Basic properties of insulating materials

In order to prevent accidents caused by damage to the insulation performance of insulating materials, the insulating materials must meet the performance indexes specified in national standards. There are many performance indexes of insulating materials, and the characteristics of various insulating materials are also different. The main performance indexes of common insulating materials are breakdown strength, heat resistance, insulation resistance and mechanical strength.

(1) Breakdown strength. Under the action of electric field strength higher than a certain value, insulating materials will be damaged and lose their 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 increases, the resistance, breakdown strength, mechanical strength and other properties of insulating materials will decrease. Therefore, the insulating material is required to work for a long time at the specified temperature and ensure reliable insulation performance. The insulation materials with different components have different heat resistance. The heat resistance grades can be divided into seven grades: y, a, e, B, F, h and C, and the maximum limit working temperature is specified for each grade of insulation materials.

Class Y: the ultimate working temperature is 90 ℃, such as textiles such as wood, cotton yarn, paper fiber, acetate fiber and polyacyl, as well as plastic insulators easy to thermal decomposition and low melting point.

Class A: the limit working temperature is 105 ℃, such as enameled wire, lacquer cloth, lacquer wire, oily paint, asphalt and other insulating materials.

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

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

Grade F: the ultimate working temperature is 155 ℃, such as mica, glass filament, asbestos, glass lacquer cloth bonded or impregnated with grade F insulating resin and laminated products based on the above materials, mica and powder products, polyester and alkyd materials with good chemical and thermal stability, and composite silicone organic polyester paint.

Class H: the ultimate working temperature is 180 ℃, such as thickened class F materials, mica, silicone mica products, silicone organic paint, silicone organic rubber polyimide composite glass cloth, composite film, polyimide paint, etc.

Grade C: the ultimate working temperature is greater than 180 ℃. It refers to inorganic substances without any organic adhesive and impregnant, such as quartz, asbestos, mica, glass, ceramics and tetrafluoroethylene plastics.

(3) Insulation resistance. The resistance value of insulating materials is insulation resistance. Under normal conditions, the insulation resistance is generally more than tens of megaohms. Insulation resistance will vary greatly due to temperature, thickness and surface conditions (moisture, dirt, etc.).

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

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

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

Aging of insulating materials

Under the action of electric field, insulating materials will have physical phenomena such as polarization, conductivity, dielectric heating and breakdown. While bearing the action of electric field, they will also be affected by many factors such as machinery and chemistry for a long time_ T-work will be aged. Therefore, many faults of electrical products often occur in the insulation part.

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

(1) Electrical aging. It is common in high-voltage electrical appliances. The main reason is partial discharge of insulating materials under high voltage.

(2) Thermal aging. It is common in low-voltage electrical appliances. Its mechanism is that under the action of temperature, the internal components of insulating materials oxidize, crack and deteriorate, and gradually lose their insulating properties due to hydrolysis reaction with water.

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

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

(1) Add anti-aging agent during the production of insulating materials.

(2) For outdoor insulation materials, ultraviolet absorbent can be added, or a partition layer can be used to isolate sunlight.

(3) Anti mildew agent can be added to the insulating materials used in hot and humid areas.

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