Transformers are the first to encounter lightning and other high voltage surges. The transformer insulation has to withstand very high impulse voltages many times the power frequency operating voltages. The Applications of Power Transformers insulation is broadly divided into
Conductor or turn-to-turn insulation,
Low voltage coil-to-earth insulation,
High voltage coil-to-low voltage coil insulation, and
High voltage coil-to-ground insulation.
The low voltage coil-to-ground and the high voltage coil-to-low voltage coil insulations normally consist of solid tubes combined with liquid or gas filled spaces. The liquid or gas in the spaces help to remove the heat from the core and coil structure and also help to improve the insulation strengths. The inter-turn insulation is directly applied on the conductor as organic enamel in smaller rating transformers. In the large Applications of Power Transformers paper or glass tape is wrapped on the rectangular conductors. In the case of layer to layer, coil-to-coil and coil-toground insulations, Kraft paper is used in smaller transformers, whereas thick radial spacers made ofpressboard, glass fabric, or porcelain are used in the case of higher rating transformers.
Much of the insulation system used in oil-filled transformers consists of oil-impregnated pressboard placed around the low and high voltage windings and supported by the iron core. The thin oil layers within the multilayer pressboard structure result in high dielectric strength, which increases with pressboard density: In addition, the oil passing between the individual pressboard components of insulation structures allows the required heat dissipation in order to achieve necessary electrical and thermal performance.
Conductor insulation is usually paper. Paper by itself is not mechanically strong and dealing with copper conductors which are heavy, a much higher quantity of insulation in the form of pressboard will be required. Over the years, major developments have taken place to improve the quality of insulating materials. The most commonly used insulating board is made of wood or a mixture of wood and cotton. By using the relatively long cotton fibres the oil impregnation of the wood materials becomes easier to accomplish. Other insulation systems at lower voltage ratings use cast resin as the insulation and air as the coolant. Higher temperature systems, on the other hand, employ synthetic fluids which include silicones and these are usually used in association with high temperature solid insulating materials, such as Nomex with resin or polyester glasses. These materials are much more expensive than paper or pressboard.
Gas insulated power transformers use sheet aluminium conductors for windings, a polymer film (Mylar) for turn-to-turn insulation, self-contained annular cooling ducts containing circulating cooling gas to cool the windings, and compressed SF6 gas which insulates all the major gaps.
Transformer oil provides the required dielectric strength and insulation and also cools the transformer by circulating itself through the core and the coil structure. The transformer oil, therefore, should be in the liquid state over the complete operating range of temperatures between – 40°C and + 50°C. The oil gets oxidized when exposed to oxygen at high temperatures, and the oxidation results in the formation of peroxides,- water, organic acids and sludge. These products cause chemical deterioration of the paper insulation and the metal parts of the Applications of Power Transformers. Sludge being heavy, reduces the heat transfer capabilities of the oil, and also forms as a heat insulating layer on the coil structure, the core and the tank walls. In present-day transformers the effects of oxidation are minimized by designing them such that access to oxygen itself is limited. This is done by the use of
(a) sealed transformers,
(b) by filling the air space with nitrogen gas, and
(c) providing oxygen absorbers like activated clay or alumina.
When an arc discharge occurs inside a transformer, the oil decomposition occurs. The decomposition products consist of hydrogen and gaseous hydrocarbons which may lead to explosion. And hence, oil insulated transformers are seldom used inside buildings or other hazardous locations like mines. Under such conditions dry type or sulphur hexafluoride (SF6) gas filled transformers are used.
Dry Type Transformers
Apart form oil filled distribution transformers, dry type transformers have become important during the last few years. This type of Applications of Power Transformers are mainly used in areas where use of oil filled transformers is risky, such as in apartment complexes, cinema halls, water protection areas and some industrial enterprises.
Insulation of this type of Applications of Power Transformers is now a days done using prepregs. The advantages of these materials are good directric strength of the coil, easier production methods and elimination of expensive vacuum impregnation of the low voltage coils. Prepregs are mainly manufactured using the following baking materials: glass fibres, Nomex, aramide papers and multi-layers made of nonwoven polyester and polyester films. Esterimide resins and epoxy resins are used as resin systems. It is important that the prepregs are free from any cracks. Prepregs have excellent adhesive capability, good ageing as well as good storage properties.
However, change from vacuum impregnation technology for low voltage coils to prepreg technology is yet to become popular because of its high cost.