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Basic properties of insulating materials

2019-05-27

Main performance indexes of insulating high temperature resistant materials


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


(1) Breakdown strength. Insulation materials will be damaged and lose insulation performance under the action of electric field strength higher than a certain value, which is called breakdown. The electric field strength when insulating material is broken down is called breakdown strength, and the unit is kV/mm.


(2) heat resistance. When the temperature rises, the resistance, breakdown strength, mechanical strength and other properties of insulating materials will be reduced. Therefore, insulation materials are required to work for a long time at a specified temperature and ensure reliable insulation performance. The heat resistance of insulating materials of different components is different, and the heat resistance grade can be divided into Y, A, E, B, F, H, C and other 7 grades, and the maximum working temperature of insulating materials of each grade is specified.


Class Y: the ultimate butyl temperature of 90℃, such as wood, cotton yarn, paper fiber, acetate fiber, polyacyl and other textiles and easy to thermal decomposition and low melting point of plastic insulation.


Class A: the limit working temperature is 105℃, such as enameled wire, lacquer cloth, lacquer silk, oil paint and asphalt insulation materials.


Class E: the limit working temperature is 120℃, such as glass cloth, oil resin paint, high strength enameled wire, vinyl acetate heat-resistant enameled wire and other insulating materials.


Class B: the limit working temperature is 130℃, such as polyester thin wax, mica treated by the corresponding resin, glass fiber, asbestos, polyester paint, polyester enameled wire and other insulating materials.


F class: the limit working temperature is 155℃, such as the use of F class insulating resin adhesion or impregnation, coated mica, glass silk, asbestos, glass cloth and based on the above materials laminated products, mica, powder products, polyester and alkyd materials with better chemical thermal stability, composite silicone organic polyester paint.


Class H: limit operating temperature is 180℃, such as thickening F class materials, mica, silicone mica products, silicone organic paint, silicone organic rubber polyimide composite glass cloth, composite film, polyimide paint, etc.


Class C: the ultimate operating temperature is greater than 180℃. Refers to inorganic materials without any organic adhesives and impregnators, such as quartz, asbestos, mica, glass, ceramics and tetrafluoroethylene plastics.


(3) Insulation resistance. The resistance value presented by insulating materials is insulation resistance, usually in the state, the insulation resistance is generally up to dozens of megohm. Insulation resistance varies greatly due to temperature, thickness, surface conditions (moisture, dirt, etc.).


The resistivity of insulating materials is very high, but 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 provisions of tensile, compressive, bending, shear, tear, impact and other strength indicators, collectively referred to as mechanical strength.


(5) Other characteristics. Some insulating materials are presented in liquid form, such as various insulating paints, whose characteristic indexes include viscosity, fixed content, acid value, drying time and gelation time, etc. Some insulation properties also involve permeability, oil resistance, elongation, shrinkage, solvent resistance, arc resistance and so on.


Aging of insulation materials


Insulation materials under the action of electric field will occur polarization, conductivity, medium heating, breakdown and other physical phenomena, while bearing the action of electric field, but also to withstand the influence of mechanical, chemical and many other factors, long-term will appear aging phenomenon. Therefore, many failures of electrical products tend to occur in the insulation part.


Dielectric aging refers to the phenomenon that the electrical and mechanical properties of dielectric deteriorate gradually with the increase of time in long-term operation. The main aging forms are electrical aging, thermal aging and environmental aging.


(1) electrical aging. Common in high voltage electrical appliances, the main reason is the partial discharge of insulating materials under the action of high voltage.


(2) Thermal aging. Common in low-voltage appliances, its mechanism is under the action of temperature. Insulation material internal components oxidation, cracking, metamorphism, hydrolytic reaction with water and gradually lose insulation performance.


(3) Environmental aging. Also known as atmospheric aging, is due to ultraviolet, ozone, salt fog, acid and alkali factors caused by pollution chemical aging. One of them. Ultraviolet light is the main factor. Ozone is produced by corona or partial discharge of electrical equipment.


Once the insulation material has aging, its insulation performance usually cannot be restored. In engineering, the following methods are commonly used to prevent the aging of insulation materials.


(1) Add anti-aging agent in the production process of insulation materials.


(2) Outdoor insulation materials can add ultraviolet absorbent, or use a separation layer to isolate sunlight.


(3) Insulation materials used in hot and humid areas can be added with mildew inhibitor.


(4) Strengthen the measures to prevent corona and partial discharge of electrical equipment.


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