength, low dielectric constant, high resistivity, virtually no aging, inertness of a number of gases in relation to solid and liquid materials, non-toxicity, their ability to work at low temperatures and high pressure, incombustibility.
LIQUID DIELECTRICS
Liquid dielectrics are designed to remove heat from windings and magnetic circuits in transformers, extinguish the arc in oil circuit breakers, reinforce solid insulation in transformers, oil-filled bushings, capacitors, oil-impregnated and oil-filled cables.
Liquid dielectrics are divided into two groups:
Petroleum oils (transformer, condenser, cable);
Synthetic oils (sovtol, liquid organosilicon and organofluorine compounds).
4.1.7 Areas of use of dielectrics as ETM
Application in the electric power industry:
- linear and substation insulation - these are porcelain, glass and organosilicon rubber in overhead line insulators, porcelain in support and bushing insulators, fiberglass as load-bearing elements, polyethylene, paper in high-voltage bushings, paper, polymers in power cables;
- insulation of electrical appliances - paper, getinax, fiberglass, polymers, mica materials;
- machines, apparatus - paper, cardboard, varnishes, compounds, polymers;
- capacitors of different types- polymer films, paper, oxides, nitrides.
From a practical point of view, in each case of choosing a material for electrical insulation, it is necessary to analyze the working conditions and select the insulation material in accordance with a set of requirements. For orientation, it is advisable to divide the main dielectric materials into groups according to the conditions of use.
1. Heat resistant electrical insulation. These are primarily products from mica materials, some of which are capable of operating up to a temperature of 700 ° C. Glasses and materials based on them (fiberglass, fiberglass). Organosilicate and metal phosphate coatings. Ceramic materials, in particular boron nitride. Compositions of organosilicon with a heat-resistant binder. Of polymers, polyimide and fluoroplastic have high heat resistance.
2. Moisture proof electrical insulation. These materials must be hydrophobic (non-wetting with water) and non-hygroscopic. A striking representative of this class is ftoroplast. In principle, hydrophobization is possible by creating protective coatings.
3. Radiation resistant insulation. These are, first of all, inorganic films, ceramics, fiberglass, mica materials, some types of polymers (polyimides, polyethylene).
4. Tropic-resistant insulation.The material must be hydrophobic in order to work in high humidity and temperature conditions. In addition, it must be resistant to mold. The best materials: fluoroplastic, some other polymers, the worst - paper, cardboard.
5. Frost-resistant insulation. This requirement is typical mainly for rubbers, as when the temperature drops, all rubbers lose their elasticity. The most frost-resistant is organosilicon rubber with phenyl groups (up to -90 ° C).
6. Insulation for work in vacuum (space, vacuum devices). For these conditions, it is necessary to use vacuum-tight materials. Some specially prepared ceramic materials are suitable, polymers are of little use.
Electrical cardboard used as dielectric spacers, washers, spacers, as insulation of magnetic circuits, groove insulation of rotating machines, etc. Cardboard, as a rule, is used after impregnation with transformer oil. The electric strength of the impregnated cardboard reaches 40-50 kV / mm. Since it is higher than the strength of transformer oil, special barriers made of cardboard are often arranged in the oil environment to increase the electrical strength of transformers. Oil barrier insulation usually has a strength of E \u003d 300-400 kV / cm. The drawback of cardboard is its hygroscopicity, as a result of moisture, the mechanical strength decreases and the dielectric strength sharply decreases (by 4 or more times).
Recently, the production of insulators for overhead lines based on silicone rubber... This material refers to rubbers, the main property of which is elasticity. This makes it possible to produce from rubbers not only insulators, but also flexible cables. In the energy sector, different types of rubbers are used: natural rubbers, butadiene rubbers, styrene butadiene, ethylene propylene and organosilicon.
Electrical porcelain is an artificial mineral formed from clay minerals, feldspar and quartz as a result of heat treatment using ceramic technology. Among its most valuable properties include high resistance to atmospheric influences, positive and negative temperatures, to the effects of chemicals, high mechanical and electrical strength, low cost of the starting components. This has determined the widespread use of porcelain for the production of insulators.
Electrical glass as a material for insulators has some advantages over porcelain. In particular, it has a more stable raw material base, a simpler technology that allows greater automation, the ability to visually inspect defective insulators.
Micais the basis of a large group of electrical insulating products. The main advantage of mica is its high heat resistance along with sufficiently high electrical insulating characteristics. Mica is a natural mineral with a complex composition. In electrical engineering, two types of mica are used: muscovite KAl 2 (AlSi 3 O 10) (OH) 2 and phlogopite KMg 3 (AlSi 3 O 10 (OH) 2. High electrical insulating characteristics of mica are due to its unusual structure, namely, layering. can be split into flat plates up to submicron sizes. Destructive stresses at separation of one layer from another layer are approximately 0.1 MPa, while when stretched along the layer - 200-300 MPa. Among other properties of mica, we note a low tg, less than 10 -2; high resistivity, more than 10 12 Ohm m; sufficiently high dielectric strength, more than 100 kV / mm; heat resistance, melting point over 1200 ° C.
Mica is used as electrical insulation, in the form of plucked thin plates, incl. glued together (micanites), and in the form of mica papers, incl. impregnated with various binders (mica or mica plastics). Mica paper is produced using a technology close to that of ordinary paper. Mica is crushed, pulp is prepared, sheets of paper are rolled out on paper machines.
Mikanites have better mechanical characteristics and moisture resistance, but they are more expensive and less technological. Application - groove and coil insulation of electrical machines.
Mica - sheet materials made from muscovite-based mica paper. Sometimes they are combined with a substrate of fiberglass (fiberglass), or a polymer film (fiberglass). Papers impregnated with varnish, or another binder, have better mechanical and electrophysical characteristics than non-impregnated papers, but their heat resistance is usually lower, because it is determined by the properties of the impregnating binder.
Mica plastics - sheet materials made of phlogopite-based mica paper and impregnated with binders. Like mica, they can also be combined with other materials. Compared to mica, they have slightly worse electrophysical characteristics, but are less expensive. Application of mica and mica plastics - insulation of electrical machines, heat-resistant insulation of electrical devices.
The greatest use of gases in the power industry is air. This is due to the low cost, general availability of air, the simplicity of the creation, maintenance and repair of airborne electrical insulation systems, the possibility of visual inspection. Objects in which air is used as electrical insulation - power lines, outdoor switchgears, air circuit breakers, etc.
Of the electronegative gases with high electrical strength, the most widely used sF6 gas.... It got its name from the abbreviation "electric gas". The unique properties of SF6 gas were discovered in Russia, and its use also began in Russia. In the 30s, the famous scientist B.M. Gokhberg investigated the electrical properties of a number of gases and drew attention to some of the properties of sulfur hexafluoride SF6. The dielectric strength at atmospheric pressure and a gap of 1 cm is E \u003d 89 kV / cm. The molecular weight is 146, a very large coefficient of thermal expansion and high density are characteristic. This is important for power plantsin which the cooling of any parts of the device, as with a large coefficient of thermal expansion, a convective flow that carries away heat is easily formed. Out of heat physical properties: melting point \u003d -50 ° C at 2 atm, boiling point (sublimation) \u003d -63 ° C, which means it can be used at low temperatures.
Among other useful properties, we note the following: chemical inertness, non-toxicity, incombustibility, heat resistance (up to 800 ° C), explosion safety, weak decomposition in discharges, low liquefaction temperature. In the absence of impurities, SF6 is completely harmless to humans. However, the decomposition products of SF6 as a result of the action of discharges (for example, in a spark gap or circuit breaker) are toxic and chemically active. The complex of properties of SF6 gas provided a fairly widespread use of SF6 insulation. In devices, SF6 gas is usually used at a pressure of several atmospheres for greater compactness of power plants, since dielectric strength increases with increasing pressure. On the basis of gas-insulated insulation, a number of electrical devices have been created and are being used, of which cables, capacitors, circuit breakers, compact switchgear (indoor switchgear).
The most common liquid dielectric in power engineering is transformer oil.
Transformer oil - the refined fraction of oil obtained by distillation, boiling at a temperature of 300 ° C to 400 ° C. Depending on the origin of the oil, they have different properties and these distinctive properties of the feedstock are reflected in the properties of the oil. It has a complex hydrocarbon composition with an average molecular weight of 220-340 a.u. and contains the following main components.
Capacitor and cable oils should be noted among the properties and application of liquid dielectrics related to transformer oil.
Condenser oils. Under this term, a group of different dielectrics is combined used for impregnation of paper-oil and paper-film insulation of capacitors. Most common condenser oil according to GOST 5775-68, they are produced from transformer oil by deeper purification. It differs from conventional oils in greater transparency, lower tan value (more than ten times). Castor oil plant origin, it is obtained from castor seeds. The main area of \u200b\u200buse is impregnation of paper capacitors for operation in pulsed conditions.
The density of castor oil is 0.95-0.97 t / m3, the pour point is from -10 ° C to -18 ° C. Its dielectric constant at 20 ° C is 4.0 - 4.5, and at 90 ° C - \u003d 3.5 - 4.0; tg at 20 ° C is 0.01-0.03, and at 100 ° C tg \u003d 0.2-0.8; Ep at 20 ° C is equal to 15-20 MV / m. Castor oil does not dissolve in gasoline, but it does dissolve in ethyl alcohol. Unlike petroleum oils, castor oil does not swell in conventional rubber. This dielectric belongs to weakly polar liquid dielectrics, its resistivity under normal conditions is 108 - 1010 Ohm · m.
Cable oils designed for impregnation of paper insulation of power cables. They are also based on petroleum oils. They are distinguished from transformer oil by increased viscosity, increased flash point and reduced dielectric loss. Among the brands of oils, we note MN-4 (low-viscosity, for filling low-pressure cables), S-220 (high-viscosity for filling high-pressure cables), KM-25 (the most viscous).
The second type of liquid dielectrics is hardly combustible and non-combustible liquids. There are a lot of liquid dielectrics with such properties. The most widespread in energy and electrical engineering are chlorodiphenyls... In foreign literature they are called chlorobiphenyls... These are substances that contain a double benzene ring, the so-called. di (bi) phenyl ring and one or more chlorine atoms attached to it. In Russia, dielectrics of this group are used in the form of mixtures, mainly mixtures of pentachlorobiphenyl with trichlorobiphenyl. The commercial names of some of them are “sovol”, “sovtol”, “calorie-2”.
Dielectric materials are also classified according to a number of intraspecific characteristics, which are determined by their main characteristics: electrical, mechanical, physicochemical, and thermal.
4.2.1 The electrical characteristics of dielectric materials include:
Specific volume electric resistivity ρ, Ohm * m or specific volume conductivity σ, S / m;
Specific surface electrical resistance ρ s, Ohm, or specific surface conductivity σ s cm;
Temperature coefficient of electrical resistivity TK ρ, ˚С -1;
Dielectric constant ε;
Temperature coefficient of dielectric constant TKε;
Dielectric loss tangent δ;
Dielectric strength of material E pr, MV / m.
4.2.2 Thermal characteristics determine the thermal properties of dielectrics.
Thermal characteristics include:
Heat capacity;
The melting temperature;
Softening temperature;
Dropping point;
Heat resistance;
Heat resistance;
Cold resistance - the ability of dielectrics to withstand low temperatures, while maintaining electrical insulating properties;
Tropic resistance - the resistance of dielectrics to a complex of external influences in a tropical climate (sharp temperature drop, high humidity, solar radiation);
Thermoelasticity;
Flash point of vapors of insulating liquids.
Heat resistance is one of the most important characteristics of dielectrics. In accordance with GOST 21515-76, heat resistance is the ability of a dielectric to withstand long-term exposure to elevated temperatures for a time comparable to the period of normal operation, without unacceptable deterioration of its properties.
Heat resistance classes. Only seven. They are characterized by the temperature index TI. This is the temperature at which the service life of the material is 20 thousand hours.
4.2.3 Moisture properties of dielectrics
Moisture resistance is the reliability of operation of the insulation when it is in an atmosphere of water vapor close to saturation. Moisture resistance is estimated by the change in electrical, mechanical and other physical properties after the material is in an atmosphere with high and high humidity; by moisture and water permeability; by moisture and water absorption.
Moisture permeability - the ability of a material to pass moisture vapor in the presence of a difference in relative humidity on both sides of the material.
Moisture absorption - the ability of a material to absorb water during prolonged exposure to a humid atmosphere close to saturation.
Water absorption - the ability of a material to absorb water when it is immersed in water for a long time.
Tropical resistance and tropicalization of equipment - protection of electrical equipment from moisture, mold, rodents.
4.2.4 The mechanical properties of dielectrics determine the following characteristics:
Tensile stress under static tension;
Destructive stress during static compression;
Breaking stress in static bending;
Hardness;
Impact strength;
Splitting resistance;
Resistance to tearing (for flexible materials);
Flexibility for the number of double bends;
Plastoelastic properties.
The mechanical characteristics of dielectrics are determined by the relevant GOSTs.
4.2.5 Physical and chemical characteristics:
An acid number that determines the amount of free acids in a dielectric that degrades the dielectric properties of liquid dielectrics, compounds and varnishes;
Kinematic and conditional viscosity;
Water absorption;
Water resistance;
Moisture resistance;
Arc resistance;
Tracking resistance;
Radiation resistance, etc.
