What is the dielectric strength? Let's try to deal with this term, identify the features of this indicator.
Definitions
Dielectrics are substances that do not conduct electrical current poorly or completely. 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. A chemically pure insulator is water.
Dielectric Characteristics
This group includes pyroelectrics, ferroelectrics, relaxors, piezoelectrics. In modern technology, the passive and active properties of such materials are actively used, so we will dwell on them in more detail.
The passive properties of insulators are used when they are used in conventional capacitors.
Dielectric materials that prevent the loss of electric charges are considered insulating materials. With their help it is possible to separate electrical circuits, parts of devices from conductive parts. In such situations, the dielectric constant does not have a special role.
Active (controlled) dielectrics are pyroelectrics, ferroelectrics, electroluminophores, materials for gates and emitters in laser technology.
Demand for dielectric materials is increasing annually. The reason is the increase in capacity of industrial enterprises and commercial institutions.
In addition, the increased demand for dielectrics can be explained by an increase in the number of communications and various electrical devices.
In technology, the electric strength of insulators, associated with the arrangement of molecules and atoms in the crystal lattice, is of particular importance.
Classification
Under various conditions, the dielectric material can exhibit various insulating characteristics, which determines the scope of its application. For example, depending on the temperature, the dielectric strength changes.
Depending on the structure, organic and inorganic electrical insulating materials are isolated.
With the development of the electrical industry, the production of dielectric materials from minerals was also formed. The technology has recently improved so much that it was possible to significantly reduce the cost of production, as a result, mineral dielectrics supplanted chemical and natural materials.
Mineral Dielectric Materials
These compounds include:
- Installation, alkaline, lamp, capacitor glasses, consisting of a mixture of different oxides. When making oxides of aluminum, calcium, silicon, the electric strength of the material increases.
- Glass enamels are materials in which a thin layer of enamel is applied to a metal surface.
- Optical fibers, which are a special type of fiberglass.
- Ceramic products.
- Mica.
- Asbestos.
Despite such a variety of electrical insulating materials, it is far from always possible to replace one with one dielectric.
The electrical strength of the insulation is an important property, but not only it is paid attention to when choosing 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 maximize the safety of the operation of instruments and devices.
Electrical Insulating Oil
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, promotes heat dissipation. In addition, transformer oil is actively used in high voltage oil circuit breakers. In such devices, between the divergent contacts of the circuit breaker, an electric arc breaks, as a result of which the arc channel is rapidly cooled and quenched. To obtain petroleum mineral electrical insulating oils, oil is used, carrying out its step-wise distillation with stage-by-stage separation at each stage of the fraction and detailed purification of impurities using sulfuric acid, followed by washing and drying.
The dielectric strength of such an oil is a quantity 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 in which the field strength has a maximum value, as a result of which a breakdown develops.
With a sharp decrease in the electric strength of the oil, not only water molecules, but also fibrous impurities are present in it. They absorb water, which significantly affects the electrical characteristics of a liquid dielectric.
Cable oils
They are used in the manufacture of electric power cables. When impregnating their paper insulation with oils, the removal of heat loss increases.
There are different types of cable oils. For example, for the impregnation of power cables of aluminum and lead shells, KM-25 brand oil is used, which has a kinematic viscosity of at least 23 millimeters per second and a pour point of not more than 1000 degrees. In order to increase the viscosity of the oil, rosin or a synthetic thickener is added to it.
Before using a dielectric, conduct the test of dielectric strength of insulation.
Liquid Synthetic Dielectrics
These electrical insulating materials are superior in some characteristics to petroleum oils. They have a tendency to electric aging, which negatively affects the properties under the influence of an electric field of increased tension.
In order to cope with a similar problem, the impregnation of capacitors is carried out by a polar liquid dielectric.
Checking the electric strength is a mandatory measure, allowing you to choose 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 diphenyl. It has a high viscosity, has the main characteristics corresponding to GOST. The dielectric strength of this insulator is higher than other non-polar petroleum oils, so when it is used, the capacitor volume is almost halved. Among the advantages of chlorinated biphenyls, we emphasize their incombustibility, and the disadvantages are toxicity and high cost.
Among the inexpensive domestic materials with excellent insulating characteristics, let us single out 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 gum, has organic acids in its composition. It is highly soluble in petroleum oils, and is used as casting 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.
The reasons for the loss of electrical strength
In those dielectrics that are used in practice, there are free charges. When electrons move, electrical conductivity increases . Since there are few charges, the insulators pass this test successfully. The dielectric strength of insulators determines the main areas of their industrial application.
Isolation is necessary for isolating current, adjusting temperature, electric field strength, and other characteristics possessed by instruments and devices.
If a piezoelectric is used as a dielectric in a capacitor, it changes its linear characteristics under the influence of an alternating voltage and turns into a generator of ultrasonic vibrations.
Conclusion
The technology and operating characteristics of electronic and electrical equipment determine different requirements for the parameters of dielectric materials.
Insulators used for practical purposes have few electrons in their volume; therefore, at constant voltage, they pass a minimum current, called the leakage current.
If, with increasing voltage applied to the insulation, the value of the field strength in the dielectric exceeds a certain value, the insulator loses its electrical insulating characteristics.
The through current that flows through the insulator increases and its resistance decreases, resulting in a short circuit of the electrodes.
A similar phenomenon is called dielectric breakdown. In the case when the voltage that is applied to the dielectric reaches a critical value, a sharp increase in the through current is observed, the voltage at 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 arises after breakdown, which leads to melting, burning, cracking, as well as other changes in both the dielectric and the electrodes.
With the correct selection of electrical insulating materials, it is possible to ensure uninterrupted operation of electrical appliances and technical devices.