What is Dielectric?
A dielectric is a material with low electrical conductivity and so is classified as an insulator. Dielectric materials are used to block the flow of electricity through them, allowing them to behave as an insulators. They can also be used to store electric charge or improve capacitance.
Dielectrics have no free electrons for conduction because electrons are tightly bound to the nucleus. Because dielectrics contain a small number of electrons for electrical conductivity, they have a dipole. The two primary roles of dielectrics in electrical devices are insulation and electrical energy storage. An electric field polarizes the dielectric, and the polarization of the molecules allows dielectric materials to store charge.
What is Dielectric Breakdown?
An ordinary material, an electrical insulator, may begin to conduct electricity when exposed to high electric fields and in this way it loses its dielectric properties. The dielectric breakdown is the term for this occurrence.
A dielectric can lose its insulation properties if an electromagnetic field that surrounds it exceeds a certain critical value. This is done by a conductivity channel in a dielectric in an electric field and this is known as a dielectric breakdown. Hence, in short the breakdown voltage is the lowest voltage that causes a dielectric to break down. When the current in the dielectric state continuously increases before the break, the pre-breakdown state occur.
The dielectric width, electric field, and electrode arrangement influences the breakdown voltage. It is defined by the dielectric's capacity to withstand an electric field. The electric durability of a dielectric is a small field when it breaks down.
The dielectric breakdown can occur in an insulator in the electric field due to electric, thermal, or electrochemical reactions. If a dielectric gas breaks down and the power is switched off, the gas might regain its insulating properties. Solid dielectrics cannot recover their insulating capabilities after a breakdown, since their insulation has been destroyed.
Consider that case of two electrodes that are used to make contacts in supply system. When a fault occurs an electric spark occurs first between the electrodes, which then changes into an electric arc. Also it is to be noted that a non-uniform field breakdown differs from uniform field breakdown. A non-uniform field occurs between the two electrodes of different forms, such as cutting-edge and plane, between wires, or between spheres with ample space between them. In non-uniform electric fields, gas breakdown occurs when the electric field is at its maximum, a partial crown shock emerges between electrodes in the electric field spots, which subsequently turns into a spark and finally an electric arc as the field grows.
Gases with a higher electrical strength characterize liquid dielectrics. Water, gases, and microscopic solids are common contaminants in liquids. It slows down the disintegration process. Because electrons in a liquid have a shorter travel route than in gas, dielectric fluids have a higher electrical strength. Overheating and liquid boiling in areas with the highest concentration of contaminants can trigger a breakdown in clean liquids. Overheating due to dielectric losses can cause a breakdown in a liquid at radio frequencies, resulting in dielectric liquid annihilation. That is why dielectric liquids are employed at lower frequencies than those utilized in industry.
Three types of breakdowns can occur in solid dielectrics: electric, thermal, and electrochemical. Depending on the electric field, voltage, intensity and duration, frequency, and other characteristics, each breakdown occurs in the same dielectrics at different temperatures and pressures.
Types of Dielectrical breakdown
Electrical Dielectrical Breakdown
For homogeneous solid dielectrics in a uniform electric field, an electric breakdown can be used to calculate the electrical strength. Thin films may have higher electric strengths than bulk materials. In microelectronics, this feature of thin dielectric films is often used for insulation. When thermal energy in a solid dielectric surpasses the dielectric's ability to dissipate it, the solid overheats, this process begins to resemble an avalanche.
Thermal Dielectrical Breakdown
The thermal breakdown is a kind of ionization breakdown. It is common in porous dielectrics, and the breakdown is caused by gas ionization in the pores. The surfaces of the pores are heated, resulting in a local temperature difference across the solid. The solid may be destroyed as a result of this procedure. At high temperatures and humidity, electrochemical breakdown makes sense. When the processes of declining isolation develop in a solid, the sort of breakdown occurs at AC and DC voltages at low frequencies.
Thermal breakdown mechanisms can be continued by heating the solid to its melting temperature. The breakdown voltage caused by dielectric heating is affected by temperature, frequency, and other factors. The indicators of thermal breakdown in a solid are exponential, with the breakdown voltage decreasing with increasing temperature and the solid electric strength decreasing in the electric field.
Electrochemical Dielectrical Breakdown
The electrochemical breakdown is time-consuming and is connected to solid conductivity. The Surface breakdown is conceivable in solids with high electric strength, a conducting channel emerges when liquid or gas on the bulk solid surface breaks down. The surface breakdown voltage is determined by the shape of the solid and the design and size of the electrodes.
Context and Applications
This topic is taught in many undergraduate and postgraduate degree courses like:
- Bachelors of Technology Electrical engineering
- Bachelors of Technology Electronics engineering
- Masters of Technology Digital Electronics engineering
- Masters of Technology Very Large Scale Integration Engineering
- Application in power system engineering
- High voltage engineering
- Application in Transmission lines
- Application in Switch-gear and protective devices
Dielectric strength is one of the essential features of a dielectric and is defined as the ability of the material to withstand an electric field without breaking down and enabling current flow through it. It's a metric for a material's resistance to deterioration. When a dielectric breakdown occurs, a considerable discharge current flows across it. The dielectric strength of solids is affected by molecular structure, impurities, temperature, and frequency. The dielectric strength diminishes as temperature, humidity, and age rise.
Dielectric strength= Breakdown voltage/ thickness of dielectric material
- Deterioration of oil : These normally cause the oil's physical, chemical, and operational qualities to deteriorate.
- Deterioration of contacts : Contacts are damaged.
- Faulty operating relay for breaking contacts when there is no fault.
Q1 Which are the Types of breakdowns in solid dielectrics?
D. All of the above
Explanation: Three types of breakdowns can occur in solid dielectrics: electric, thermal, and electrochemical. Depending on the electric field, voltage, intensity and duration, frequency, and other characteristics.
Q2. The indicators of thermal breakdown in a solid are exponential, with the breakdown voltage decreasing with ................................... temperature and the solid electric strength decreasing in the electric field.
Explanation : Thermal breakdown processes can be continued by heating the solid to its melting temperature. The breakdown voltage caused by dielectric heating is affected by temperature, frequency, and other factors. The indicators of thermal breakdown in a solid are exponential, with the breakdown voltage decreasing with increasing temperature and the solid electric strength decreasing in the electric field.
Q3. The surface breakdown voltage is determined by the ....... of the solid and the .......... of the electrodes.
A. Shape, design
B. Design, applications
C. Size and shape
D. All of above
Explanation: The surface breakdown is possible in solids with high electric strength—the conducting channel forms due to the breakdown of liquid or gas on the bulk solid surface. So,the surface breakdown voltage is determined by the shape of the solid and the design of the electrodes.
Q4. Dielectric strength= ........................ / thickness of dielectric material
A. Breakdown Strength
B. Breakdown Voltage
C. Electric Strength
D. Electric Field
Explanation: The dielectric strength is directly proportional to breakdown voltage.
Q5. Dielectrical breakdown means, material........
A. loses its dielectric properties
B. have low conductivity
C. have high resistivity
D. None of these
Explanation: A material that is ordinarily an electrical insulator may begin to conduct electricity when exposed to high electric fields, i.e., it loses its dielectric properties.
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