Other than active materials, for example, copper and grain oriented silicon steel, a large number of ferrous, non-ferrous and insulating materials are used in power transformers. Optimum utilization of these materials according to their electrical, mechanical, physical, chemical and thermal characteristics is important for using an appropriate insulating material. The insulation class of a power transformer is decided on the test level of which insulation is capable of withstanding.
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Transformer insulation is based on basic impulse insulation level together with voltage rating. Insulating media in up to 11KV transformer is mostly comprised of paper wrapped around the conductor in transformer coils, mineral oil and pressboard to insulate the coil from the ground. One basic requirement of all insulating materials is that they should be compatible with insulating oil and should not react with oil. The types of materials used for insulations are and their applications:
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1.Insulating Oil: Insulation oil plays a very important in transformer insulation system. In low voltage transformer, for example, transformers used in the range of 12-1000V or low power rating transformers there is no need of insulating oil in such transformers. Heat dissipation is very low in low voltage transformers. In 11KV transformers, insulating oil has the important rule of acting as an electrical insulation as well as a coolant to dissipate heat losses. Transformer oil is basically obtained by fractional distillation and subsequent treatment of crude petroleum. So Transformer oil act as liquid dielectric and coolant and it is placed in a tank in which core of the transformer is placed. 2.Insulating paper: Paper is a fabric made from vegetables fibers which are felted to form a web or sheet. The fibrous raw materials are obtained from plants including cotton, hemp, manila, straw, and coniferous trees. It attains a very high value of electric strength when emerged in oil under vacuum. ”Craft insulating paper of medium air permeability” is used in layer winding insulation, condenser core of oil impregnated bushing. Craft insulating paper of high air permeability” is used in covering over rectangular copper conductor and continuously transposed copper conductor. “Crepe Kraft paper” is used in covering over flexible copper cable insulation of winding lead. “Press paper” is used as backing paper for axial cooling duct.
3. Pressboard: Pressboard is a widely used insulating material for making a variety of components used in electrical, mechanical and thermal design of transformers. Pressboard id also made from vegetable fibers, whose cells contains much cellulose. The most difficult practical insulation in power transformers occur at the end of the windings and the lead outs from the windings. Pressboard molded components can be made to any required shape. Angle rings and caps are the widely used mouldings. There are many kind of pressboards use in high voltage transformers but “soft pressboard –laminated” is used in 11KV transformers as a block washer, terminal gear cleat and support and spacer etc. “Pressboard moulding from wet sheet or wet wood pulp” is used in angle ring, cap, sector, snouts, square tube, lead out, for insulating ends of winding, insulation between windings and numerous other applications.
4. Wood: Wood based laminates are manufactured from selected veneers obtained from various timbers. The veneers are dried and partially or fully impregnated with natural phenomenon. The areas which required higher mechanical and lower electric strength, densities laminated wood is used for making a variety of insulation components like coil clamping ring, cleat, support, core and yoke etc.
5. Insulated copper conductor for winding: Different type of insulated copper conductor windings are used in power transformers for example paper covered rectangular copper conductor, twin paper covered rectangular copper conductor bunched together, paper covered continuously transposed copper conductor, twin transposed copper conductor bunched together, twin rectangular copper bunched together and provided with a common paper strip between the two conductors and epoxy coated continuously transposed conductor. These are used to the winding space factor and mechanical strength of windings.” paper covered rectangular copper conductor” is used for making different kind of windings. “Paper covered standard copper cable” is used for making lead and terminal. “Crepe paper covered flexible copper cable” is used for making lead and terminal required to be bent to a small radius. “PVC insulated copper cable-single and multi core” is used to control wiring in marshalling box, nitrogen, sealing system.
7.Insulating tape: Insulating tape is used for various taping purposes .For example cotton tape, cotton newar tape, glass woven tape, woven tape and phenol laminated paper base sheet. These tapes are used in taping, banding, core bolt insulation, places where required high strength and in banding of transformer cores.