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Basic properties of insulating materials

2019-05-27

Main performance indicators of insulating high temperature resistant materials


In order to prevent accidents caused by insulation damage of insulating materials, it is necessary to make the insulating materials meet the performance specifications stipulated by national standards. The performance index of insulating materials is many, and the characteristics of various insulating materials are also different. The main performance indexes of commonly used insulating materials are breakdown strength, heat resistance, insulation resistance and mechanical strength.


(1) Breakdown strength. Under the action of the electric field strength above a certain value, the insulating material will be damaged and lose its insulating properties. This phenomenon is called breakdown. The electric field strength when the insulating material is broken down is called the breakdown strength and the unit is: kV/mm.


(2) Heat resistance. When the temperature rises, the electrical resistance, breakdown strength, mechanical strength and the like of the insulating material are lowered. Therefore, it is required that the insulating material can work for a long period of time at a prescribed temperature and the insulation performance is ensured to be reliable. Insulation materials with different compositions have different heat resistance levels, and the heat resistance grades can be divided into seven grades of Y, A, E, B, F, H, and C, and the maximum limit operating temperature is specified for each grade of insulating material.


Grade Y: The limit temperature is 90 ° C, such as wood, cotton yarn, paper fiber, acetate fiber, polyamide and other textiles and plastic insulation which is easy to thermally decompose and has low melting point.


Class A: The ultimate working temperature is 105 ° C, such as enameled wire, varnish, lacquer, oil paint and asphalt insulation.


Class E: The limit working temperature is 120 °C, such as glass cloth, oily resin paint, high-strength enameled wire, vinyl acetate heat-resistant enameled wire and other insulation.


Grade B: The ultimate working temperature is 130 ° C, such as polyester thin wax, mica, glass fiber, asbestos, polyester paint, polyester enameled wire and other insulation treated with the corresponding resin.


Class F: Extreme working temperature is 155 ° C, such as mica bonded or impregnated with grade F insulating resin, coated mica, glass wool, asbestos, glass varnish and laminates based on the above materials, mica, powder products, chemistry Polyester and alkyd materials with good thermal stability, composite silicone organic polyester paint.


Class H: The ultimate working temperature is 180 °C, such as thick F-grade materials, mica, silicone mica products, silicon organic paint, silicone rubber polyimide composite glass cloth, composite film, polyimide paint, etc.


Class C: The limit operating temperature is greater than 180 °C. Refers to inorganic materials that do not use any organic binders and impregnating agents, such as quartz, asbestos, mica, glass, ceramics and tetrafluoroethylene plastics.


(3) Insulation resistance. The resistance value exhibited by the insulating material is the insulation resistance. Generally, the insulation resistance is generally several tens of megaohms or more. Insulation resistance varies greatly depending on temperature, thickness, and surface condition (moisture, dirt, etc.).


Although the electrical resistivity of the insulating material is very high, it is under a certain voltage. There is always a small current passing through, and this current is called leakage current.


(4) Mechanical strength. According to the specific requirements of various insulating materials, the corresponding specified tensile strength, compression resistance, bending resistance, shear resistance, tear resistance, impact resistance and other strength indicators are collectively referred to as mechanical strength.


(5) Other characteristic indicators. Some insulating materials are presented in liquid form, such as various insulating varnishes, and their characteristics include viscosity, fixed content, acid value, drying time and gelation time. Some insulating material properties also refer to permeability, oil resistance, elongation, shrinkage, solvent resistance, arc resistance, and the like.


Aging of insulating materials


Under the action of electric field, the insulating material will undergo physical phenomena such as polarization, conductance, medium heating, and breakdown. When subjected to electric field, it will also be affected by many factors such as mechanical and chemical factors. Long-term _T will appear aging. . Therefore, many failures of electrical products often occur in the insulating portion.


The aging of the dielectric refers to a phenomenon in which the electrical properties, mechanical properties, and the like of the dielectric gradually deteriorate over time in the long-term operation. The main forms of aging are electrical aging, heat aging and environmental aging.


(1) Electrical aging. More common in high-voltage electrical appliances, the main reason is that the insulation material is partially discharged under the action of high pressure.


(2) Heat aging. More common in low-voltage electrical appliances, the mechanism is under the action of temperature. The internal components of the insulating material are oxidized, cracked, and deteriorated, and the water is hydrolyzed to gradually lose the insulating properties.


(3) Environmental aging. Also known as atmospheric aging, it is a polluted chemical aging caused by ultraviolet light, ozone, salt spray, acid and alkali. among them. Ultraviolet light is the main factor. Ozone is produced by corona or partial discharge of electrical equipment.


Once the insulating material has aged, its insulation performance is usually unrecoverable. The following methods are commonly used in engineering to prevent the aging of the insulating material.


(1) Add anti-aging agent in the process of making insulating materials.


(2) Outdoor insulation materials may be added with UV absorbers or separated by a barrier.


(3) The insulating material used in the hot and humid zone can be added with anti-fungal agents.


(4) Measures to strengthen local anti-corona and partial discharge prevention of electrical equipment.


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