Before considering the breakdown mechanisms of dielectrics, let's try to find out the features of these materials. Electrical insulating materials are substances that allow you to isolate parts of electrical equipment or circuit elements that have different electrical potentials.
Features of materials
In comparison with conductive materials, insulators have a significantly higher electrical resistance. A typical property of these materials is the creation of powerful electric fields, as well as the accumulation of energy. This property is widely used in capacitors.
Classification
According to the state of aggregation, all electrical insulating materials are divided into liquid, gaseous, solid. The largest is the last group of dielectrics. These include plastics, ceramics, high-polymer materials.
Depending on the chemical composition, electrical insulating materials are divided into inorganic and organic.
Carbon acts as the main chemical element in organic insulators. Maximum temperatures withstandinorganic materials: ceramics, mica.
Depending on the method of obtaining dielectrics, it is customary to divide them into synthetic and natural (natural). Each type has certain features. Currently, synthetic substances are a large group.
Solid dielectric materials are further subdivided into separate subcategories according to the structure, composition, technological characteristics of materials. For example, there are wax, ceramic, mineral, film insulators.
All these materials are characterized by electrical conductivity. Over time, such substances show a change in the current value due to a decrease in the absorption current. From a certain moment in the electrical insulating material there is only a conduction current, on the value of which the properties of this material depend.
Process Features
If the electric field strength is greater than the electrical strength limit, dielectric breakdown occurs. This is the process of its destruction. It leads to the loss in the place of breakdown by such material of its initial electrical insulating characteristics.
Breakdown voltage is the value at which a dielectric breakdown occurs.
Dielectric strength is characterized by the value of the field strength.
The breakdown of solid dielectrics is an electrical or thermal process. It is based on phenomena that lead to an avalanche increase in solid insulating materials of the valueelectric current.
The breakdown of solid dielectrics has characteristic features:
- absence or weak dependence on temperature and voltage of conductivity value;
- electrical strength of a material in a uniform field, regardless of the thickness of the dielectric material used;
- narrow limits of mechanical strength;
- first, the current increases exponentially, and breakdowns of solid dielectrics are accompanied by an abrupt increase in current;
- in an inhomogeneous field, this process occurs in a place with a maximum field strength.
Thermal breakdown
It appears when there are large dielectric losses, when the material is heated by other heat sources, when the heat energy is poorly removed. Such a breakdown of the dielectric is accompanied by an increase in electric current as a result of a sharp decrease in resistance in the area where heat conduction is impaired. A similar process is observed until the complete thermal destruction of the dielectric occurs in the weakened place. For example, the original solid electrical insulating material will melt.
Signs
Dielectric breakdown has characteristic features:
- occurs in a place of poor-quality heat removal to the environment;
- breakdown voltage decreases with increasing ambient temperature;
- electrical strength is inversely proportional to the thickness of the dielectriclayer.
General characteristics
Let's characterize the main types of breakdown of dielectrics. The essence of the process lies in the loss of electrical insulating material of its characteristics when the critical value of the electric field strength is exceeded. There are several types of this process:
- electric breakdown of dielectric;
- thermal process;
- electrochemical aging.
Electric variant occurs as a result of impact ionization by negative electrons, appearing in a powerful electric field. This process is accompanied by a sharp increase in current density.
The reason for the thermal process in the insulator is an increase in the amount of heat generated by the system due to the effects of electrical conductivity or as a result of dielectric losses. The result of such a breakdown is the thermal destruction of the electrical insulating material.
When the breakdown voltage of dielectrics changes, transformations occur in the structure of the electrical insulating material, and the chemical composition of the dielectric also changes. As a result, an irreversible decrease in the insulation resistance is observed. In this case, electrical aging of the dielectric occurs.
In a gaseous medium
How does the breakdown of gaseous dielectrics occur? Due to cosmic and radioactive radiation, there are a small number of charged particles in the air gaps. There is an acceleration of negative electrons in the field, as a result of which they acquire additional energy, the value of which directly depends on the field strength andmean path length of the particle before the collision. At a significant intensity value, an increase in the electron flow is observed, which causes a breakdown of the gap. This process is influenced by several factors. The most important of these is the field option. There is a direct relationship between the electrical strength of gas and pressure and temperature.
Liquid medium
The breakdown of liquid dielectrics is related to the purity of the electrical insulating material. There are three degrees:
- content of solid mechanical impurities and emulsion water in the dielectric;
- technically clean;
- thoroughly cleaned and degassed.
In carefully cleaned liquid dielectrics, there is only an electrical version of breakdown. Due to the significant difference in the densities of the liquid and gas, the electron path length decreases, which leads to an increase in the breakdown voltage.
In modern electric power industry, technically pure types of liquid dielectrics are used, only a slight presence of impurities in them is allowed.
It must be taken into account that even the minimum amount of emulsion water in the liquid electrical insulating material causes a strong reduction in electrical strength.
Thus, dielectric strength and breakdown of dielectrics are related quantities. Let us consider the breakdown mechanism in a liquid medium. Drops of emulsion water are polarized in an electric field, then they fall into the space between the polar electrodes. Here they are deformed, merged, and bridges are formed,with little electrical resistance. It is on them that the test occurs. The appearance of bridges causes a significant reduction in the strength of the oil.
Features of electrical insulating materials
The considered types of breakdown of solid dielectrics have found their application in modern electrical engineering.
Among the liquid and semi-liquid dielectric materials currently used in technology, transformer and capacitor oils, as well as synthetic fluids: sovtol, sovol.
Mineral oils are obtained from the fractional distillation of crude oil. Between their individual types there are differences in viscosity, electrical characteristics.
For example, cable and capacitor oils are highly refined, so they have excellent dielectric characteristics. Non-flammable synthetic liquids are sovtol and sovol. To obtain the first, a chlorination reaction of crystalline diphenyl is carried out. This transparent viscous liquid is toxic and can irritate the mucous membrane, therefore, when working with such a dielectric, precautions must be carefully observed.
Sovtol is a mixture of trichlorobenzene and sovol, so this electrical insulating material is characterized by a lower viscosity.
Both synthetic fluids are used to impregnate modern paper capacitors installed in industrial AC and DC devices.
Organichigh-polymer dielectric materials are composed of many monomer molecules. Amber, natural rubber, has high dielectric characteristics.
Waxy materials such as ceresin and paraffin have a distinct melting point. Such dielectrics have a polycrystalline structure.
In modern electrical engineering, plastics, which are composite materials, are in demand. They contain polymers, resins, dyes, stabilizing agents, as well as plasticizing components. According to their relationship to heat, they are classified into thermoplastic and thermoset materials.
For work in the air, electric cardboard is used, which has a denser structure compared to conventional material.
Among the layered electrical insulating materials with dielectric characteristics, we highlight textolite, getinaks, fiberglass. These laminates, which use silicone or resole resins as the binder, are excellent dielectrics.
Causes of the phenomenon
There are various reasons for the breakdown of dielectrics. Therefore, there is still no universal theory that would fully explain this physical process. Regardless of the insulation option, in the event of a breakdown, a channel of special conductivity is formed, the magnitude of which leads to a short circuit in this electrical device. What are the consequences of such a process? There is a high probability of an emergency, as a result of whichelectrical device will be taken out of service.
Depending on the insulation system, the breakdown may have different manifestations. For solid dielectrics, the channel retains significant conductivity even after the current is turned off. Gaseous and liquid electrical insulating materials are characterized by high mobility of charged electrons. Therefore, there is an instant recovery of the breakdown channel due to a change in voltage.
In liquids, breakdown is caused by various processes. First, optical inhomogeneities are formed in the space between the electrodes, in these places the liquid loses its transparency. A. Gemant's theory considers the breakdown of a liquid dielectric as an emulsion. According to calculations carried out by scientists, due to the action of an electric field, moisture drops take the form of an elongated dipole. In the case of a high field strength, they combine, which contributes to the discharge in the formed channel.
When conducting numerous experiments, it was found that if there is a gas in the liquid, then with a sharp increase in voltage, bubbles will appear before breakdown. In this case, the breakdown voltage of such liquids decreases with a decrease in pressure or with an increase in temperature.
Conclusion
Modern dielectric materials are improving as the electrical industry develops. At present, the technology for creating various types of dielectrics has been so modernized that it is possible to create inexpensive dielectrics with high performance.
AmongThe most demanded materials with the corresponding characteristics are of particular interest to glass and glass enamels. Installation, alkaline, lamp, capacitor, other types of this material are substances of an amorphous structure. By adding calcium and aluminum oxides to the mixture, it is possible to improve the dielectric properties of the material and reduce the likelihood of breakdown.
Glass enamels are materials in which a thin layer of glass is deposited on the metal surface. This technology provides reliable protection against corrosion.
All materials with electrical insulating characteristics are widely used in modern technology. If dielectric breakdown is prevented in time, it is quite possible to prevent damage to expensive equipment.
