What is the dielectric strength of a dielectric? Let's try to understand this term, to identify the features of this indicator.
Definitions
Dielectrics are substances that do not conduct electricity well or completely. The value of the density in such a substance of charge carriers (electrons) does not exceed 108 pieces per cubic centimeter. The main characteristic of electrical insulating materials is their ability to polarize in an external field. Dielectrics include gaseous substances, various resins, glass, and polymeric materials. The chemically pure insulator is water.
Dielectric Characteristics
This group includes pyroelectrics, ferroelectrics, relaxors, piezoelectrics. Passive and active properties of such materials are actively used in modern technology, so we will dwell on them in more detail.
The passive properties of insulators apply when they are used in conventional capacitors.
Electrical insulating materials are dielectrics that do not allow the loss of electric charges. With their help, it is possible to separate electrical circuits from each other, parts of devices from conductive parts. In such situationspermittivity has no special role.
Active (controlled) dielectrics are pyroelectrics, ferroelectrics, electroluminophores, materials for shutters and emitters in laser technology.
Demand for dielectric materials is increasing every year. The reason is the increase in the capacity of industrial enterprises and commercial institutions.
In addition, the increased demand for dielectrics can be explained by the increase in the number of communications and various electrical appliances.
In technology, the electrical strength of insulators is of particular importance, associated with the arrangement of molecules and atoms in the crystal lattice.
Classification
Under different conditions, the dielectric material can exhibit different insulating characteristics, which determines the scope of its application. For example, dielectric strength changes with temperature.
Depending on the structure, organic and inorganic electrical insulating materials are distinguished.
As the electrical industry developed, so did the production of dielectric materials from minerals. The technology has recently improved so much that it has been possible to significantly reduce the cost of production, as a result, mineral dielectrics have replaced chemical and natural materials.
Mineral dielectric materials
Such compounds include:
- Installation, alkaline, lamp,capacitor glasses, consisting of a mixture of different oxides. When making oxides of aluminum, calcium, silicon, the electrical strength of the material increases.
- Glass enamels are materials in which a thin layer of enamel is applied to a metal surface.
- Light guides, which are a special type of light-conducting fiberglass.
- Ceramic items.
- Mica.
- Asbestos.
Despite such a variety of electrical insulating materials, it is far from always possible to replace one dielectric with another.
The electrical strength of the insulation is an important property, but it is not the only thing to pay attention to when selecting such materials.
Particular attention is also paid to thermal, mechanical, other physical and chemical properties, including the ability to various types of processing, cost, availability of materials.
Checking the electrical strength of the insulation is carried out in order to ensure the maximum safety of the operation of instruments and devices.
Electrical insulating petroleum oils
Transformer oil, used for power transformers, has the maximum distribution in electrical engineering among liquid insulating materials. They fill the pores in the fibrous insulation, the distance between the windings, increases the dielectric strength of the insulation, and promotes heat removal. In addition, transformer oil is actively used in high voltage oil circuit breakers. In such devices, between divergentthe contacts of the switch break the electric arc, as a result of which the arc channel is quickly cooled and extinguished. To obtain petroleum mineral electrical insulating oils, oil is used, carrying out its staged distillation with a phased separation of a fraction at each stage and detailed purification from impurities with sulfuric acid, followed by washing and drying.
The electrical strength of such an oil is a value that is very sensitive to moisture. Even with a slight admixture of water in the oil, a significant decrease in this physical quantity is observed. Under the action of an electric field, droplets of emulsified water are drawn into those places where the field strength has a maximum value, as a result of which a breakdown develops.
With a sharp decrease in the electrical strength of the oil, it contains not only water molecules, but also fibrous impurities. They absorb water, which significantly affects the electrical characteristics of the liquid dielectric.
Cable oils
They are used in the production of electrical power cables. When their paper insulation is impregnated with oils, heat loss is increased.
There are different types of cable oils. For example, for the impregnation of power cables of aluminum and lead sheaths, oil of the KM-25 brand is used, which has a kinematic viscosity of at least 23 millimeters per second, a pour point of no more than 1000 degrees. In order to increase the viscosity of the oil, rosin is added to it orsynthetic thickener.
Before using a dielectric, test the dielectric strength of the insulation.
Liquid synthetic dielectrics
These electrical insulating materials are superior in some respects to petroleum oils. They have a tendency to electrical aging, which negatively affects the properties under the influence of an electric field of increased intensity.
To deal with this problem, capacitors are impregnated with a polar liquid dielectric.
Checking the electrical strength is a must to select the most effective type of insulator.
Chlorinated hydrocarbons
They are obtained from various hydrocarbons by replacing one or more hydrogen atoms with chlorine. The most common type of such dielectrics is chlorinated biphenyl. It has high viscosity, has the main characteristics corresponding to GOST. The electrical strength of this insulator is higher than other non-polar petroleum oils, therefore, when using it, the volume of the capacitor is almost halved. Among the advantages of chlorinated biphenyls, we highlight their incombustibility, and the disadvantages are toxicity and high cost.
Among inexpensive domestic materials with excellent insulating characteristics, we highlight a mixture of isobutene and its isomers (octol), obtained as a result of oil cracking.
Natural insulators
Rosin,which is a brittle resin obtained from resin, contains organic acids in its composition. It dissolves well in petroleum oils and is used as sealing and impregnating cable compounds.
A thin layer of vegetable oil, falling on the surface of the material, forms a thin film, increasing the insulating characteristics of the part.
Causes of loss of electrical strength
In those dielectrics that are used in practice, there are free charges. As electrons move, the electrical conductivity increases. Since there are few charges, the insulators successfully pass this test. The electrical strength of insulators determines the main areas of their industrial application.
Insulation is necessary for current isolation, temperature control, electric field strength, other characteristics that devices and devices have.
If a piezoelectric is used as a dielectric in a capacitor, it changes its linear characteristics under the influence of an alternating voltage, turns into a generator of ultrasonic vibrations.
Conclusion
Technology and features of operation of radio-electronic and electrical equipment determine different requirements for the parameters of dielectric materials.
Insulators used for practical purposes have few electrons in their volume, so at a constant voltage they pass a minimum current, called leakage current.
If the voltage rises,applied to the insulation, the value of the field strength in the dielectric will exceed a certain value, the insulator will lose its electrical insulating characteristics.
The through current that flows through the insulator increases and its resistance decreases, causing the electrodes to short circuit.
This phenomenon is called dielectric breakdown. In the case when the voltage applied to the dielectric reaches a critical value, a sharp increase in the through current is observed, the voltage on the electrodes decreases, as a result of irreversible changes, the electrical resistance of the insulator decreases.
Depending on the parameters of power and energy isolation, a spark occurs after a breakdown, which leads to melting, burning, cracking, and other changes in both the dielectric and the electrodes.
With the right selection of electrical insulating materials, you can ensure the smooth operation of electrical appliances and technical devices.
