| CLASSIFICATION OF SOLID INSULATING MATERIALS
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A vast number of solid insulating materials are used in electrical power engineering. With the invention of modem insulating materials, many earlier conventional materials have been discarded, especially in case of electrical machines, capacitors and power cables. From the usage point of view, the solid insulating materials can be divided into the following three broad categories: |
Moulding materials: These are used for providing mechanically rigid forms of insulation, for example, insulators and bushings etc. Such materials are required to have good electrical insulating properties besides having high mechanical strength. These are usually made out of ceramics, glass (toughened glass), fibre glass reinforced plastics and epoxyresins.
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Jacketing materials: Polymers have been found suitable for providing insulating jackets to the conductors. For example, polyethylene (PE), polyvinylchloride (PVC), natural and synthetic rubber and paper used in power cables, capacitors and transformers. Mica and fibre glass based plastic tapes are used in electrical machines.
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Filling materials: Beside oils, wax based draining land non draining impregnating compounds of different types are used to impregnate paper in power cables, transformers, capacitors, and instrument transformers.
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Insulating mechanical support: In the form of plates, pipes and ledges insulating supports are required in transformers, GIS, circuit breakers and isolators. The products, such as press boards, hard paper (thin paper laminates), wood (yellow teak) are used in transformers. Ebonite, also known as vulcanite, is a form of cross linked rubber containing large proportion of sulphur, bakelite, a hard synthetic material, and acrylic resin plates are used for insulating supports in other equipment.
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However, the most common method of classifying the solid insulating materials is based upon their chemical compositions, distinguished mainly between inorganic and organic materials.
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| Among the inorganic materials; ceramics, glass, fibre glass, enamel, mica and asbestos are important and have found their wide application as dielectrics in the order of the mentioned sequence. They distinguish themselves in their unique ability to withstand high temperatures in addition to their being highly chemical resistant. There is practically negligible sign of ageing in these materials. However, it is difficult to machine or process them. These materials are basically inhomogeneous both microscopically as well as macroscopically. |
| Solid organic materials used in electrical engineering are: paper, wood, wax, leather, cotton besides a number of natural and synthetic resins, rubbers and plastics, also known as polymers. Wood being so hygroscopic, is rarely used as such, but wooden poles supporting overhead cables and distribution lines are being widely used in North America. |
The polymeric organic insulating materials used in electrical engineering have a very high molecular weight and consist of two or more polymeric compounds of several structural units normally bound together by covalent bonds. The individual structural units may consist of single atoms or may be molecular in nature, which repeat in a regular order. |
A polymer can be obtained by the reaction of compounds having at least two reactive functional groups which can react under suitable conditions. These chemical compounds are known as 'monomers'. There can be one or a mixture of reacting monomers and in the latter case, the resulting material is called 'copolymer'. |
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