Insulation Materials Used in Transformers

In this tutorial, we will discuss the insulation material of transformers. Transformers can have active or non-active materials as insulators. For example, in power transformers with copper and grain-oriented silicon steel, a large number of ferrous, non-ferrous, and insulating materials are used. Optimum utilization of these materials according to their electrical, mechanical, physical, chemical, and thermal characteristics is important for using an appropriate insulating material. Hence, the decision for the insulation class of a power transformer depends on the test level, which insulation is capable of withstanding.

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Transformer Insulation

Transformer insulation has a basic impulse insulation level along with a voltage rating. Insulating media in up to 11KV transformers mostly consists of paper wrap around the conductor in transformer coils, mineral oil, and a pressboard to insulate the coil from the ground. So, the one basic requirement of all insulating materials is that they should be compatible with insulating oil and should not react with oil.

Types of Transformer Insulation

There are different types of materials for insulation, and their applications are listed below:

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insulation materials used in transformers-min

1. Insulating Oil

Insulation oil plays a very important role in transformer insulation systems. In low-voltage transformers, for example, transformers with a range of 12-1000V or low power rating, there is no need for 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. We obtain the transformer oil by fractional distillation and subsequent treatment of crude petroleum. So transformer oil acts as a liquid dielectric and coolant for the core of the transformer.

2. Insulating Paper

Paper is a fabric made from vegetable fibers that are felted to form a web or sheet. The fibrous raw materials are obtainable from plants including cotton, hemp, manila, straw, and coniferous trees. It attains a very high value of electric strength when it emerges in oil under vacuum. ”Craft insulating paper of medium air permeability” is useful in layer winding insulation and condenser cores of oil-impregnated bushings. Craft insulating paper of high air permeability” is useful in covering rectangular copper conductors and continuously transposing copper conductors. The use of “Crepe Kraft paper” in covering over flexible copper cable insulation of winding leads. “Press paper” is useful as backing paper for axial cooling ducts.

3. Pressboard

This is an insulating material for making a variety of components in the electrical, mechanical, and thermal design of transformers. The pressboard also has vegetable fibers, whose cells contain a lot of cellulose. Hence, the most difficult practical insulation in power transformers occurs at the end of the windings and the lead-outs from the windings. The components of the pressboard mold are easily adaptable to any shape. Angle rings and caps are widely used moldings. There are many kinds of pressboards in high-voltage transformers, but “soft pressboard laminated” has applications in 11KV transformers as a block washer, terminal gear cleat, support, spacer, etc. “Pressboard molding from wet sheet or wet wood pulp” is useful in angle rings, caps, sectors, snouts, square tubes, lead-outs, insulating ends of windings, insulation between windings, and numerous other applications.

4. Wood

Wood-based laminates are manufactured from selected veneers obtained from various timbers. The process includes the drying of veneers and partial or full impregnation with natural phenomena. Thus, the areas that require higher mechanical and lower electric strengths and densities, laminated wood is useful for making a variety of insulation components like coil clamping rings, cleats, supports, cores, yokes, etc.

5. Insulated Copper Conductor for Winding

Different types of insulated copper conductor windings have uses in power transformers, for example, paper-covered rectangular copper conductors, twin paper-covered rectangular copper conductors bunched together, paper-covered continuously transposed copper conductors, twin transposed copper conductors bunched together, twin rectangular copper conductors bunched together, and those with a common paper strip between the two conductors and an epoxy-coated continuously transposed conductor. Their uses include the winding space factor and the mechanical strength of windings. “Paper-covered rectangular copper conductor” is useful for making different kinds of windings. “Paper-covered standard copper cable” is useful for making lead and terminals. “Crepe paper-covered flexible copper cable” is useful for making lead and terminals that require to be bent to a small radius. “PVC-insulated copper cable—single and multi-core” is useful to control wiring in the marshalling box, nitrogen, and sealing systems.

6. Insulating Tape

Insulating tapes are useful for various taping purposes. For example, cotton tape, cotton newar tape, glass woven tape, woven tape, and phenol laminated paper base sheets These tapes are helpful in taping, banding, core bolt insulation, places where the requirement is high strength, and in banding transformer cores.


In conclusion, this tutorial provides an in-depth overview of the insulating materials of power transformers. It covers a basic introduction along with different types of insulating materials. A few materials that we discuss as insulating materials are insulating oil, paper, copper conducting wires and tapes, along with wood and pressboard. Hopefully, this was helpful in expanding your knowledge.

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This concludes today’s article. If you face any issues or difficulties, let us know in the comment section below.

3 thoughts on “Insulation Materials Used in Transformers”

  1. Thanks for elaborating upon the insulation materials used in transformers. They materials are the key responsible ones for controlling the voltage levels and providing steady flow of current in an AC circuit.

  2. How about in winding coil?insulation is common problem now in our company how can you help us
    What is the best materials used to avoid insulation no good


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