Solid insulating materials may be natural organic substances like paper, cloth, rubber, etc. or inorganic materials like Mica, Glass, or synthetic materials like plastics.
The different solid insulating materials are:
- Paper and Pressboards
- Fibrous materials (unimpregnated and impregnated)
- Impregnated coating, filling, and bonding materials
- Resins (Polymers)
- Natural and synthetic rubbers
- Inorganic Insulating Materials:
c) Mica and Micanite
- Composite Materials
1. Paper and pressboard
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These are produced from organic, synthetic fibres, and inorganic fibres (glass, asbestos). The manufacture of paper for use as an insulant requires cellulose fibres of a high degree of chemical purity and possessing high mechanical strength. Cellulose is a high molecular substance with formula (C6H10O5)n, where n is a very large number. For the manufacture of paper used as insulating material, wood fibres constitute the main ingredient. Generally, soft wood fibres are used. Paper and card board made from alkaline cellulose is stronger mechanically and more heat resistant, which is extremely important in engineering applications. Paper is sometimes made from raw materials other than wood, for example, rags, cotton, linen and various types of hemp. For paper to be at its best for most applications, it should have a certain minimum moisture content. Paper being hygroscopic, is dried and impregnated with impregnants. The common impregnants are mineral oil and vegetable oils.
These are thick, denser and less flexible in comparison to paper. It is used for making slot wedges, and liners for stator and rotor core stacks, separators in transformer windings, and slot lining.
The paper and press-boards are used for winding and cable conductor insulation, primary dielectrics in capacitors, backing for mica insulation, slot insulation of electrical machines, transformer insulation, etc.
2. Fibrous Materials
Some of the important fibres available today include cotton, silk, wool, jute, viscous rayons, acetate, nylon, Terylene, Teflon, fibre glass, and many others. The advantages of these materials are the high mechanical strength, flexibility, cheapness and easy processing. The drawbacks include hygroscopicity and low electrical strength. The properties of these materials may be improved by impregnation. Vulcanized fibre paper treated with zinc chloride is used for bushings, washers, and low-voltage insulation.
3. Impregnated coating, filling, and bonding materials
These materials are produced from artificial and synthetic high molecular organic and organic-o-metallic compounds in the form of various resins and solid cellulose esters. They are also produced from vegetable oils and materials of petroleum origin. These materials have been classified as waxes, insulating varnishes, filling and bonding materials.
4. Resins (polymers)
Resins are-organic substances with a very high molecular weight. They are available in nature as well as artificially made. For an electrical insulating purpose, the natural resins these days have been replaced by synthetic resins. Synthetic resins are synthetic polymers sometimes called plastics and form an important group of insulating materials.
Resins can be classified on the basis of their behavior under heat, as thermoplastic resins and thermosetting resins.
The different resins are
- Natural resins
- Polyvinyl chloride (P.V.C)
- The Acrylic Resins (Polymethyl methacrylate)
- Polytetrafluoroethylene (Teflon)
- Polyamide Resins (Nylons)
- Resins derived from Cellulose
- Polyester Resins
- Epoxy Resins
- Phenolic Resins
- Silicon Resins
5. Epoxy Resins
Epoxy resins belong to the group of thermosetting insulating materials. Typical epoxy resins are obtained from the alkaline condensation of epichlorohydrin with dioxydiphenyl propane. Such polymers contain the reactive epoxy group. They possess excellent electrical and mechanical properties and are ideally suitable for casting of insulators, bushings etc for high voltages. They are also used to produce laminated and insulating varnishes. Dielectric constant and loss tangents of such materials are 2.5-3.8 and 0.003-0.03, respectively, a typical value of resistivity being 1013ohm-cm. The commercial epoxy resins include Araldite, Debeckote, Epikot, etc.
Epoxy resins are surely superior to all other plastics and resins. There is hardly any industry in which these resins are not used. They find applications in electrical and electronic devices, in space satellites, in supersonic aircraft, in oil wells, stained glass window, on roads and bridges and in computers.
In switchgear, they are fully incorporated in the 5-30 kV range. In bushings, they are used above 15 kV. They are also used in transformers and busbars.
6. Inorganic Insulating Materials
Inorganic insulating materials are used when the application demands physical and dielectric stability at higher temperatures where the organic insulation fails to operate. They can withstand severe arcing conditions. Important inorganic insulating materials are Glass, Ceramics, Mica, Asbestos, and various metallic oxides and combination thereof.
The ceramics used as dielectric may be broadly described as clay containing porcelains, alumina, and ceramics talc containing steatites and titanates (barium titanate). Of the many different compositions of ceramics, those suitable for electric use are made from the combination involving silica (SiO2); alumina (Al2O3); magnesia (MgO); boron oxide (B2O3); Titania (TiO2); or zirconia (ZrO2).
The different classifications of ceramics are:
Porcelain is common ceramics for low and high voltage insulators. It contains clays, filler agents-Quartz (SiO2), a fluxing agent like feldspar and glazes producing agents like chalk, and dolomite.
Steatite is made from talc and small quantities of clay and feldspar. They are used for high-frequency applications.
Alumina is used in the furnace. It can withstand very high temperature.
Titanite ceramics are made from metal oxides and titanium dioxide. They have high dielectric constant and positive temperature coefficient of resistance. It used for photography pickups accelerometers and capacitors for electronic circuits.
Some of the applications of low permittivity ceramics (porcelain etc.) include suspension insulators for high voltage lines and pin insulators for low voltage lines, station insulators and bushings installed in transformers, oil breakers, disconnecting switches; and other porcelain parts such as rollers, parts of sockets, switches, plugs, fuse holders, antenna guy insulators and telephone insulators.